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RFC 3830 - MIKEY: Multimedia Internet KEYing


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Network Working Group                                           J. Arkko
Request for Comments: 3830                                    E. Carrara
Category: Standards Track                                    F. Lindholm
                                                              M. Naslund
                                                              K. Norrman
                                                       Ericsson Research
                                                             August 2004

                   MIKEY: Multimedia Internet KEYing

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes a key management scheme that can be used for
   real-time applications (both for peer-to-peer communication and group
   communication).  In particular, its use to support the Secure Real-
   time Transport Protocol is described in detail.

   Security protocols for real-time multimedia applications have started
   to appear.  This has brought forward the need for a key management
   solution to support these protocols.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.  Existing Solutions . . . . . . . . . . . . . . . . . . .  4
       1.2.  Notational Conventions . . . . . . . . . . . . . . . . .  4
       1.3.  Definitions. . . . . . . . . . . . . . . . . . . . . . .  4
       1.4.  Abbreviations. . . . . . . . . . . . . . . . . . . . . .  6
       1.5.  Outline. . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Basic Overview . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.  Scenarios. . . . . . . . . . . . . . . . . . . . . . . .  7
       2.2.  Design Goals . . . . . . . . . . . . . . . . . . . . . .  8
       2.3.  System Overview. . . . . . . . . . . . . . . . . . . . .  8
       2.4.  Relation to GKMARCH. . . . . . . . . . . . . . . . . . . 10
   3.  Basic Key Transport and Exchange Methods . . . . . . . . . . . 10
       3.1.  Pre-shared Key . . . . . . . . . . . . . . . . . . . . . 12
       3.2.  Public-Key Encryption. . . . . . . . . . . . . . . . . . 13
       3.3.  Diffie-Hellman Key Exchange. . . . . . . . . . . . . . . 14
   4.  Selected Key Management Functions. . . . . . . . . . . . . . . 15
       4.1.  Key Calculation. . . . . . . . . . . . . . . . . . . . . 16
             4.1.1.  Assumptions. . . . . . . . . . . . . . . . . . . 16
             4.1.2.  Default PRF Description. . . . . . . . . . . . . 17
             4.1.3.  Generating keys from TGK . . . . . . . . . . . . 18
             4.1.4.  Generating keys for MIKEY Messages from
                     an Envelope/Pre-Shared Key . . . . . . . . . . . 19
       4.2 Pre-defined Transforms and Timestamp Formats . . . . . . . 19
             4.2.1.  Hash Functions . . . . . . . . . . . . . . . . . 19
             4.2.2.  Pseudo-Random Number Generator and PRF . . . . . 20
             4.2.3.  Key Data Transport Encryption. . . . . . . . . . 20
             4.2.4.  MAC and Verification Message Function. . . . . . 21
             4.2.5.  Envelope Key Encryption. . . . . . . . . . . . . 21
             4.2.6.  Digital Signatures . . . . . . . . . . . . . . . 21
             4.2.7.  Diffie-Hellman Groups. . . . . . . . . . . . . . 21
             4.2.8.  Timestamps . . . . . . . . . . . . . . . . . . . 21
             4.2.9.  Adding New Parameters to MIKEY . . . . . . . . . 22
       4.3.  Certificates, Policies and Authorization . . . . . . . . 22
             4.3.1.  Certificate Handling . . . . . . . . . . . . . . 22
             4.3.2.  Authorization. . . . . . . . . . . . . . . . . . 23
             4.3.3.  Data Policies. . . . . . . . . . . . . . . . . . 24
       4.4.  Retrieving the Data SA . . . . . . . . . . . . . . . . . 24
       4.5.  TGK Re-Keying and CSB Updating . . . . . . . . . . . . . 25
   5.  Behavior and Message Handling. . . . . . . . . . . . . . . . . 26
       5.1.  General. . . . . . . . . . . . . . . . . . . . . . . . . 26
             5.1.1.  Capability Discovery . . . . . . . . . . . . . . 26
             5.1.2.  Error Handling . . . . . . . . . . . . . . . . . 27
       5.2.  Creating a Message . . . . . . . . . . . . . . . . . . . 28
       5.3.  Parsing a Message. . . . . . . . . . . . . . . . . . . . 29
       5.4.  Replay Handling and Timestamp Usage. . . . . . . . . . . 30
   6.  Payload Encoding . . . . . . . . . . . . . . . . . . . . . . . 32

       6.1.  Common Header Payload (HDR). . . . . . . . . . . . . . . 32
             6.1.1.  SRTP ID. . . . . . . . . . . . . . . . . . . . . 35
       6.2.  Key Data Transport Payload (KEMAC) . . . . . . . . . . . 36
       6.3.  Envelope Data Payload (PKE). . . . . . . . . . . . . . . 37
       6.4.  DH Data Payload (DH) . . . . . . . . . . . . . . . . . . 38
       6.5.  Signature Payload (SIGN) . . . . . . . . . . . . . . . . 39
       6.6.  Timestamp Payload (T). . . . . . . . . . . . . . . . . . 39
       6.7.  ID Payload (ID) / Certificate Payload (CERT) . . . . . . 40
       6.8.  Cert Hash Payload (CHASH). . . . . . . . . . . . . . . . 41
       6.9.  Ver msg payload (V). . . . . . . . . . . . . . . . . . . 42
       6.10. Security Policy Payload (SP) . . . . . . . . . . . . . . 42
             6.10.1. SRTP Policy. . . . . . . . . . . . . . . . . . . 44
       6.11. RAND Payload (RAND). . . . . . . . . . . . . . . . . . . 45
       6.12. Error Payload (ERR). . . . . . . . . . . . . . . . . . . 46
       6.13. Key Data Sub-Payload . . . . . . . . . . . . . . . . . . 46
       6.14. Key Validity Data. . . . . . . . . . . . . . . . . . . . 48
       6.15. General Extension Payload. . . . . . . . . . . . . . . . 50
   7.  Transport Protocols. . . . . . . . . . . . . . . . . . . . . . 50
   8.  Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
       8.1.  Simple One-to-Many . . . . . . . . . . . . . . . . . . . 51
       8.2.  Small-Size Interactive Group . . . . . . . . . . . . . . 51
   9.  Security Considerations. . . . . . . . . . . . . . . . . . . . 52
       9.1.  General. . . . . . . . . . . . . . . . . . . . . . . . . 52
       9.2.  Key Lifetime . . . . . . . . . . . . . . . . . . . . . . 54
       9.3.  Timestamps . . . . . . . . . . . . . . . . . . . . . . . 55
       9.4.  Identity Protection. . . . . . . . . . . . . . . . . . . 55
       9.5.  Denial of Service. . . . . . . . . . . . . . . . . . . . 56
       9.6.  Session Establishment. . . . . . . . . . . . . . . . . . 56
   10. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 57
       10.1. MIME Registration. . . . . . . . . . . . . . . . . . . . 59
   11. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 59
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 60
       12.1. Normative References . . . . . . . . . . . . . . . . . . 60
       12.2. Informative References . . . . . . . . . . . . . . . . . 61
   Appendix A. - MIKEY - SRTP Relation. . . . . . . . . . . . . . . . 63
   Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 65
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 66

1.  Introduction

   There has recently been work to define a security protocol for the
   protection of real-time applications running over RTP, [SRTP].
   However, a security protocol needs a key management solution to
   exchange keys and related security parameters.  There are some
   fundamental properties that such a key management scheme has to
   fulfill to serve streaming and real-time applications (such as
   unicast and multicast), particularly in heterogeneous (mix of wired
   and wireless) networks.

   This document describes a key management solution that addresses
   multimedia scenarios (e.g., SIP [SIP] calls and RTSP [RTSP]
   sessions).  The focus is on how to set up key management for secure
   multimedia sessions such that requirements in a heterogeneous
   environment are fulfilled.

1.1.  Existing Solutions

   There is work done in the IETF to develop key management schemes.
   For example, IKE [IKE] is a widely accepted unicast scheme for IPsec,
   and the MSEC WG is developing other schemes to address group
   communication [GDOI, GSAKMP].  However, for reasons discussed below,
   there is a need for a scheme with lower latency, suitable for
   demanding cases such as real-time data over heterogeneous networks
   and small interactive groups.

   An option in some cases might be to use [SDP], as SDP defines one
   field to transport keys, the "k=" field.  However, this field cannot
   be used for more general key management purposes, as it cannot be
   extended from the current definition.

1.2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119].

1.3.  Definitions

   (Data) Security Protocol: the security protocol used to protect the
   actual data traffic.  Examples of security protocols are IPsec and
   SRTP.

   Data Security Association (Data SA): information for the security
   protocol, including a TEK and a set of parameters/policies.

   Crypto Session (CS): uni- or bi-directional data stream(s), protected
   by a single instance of a security protocol.  For example, when SRTP
   is used, the Crypto Session will often contain two streams, an RTP
   stream and the corresponding RTCP, which are both protected by a
   single SRTP Cryptographic Context, i.e., they share key data and the
   bulk of security parameters in the SRTP Cryptographic Context
   (default behavior in [SRTP]).  In the case of IPsec, a Crypto Session
   would represent an instantiation of an IPsec SA.  A Crypto Session
   can be viewed as a Data SA (as defined in [GKMARCH]) and could
   therefore be mapped to other security protocols if necessary.

   Crypto Session Bundle (CSB): collection of one or more Crypto
   Sessions, which can have common TGKs (see below) and security
   parameters.

   Crypto Session ID: unique identifier for the CS within a CSB.

   Crypto Session Bundle ID (CSB ID): unique identifier for the CSB.

   TEK Generation Key (TGK): a bit-string agreed upon by two or more
   parties, associated with CSB.  From the TGK, Traffic-encrypting Keys
   can then be generated without needing further communication.

   Traffic-Encrypting Key (TEK): the key used by the security protocol
   to protect the CS (this key may be used directly by the security
   protocol or may be used to derive further keys depending on the
   security protocol).  The TEKs are derived from the CSB's TGK.

   TGK re-keying: the process of re-negotiating/updating the TGK (and
   consequently future TEK(s)).

   Initiator: the initiator of the key management protocol, not
   necessarily the initiator of the communication.

   Responder: the responder in the key management protocol.

   Salting key: a random or pseudo-random (see [RAND, HAC]) string used
   to protect against some off-line pre-computation attacks on the
   underlying security protocol.

   PRF(k,x):  a keyed pseudo-random function (see [HAC]).
   E(k,m):    encryption of m with the key k.
   PKx:       the public key of x
   []         an optional piece of information
   {}         denotes zero or more occurrences
   ||         concatenation
   |          OR (selection operator)
   ^          exponentiation
   XOR        exclusive or

   Bit and byte ordering: throughout the document bits and bytes are
   indexed, as usual, from left to right, with the leftmost bits/bytes
   being the most significant.

1.4.  Abbreviations

   AES    Advanced Encryption Standard
   CM     Counter Mode (as defined in [SRTP])
   CS     Crypto Session
   CSB    Crypto Session Bundle
   DH     Diffie-Hellman
   DoS    Denial of Service
   MAC    Message Authentication Code
   MIKEY  Multimedia Internet KEYing
   PK     Public-Key
   PSK    Pre-Shared key
   RTP    Real-time Transport Protocol
   RTSP   Real Time Streaming Protocol
   SDP    Session Description Protocol
   SIP    Session Initiation Protocol
   SRTP   Secure RTP
   TEK    Traffic-encrypting key
   TGK    TEK Generation Key

1.5.  Outline

   Section 2 describes the basic scenarios and the design goals for
   which MIKEY is intended.  It also gives a brief overview of the
   entire solution and its relation to the group key management
   architecture [GKMARCH].

   The basic key transport/exchange mechanisms are explained in detail
   in Section 3.  The key derivation, and other general key management
   procedures are described in Section 4.

   Section 5 describes the expected behavior of the involved parties.
   This also includes message creation and parsing.

   All definitions of the payloads in MIKEY are described in Section 6.

   Section 7 deals with transport considerations, while Section 8
   focuses on how MIKEY is used in group scenarios.

   The Security Considerations section (Section 9), gives a deeper
   explanation of important security related topics.

2.  Basic Overview

2.1.  Scenarios

   MIKEY is mainly intended to be used for peer-to-peer, simple one-to-
   many, and small-size (interactive) groups.  One of the main
   multimedia scenarios considered when designing MIKEY has been the
   conversational multimedia scenario, where users may interact and
   communicate in real-time.  In these scenarios it can be expected that
   peers set up multimedia sessions between each other, where a
   multimedia session may consist of one or more secured multimedia
   streams (e.g., SRTP streams).

   peer-to-peer/         many-to-many           many-to-many
    simple one-to-many           (distributed)          (centralized)
              ++++        ++++          ++++     ++++           ++++
              |. |        |A |          |B |     |A |----   ----|B |
            --| ++++      |  |----------|  |     |  |    \ /    |  |
   ++++    /  ++|. |      ++++          ++++     ++++    (S)    ++++
   |A |---------| ++++       \          /                 |
   |  |    \    ++|B |        \        /                  |
   ++++     \-----|  |         \ ++++ /                  ++++
                  ++++          \|C |/                   |C |
                                 |  |                    |  |
                                 ++++                    ++++

   Figure 2.1: Examples of the four scenarios: peer-to-peer, simple
   one-to-many, many-to-many without a centralized server (also denoted
   as small interactive group), and many-to-many with a centralized
   server.

   We identify in the following some typical scenarios which involve the
   multimedia applications we are dealing with (see also Figure 2.1).

   a) peer-to-peer (unicast), e.g., a SIP-based [SIP] call between two
      parties, where it may be desirable that the security is either set
      up by mutual agreement or that each party sets up the security for
      its own outgoing streams.

   b) simple one-to-many (multicast), e.g., real-time presentations,
      where the sender is in charge of setting up the security.

   c) many-to-many, without a centralized control unit, e.g., for
      small-size interactive groups where each party may set up the
      security for its own outgoing media.  Two basic models may be used
      here.  In the first model, the Initiator of the group acts as the

      group server (and is the only one authorized to include new
      members).  In the second model, authorization information to
      include new members can be delegated to other participants.

   d) many-to-many, with a centralized control unit, e.g., for larger
      groups with some kind of Group Controller that sets up the
      security.

   The key management solutions may be different in the above scenarios.
   When designing MIKEY, the main focus has been on case a, b, and c.
   For scenario c, only the first model is covered by this document.

2.2.  Design Goals

   The key management protocol is designed to have the following
   characteristics:

   *  End-to-end security.  Only the participants involved in the
      communication have access to the generated key(s).

   *  Simplicity.

   *  Efficiency.  Designed to have:
      - low bandwidth consumption,
      - low computational workload,
      - small code size, and
      - minimal number of roundtrips.

   *  Tunneling.  Possibility to "tunnel"/integrate MIKEY in session
      establishment protocols (e.g., SDP and RTSP).

   *  Independence from any specific security functionality of the
      underlying transport.

2.3.  System Overview

   One objective of MIKEY is to produce a Data SA for the security
   protocol, including a traffic-encrypting key (TEK), which is derived
   from a TEK Generation Key (TGK), and used as input for the security
   protocol.

   MIKEY supports the possibility of establishing keys and parameters
   for more than one security protocol (or for several instances of the
   same security protocol) at the same time.  The concept of Crypto
   Session Bundle (CSB) is used to denote a collection of one or more
   Crypto Sessions that can have common TGK and security parameters, but
   which obtain distinct TEKs from MIKEY.

   The procedure of setting up a CSB and creating a TEK (and Data SA),
   is done in accordance with Figure 2.2:

   1. A set of security parameters and TGK(s) are agreed upon for the
      Crypto Session Bundle (this is done by one of the three
      alternative key transport/exchange mechanisms, see Section 3).

   2. The TGK(s) is used to derive (in a cryptographically secure way) a
      TEK for each Crypto Session.

   3. The TEK, together with the security protocol parameters, represent
      the Data SA, which is used as the input to the security protocol.

        +-----------------+
        |       CSB       |
        |  Key transport  |                      (see Section 3)
        |    /exchange    |
        +-----------------+
                 |      :
                 | TGK  :
                 v      :
           +----------+ :
   CS ID ->|   TEK    | : Security protocol      (see Section 4)
           |derivation| : parameters (policies)
           +----------+ :
              TEK |     :
                  v     v
                  Data SA
                    |
                    v
           +-------------------+
           |  Crypto Session   |
           |(Security Protocol)|
           +-------------------+

   Figure 2.2: Overview of MIKEY key management procedure.

   The security protocol can then either use the TEK directly, or, if
   supported, derive further session keys from the TEK (e.g., see SRTP
   [SRTP]).  It is however up to the security protocol to define how the
   TEK is used.

   MIKEY can be used to update TEKs and the Crypto Sessions in a current
   Crypto Session Bundle (see Section 4.5).  This is done by executing
   the transport/exchange phase once again to obtain a new TGK (and
   consequently derive new TEKs) or to update some other specific CS
   parameters.

2.4.  Relation to GKMARCH

   The Group key management architecture (GKMARCH) [GKMARCH] describes a
   general architecture for group key management protocols.  MIKEY is a
   part of this architecture, and can be used as a so-called
   Registration protocol.  The main entities involved in the
   architecture are the group controller/key server (GCKS), the
   receiver(s), and the sender(s).

   In MIKEY, the sender could act as GCKS and push keys down to the
   receiver(s).

   Note that, for example, in a SIP-initiated call, the sender may also
   be a receiver.  As MIKEY addresses small interactive groups, a member
   may dynamically change between being a sender and receiver (or being
   both simultaneously).

3.  Basic Key Transport and Exchange Methods

   The following sub-sections define three different methods of
   transporting/establishing a TGK: with the use of a pre-shared key,
   public-key encryption, and Diffie-Hellman (DH) key exchange.  In the
   following, we assume unicast communication for simplicity.  In
   addition to the TGK, a random "nonce", denoted RAND, is also
   transported.  In all three cases, the TGK and RAND values are then
   used to derive TEKs as described in Section 4.1.3.  A timestamp is
   also sent to avoid replay attacks (see Section 5.4).

   The pre-shared key method and the public-key method are both based on
   key transport mechanisms, where the actual TGK is pushed (securely)
   to the recipient(s).  In the Diffie-Hellman method, the actual TGK is
   instead derived from the Diffie-Hellman values exchanged between the
   peers.

   The pre-shared case is, by far, the most efficient way to handle the
   key transport due to the use of symmetric cryptography only.  This
   approach also has the advantage that only a small amount of data has
   to be exchanged.  Of course, the problematic issue is scalability as
   it is not always feasible to share individual keys with a large group
   of peers.  Therefore, this case mainly addresses scenarios such as
   server-to-client and also those cases where the public-key modes have
   already been used, thus allowing for the "cache" of a symmetric key
   (see below and Section 3.2).

   Public-key cryptography can be used to create a scalable system.  A
   disadvantage with this approach is that it is more resource consuming
   than the pre-shared key approach.  Another disadvantage is that in
   most cases, a PKI (Public Key Infrastructure) is needed to handle the

   distribution of public keys.  Of course, it is possible to use public
   keys as pre-shared keys (e.g., by using self-signed certificates).
   It should also be noted that, as mentioned above, this method may be
   used to establish a "cached" symmetric key that later can be used to
   establish subsequent TGKs by using the pre-shared key method (hence,
   the subsequent request can be executed more efficiently).

   In general, the Diffie-Hellman (DH) key agreement method has a higher
   resource consumption (both computationally and in bandwidth) than the
   previous ones, and needs certificates as in the public-key case.
   However, it has the advantage of providing perfect forward secrecy
   (PFS) and flexibility by allowing implementation in several different
   finite groups.

   Note that by using the DH method, the two involved parties will
   generate a unique unpredictable random key.  Therefore, it is not
   possible to use this DH method to establish a group TEK (as the
   different parties in the group would end up with different TEKs).  It
   is not the intention of the DH method to work in this scenario, but
   to be a good alternative in the special peer-to-peer case.

   The following general notation is used:

   HDR:  The general MIKEY header, which includes MIKEY CSB related data
   (e.g., CSB ID) and information mapping to the specific security
   protocol used.  See Section 6.1 for payload definition.

   T:    The timestamp, used mainly to prevent replay attacks.  See
   Section 6.6 for payload definition and also Section 5.4 for other
   timestamp related information.

   IDx:  The identity of entity x (IDi=Initiator, IDr=Responder).  See
   Section 6.7 for payload definition.

   RAND: Random/pseudo-random byte-string, which is always included in
   the first message from the Initiator.  RAND is used as a freshness
   value for the key generation.  It is not included in update messages
   of a CSB.  See Section 6.11 for payload definition.  For randomness
   recommendations for security, see [RAND].

   SP:   The security policies for the data security protocol.  See
   Section 6.10 for payload definition.

3.1.  Pre-shared key

   In this method, the pre-shared secret key, s, is used to derive key
   material for both the encryption (encr_key) and the integrity
   protection (auth_key) of the MIKEY messages, as described in Section
   4.1.4.  The encryption and authentication transforms are described in
   Section 4.2.

   Initiator                                   Responder

      I_MESSAGE =
      HDR, T, RAND, [IDi],[IDr],
           {SP}, KEMAC                --->
                                                  R_MESSAGE =
                                     [<---]       HDR, T, [IDr], V

   The main objective of the Initiator's message (I_MESSAGE) is to
   transport one or more TGKs (carried into KEMAC) and a set of security
   parameters (SPs) to the Responder in a secure manner.  As the
   verification message from the Responder is optional, the Initiator
   indicates in the HDR whether it requires a verification message or
   not from the Responder.

   KEMAC = E(encr_key, {TGK}) || MAC

   The KEMAC payload contains a set of encrypted sub-payloads and a MAC.
   Each sub-payload includes a TGK randomly and independently chosen by
   the Initiator (and other possible related parameters, e.g., the key
   lifetime).  The MAC is a Message Authentication Code covering the
   entire MIKEY message using the authentication key, auth_key.  See
   Section 6.2 for payload definition and Section 5.2 for an exact
   definition of the MAC calculation.

   The main objective of the verification message from the Responder is
   to obtain mutual authentication.  The verification message, V, is a
   MAC computed over the Responder's entire message, the timestamp (the
   same as the one that was included in the Initiator's message), and
   the two parties identities, using the authentication key.  See also
   Section 5.2 for the exact definition of the Verification MAC
   calculation and Section 6.9 for payload definition.

   The ID fields SHOULD be included, but they MAY be left out when it
   can be expected that the peer already knows the other party's ID
   (otherwise it cannot look up the pre-shared key).  For example, this
   could be the case if the ID is extracted from SIP.

   It is MANDATORY to implement this method.

3.2.  Public-key encryption

   Initiator                                        Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi|CERTi], [IDr], {SP},
       KEMAC, [CHASH], PKE, SIGNi         --->
                                                   R_MESSAGE =
                                         [<---]    HDR, T, [IDr], V

   As in the previous case, the main objective of the Initiator's
   message is to transport one or more TGKs and a set of security
   parameters to the Responder in a secure manner.  This is done using
   an envelope approach where the TGKs are encrypted (and integrity
   protected) with keys derived from a randomly/pseudo-randomly chosen
   "envelope key".  The envelope key is sent to the Responder encrypted
   with the public key of the Responder.

   The PKE contains the encrypted envelope key: PKE = E(PKr, env_key).
   It is encrypted using the Responder's public key (PKr).  If the
   Responder possesses several public keys, the Initiator can indicate
   the key used in the CHASH payload (see Section 6.8).

   The KEMAC contains a set of encrypted sub-payloads and a MAC:

   KEMAC = E(encr_key, IDi || {TGK}) || MAC

   The first payload (IDi) in KEMAC is the identity of the Initiator
   (not a certificate, but generally the same ID as the one specified in
   the certificate).  Each of the following payloads (TGK) includes a
   TGK randomly and independently chosen by the Initiator (and possible
   other related parameters, e.g., the key lifetime).  The encrypted
   part is then followed by a MAC, which is calculated over the KEMAC
   payload.  The encr_key and the auth_key are derived from the envelope
   key, env_key, as specified in Section 4.1.4.  See also Section 6.2
   for payload definition.

   The SIGNi is a signature covering the entire MIKEY message, using the
   Initiator's signature key (see also Section 5.2 for the exact
   definition).

   The main objective of the verification message from the Responder is
   to obtain mutual authentication.  As the verification message V from
   the Responder is optional, the Initiator indicates in the HDR whether
   it requires a verification message or not from the Responder.  V is
   calculated in the same way as in the pre-shared key mode (see also
   Section 5.2 for the exact definition).  See Section 6.9 for payload
   definition.

   Note that there will be one encrypted IDi and possibly also one
   unencrypted IDi.  The encrypted one is used together with the MAC as
   a countermeasure for certain man-in-the-middle attacks, while the
   unencrypted one is always useful for the Responder to immediately
   identify the Initiator.  The encrypted IDi MUST always be verified to
   be equal with the expected IDi.

   It is possible to cache the envelope key, so that it can be used as a
   pre-shared key.  It is not recommended for this key to be cached
   indefinitely (however it is up to the local policy to decide this).
   This function may be very convenient during the lifetime of a CSB, if
   a new crypto session needs to be added (or an expired one removed).
   Then, the pre-shared key can be used, instead of the public keys (see
   also Section 4.5).  If the Initiator indicates that the envelope key
   should be cached, the key is at least to be cached during the
   lifetime of the entire CSB.

   The cleartext ID fields and certificate SHOULD be included, but they
   MAY be left out when it can be expected that the peer already knows
   the other party's ID, or can obtain the certificate in some other
   manner.  For example, this could be the case if the ID is extracted
   from SIP.

   For certificate handling, authorization, and policies, see Section
   4.3.

   It is MANDATORY to implement this method.

3.3.  Diffie-Hellman key exchange

   For a fixed, agreed upon, cyclic group, (G,*), we let g denote a
   generator for this group.  Choices for the parameters are given in
   Section 4.2.7.  The other transforms below are described in Section
   4.2.

   This method creates a DH-key, which is used as the TGK.  This method
   cannot be used to create group keys; it can only be used to create
   single peer-to-peer keys.  It is OPTIONAL to implement this method.

   Initiator                                          Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi|CERTi],[IDr]
        {SP}, DHi, SIGNi           --->
                                              R_MESSAGE =
                                   <---       HDR, T, [IDr|CERTr], IDi,
                                              DHr, DHi, SIGNr

   The main objective of the Initiator's message is to, in a secure way,
   provide the Responder with its DH value (DHi) g^(xi), where xi MUST
   be randomly/pseudo-randomly and secretly chosen, and a set of
   security protocol parameters.

   The SIGNi is a signature covering the Initiator's MIKEY message,
   I_MESSAGE, using the Initiator's signature key (see Section 5.2 for
   the exact definition).

   The main objective of the Responder's message is to, in a secure way,
   provide the Initiator with the Responder's value (DHr) g^(xr), where
   xr MUST be randomly/pseudo-randomly and secretly chosen.  The
   timestamp that is included in the answer is the same as the one
   included in the Initiator's message.

   The SIGNr is a signature covering the Responder's MIKEY message,
   R_MESSAGE, using the Responder's signature key (see Section 5.2 for
   the exact definition).

   The DH group parameters (e.g., the group G, the generator g) are
   chosen by the Initiator and signaled to the Responder.  Both parties
   calculate the TGK, g^(xi*xr) from the exchanged DH-values.

   Note that this approach does not require that the Initiator has to
   possess any of the Responder's certificates before the setup.
   Instead, it is sufficient that the Responder includes its signing
   certificate in the response.

   The ID fields and certificate SHOULD be included, but they MAY be
   left out when it can be expected that the peer already knows the
   other party's ID (or can obtain the certificate in some other
   manner).  For example, this could be the case if the ID is extracted
   from SIP.

   For certificate handling, authorization, and policies, see Section
   4.3.

4.  Selected Key Management Functions

   MIKEY manages symmetric keys in two main ways.  First, following key
   transport or key exchange of TGK(s) (and other parameters) as defined
   by any of the above three methods, MIKEY maintains a mapping between
   Data SA identifiers and Data SAs, where the identifiers used depend
   on the security protocol in question, see Section 4.4.  Thus, when
   the security protocol requests a Data SA, given such a Data SA
   identifier, an up-to-date Data SA will be obtained.  In particular,

   correct keying material, TEK(s), might need to be derived.  The
   derivation of TEK(s) (and other keying material) is done from a TGK
   and is described in Section 4.1.3.

   Second, for use within MIKEY itself, two key management procedures
   are needed:

   *  in the pre-shared case, deriving encryption and authentication key
      material from a single pre-shared key, and

   *  in the public key case, deriving similar key material from the
      transported envelope key.

   These two key derivation methods are specified in section 4.1.4.

   All the key derivation functionality mentioned above is based on a
   pseudo-random function, defined next.

4.1.  Key Calculation

   In the following, we define a general method (pseudo-random function)
   to derive one or more keys from a "master" key.  This method is used
   to derive:

   *  TEKs from a TGK and the RAND value,

   *  encryption, authentication, or salting key from a pre-shared/
      envelope key and the RAND value.

4.1.1.  Assumptions

   We assume that the following parameters are in place:

   csb_id : Crypto Session Bundle ID (32-bits unsigned integer)
   cs_id  : the Crypto Session ID (8-bits unsigned integer)
   RAND   : (at least) 128-bit (pseudo-)random bit-string sent by the
            Initiator in the initial exchange.

   The key derivation method has the following input parameters:

   inkey     : the input key to the derivation function
   inkey_len : the length in bits of the input key
   label     : a specific label, dependent on the type of the key to be
               derived, the RAND, and the session IDs
   outkey_len: desired length in bits of the output key.

   The key derivation method has the following output:

   outkey: the output key of desired length.

4.1.2.  Default PRF Description

   Let HMAC be the SHA-1 based message authentication function, see
   [HMAC] [SHA-1].  Similarly to [TLS], we define:

      P (s, label, m) = HMAC (s, A_1 || label) ||
                        HMAC (s, A_2 || label) || ...
                        HMAC (s, A_m || label)
   where

      A_0 = label,
      A_i = HMAC (s, A_(i-1))
      s is a key (defined below)
      m is a positive integer (also defined below).

   Values of label depend on the case in which the PRF is invoked, and
   values are specified in the following for the default PRF.  Thus,
   note that other PRFs later added to MIKEY MAY specify different input
   parameters.

   The following procedure describes a pseudo-random function, denoted
   PRF(inkey,label), based on the above P-function, applied to compute
   the output key, outkey:

   *  let n = inkey_len / 256, rounded up to the nearest integer if not
      already an integer

   *   split the inkey into n blocks, inkey = s_1 || ... || s_n, where *
      all s_i, except possibly s_n, are 256 bits each

   *  let m = outkey_len / 160, rounded up to the nearest integer if not
      already an integer

   (The values "256" and "160" equals half the input block-size and full
   output hash size, respectively, of the SHA-1 hash as part of the P-
   function.)

   Then, the output key, outkey, is obtained as the outkey_len most
   significant bits of

   PRF(inkey, label) = P(s_1, label, m) XOR P(s_2, label, m) XOR ...
                       XOR P(s_n, label, m).

4.1.3.  Generating keys from TGK

   In the following, we describe how keying material is derived from a
   TGK, thus assuming that a mapping of the Data SA identifier to the
   correct TGK has already been done according to Section 4.4.

   The key derivation method SHALL be executed using the above PRF with
   the following input parameters:

   inkey       : TGK
   inkey_len   : bit length of TGK
   label       : constant || cs_id || csb_id || RAND
   outkey_len  : bit length of the output key.

   The constant part of label depends on the type of key that is to be
   generated.  The constant 0x2AD01C64 is used to generate a TEK from
   TGK.  If the security protocol itself does not support key derivation
   for authentication and encryption from the TEK, separate
   authentication and encryption keys MAY be created directly for the
   security protocol by replacing 0x2AD01C64 with 0x1B5C7973 and
   0x15798CEF respectively, and outkey_len by the desired key-length(s)
   in each case.

   A salt key can be derived from the TGK as well, by using the constant
   0x39A2C14B.  Note that the Key data sub-payload (Section 6.13) can
   carry a salt.  The security protocol in need of the salt key SHALL
   use the salt key carried in the Key data sub-payload (in the pre-
   shared and public-key case), when present.  If that is not sent, then
   it is possible to derive the salt key via the key derivation
   function, as described above.

   The table below summarizes the constant values, used to generate keys
   from a TGK.

   constant    | derived key from the TGK
   --------------------------------------
   0x2AD01C64  | TEK
   0x1B5C7973  | authentication key
   0x15798CEF  | encryption key
   0x39A2C14B  | salting key

   Table 4.1.3: Constant values for the derivation of keys from TGK.

   Note that these 32-bit constant values (listed in the table above)
   are taken from the decimal digits of e (i.e., 2.7182...), where each
   constant consists of nine decimal digits (e.g., the first nine
   decimal digits 718281828 = 0x2AD01C64).  The strings of nine

   decimal digits are not chosen at random, but as consecutive "chunks"
   from the decimal digits of e.

4.1.4.  Generating keys for MIKEY messages from an envelope/pre-shared
        key

   This derivation is to form the symmetric encryption key (and salting
   key) for the encryption of the TGK in the pre-shared key and public
   key methods.  This is also used to derive the symmetric key used for
   the message authentication code in these messages, and the
   corresponding verification messages.  Hence, this derivation is
   needed in order to get different keys for the encryption and the MAC
   (and in the case of the pre-shared key, it will result in fresh key
   material for each new CSB).  The parameters for the default PRF are
   here:

   inkey      : the envelope key or the pre-shared key
   inkey_len  : the bit length of inkey
   label      : constant || 0xFF || csb_id || RAND
   outkey_len : desired bit length of the output key.

   The constant part of label depends on the type of key that is to be
   generated from an envelope/pre-shared key, as summarized below.

   constant    | derived key
   --------------------------------------
   0x150533E1  | encryption key
   0x2D22AC75  | authentication key
   0x29B88916  | salt key

   Table 4.1.4: Constant values for the derivation of keys from an
   envelope/pre-shared key.

4.2.  Pre-defined Transforms and Timestamp Formats

   This section identifies default transforms for MIKEY.  It is
   mandatory to implement and support the following transforms in the
   respective case.  New transforms can be added in the future (see
   Section 4.2.9 for further guidelines).

4.2.1.  Hash functions

   In MIKEY, it is MANDATORY to implement SHA-1 as the default hash
   function.

4.2.2.  Pseudo-random number generator and PRF

   A cryptographically secure random or pseudo-random number generator
   MUST be used for the generation of the keying material and nonces,
   e.g., [BMGL].  However, which one to use is implementation specific
   (as the choice will not affect the interoperability).

   For the key derivations, it is MANDATORY to implement the PRF
   specified in Section 4.1.  Other PRFs MAY be added by writing
   standard-track RFCs specifying the PRF constructions and their exact
   use within MIKEY.

4.2.3.  Key data transport encryption

   The default and mandatory-to-implement key transport encryption is
   AES in counter mode, as defined in [SRTP], using a 128-bit key as
   derived in Section 4.1.4, SRTP_PREFIX_LENGTH set to zero, and using
   the initialization vector

   IV = (S XOR (0x0000 || CSB ID || T)) || 0x0000,

   where S is a 112-bit salting key, also derived as in Section 4.1.4,
   and where T is the 64-bit timestamp sent by the Initiator.

   Note: this restricts the maximum size that can be encrypted to 2^23
   bits, which is still enough for all practical purposes [SRTP].

   The NULL encryption algorithm (i.e., no encryption) can be used (but
   implementation is OPTIONAL).  Note that this MUST NOT be used unless
   the underlying protocols can guarantee security.  The main reason for
   including this is for specific SIP scenarios, where SDP is protected
   end-to-end.  For this scenario, MIKEY MAY be used with the pre-shared
   key method, the NULL encryption, and NULL authentication algorithm
   (see Section 4.2.4) while relying on the security of SIP.  Use this
   option with caution!

   The AES key wrap function [AESKW] is included as an OPTIONAL
   implementation method.  If the key wrap function is used in the
   public key method, the NULL MAC is RECOMMENDED to be used, as the key
   wrap itself will provide integrity of the encrypted content (note
   though that the NULL MAC SHOULD NOT be used in the pre-shared key
   case, as the MAC in that case covers the entire message).  The 128-
   bit key and a 64-bit salt, S, are derived in accordance to Section
   4.1.4 and the key wrap IV is then set to S.

4.2.4.  MAC and Verification Message function

   MIKEY uses a 160-bit authentication tag, generated by HMAC with SHA-1
   as the MANDATORY implementation method, see [HMAC].  Authentication
   keys are derived according to Section 4.1.4.  Note that the
   authentication key size SHOULD be equal to the size of the hash
   function's output (e.g., for HMAC-SHA-1, a 160-bit authentication key
   is used) [HMAC].

   The NULL authentication algorithm (i.e., no MAC) can be used together
   with the NULL encryption algorithm (but implementation is OPTIONAL).
   Note that this MUST NOT be used unless the underlying protocols can
   guarantee security.  The main reason for including this is for
   specific SIP scenarios, where SDP is protected end-to-end.  For this
   scenario, MIKEY MAY be used with the pre-shared key method and the
   NULL encryption and authentication algorithm, while relying on the
   security of SIP.  Use this option with caution!

4.2.5.  Envelope Key encryption

   The public key encryption algorithm applied is defined by, and
   dependent on the certificate used. It is MANDATORY to support RSA
   PKCS#1, v1.5, and it is RECOMMENDED to also support RSA OAEP [PSS].

4.2.6.  Digital Signatures

   The signature algorithm applied is defined by, and dependent on the
   certificate used. It is MANDATORY to support RSA PKCS#1, v1.5, and it
   is RECOMMENDED to also support RSA PSS [PSS].

4.2.7.  Diffie-Hellman Groups

   The Diffie-Hellman key exchange, when supported, uses OAKLEY 5
   [OAKLEY] as a mandatory implementation.  Both OAKLEY 1 and OAKLEY 2
   MAY be used (but these are OPTIONAL implementations).

   See Section 4.2.9 for the guidelines on specifying a new DH Group to
   be used within MIKEY.

4.2.8.  Timestamps

   The timestamp is as defined in NTP [NTP], i.e., a 64-bit number in
   seconds relative to 0h on 1 January 1900.  An implementation MUST be
   aware of (and take into account) the fact that the counter will
   overflow approximately every 136th year.  It is RECOMMENDED that the
   time always be specified in UTC.

4.2.9.  Adding new parameters to MIKEY

   There are two different parameter sets that can be added to MIKEY.
   The first is a set of MIKEY transforms (needed for the exchange
   itself), and the second is the Data SAs.

   New transforms and parameters (including new policies) SHALL be added
   by registering with IANA (according to [RFC2434], see also Section
   10) a new number for the concerned payload, and also if necessary,
   documenting how the new transform/parameter is used.  Sometimes it
   might be enough to point to an already specified document for the
   usage, e.g., when adding a new, already standardized, hash function.

   In the case of adding a new DH group, the group MUST be specified in
   a companion standards-track RFC (it is RECOMMENDED that the specified
   group use the same format as used in [OAKLEY]).  A number can then be
   assigned by IANA for such a group to be used in MIKEY.

   When adding support for a new data security protocol, the following
   MUST be specified:

   *  A map sub-payload (see Section 6.1).  This is used to be able to
      map a crypto session to the right instance of the data security
      protocol and possibly also to provide individual parameters for
      each data security protocol.

   *  A policy payload, i.e., specification of parameters and supported
      values.

   *  General guidelines of usage.

4.3.  Certificates, Policies and Authorization

4.3.1.  Certificate handling

   Certificate handling may involve a number of additional tasks not
   shown here, and effect the inclusion of certain parts of the message
   (c.f. [X.509]).  However, the following observations can be made:

   *  The Initiator typically has to find the certificate of the
      Responder in order to send the first message.  If the Initiator
      does not already have the Responder's certificate, this may
      involve one or more roundtrips to a central directory agent.

   *  It will be possible for the Initiator to omit its own certificate
      and rely on the Responder getting this certificate using other
      means.  However, we only recommend doing this when it is
      reasonable to expect that the Responder has cached the certificate

      from a previous connection.  Otherwise accessing the certificate
      would mean additional roundtrips for the Responder as well.

   *  Verification of the certificates using Certificate Revocation
      Lists (CRLs) [X.509] or protocols such as OCSP [OCSP] may be
      necessary.  All parties in a MIKEY exchange should have a local
      policy which dictates whether such checks are made, how they are
      made, and how often they are made.  Note that performing the
      checks may imply additional messaging.

4.3.2.  Authorization

   In general, there are two different models for making authorization
   decisions for both the Initiator and the Responder, in the context of
   the applications targeted by MIKEY:

   *  Specific peer-to-peer configuration.  The user has configured the
      application to trust a specific peer.

      When pre-shared secrets are used, this is pretty much the only
      available scheme.  Typically, the configuration/entering of the
      pre-shared secret is taken to mean that authorization is implied.

      In some cases, one could also use this with public keys, e.g., if
      two peers exchange keys offline and configure them to be used for
      the purpose of running MIKEY.

   *  Trusted root.  The user accepts all peers that prove to have a
      certificate issued by a specific CA.  The granularity of
      authorization decisions is not very precise in this method.

      In order to make this method possible, all participants in the
      MIKEY protocol need to configure one or more trusted roots.  The
      participants also need to be capable of performing certificate
      chain validation, and possibly transfer more than a single
      certificate in the MIKEY messages (see also Section 6.7).

   In practice, a combination of both mentioned methods might be
   advantageous.  Also, the possibility for a user to explicitly exclude
   a specific peer (or sub-tree) in a trust chain might be needed.

   These authorization policies address the MIKEY scenarios a-c of
   Section 2.1, where the Initiator acts as the group owner and is also
   the only one that can invite others.  This implies that for each
   Responder, the distributed keys MUST NOT be re-distributed to other
   parties.

   In a many-to-many situation, where the group control functions are
   distributed (and/or where it is possible to delegate the group
   control function to others), a means of distributing authorization
   information about who may be added to the group MUST exist.  However,
   it is out of scope of this document to specify how this should be
   done.

   For any broader communication situation, an external authorization
   infrastructure may be used (following the assumptions of [GKMARCH]).

4.3.3.  Data Policies

   Included in the message exchange, policies (i.e., security
   parameters) for the Data security protocol are transmitted.  The
   policies are defined in a separate payload and are specific to the
   security protocol (see also Section 6.10).  Together with the keys,
   the validity period of these can also be specified.  For example,
   this can be done with an SPI (or SRTP MKI) or with an Interval (e.g.,
   a sequence number interval for SRTP), depending on the security
   protocol.

   New parameters can be added to a policy by documenting how they
   should be interpreted by MIKEY and by also registering new values in
   the appropriate name space in IANA.  If a completely new policy is
   needed, see Section 4.2.9 for guidelines.

4.4.  Retrieving the Data SA

   The retrieval of a Data SA will depend on the security protocol, as
   different security protocols will have different characteristics.
   When adding support for a security protocol to MIKEY, some interface
   of how the security protocol retrieves the Data SA from MIKEY MUST be
   specified (together with policies that can be negotiated).

   For SRTP, the SSRC (see [SRTP]) is one of the parameters used to
   retrieve the Data SA (while the MKI may be used to indicate the
   TGK/TEK used for the Data SA).  However, the SSRC is not sufficient.
   For the retrieval of the Data SA from MIKEY, it is RECOMMENDED that
   the MIKEY implementation support a lookup using destination network
   address and port together with SSRC.  Note that MIKEY does not send
   network addresses or ports.  One reason for this is that they may not
   be known in advance.  Also, if a NAT exists in-between, problems may
   arise.  When SIP or RTSP is used, the local view of the destination
   address and port can be obtained from either SIP or RTSP.  MIKEY can
   then use these addresses as the index for the Data SA lookup.

4.5.  TGK re-keying and CSB updating

   MIKEY provides a means of updating the CSB (e.g., transporting a new
   TGK/TEK or adding a new Crypto Session to the CSB).  The updating of
   the CSB is done by executing MIKEY again, for example, before a TEK
   expires, or when a new Crypto Session is added to the CSB.  Note that
   MIKEY does not provide re-keying in the GKMARCH sense, only updating
   of the keys by normal unicast messages.

   When MIKEY is executed again to update the CSB, it is not necessary
   to include certificates and other information that was provided in
   the first exchange, for example, all payloads that are static or
   optionally included may be left out (see Figure 4.1).

   The new message exchange MUST use the same CSB ID as the initial
   exchange, but MUST use a new timestamp.  A new RAND MUST NOT be
   included in the message exchange (the RAND will only have effect in
   the Initial exchange).  If desired, new Crypto Sessions are added in
   the update message.  Note that a MIKEY update message does not need
   to contain new keying material (e.g., new TGK).  In this case, the
   crypto session continues to use the previously established keying
   material, while updating the new information.

   As explained in Section 3.2, the envelope key can be "cached" as a
   pre-shared key (this is indicated by the Initiator in the first
   message sent).  If so, the update message is a pre-shared key message
   with the cached envelope key as the pre-shared key; it MUST NOT be a
   public key message.  If the public key message is used, but the
   envelope key is not cached, the Initiator MUST provide a new
   encrypted envelope key that can be used in the verification message.
   However, the Initiator does not need to provide any other keys.

   Figure 4.1 visualizes the update messages that can be sent, including
   the optional parts.  The main difference from the original message is
   that it is optional to include TGKs (or DH values in the DH method).
   Also see Section 3 for more details on the specific methods.

   By definition, a CSB can contain several CSs.  A problem that then
   might occur is to synchronize the TGK re-keying if an SPI (or similar
   functionality, e.g., MKI in [SRTP]) is not used.  It is therefore
   RECOMMENDED that an SPI or MKI be used, if more than one CS is
   present.

     Initiator                                       Responder

     Pre-shared key method:

     I_MESSAGE =
     HDR, T, [IDi], [IDr], {SP}, KEMAC   --->
                                                    R_MESSAGE =
                                        [<---]     HDR, T, [IDr], V

     Public key method:

     I_MESSAGE =
     HDR, T, [IDi|CERTi], [IDr], {SP},
          [KEMAC], [CHASH], PKE, SIGNi   --->
                                                 R_MESSAGE =
                                        [<---]   HDR, T, [IDr], V

     DH method:

     I_MESSAGE =
     HDR, T, [IDi|CERTi], [IDr], {SP},
          [DHi], SIGNi                   --->
                                               R_MESSAGE =
                                         <---  HDR, T, [IDr|CERTr], IDi,
                                                   [DHr, DHi], SIGNr

   Figure 4.1: Update messages.

   Note that for the DH method, if the Initiator includes the DHi
   payload, then the Responder MUST include DHr and DHi.  If the
   Initiator does not include DHi, the Responder MUST NOT include DHr or
   DHi.

5.  Behavior and message handling

   Each message that is sent by the Initiator or the Responder is built
   by a set of payloads.  This section describes how messages are
   created and also when they can be used.

5.1.  General

5.1.1.  Capability Discovery

   The Initiator indicates the security policy to be used (i.e., in
   terms of security protocol algorithms).  If the Responder does not
   support it (for some reason), the Responder can together with an
   error message (indicating that it does not support the parameters),
   send back its own capabilities (negotiation) to let the Initiator

   choose a common set of parameters.  This is done by including one or
   more security policy payloads in the error message sent in response
   (see Section 5.1.2.).  Multiple attributes can be provided in
   sequence in the response.  This is done to reduce the number of
   roundtrips as much as possible (i.e., in most cases, where the policy
   is accepted the first time, one roundtrip is enough).  If the
   Responder does not accept the offer, the Initiator must go out with a
   new MIKEY message.

   If the Responder is not willing/capable of providing security or the
   parties simply cannot agree, it is up to the parties' policies how to
   behave, for example, accepting or rejecting an insecure
   communication.

   Note that it is not the intention of this protocol to have a broad
   variety of options, as it is assumed that a denied offer should
   rarely occur.

   In the one-to-many and many-to-many scenarios using multicast
   communication, one issue is of course that there MUST be a common
   security policy for all the receivers.  This limits the possibility
   of negotiation.

5.1.2.  Error Handling

   Due to the key management protocol, all errors SHOULD be reported to
   the peer(s) by an error message.  The Initiator SHOULD therefore
   always be prepared to receive such a message from the Responder.

   If the Responder does not support the set of parameters suggested by
   the Initiator, the error message SHOULD include the supported
   parameters (see also Section 5.1.1).

   The error message is formed as:

   HDR, T, {ERR}, {SP}, [V|SIGNr]

   Note that if failure is due to the inability to authenticate the
   peer, the error message is OPTIONAL, and does not need to be
   authenticated.  It is up to local policy to determine how to treat
   this kind of message.  However, if in response to a failed
   authentication a signed error message is returned, this can be used
   for DoS purposes (against the Responder).  Similarly, an
   unauthenticated error message could be sent to the Initiator in order
   to fool the Initiator into tearing down the CSB.  It is highly
   RECOMMENDED that the local policy take this into consideration.
   Therefore, in case of authentication failure, one recommendation
   would be not to authenticate such an error message, and when

   receiving an unauthenticated error message view it only as a
   recommendation of what may have gone wrong.

5.2.  Creating a message

   To create a MIKEY message, a Common Header payload is first created.
   This payload is then followed, depending on the message type, by a
   set of information payloads (e.g., DH-value payload, Signature
   payload, Security Policy payload).  The defined payloads and the
   exact encoding of each payload are described in Section 6.

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  version      !  data type    ! next payload  !               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...            +
   ~                   Common Header...                            ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! next payload  !   Payload 1 ...                               !
   +-+-+-+-+-+-+-+-+                                               +
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                             :                                 :
   :                             :                                 :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! next payload  !   Payload x ...                               !
   +-+-+-+-+-+-+-+-+                                               +
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                   MAC/Signature                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5.1. MIKEY payload message example.  Note that the payloads
   are byte aligned and not 32-bit aligned.

   The process of generating a MIKEY message consists of the following
   steps:

   *  Create an initial MIKEY message starting with the Common Header
      payload.

   *  Concatenate necessary payloads of the MIKEY message (see the
      exchange definitions for payloads that may be included, and the
      recommended order).

   *  As a last step (for messages that must be authenticated, this also
      includes the verification message), create and concatenate the
      MAC/signature payload without the MAC/signature field filled in

      (if a Next payload field is included in this payload, it is set to
      Last payload).

   *  Calculate the MAC/signature over the entire MIKEY message, except
      the MAC/Signature field, and add the MAC/signature in the field.
      In the case of the verification message, the Identity_i ||
      Identity_r || Timestamp MUST directly follow the MIKEY message in
      the Verification MAC calculation.  Note that the added identities
      and timestamp are identical to those transported in the ID and T
      payloads.

   In the public key case, the Key data transport payload is generated
   by concatenating the IDi with the TGKs.  This is then encrypted and
   placed in the data field.  The MAC is calculated over the entire Key
   data transport payload except the MAC field.  Before calculating the
   MAC, the Next payload field is set to zero.

   Note that all messages from the Initiator MUST use a unique
   timestamp.  The Responder does not create a new timestamp, but uses
   the timestamp used by the Initiator.

5.3.  Parsing a message

   In general, parsing of a MIKEY message is done by extracting payload
   by payload and checking that no errors occur.  The exact procedure is
   implementation specific; however, for the Responder, it is
   RECOMMENDED that the following procedure be followed:

   *  Extract the Timestamp and check that it is within the allowable
      clock skew (if not, discard the message).  Also check the replay
      cache (Section 5.4) so that the message is not replayed (see
      Section 5.4).  If the message is replayed, discard it.

   *  Extract the ID and authentication algorithm (if not included,
      assume the default).

   *  Verify the MAC/signature.

   *  If the authentication is not successful, an Auth failure Error
      message MAY be sent to the Initiator.  The message is then
      discarded from further processing.  See also Section 5.1.2 for
      treatment of errors.

   *  If the authentication is successful, the message is processed and
      also added to the replay cache; processing is implementation
      specific.  Note also that only successfully authenticated messages
      are stored in the replay cache.

   *  If any unsupported parameters or errors occur during the
      processing, these MAY be reported to the Initiator by sending an
      error message.  The processing is then aborted.  The error message
      can also include payloads to describe the supported parameters.

   *  If the processing was successful and in case the Initiator
      requested it, a verification/response message MAY be created and
      sent to the Initiator.

5.4.  Replay handling and timestamp usage

   MIKEY does not use a challenge-response mechanism for replay
   handling; instead, timestamps are used.  This requires that the
   clocks are synchronized.  The required synchronization is dependent
   on the number of messages that can be cached (note though, that the
   replay cache only contains messages that have been successfully
   authenticated).  If we could assume an unlimited cache, the terminals
   would not need to be synchronized at all (as the cache could then
   contain all previous messages).  However, if there are restrictions
   on the size of the replay cache, the clocks will need to be
   synchronized to some extent.  In short, one can in general say that
   it is a tradeoff between the size of the replay cache and the
   required synchronization.

   Timestamp usage prevents replay attacks under the following
   assumptions:

   *  Each host has a clock which is at least "loosely synchronized"
      with the clocks of the other hosts.

   *  If the clocks are to be synchronized over the network, a secure
      network clock synchronization protocol SHOULD be used, e.g.,
      [ISO3].

   *  Each Responder utilizes a replay cache in order to remember the
      successfully authenticated messages presented within an allowable
      clock skew (which is set by the local policy).

   *  Replayed and outdated messages, for example, messages that can be
      found in the replay cache or which have an outdated timestamp are
      discarded and not processed.

   *  If the host loses track of the incoming requests (e.g., due to
      overload), it rejects all incoming requests until the clock skew
      interval has passed.

   In a client-server scenario, servers may encounter a high workload,
   especially if a replay cache is necessary.  However, servers that
   assume the role of MIKEY Initiators will not need to manage any
   significant replay cache as they will refuse all incoming messages
   that are not a response to a message previously sent by the server.

   In general, a client may not expect a very high load of incoming
   messages and may therefore allow the degree of looseness to be on the
   order of several minutes to hours.  If a (D)DoS attack is launched
   and the replay cache grows too large, MIKEY MAY dynamically decrease
   the looseness so that the replay cache becomes manageable.  However,
   note that such (D)DoS attacks can only be performed by peers that can
   authenticate themselves.  Hence, such an attack is very easy to trace
   and mitigate.

   The maximum number of messages that a client will need to cache may
   vary depending on the capacity of the client itself and the network.
   The number of expected messages should be taken into account.

   For example, assume that we can at most spend 6kB on a replay cache.
   Assume further that we need to store 30 bytes for each incoming
   authenticated message (the hash of the message is 20 bytes).  This
   implies that it is possible to cache approximately 204 messages.  If
   the expected number of messages per minute can be estimated, the
   clock skew can easily be calculated.  For example, in a SIP scenario
   where the client is expected, in the most extreme case, to receive 10
   calls per minute, the clock skew needed is then approximately 20
   minutes.  In a not so extreme setting, where one could expect an
   incoming call every 5th minute, this would result in a clock skew on
   the order of 16.5 hours (approx 1000 minutes).

   Consider a very extreme case, where the maximum number of incoming
   messages are assumed to be on the order of 120 messages per minute,
   and a requirement that the clock skew is on the order of 10 minutes,
   a 48kB replay cache would be required.

   Hence, one can note that the required clock skew will depend largely
   on the setting in which MIKEY is used.  One recommendation is to fix
   a size for the replay cache, allowing the clock skew to be large (the
   initial clock skew can be set depending on the application in which
   it is used).  As the replay cache grows, the clock skew is decreased
   depending on the percentage of the used replay cache.  Note that this
   is locally handled, which will not require interaction with the peer
   (even though it may indirectly effect the peer).  However, exactly
   how to implement such functionality is out of the scope of this
   document and considered implementation specific.

   In case of a DoS attack, the client will most likely be able to
   handle the replay cache.  A more likely (and serious) DoS attack is a
   CPU DoS attack where the attacker sends messages to the peer, which
   then needs to expend resources on verifying the MACs/signatures of
   the incoming messages.

6.  Payload Encoding

   This section describes, in detail, all the payloads.  For all
   encoding, network byte order is always used.  While defining
   supported types (e.g., which hash functions are supported) the
   mandatory-to-implement types are indicated (as Mandatory), as well as
   the default types (note, default also implies mandatory
   implementation).  Support for the other types are implicitly assumed
   to be optional.

   In the following, note that the support for SRTP [SRTP] as a security
   protocol is defined.  This will help us better understand the purpose
   of the different payloads and fields.  Other security protocols MAY
   be specified for use within MIKEY, see Section 10.

   In the following, the sign ~ indicates variable length field.

6.1.  Common Header payload (HDR)

   The Common Header payload MUST always be present as the first payload
   in each message.  The Common Header includes a general description of
   the exchange message.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  version      !  data type    ! next payload  !V! PRF func    !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                         CSB ID                                !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! #CS           ! CS ID map type! CS ID map info                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  version (8 bits): the version number of MIKEY.

      version = 0x01 refers to MIKEY as defined in this document.

   *  data type (8 bits): describes the type of message (e.g., public-
      key transport message, verification message, error message).

      Data type     | Value | Comment
      --------------------------------------
      Pre-shared    |     0 | Initiator's pre-shared key message
      PSK ver msg   |     1 | Verification message of a Pre-shared
                    |       | key message
      Public key    |     2 | Initiator's public-key transport message
      PK ver msg    |     3 | Verification message of a public-key
                    |       | message
      D-H init      |     4 | Initiator's DH exchange message
      D-H resp      |     5 | Responder's DH exchange message
      Error         |     6 | Error message

      Table 6.1.a

   *  next payload (8 bits): identifies the payload that is added after
      this payload.

      Next payload  | Value | Section
      ------------------------------
      Last payload  |     0 | -
      KEMAC         |     1 | 6.2
      PKE           |     2 | 6.3
      DH            |     3 | 6.4
      SIGN          |     4 | 6.5
      T             |     5 | 6.6
      ID            |     6 | 6.7
      CERT          |     7 | 6.7
      CHASH         |     8 | 6.8
      V             |     9 | 6.9
      SP            |    10 | 6.10
      RAND          |    11 | 6.11
      ERR           |    12 | 6.12
      Key data      |    20 | 6.13
      General Ext.  |    21 | 6.15

      Table 6.1.b

      Note that some of the payloads cannot directly follow the header
      (such as "Last payload", "Signature").  However, the Next payload
      field is generic for all payloads.  Therefore, a value is
      allocated for each payload.  The Next payload field is set to zero
      (Last payload) if the current payload is the last payload.

   *  V (1 bit): flag to indicate whether a verification message is
      expected or not (this only has meaning when it is set by the
      Initiator).  The V flag SHALL be ignored by the receiver in the DH
      method (as the response is MANDATORY).

      V = 0  ==> no response expected
      V = 1  ==> response expected

   *  PRF func (7 bits): indicates the PRF function that has been/will
      be used for key derivation.

      PRF func      | Value | Comments
      --------------------------------------------------------
      MIKEY-1       |     0 | Mandatory (see Section 4.1.2)

      Table 6.1.c

   *  CSB ID (32 bits): identifies the CSB.  It is RECOMMENDED that the
      CSB ID be chosen at random by the Initiator.  This ID MUST be
      unique between each Initiator-Responder pair, i.e., not globally
      unique.  An Initiator MUST check for collisions when choosing the
      ID (if the Initiator already has one or more established CSBs with
      the Responder).  The Responder uses the same CSB ID in the
      response.

   *  #CS (8 bits): indicates the number of Crypto Sessions that will be
      handled within the CBS.  Note that even though it is possible to
      use 255 CSs, it is not likely that a CSB will include this many
      CSs.  The integer 0 is interpreted as no CS included.  This may be
      the case in an initial setup message.

   *  CS ID map type (8 bits): specifies the method of uniquely mapping
      Crypto Sessions to the security protocol sessions.

      CS ID map type | Value
      -----------------------
      SRTP-ID        |     0

      Table 6.1.d

   *  CS ID map info (16 bits): identifies the crypto session(s) for
      which the SA should be created.  The currently defined map type is
      the SRTP-ID (defined in Section 6.1.1).

6.1.1.  SRTP ID

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Policy_no_1   ! SSRC_1                                        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! SSRC_1 (cont) ! ROC_1                                         !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! ROC_1 (cont)  ! Policy_no_2   ! SSRC_2                        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! SSRC_2 (cont)                 ! ROC_2                         !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! ROC_2 (cont)                  !                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
   :                               :                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Policy_no_#CS !           SSRC_#CS                            !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !SSRC_#CS (cont)!           ROC_#CS                             !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! ROC_#CS (cont)!
   +-+-+-+-+-+-+-+-+

   *  Policy_no_i (8 bits): The security policy applied for the stream
      with SSRC_i.  The same security policy may apply for all CSs.

   *  SSRC_i (32 bits): specifies the SSRC that MUST be used for the
      i-th SRTP stream.  Note that it is the sender of the streams that
      chooses the SSRC.  Therefore, it is possible that the Initiator of
      MIKEY cannot fill in all fields.  In this case, SSRCs that are not
      chosen by the Initiator are set to zero and the Responder fills in
      these fields in the response message.  Note that SRTP specifies
      requirements on the uniqueness of the SSRCs (to avoid two-time pad
      problems if the same TEK is used for more than one stream) [SRTP].

   *  ROC_i (32 bits): Current rollover counter used in SRTP.  If the
      SRTP session has not started, this field is set to 0.  This field
      is used to enable a member to join and synchronize with an already
      started stream.

   NOTE: The stream using SSRC_i will also have Crypto Session ID equal
   to no i (NOT to the SSRC).

6.2.  Key data transport payload (KEMAC)

   The Key data transport payload contains encrypted Key data sub-
   payloads (see Section 6.13 for the definition of the Key data sub-
   payload).  It may contain one or more Key data payloads, each
   including, for example, a TGK.  The last Key data payload has its
   Next payload field set to Last payload.  For an update message (see
   also Section 4.5), it is allowed to skip the Key data sub-payloads
   (which will result in the Encr data len being equal to 0).

   Note that the MAC coverage depends on the method used, i.e., pre-
   shared vs public key, see below.

   If the transport method used is the pre-shared key method, this Key
   data transport payload is the last payload in the message (note that
   the Next payload field is set to Last payload).  The MAC is then
   calculated over the entire MIKEY message following the directives in
   Section 5.2.

   If the transport method used is the public-key method, the
   Initiator's identity is added in the encrypted data.  This is done by
   adding the ID payload as the first payload, which is then followed by
   the Key data sub-payloads.  Note that for an update message, the ID
   is still sent encrypted to the Responder (this is to avoid certain
   re-direction attacks) even though no Key data sub-payload is added
   after.

   In the public-key case, the coverage of the MAC field is over the Key
   data transport payload only, instead of the complete MIKEY message,
   as in the pre-shared case.  The MAC is therefore calculated over the
   Key data transport payload, except for the MAC field and where the
   Next payload field has been set to zero (see also Section 5.2).

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next payload  ! Encr alg      ! Encr data len                 !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                        Encr data                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Mac alg       !        MAC                                    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for defined values.

   *  Encr alg (8 bits): the encryption algorithm used to encrypt the
      Encr data field.

      Encr alg      | Value | Comment
      -------------------------------------------
      NULL          |     0 | Very restricted usage, see Section 4.2.3!
      AES-CM-128    |     1 | Mandatory; AES-CM using a 128-bit key, see
                               Section 4.2.3)
      AES-KW-128    |     2 | AES Key Wrap using a 128-bit key, see
                               Section 4.2.3

      Table 6.2.a

   *  Encr data len (16 bits): length of Encr data (in bytes).

   *  Encr data (variable length): the encrypted key sub-payloads (see
      Section 6.13).

   *  MAC alg (8 bits): specifies the authentication algorithm used.

      MAC alg        | Value | Comments          | Length (bits)
      ----------------------------------------------------------
      NULL           |     0 | restricted usage  | 0
                     |       | Section 4.2.4     |
      HMAC-SHA-1-160 |     1 | Mandatory,        | 160
                     |       | Section 4.2.4     |

      Table 6.2.b

   *  MAC (variable length): the message authentication code of the
      entire message.

6.3.  Envelope data payload (PKE)

   The Envelope data payload contains the encrypted envelope key that is
   used in the public-key transport to protect the data in the Key data
   transport payload.  The encryption algorithm used is implicit from
   the certificate/public key used.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  ! C ! Data len                  ! Data          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  C (2 bits): envelope key cache indicator (Section 3.2).

      Cache type    | Value | Comments
      --------------------------------------
      No cache      |     0 | The envelope key MUST NOT be cached
      Cache         |     1 | The envelope key MUST be cached
      Cache for CSB |     2 | The envelope key MUST be cached, but only
                    |       | to be used for the specific CSB.
      Table 6.3

   *  Data len (14 bits): the length of the data field (in bytes).

   *  Data (variable length): the encrypted envelope key.

6.4.  DH data payload (DH)

   The DH data payload carries the DH-value and indicates the DH-group
   used.  Notice that in this sub-section, "MANDATORY" is conditioned
   upon DH being supported.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Next Payload ! DH-Group      !  DH-value                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Reserv! KV    ! KV data (optional)                            ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  DH-Group (8 bits): identifies the DH group used.

      DH-Group      | Value | Comment       | DH Value length (bits)
      --------------------------------------|---------------------
      OAKLEY 5      |     0 | Mandatory     |  1536
      OAKLEY 1      |     1 |               |   768
      OAKLEY 2      |     2 |               |  1024

      Table 6.4

   *  DH-value (variable length): the public DH-value (the length is
      implicit from the group used).

   *  KV (4 bits): indicates the type of key validity period specified.
      This may be done by using an SPI (alternatively an MKI in SRTP) or
      by providing an interval in which the key is valid (e.g., in the
      latter case, for SRTP this will be the index range where the key
      is valid).  See Section 6.13 for pre-defined values.

   *  KV data (variable length): This includes either the SPI/MKI or an
      interval (see Section 6.14).  If KV is NULL, this field is not
      included.

6.5.  Signature payload (SIGN)

   The Signature payload carries the signature and its related data.
   The signature payload is always the last payload in the PK transport
   and DH exchange messages.  The signature algorithm used is implicit
   from the certificate/public key used.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! S type| Signature len         ! Signature                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  S type (4 bits): indicates the signature algorithm applied by the
      signer.

      S type        | Value | Comments
      -------------------------------------
      RSA/PKCS#1/1.5|     0 | Mandatory, PKCS #1 version 1.5 signature
                               [PSS]
      RSA/PSS       |     1 | RSASSA-PSS signature [PSS]

      Table 6.5

   *  Signature len (12 bits): the length of the signature field (in
      bytes).

   *  Signature (variable length): the signature (its formatting and
      padding depend on the type of signature).

6.6.  Timestamp payload (T)

   The timestamp payload carries the timestamp information.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  !   TS type     ! TS value                      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  TS type (8 bits): specifies the timestamp type used.

      TS type       | Value | Comments     | length of TS value
      -------------------------------------|-------------------
      NTP-UTC       |     0 | Mandatory    |   64-bits
      NTP           |     1 | Mandatory    |   64-bits
      COUNTER       |     2 | Optional     |   32-bits

      Table 6.6

      Note: COUNTER SHALL be padded (with leading zeros) to a 64-bit
      value when used as input for the default PRF.

   *  TS-value (variable length): The timestamp value of the specified
      TS type.

6.7.  ID payload (ID) / Certificate Payload (CERT)

   Note that the ID payload and the Certificate payload are two
   completely different payloads (having different payload identifiers).
   However, as they share the same payload structure, they are described
   in the same section.

   The ID payload carries a uniquely defined identifier.

   The certificate payload contains an indicator of the certificate
   provided as well as the certificate data.  If a certificate chain is
   to be provided, each certificate in the chain should be included in a
   separate CERT payload.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Next Payload ! ID/Cert Type  ! ID/Cert len                   !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                       ID/Certificate Data                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   If the payload is an ID payload, the following values apply for the
   ID type field:

   *  ID Type (8 bits): specifies the identifier type used.

      ID Type       | Value | Comments
      ----------------------------------------------
      NAI           |     0 | Mandatory (see [NAI])
      URI           |     1 | Mandatory (see [URI])

      Table 6.7.a

   If the payload is a Certificate payload, the following values applies
   for the Cert type field:

   *  Cert Type (8 bits): specifies the certificate type used.

     Cert Type     | Value | Comments
     ----------------------------------------------
     X.509v3       |     0 | Mandatory
     X.509v3 URL   |     1 | plain ASCII URL to the location of the Cert
     X.509v3 Sign  |     2 | Mandatory (used for signatures only)
     X.509v3 Encr  |     3 | Mandatory (used for encryption only)

     Table 6.7.b

   *  ID/Cert len (16 bits): the length of the ID or Certificate field
      (in bytes).

   *  ID/Certificate (variable length): The ID or Certificate data.  The
      X.509 [X.509] certificates are included as a bytes string using
      DER encoding as specified in X.509.

6.8.  Cert hash payload (CHASH)

   The Cert hash payload contains the hash of the certificate used.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  ! Hash func     ! Hash                          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  Hash func (8 bits): indicates the hash function that is used (see
      also Section 4.2.1).

      Hash func     | Value | Comment     | hash length (bits)
      -------------------------------------------------
      SHA-1         |     0 | Mandatory   |  160
      MD5           |     1 |             |  128

      Table 6.8

   *  Hash (variable length): the hash data.  The hash length is
      implicit from the hash function used.

6.9.  Ver msg payload (V)

   The Ver msg payload contains the calculated verification message in
   the pre-shared key and the public-key transport methods.  Note that
   the MAC is calculated over the entire MIKEY message, as well as the
   IDs and Timestamp (see also Section 5.2).

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  ! Auth alg      ! Ver data                      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  Auth alg (8 bits): specifies the MAC algorithm used for the
      verification message.  See Section 6.2 for defined values.

   *  Ver data (variable length): the verification message data.  The
      length is implicit from the authentication algorithm used.

6.10.  Security Policy payload (SP)

   The Security Policy payload defines a set of policies that apply to a
   specific security protocol.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next payload  ! Policy no     ! Prot type     ! Policy param  ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~ length (cont) ! Policy param                                  ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  Policy no (8 bits): each security policy payload must be given a
      distinct number for the current MIKEY session by the local peer.
      This number is used to map a crypto session to a specific policy
      (see also Section 6.1.1).

   *  Prot type (8 bits): defines the security protocol.

      Prot type     | Value |
      ---------------------------
      SRTP          |     0 |

      Table 6.10

   *  Policy param length (16 bits): defines the total length of the
      policy parameters for the specific security protocol.

   *  Policy param (variable length): defines the policy for the
      specific security protocol.

      The Policy param part is built up by a set of Type/Length/Value
      fields.  For each security protocol, a set of possible
      types/values that can be negotiated is defined.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Type          ! Length        ! Value                         ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Type (8 bits): specifies the type of the parameter.

   *  Length (8 bits): specifies the length of the Value field (in
      bytes).

   *  Value (variable length): specifies the value of the parameter.

6.10.1.  SRTP policy

   This policy specifies the parameters for SRTP and SRTCP.  The
   types/values that can be negotiated are defined by the following
   table:

   Type | Meaning                     | Possible values
   ----------------------------------------------------
      0 | Encryption algorithm        | see below
      1 | Session Encr. key length    | depends on cipher used
      2 | Authentication algorithm    | see below
      3 | Session Auth. key length    | depends on MAC used
      4 | Session Salt key length     | see [SRTP] for recommendations
      5 | SRTP Pseudo Random Function | see below
      6 | Key derivation rate         | see [SRTP] for recommendations
      7 | SRTP encryption off/on      | 0 if off, 1 if on
      8 | SRTCP encryption off/on     | 0 if off, 1 if on
      9 | sender's FEC order          | see below
     10 | SRTP authentication off/on  | 0 if off, 1 if on
     11 | Authentication tag length   | in bytes
     12 | SRTP prefix length          | in bytes

   Table 6.10.1.a

   Note that if a Type/Value is not set, the default is used (according
   to SRTP's own criteria). Note also that, if "Session Encr. key
   length" is set, this should also be seen as the Master key length
   (otherwise, the SRTP default Master key length is used).

   For the Encryption algorithm, a one byte length is enough.  The
   currently defined possible Values are:

     SRTP encr alg | Value
     ---------------------
     NULL          |     0
     AES-CM        |     1
     AES-F8        |     2

     Table 6.10.1.b

   where AES-CM is AES in CM, and AES-F8 is AES in f8 mode [SRTP].

   For the Authentication algorithm, a one byte length is enough.  The
   currently defined possible Values are:

     SRTP auth alg | Value
     ---------------------
     NULL          |     0
     HMAC-SHA-1    |     1

     Table 6.10.1.c

   For the SRTP pseudo-random function, a one byte length is also
   enough. The currently defined possible Values are:

     SRTP PRF      | Value
     ---------------------
     AES-CM        |     0

     Table 6.10.1.d

   If FEC is used at the same time SRTP is used, MIKEY can negotiate the
   order in which these should be applied at the sender side.

      FEC order     | Value | Comments
      --------------------------------
      FEC-SRTP      |     0 | First FEC, then SRTP

      Table 6.10.1.e

6.11.  RAND payload (RAND)

   The RAND payload consists of a (pseudo-)random bit-string.  The RAND
   MUST be independently generated per CSB (note that if the CSB has
   several members, the Initiator MUST use the same RAND for all the
   members).  For randomness recommendations for security, see [RAND].

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next payload  ! RAND len      ! RAND                          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  RAND len (8 bits): length of the RAND (in bytes).  It SHOULD be at
      least 16.

   *  RAND (variable length): a (pseudo-)randomly chosen bit-string.

6.12.  Error payload (ERR)

   The Error payload is used to specify the error(s) that may have
   occurred.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Next Payload ! Error no      !           Reserved            !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  Error no (8 bits): indicates the type of error that was
      encountered.

      Error no          | Value | Comment
      -------------------------------------------------------
      Auth failure      |     0 | Authentication failure
      Invalid TS        |     1 | Invalid timestamp
      Invalid PRF       |     2 | PRF function not supported
      Invalid MAC       |     3 | MAC algorithm not supported
      Invalid EA        |     4 | Encryption algorithm not supported
      Invalid HA        |     5 | Hash function not supported
      Invalid DH        |     6 | DH group not supported
      Invalid ID        |     7 | ID not supported
      Invalid Cert      |     8 | Certificate not supported
      Invalid SP        |     9 | SP type not supported
      Invalid SPpar     |    10 | SP parameters not supported
      Invalid DT        |    11 | not supported Data type
      Unspecified error |    12 | an unspecified error occurred

      Table 6.12

6.13.  Key data sub-payload

   The Key data payload contains key material, e.g., TGKs.  The Key data
   payloads are never included in clear, but as an encrypted part of the
   Key data transport payload.

   Note that a Key data transport payload can contain multiple Key data
   sub-payloads.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Next Payload ! Type  ! KV    ! Key data len                  !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                         Key data                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Salt len (optional)           ! Salt data (optional)          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                        KV data (optional)                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 for values.

   *  Type (4 bits): indicates the type of key included in the payload.

      Type     | Value
      -----------------
      TGK      |     0
      TGK+SALT |     1
      TEK      |     2
      TEK+SALT |     3

      Table 6.13.a

      Note that the possibility of including a TEK (instead of using the
      TGK) is provided.  When sent directly, the TEK can generally not
      be shared between more than one Crypto Session (unless the
      Security protocol allows for this, e.g., [SRTP]).  The recommended
      use of sending a TEK, instead of a TGK, is when pre-encrypted
      material exists and therefore, the TEK must be known in advance.

   *  KV (4 bits): indicates the type of key validity period specified.
      This may be done by using an SPI (or MKI in the case of [SRTP]) or
      by providing an interval in which the key is valid (e.g., in the
      latter case, for SRTP this will be the index range where the key
      is valid).

      KV            | Value | Comments
      -------------------------------------------
      Null          |     0 | No specific usage rule (e.g., a TEK
                    |       | that has no specific lifetime)
      SPI           |     1 | The key is associated with the SPI/MKI
      Interval      |     2 | The key has a start and expiration time
                    |       | (e.g., an SRTP TEK)

      Table 6.13.b

      Note that when NULL is specified, any SPI or Interval is valid.
      For an Interval, this means that the key is valid from the first
      observed sequence number until the key is replaced (or the
      security protocol is shutdown).

   *  Key data len (16 bits): the length of the Key data field (in
      bytes).  Note that the sum of the overall length of all the Key
      data payloads contained in a single Key data transport payload
      (KEMAC) MUST be such that the KEMAC payload does not exceed a
      length of 2^16 bytes (total length of KEMAC, see Section 6.2).

   *  Key data (variable length): The TGK or TEK data.

   *  Salt len (16 bits): The salt key length in bytes.  Note that this
      field is only included if the salt is specified in the Type-field.

   *  Salt data (variable length): The salt key data.  Note that this
      field is only included if the salt is specified in the Type-field.
      (For SRTP, this is the so-called master salt.)

   *  KV data (variable length): This includes either the SPI or an
      interval (see Section 6.14).  If KV is NULL, this field is not
      included.

6.14.  Key validity data

   The Key validity data is not a standalone payload, but part of either
   the Key data payload (see Section 6.13) or the DH payload (see
   Section 6.4).  The Key validity data gives a guideline of when the
   key should be used.  There are two KV types defined (see Section
   6.13), SPI/MKI (SPI) or a lifetime range (interval).

   SPI/MKI
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! SPI Length    ! SPI                                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  SPI Length (8 bits): the length of the SPI (or MKI) in bytes.

   *  SPI (variable length): the SPI (or MKI) value.

   Interval
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! VF Length     ! Valid From                                    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! VT Length     ! Valid To (expires)                            ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  VF Length (8 bits): length of the Valid From field in bytes.

   *  Valid From (variable length): sequence number, index, timestamp,
      or other start value that the security protocol uses to identify
      the start position of the key usage.

   *  VT Length (8 bits): length of the Valid To field in bytes.

   *  Valid To (variable length): sequence number, index, timestamp, or
      other expiration value that the security protocol can use to
      identify the expiration of the key usage.

      Note that for SRTP usage, the key validity period for a TGK/TEK
      should be specified with either an interval, where the VF/VT
      Length is equal to 6 bytes (i.e., the size of the index), or with
      an MKI.  It is RECOMMENDED that if more than one SRTP stream is
      sharing the same keys and key update/re-keying is desired, this is
      handled using MKI rather than the From-To method.

6.15.  General Extension Payload

   The General extensions payload is included to allow possible
   extensions to MIKEY without the need for defining a completely new
   payload each time.  This payload can be used in any MIKEY message and
   is part of the authenticated/signed data part.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next payload  ! Type          ! Length                        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Data                                                          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Next payload (8 bits): identifies the payload that is added after
      this payload.

   *  Type (8 bits): identifies the type of general payload.

      Type      | Value | Comments
      ---------------------------------------
      Vendor ID |     0 | Vendor specific byte string
      SDP IDs   |     1 | List of SDP key mgmt IDs (allocated for use in
                           [KMASDP])

      Table 6.15

   *  Length (16 bits): the length in bytes of the Data field.

   *  Data (variable length): the general payload data.

7.  Transport protocols

   MIKEY MAY be integrated within session establishment protocols.
   Currently, integration of MIKEY within SIP/SDP and RTSP is defined in
   [KMASDP].  MIKEY MAY use other transports, in which case how MIKEY is
   transported over such a transport protocol has to be defined.

8.  Groups

   What has been discussed up to now is not limited to single peer-to-
   peer communication (except for the DH method), but can be used to
   distribute group keys for small-size interactive groups and simple
   one-to-many scenarios.  Section 2.1. describes the scenarios in the
   focus of MIKEY.  This section describes how MIKEY is used in a group
   scenario (though, see also Section 4.3 for issues related to
   authorization).

8.1.  Simple one-to-many

                            ++++
                            |S |
                            |  |
                            ++++
                              |
                      --------+-------------- - -
                      |       |      |
                      v       v      v
                    ++++    ++++   ++++
                    |A |    |B |   |C |
                    |  |    |  |   |  |
                    ++++    ++++   ++++

   Figure 8.1. Simple one-to-many scenario.

   In the simple one-to-many scenario, a server is streaming to a small
   group of clients.  RTSP or SIP is used for the registration and the
   key management set up.  The streaming server acts as the Initiator of
   MIKEY.  In this scenario, the pre-shared key or public key transport
   mechanism will be appropriate in transporting the same TGK to all the
   clients (which will result in common TEKs for the group).

   Note, if the same TGK/TEK(s) should be used by all the group members,
   the streaming server MUST specify the same CSB_ID and CS_ID(s) for
   the session to all the group members.

   As the communication may be performed using multicast, the members
   need a common security policy if they want to be part of the group.
   This limits the possibility of negotiation.

   Furthermore, the Initiator should carefully consider whether to
   request the verification message in reply from each receiver, as this
   may result in a certain load for the Initiator itself as the group
   size increases.

8.2.  Small-size interactive group

   As described in the overview section, for small-size interactive
   groups, one may expect that each client will be in charge for setting
   up the security for its outgoing streams.  In these scenarios, the
   pre-shared key or the public-key transport method is used.

                       ++++          ++++
                       |A | -------> |B |
                       |  | <------- |  |
                       ++++          ++++
                        ^ |          | ^
                        | |          | |
                        | |   ++++   | |
                        | --->|C |<--- |
                        ------|  |------
                              ++++

   Figure 8.2. Small-size group without a centralized controller.

   One scenario may then be that the client sets up a three-part call,
   using SIP.  Due to the small size of the group, unicast SRTP is used
   between the clients.  Each client sets up the security for its
   outgoing stream(s) to the others.

   As for the simple one-to-many case, the streaming client specifies
   the same CSB_ID and CS_ID(s) for its outgoing sessions if the same
   TGK/TEK(s) is used for all the group members.

9.  Security Considerations

9.1.  General

   Key management protocols based on timestamps/counters and one-
   roundtrip key transport have previously been standardized, for
   example ISO [ISO1, ISO2].  The general security of these types of
   protocols can be found in various articles and literature, c.f. [HAC,
   AKE, LOA].

   No chain is stronger than its weakest link.  If a given level of
   protection is wanted, then the cryptographic functions protecting the
   keys during transport/exchange MUST offer a security corresponding to
   at least that level.

   For instance, if a security against attacks with a complexity 2^96 is
   wanted, then one should choose a secure symmetric cipher supporting
   at least 96 bit keys (128 bits may be a practical choice) for the
   actual media protection, and a key transport mechanism that provides
   equivalent protection, e.g., MIKEY's pre-shared key transport with
   128 bit TGK, or RSA with 1024 bit keys (which according to [LV]
   corresponds to the desired 96 bit level, with some margin).

   In summary, key size for the key-exchange mechanism MUST be weighed
   against the size of the exchanged TGK so that it at least offers the
   required level.  For efficiency reasons, one SHOULD also avoid a

   security overkill, e.g., by not using a public key transport with
   public keys giving a security level that is orders of magnitude
   higher than length of the transported TGK.  We refer to [LV] for
   concrete key size recommendations.

   Moreover, if the TGKs are not random (or pseudo-random), a brute
   force search may be facilitated, again lowering the effective key
   size.  Therefore, care MUST be taken when designing the (pseudo-)
   random generators for TGK generation, see [FIPS][RAND].

   For the selection of the hash function, SHA-1 with 160-bit output is
   the default one.  In general, hash sizes should be twice the
   "security level", indicating that SHA-1-256, [SHA256], should be used
   for the default 128-bit level.  However, due to the real-time aspects
   in the scenarios we are treating, hash sizes slightly below 256 are
   acceptable, as the normal "existential" collision probabilities would
   be of secondary importance.

   In a Crypto Session Bundle, the Crypto Sessions can share the same
   TGK as discussed earlier.  From a security point of view, to satisfy
   the criterion in case the TGK is shared, the encryption of the
   individual Crypto Sessions are performed "independently".  In MIKEY,
   this is accomplished by having unique Crypto Session identifiers (see
   also Section 4.1) and a TEK derivation method that provides
   cryptographically independent TEKs to distinct Crypto Sessions
   (within the Crypto Session Bundle), regardless of the security
   protocol used.

   Specifically, the key derivations, as specified in Section 4.1, are
   implemented by a pseudo-random function.  The one used here is a
   simplified version of that used in TLS [TLS].  Here, only one single
   hash function is used, whereas TLS uses two different functions.
   This choice is motivated by the high confidence in the SHA-1 hash
   function, and by efficiency and simplicity of design (complexity does
   not imply security).  Indeed, as shown in [DBJ], if one of the two
   hashes is severely broken, the TLS PRF is actually less secure than
   as if a single hash had been used on the whole key, as is done in
   MIKEY.

   In the pre-shared key and public-key schemes, the TGK is generated by
   a single party (Initiator).  This makes MIKEY somewhat more sensitive
   if the Initiator uses a bad random number generator.  It should also
   be noted that neither the pre-shared nor the public-key scheme
   provides perfect forward secrecy.  If mutual contribution or perfect
   forward secrecy is desired, the Diffie-Hellman method is to be used.
   Authentication (e.g., signatures) in the Diffie-Hellman method is
   required to prevent man-in-the-middle attacks.

   Forward/backward security: if the TGK is exposed, all generated TEKs
   are compromised.  However, under the assumption that the derivation
   function is a pseudo-random function, disclosure of an individual TEK
   does not compromise other (previous or later) TEKs derived from the
   same TGK.  The Diffie-Hellman mode can be considered by cautious
   users, as it is the only one that supports so called perfect forward
   secrecy (PFS).  This is in contrast to a compromise of the pre-shared
   key (or the secret key of the public key mode), where future sessions
   and recorded sessions from the past are then also compromised.

   The use of random nonces (RANDs) in the key derivation is of utmost
   importance to counter off-line pre-computation attacks.  Note however
   that update messages re-use the old RAND.  This means that the total
   effective key entropy (relative to pre-computation attacks) for k
   consecutive key updates, assuming the TGKs and RAND are each n bits
   long, is about L = n*(k+1)/2 bits, compared to the theoretical
   maximum of n*k bits.  In other words, a 2^L work effort MAY enable an
   attacker to get all k n-bit keys, which is better than brute force
   (except when k = 1).  While this might seem like a defect, first note
   that for a proper choice of n, the 2^L complexity of the attack is
   way out of reach.  Moreover, the fact that more than one key can be
   compromised in a single attack is inherent to the key exchange
   problem.  Consider for instance a user who, using a fixed 1024-bit
   RSA key, exchanges keys and communicates during a one or two year
   lifetime of the public key.  Breaking this single RSA key will enable
   access to all exchanged keys and consequently the entire
   communication of that user over the whole period.

   All the pre-defined transforms in MIKEY use state-of-the-art
   algorithms that have undergone large amounts of public evaluation.
   One of the reasons for using the AES-CM from SRTP [SRTP], is to have
   the possibility of limiting the overall number of different
   encryption modes and algorithms, while offering a high level of
   security at the same time.

9.2.  Key lifetime

   Even if the lifetime of a TGK (or TEK) is not specified, it MUST be
   taken into account that the encryption transform in the underlying
   security protocol can in some way degenerate after a certain amount
   of encrypted data.  It is not possible to here state universally
   applicable, general key lifetime bounds; each security protocol
   should define such maximum amount and trigger a re-keying procedure
   before the "exhaustion" of the key.  For example, according to SRTP
   [SRTP] the TEK, together with the corresponding TGK, MUST be changed
   at least every 2^48 SRTP packet.

   Still, the following can be said as a rule of thumb.  If the security
   protocol uses an "ideal" b-bit block cipher (in CBC mode, counter
   mode, or a feedback mode, e.g., OFB, with full b-bit feedback),
   degenerate behavior in the crypto stream, possibly useful for an
   attacker, is (with constant probability) expected to occur after a
   total of roughly 2^(b/2) encrypted b-bit blocks (using random IVs).
   For security margin, re-keying MUST be triggered well in advance
   compared to the above bound.  See [BDJR] for more details.

   For use of a dedicated stream cipher, we refer to the analysis and
   documentation of said cipher in each specific case.

9.3.  Timestamps

   The use of timestamps, instead of challenge-responses, requires the
   systems to have synchronized clocks.  Of course, if two clients are
   not synchronized, they will have difficulties in setting up the
   security.  The current timestamp based solution has been selected to
   allow a maximum of one roundtrip (i.e., two messages), but still
   provide a reasonable replay protection.  A (secure) challenge-
   response based version would require at least three messages.  For a
   detailed description of the timestamp and replay handling in MIKEY,
   see Section 5.4.

   Practical experiences of Kerberos and other timestamp-based systems
   indicate that it is not always necessary to synchronize the terminals
   over the network.  Manual configuration could be a feasible
   alternative in many cases (especially in scenarios where the degree
   of looseness is high).  However, the choice must be made carefully
   with respect to the usage scenario.

9.4.  Identity Protection

   User privacy is a complex matter that to some extent can be enforced
   by cryptographic mechanisms, but also requires policy enforcement and
   various other functionalities.  One particular facet of privacy is
   user identity protection.  However, identity protection was not a
   main design goal for MIKEY.  Such a feature will add more complexity
   to the protocol and was therefore not chosen to be included.  As
   MIKEY is anyway proposed to be transported over, e.g., SIP, the
   identity may be exposed by this.  However, if the transporting
   protocol is secured and also provides identity protection, MIKEY
   might inherit the same feature.  How this should be done is for
   future study.

9.5.  Denial of Service

   This protocol is resistant to Denial of Service attacks in the sense
   that a Responder does not construct any state (at the key management
   protocol level) before it has authenticated the Initiator.  However,
   this protocol, like many others, is open to attacks that use spoofed
   IP addresses to create a large number of fake requests.  This may for
   example, be solved by letting the protocol transporting MIKEY do an
   IP address validity test.  The SIP protocol can provide this using
   the anonymous authentication challenge mechanism (specified in
   Section 22.1 of [SIP]).

   It is highly RECOMMENDED to include IDr in the Initiator's message.
   If not included, its absence can be used for DoS purposes (the
   largest DoS-impact being on the public key and DH methods), where a
   message intended for other entities is sent to the target.  In fact,
   the target may verify the signature correctly due to the fact that
   the Initiator's ID is correct and the message is actually signed by
   the claimed Initiator (e.g., by re-directing traffic from another
   session).

   However, in the public key method, the envelop key and the MAC will
   ensure that the message is not accepted (still, compared to a normal
   faked message, where the signature verification would detect the
   problem, one extra public key decryption is needed to detect the
   problem in this case).

   In the DH method, a message would be accepted (without detecting the
   error) and a response (and state) would be created for the malicious
   request.

   As also discussed in Section 5.4, the tradeoff between time
   synchronization and the size of the replay cache may be affected in
   case of for example, a flooding DoS attack.  However, if the
   recommendations of using a dynamic size of the replay cache are
   followed, it is believed that the client will in most cases be able
   to handle the replay cache.  Of course, as the replay cache decreases
   in size, the required time synchronization is more restricted.
   However, a bigger problem during such an attack would probably be to
   process the messages (e.g., verify signatures/MACs) due to the
   computational workload this implies.

9.6.  Session Establishment

   It should be noted that if the session establishment protocol is
   insecure, there may be attacks on this that will have indirect
   security implications on the secured media streams.  This however
   only applies to groups (and is not specific to MIKEY).  The threat is

   that one group member may re-direct a stream from one group member to
   another.  This will have the same implication as when a member tries
   to impersonate another member, e.g., by changing its IP address.  If
   this is seen as a problem, it is RECOMMENDED that a Data Origin
   Authentication (DOA) scheme (e.g., digital signatures) be applied to
   the security protocol.

   Re-direction of streams can of course be done even if it is not a
   group.  However, the effect will not be the same as compared to a
   group where impersonation can be done if DOA is not used.  Instead,
   re-direction will only deny the receiver the possibility of receiving
   (or just delay) the data.

10.  IANA Considerations

   This document defines several new name spaces associated with the
   MIKEY payloads.  This section summarizes the name spaces for which
   IANA is requested to manage the allocation of values.  IANA is
   requested to record the pre-defined values defined in the given
   sections for each name space.  IANA is also requested to manage the
   definition of additional values in the future.  Unless explicitly
   stated otherwise, values in the range 0-240 for each name space
   SHOULD be approved by the process of IETF consensus and values in the
   range 241-255 are reserved for Private Use, according to [RFC2434].

   The name spaces for the following fields in the Common header payload
   (from Section 6.1) are requested to be managed by IANA (in bracket is
   the reference to the table with the initially registered values):

   *  version

   *  data type (Table 6.1.a)

   *  Next payload (Table 6.1.b)

   *  PRF func (Table 6.1.c).  This name space is between 0-127, where
      values between 0-111 should be approved by the process of IETF
      consensus and values between 112-127 are reserved for Private Use.

   *  CS ID map type (Table 6.1.d)

   The name spaces for the following fields in the Key data transport
   payload (from Section 6.2) are requested to be managed by IANA:

   *  Encr alg (Table 6.2.a)

   *  MAC alg (Table 6.2.b)

   The name spaces for the following fields in the Envelope data payload
   (from Section 6.3) are requested to be managed by IANA:

   *  C (Table 6.3)

   The name spaces for the following fields in the DH data payload (from
   Section 6.4) are requested to be managed by IANA:

   *  DH-Group (Table 6.4)

   The name spaces for the following fields in the Signature payload
   (from Section 6.5) are requested to be managed by IANA:

   *  S type (Table 6.5)

   The name spaces for the following fields in the Timestamp payload
   (from Section 6.6) are requested to be managed by IANA:

   *  TS type (Table 6.6)

   The name spaces for the following fields in the ID payload and the
   Certificate payload (from Section 6.7) are requested to be managed by
   IANA:

   *  ID type (Table 6.7.a)

   *  Cert type (Table 6.7.b)

   The name spaces for the following fields in the Cert hash payload
   (from Section 6.8) are requested to be managed by IANA:

   *  Hash func (Table 6.8)

   The name spaces for the following fields in the Security policy
   payload (from Section 6.10) are requested to be managed by IANA:

   *  Prot type (Table 6.10)

   For each security protocol that uses MIKEY, a set of unique
   parameters MAY be registered.

   From Section 6.10.1.

   *  SRTP Type (Table 6.10.1.a)

   * SRTP encr alg (Table 6.10.1.b)

   * SRTP auth alg (Table 6.10.1.c)

   * SRTP PRF (Table 6.10.1.d)

   * FEC order (Table 6.10.1.e)

   The name spaces for the following fields in the Error payload (from
   Section 6.12) are requested to be managed by IANA:

   *  Error no  (Table 6.12)

   The name spaces for the following fields in the Key data payload
   (from Section 6.13) are requested to be managed by IANA:

   *  Type (Table 6.13.a).  This name space is between 0-16, which
      should be approved by the process of IETF consensus.

   *  KV (Table 6.13.b).  This name space is between 0-16, which should
      be approved by the process of IETF consensus.

   The name spaces for the following fields in the General Extensions
   payload (from Section 6.15) are requested to be managed by IANA:

   *  Type (Table 6.15).

10.1.  MIME Registration

   This section gives instructions to IANA to register the
   application/mikey MIME media type.  This registration is as follows:

   MIME media type name              : application
   MIME subtype name                 : mikey
   Required parameters               : none
   Optional parameters               : version
             version: The MIKEY version number of the enclosed message
                (e.g., 1). If not present, the version defaults to 1.
   Encoding Considerations           : binary, base64 encoded
   Security Considerations           : see section 9 in this memo
   Interoperability considerations   : none
   Published specification           : this memo

11.  Acknowledgments

   The authors would like to thank Mark Baugher, Ran Canetti, Martin
   Euchner, Steffen Fries, Peter Barany, Russ Housley, Pasi Ahonen (with
   his group), Rolf Blom, Magnus Westerlund, Johan Bilien, Jon-Olov
   Vatn, Erik Eliasson, and Gerhard Strangar for their valuable
   feedback.

12.  References

12.1.  Normative References

   [HMAC]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:  Keyed-
             Hashing for Message Authentication", RFC 2104, February
             1997.

   [NAI]     Aboba, B. and M. Beadles, "The Network Access Identifier",
             RFC 2486, January 1999.

   [OAKLEY]  Orman, H., "The OAKLEY Key Determination Protocol", RFC
             2412, November 1998.

   [PSS]     PKCS #1 v2.1 - RSA Cryptography Standard, RSA Laboratories,
             June 14, 2002, www.rsalabs.com

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.

   [SHA-1]   NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.

   [SRTP]    Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
             Norrman, "The Secure Real Time Transport Protocol", RFC
             3711, March 2004.

   [URI]     Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
             Resource Identifiers (URI): Generic Syntax", RFC 2396,
             August 1998.

   [X.509]   Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
             X.509 Public Key Infrastructure Certificate and Certificate
             Revocation List (CRL) Profile", RFC 3280, April 2002.

   [AESKW]   Schaad, J. and R. Housley, "Advanced Encryption Standard
             (AES) Key Wrap Algorithm", RFC 3394, September 2002.

12.2.  Informative References

   [AKE]     Canetti, R. and H. Krawczyk, "Analysis of Key-Exchange
             Protocols and their use for Building Secure Channels",
             Eurocrypt 2001, LNCS 2054, pp. 453-474, 2001.

   [BDJR]    Bellare, M., Desai, A., Jokipii, E., and P. Rogaway, "A
             Concrete Analysis of Symmetric Encryption: Analysis of the
             DES Modes of Operation", in Proceedings of the 38th
             Symposium on Foundations of Computer Science, IEEE, 1997,
             pp. 394-403.

   [BMGL]    Hastad, J. and M. Naslund: "Practical Construction and
             Analysis of Pseduo-randomness Primitives", Proceedings of
             Asiacrypt 2001, LNCS. vol 2248, pp. 442-459, 2001.

   [DBJ]     Johnson, D.B., "Theoretical Security Concerns with TLS use
             of MD5", Contribution to ANSI X9F1 WG, 2001.

   [FIPS]    "Security Requirements for Cryptographic Modules", Federal
             Information Processing Standard Publications (FIPS PUBS)
             140-2, December 2002.

   [GKMARCH] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
             "Group Key Management Architecture", Work in Progress.

   [GDOI]    Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
             Group Domain of Interpretation", RFC 3547, July 2003.

   [GSAKMP]  Harney, H., Colegrove, A., Harder, E., Meth, U., and R.
             Fleischer, "Group Secure Association Key Management
             Protocol", Work in Progress.

   [HAC]     Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook
             of Applied Cryptography", CRC press, 1996.

   [IKE]     Harkins, D. and D. Carrel, "The Internet Key Exchange
             (IKE)", RFC 2409, November 1998.

   [ISO1]    ISO/IEC 9798-3: 1997, Information technology - Security
             techniques - Entity authentication - Part 3: Mechanisms
             using digital signature techniques.

   [ISO2]    ISO/IEC 11770-3: 1997, Information technology - Security
             techniques - Key management - Part 3: Mechanisms using
             digital signature techniques.

   [ISO3]    ISO/IEC 18014 Information technology - Security techniques
             - Time-stamping services, Part 1-3.

   [KMASDP]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
             Norrman, "Key Management Extensions for SDP and RTSP", Work
             in Progress.

   [LOA]     Burrows, Abadi, and Needham, "A logic of authentication",
             ACM Transactions on Computer Systems 8 No.1 (Feb. 1990),
             18-36.

   [LV]      Lenstra, A. K. and E. R. Verheul, "Suggesting Key Sizes for
             Cryptosystems", http://www.cryptosavvy.com/suggestions.htm

   [NTP]     Mills, D., "Network Time Protocol (Version 3)
             Specification, Implementation and Analysis", RFC 1305,
             March 1992.

   [OCSP]    Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
             Adams, "X.509 Internet Public Key Infrastructure Online
             Certificate Status Protocol - OCSP", RFC 2560, June 1999.

   [RAND]    Eastlake, 3rd, D., Crocker, S., and J. Schiller,
             "Randomness Requirements for Security", RFC 1750, December
             1994.

   [RTSP]    Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
             Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [SDP]     Handley, M. and V. Jacobson, "SDP: Session Description
             Protocol", RFC 2327, April 1998.

   [SHA256]  NIST, "Description of SHA-256, SHA-384, and SHA-512",
             http://csrc.nist.gov/encryption/shs/sha256-384-512.pdf

   [SIP]     Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
             A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
             "SIP: Session Initiation Protocol", RFC 3261, June 2002.

   [TLS]     Dierks, T. and C. Allen, "The TLS Protocol - Version 1.0",
             RFC 2246, January 1999.

Appendix A.  MIKEY - SRTP Relation

   The terminology in MIKEY differs from the one used in SRTP as MIKEY
   needs to be more general, nor is tight to SRTP only.  Therefore, it
   might be hard to see the relations between keys and parameters
   generated in MIKEY and those used by SRTP.  This section provides
   some hints on their relation.

   MIKEY            | SRTP
   -------------------------------------------------
   Crypto Session   | SRTP stream (typically with related SRTCP stream)
   Data SA          | input to SRTP's crypto context
   TEK              | SRTP master key

   The Data SA is built up by a TEK and the security policy exchanged.
   SRTP may use an MKI to index the TEK or TGK (the TEK is then derived
   from the TGK that is associated with the corresponding MKI), see
   below.

A.1.  MIKEY-SRTP Interactions

   In the following, we give a brief outline of the interface between
   SRTP and MIKEY and the processing that takes place.  We describe the
   SRTP receiver side only, the sender side will require analogous
   interfacing.

   1. When an SRTP packet arrives at the receiver and is processed, the
      triple <SSRC, destination address, destination port> is extracted
      from the packet and used to retrieve the correct SRTP crypto
      context, hence the Data SA.  (The actual retrieval can, for
      example, be done by an explicit request from the SRTP
      implementation to MIKEY, or, by the SRTP implementation accessing
      a "database", maintained by MIKEY.  The application will typically
      decide which implementation is preferred.)

   2. If an MKI is present in the SRTP packet, it is used to point to
      the correct key within the SA.  Alternatively, if SRTP's <From,
      To> feature is used, the ROC||SEQ of the packet is used to
      determine the correct key.

   3. Depending on whether the key sent in MIKEY (as obtained in step 2)
      was a TEK or a TGK, there are now two cases.

      -  If the key obtained in step 2 is the TEK itself, it is used
         directly by SRTP as a master key.

      -  If the key instead is a TGK, the mapping with the CS_ID
         (internal to MIKEY, Section 6.1.1) allows MIKEY to compute the
         correct TEK from the TGK as described in Section 4.1 before
         SRTP uses it.

   If multiple TGKs (or TEKs) are sent, it is RECOMMENDED that each TGK
   (or TEK) be associated with a distinct MKI.  It is RECOMMENDED that
   the use of <From, To> in this scenario be limited to very simple
   cases, e.g., one stream only.

   Besides the actual master key, other information in the Data SA
   (e.g., transform identifiers) will of course also be communicated
   from MIKEY to SRTP.

Authors' Addresses

   Jari Arkko
   Ericsson Research
   02420 Jorvas
   Finland

   Phone:  +358 40 5079256
   EMail:  jari.arkko@ericsson.com

   Elisabetta Carrara
   Ericsson Research
   SE-16480 Stockholm
   Sweden

   Phone:  +46 8 50877040
   EMail:  elisabetta.carrara@ericsson.com

   Fredrik Lindholm
   Ericsson Research
   SE-16480 Stockholm
   Sweden

   Phone:  +46 8 58531705
   EMail:  fredrik.lindholm@ericsson.com

   Mats Naslund
   Ericsson Research
   SE-16480 Stockholm
   Sweden

   Phone:  +46 8 58533739
   EMail:  mats.naslund@ericsson.com

   Karl Norrman
   Ericsson Research
   SE-16480 Stockholm
   Sweden

   Phone:  +46 8 4044502
   EMail:  karl.norrman@ericsson.com

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