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RFC 4210 - Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)
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RFC4210 - Internet X.509 Public Key Infrastructure Certificate
Network Working Group C. Adams
Request for Comments: 4210 University of Ottawa
Obsoletes: 2510 S. Farrell
Category: Standards Track Trinity College Dublin
T. Kause
SSH
T. Mononen
SafeNet
September 2005
Internet X.509 Public Key Infrastructure
Certificate Management Protocol (CMP)
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 (2005).
Abstract
This document describes the Internet X.509 Public Key Infrastructure
(PKI) Certificate Management Protocol (CMP). Protocol messages are
defined for X.509v3 certificate creation and management. CMP
provides on-line interactions between PKI components, including an
exchange between a Certification Authority (CA) and a client system.
Table of Contents
1. Introduction ....................................................5
2. Requirements ....................................................5
3. PKI Management Overview .........................................5
3.1. PKI Management Model .......................................6
3.1.1. Definitions of PKI Entities .........................6
3.1.1.1. Subjects and End Entities ..................6
3.1.1.2. Certification Authority ....................7
3.1.1.3. Registration Authority .....................7
3.1.2. PKI Management Requirements .........................8
3.1.3. PKI Management Operations ..........................10
4. Assumptions and Restrictions ...................................14
4.1. End Entity Initialization .................................14
4.2. Initial Registration/Certification ........................14
4.2.1. Criteria Used ......................................15
4.2.1.1. Initiation of Registration/Certification ..15
4.2.1.2. End Entity Message Origin Authentication ..15
4.2.1.3. Location of Key Generation ................15
4.2.1.4. Confirmation of Successful Certification ..16
4.2.2. Mandatory Schemes ..................................16
4.2.2.1. Centralized Scheme ........................16
4.2.2.2. Basic Authenticated Scheme ................17
4.3. Proof-of-Possession (POP) of Private Key ..................17
4.3.1. Signature Keys .....................................18
4.3.2. Encryption Keys ....................................18
4.3.3. Key Agreement Keys .................................19
4.4. Root CA Key Update ........................................19
4.4.1. CA Operator Actions ................................20
4.4.2. Verifying Certificates .............................21
4.4.2.1. Verification in Cases 1, 4, 5, and 8 ......22
4.4.2.2. Verification in Case 2 ....................22
4.4.2.3. Verification in Case 3 ....................23
4.4.2.4. Failure of Verification in Case 6 .........23
4.4.2.5. Failure of Verification in Case 7 .........23
4.4.3. Revocation - Change of CA Key ......................23
5. Data Structures ................................................24
5.1. Overall PKI Message .......................................24
5.1.1. PKI Message Header .................................24
5.1.1.1. ImplicitConfirm ...........................27
5.1.1.2. ConfirmWaitTime ...........................27
5.1.2. PKI Message Body ...................................27
5.1.3. PKI Message Protection .............................28
5.1.3.1. Shared Secret Information .................29
5.1.3.2. DH Key Pairs ..............................30
5.1.3.3. Signature .................................30
5.1.3.4. Multiple Protection .......................30
5.2. Common Data Structures ....................................31
5.2.1. Requested Certificate Contents .....................31
5.2.2. Encrypted Values ...................................31
5.2.3. Status codes and Failure Information for
PKI Messages .......................................32
5.2.4. Certificate Identification .........................33
5.2.5. Out-of-band root CA Public Key .....................33
5.2.6. Archive Options ....................................34
5.2.7. Publication Information ............................34
5.2.8. Proof-of-Possession Structures .....................34
5.2.8.1. Inclusion of the Private Key ..............35
5.2.8.2. Indirect Method ...........................35
5.2.8.3. Challenge-Response Protocol ...............35
5.2.8.4. Summary of PoP Options ....................37
5.3. Operation-Specific Data Structures ........................38
5.3.1. Initialization Request .............................38
5.3.2. Initialization Response ............................39
5.3.3. Certification Request ..............................39
5.3.4. Certification Response .............................39
5.3.5. Key Update Request Content .........................40
5.3.6. Key Update Response Content ........................41
5.3.7. Key Recovery Request Content .......................41
5.3.8. Key Recovery Response Content ......................41
5.3.9. Revocation Request Content .........................41
5.3.10. Revocation Response Content .......................42
5.3.11. Cross Certification Request Content ...............42
5.3.12. Cross Certification Response Content ..............42
5.3.13. CA Key Update Announcement Content ................42
5.3.14. Certificate Announcement ..........................43
5.3.15. Revocation Announcement ...........................43
5.3.16. CRL Announcement ..................................43
5.3.17. PKI Confirmation Content ..........................43
5.3.18. Certificate Confirmation Content ..................44
5.3.19. PKI General Message Content .......................44
5.3.19.1. CA Protocol Encryption Certificate .......44
5.3.19.2. Signing Key Pair Types ...................45
5.3.19.3. Encryption/Key Agreement Key Pair Types ..45
5.3.19.4. Preferred Symmetric Algorithm ............45
5.3.19.5. Updated CA Key Pair ......................45
5.3.19.6. CRL ......................................46
5.3.19.7. Unsupported Object Identifiers ...........46
5.3.19.8. Key Pair Parameters ......................46
5.3.19.9. Revocation Passphrase ....................46
5.3.19.10. ImplicitConfirm .........................46
5.3.19.11. ConfirmWaitTime .........................47
5.3.19.12. Original PKIMessage .....................47
5.3.19.13. Supported Language Tags .................47
5.3.20. PKI General Response Content ......................47
5.3.21. Error Message Content .............................47
5.3.22. Polling Request and Response ......................48
6. Mandatory PKI Management Functions .............................51
6.1. Root CA Initialization ....................................51
6.2. Root CA Key Update ........................................51
6.3. Subordinate CA Initialization .............................51
6.4. CRL production ............................................52
6.5. PKI Information Request ...................................52
6.6. Cross Certification .......................................52
6.6.1. One-Way Request-Response Scheme: ...................52
6.7. End Entity Initialization .................................54
6.7.1. Acquisition of PKI Information .....................54
6.7.2. Out-of-Band Verification of Root-CA Key ............55
6.8. Certificate Request .......................................55
6.9. Key Update ................................................55
7. Version Negotiation ............................................56
7.1. Supporting RFC 2510 Implementations .......................56
7.1.1. Clients Talking to RFC 2510 Servers ................56
7.1.2. Servers Receiving Version cmp1999 PKIMessages ......57
8. Security Considerations ........................................57
8.1. Proof-Of-Possession with a Decryption Key .................57
8.2. Proof-Of-Possession by Exposing the Private Key ...........57
8.3. Attack Against Diffie-Hellman Key Exchange ................57
9. IANA Considerations ............................................58
Normative References ..............................................58
Informative References ............................................59
A. Reasons for the Presence of RAs ................................61
B. The Use of Revocation Passphrase ...............................61
C. Request Message Behavioral Clarifications ......................63
D. PKI Management Message Profiles (REQUIRED) .....................65
D.1. General Rules for Interpretation of These Profiles ........65
D.2. Algorithm Use Profile .....................................66
D.3. Proof-of-Possession Profile ...............................68
D.4. Initial Registration/Certification (Basic
Authenticated Scheme) .....................................68
D.5. Certificate Request .......................................74
D.6. Key Update Request ........................................75
E. PKI Management Message Profiles (OPTIONAL) .....................75
E.1. General Rules for Interpretation of These Profiles ........76
E.2. Algorithm Use Profile .....................................76
E.3. Self-Signed Certificates ..................................76
E.4. Root CA Key Update ........................................77
E.5. PKI Information Request/Response ..........................77
E.6. Cross Certification Request/Response (1-way) ..............79
E.7. In-Band Initialization Using External Identity
Certificate ..............................................82
F. Compilable ASN.1 Definitions ...................................83
G. Acknowledgements ...............................................93
1. Introduction
This document describes the Internet X.509 Public Key Infrastructure
(PKI) Certificate Management Protocol (CMP). Protocol messages are
defined for certificate creation and management. The term
"certificate" in this document refers to an X.509v3 Certificate as
defined in [X509].
This specification obsoletes RFC 2510. This specification differs
from RFC 2510 in the following areas:
The PKI management message profile section is split to two
appendices: the required profile and the optional profile. Some
of the formerly mandatory functionality is moved to the optional
profile.
The message confirmation mechanism has changed substantially.
A new polling mechanism is introduced, deprecating the old polling
method at the CMP transport level.
The CMP transport protocol issues are handled in a separate
document [CMPtrans], thus the Transports section is removed.
A new implicit confirmation method is introduced to reduce the
number of protocol messages exchanged in a transaction.
The new specification contains some less prominent protocol
enhancements and improved explanatory text on several issues.
2. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document (in uppercase,
as shown) are to be interpreted as described in [RFC2119].
3. PKI Management Overview
The PKI must be structured to be consistent with the types of
individuals who must administer it. Providing such administrators
with unbounded choices not only complicates the software required,
but also increases the chances that a subtle mistake by an
administrator or software developer will result in broader
compromise. Similarly, restricting administrators with cumbersome
mechanisms will cause them not to use the PKI.
Management protocols are REQUIRED to support on-line interactions
between Public Key Infrastructure (PKI) components. For example, a
management protocol might be used between a Certification Authority
(CA) and a client system with which a key pair is associated, or
between two CAs that issue cross-certificates for each other.
3.1. PKI Management Model
Before specifying particular message formats and procedures, we first
define the entities involved in PKI management and their interactions
(in terms of the PKI management functions required). We then group
these functions in order to accommodate different identifiable types
of end entities.
3.1.1. Definitions of PKI Entities
The entities involved in PKI management include the end entity (i.e.,
the entity to whom the certificate is issued) and the certification
authority (i.e., the entity that issues the certificate). A
registration authority MAY also be involved in PKI management.
3.1.1.1. Subjects and End Entities
The term "subject" is used here to refer to the entity to whom the
certificate is issued, typically named in the subject or
subjectAltName field of a certificate. When we wish to distinguish
the tools and/or software used by the subject (e.g., a local
certificate management module), we will use the term "subject
equipment". In general, the term "end entity" (EE), rather than
"subject", is preferred in order to avoid confusion with the field
name. It is important to note that the end entities here will
include not only human users of applications, but also applications
themselves (e.g., for IP security). This factor influences the
protocols that the PKI management operations use; for example,
application software is far more likely to know exactly which
certificate extensions are required than are human users. PKI
management entities are also end entities in the sense that they are
sometimes named in the subject or subjectAltName field of a
certificate or cross-certificate. Where appropriate, the term "end-
entity" will be used to refer to end entities who are not PKI
management entities.
All end entities require secure local access to some information --
at a minimum, their own name and private key, the name of a CA that
is directly trusted by this entity, and that CA's public key (or a
fingerprint of the public key where a self-certified version is
available elsewhere). Implementations MAY use secure local storage
for more than this minimum (e.g., the end entity's own certificate or
application-specific information). The form of storage will also
vary -- from files to tamper-resistant cryptographic tokens. The
information stored in such local, trusted storage is referred to here
as the end entity's Personal Security Environment (PSE).
Though PSE formats are beyond the scope of this document (they are
very dependent on equipment, et cetera), a generic interchange format
for PSEs is defined here: a certification response message MAY be
used.
3.1.1.2. Certification Authority
The certification authority (CA) may or may not actually be a real
"third party" from the end entity's point of view. Quite often, the
CA will actually belong to the same organization as the end entities
it supports.
Again, we use the term "CA" to refer to the entity named in the
issuer field of a certificate. When it is necessary to distinguish
the software or hardware tools used by the CA, we use the term "CA
equipment".
The CA equipment will often include both an "off-line" component and
an "on-line" component, with the CA private key only available to the
"off-line" component. This is, however, a matter for implementers
(though it is also relevant as a policy issue).
We use the term "root CA" to indicate a CA that is directly trusted
by an end entity; that is, securely acquiring the value of a root CA
public key requires some out-of-band step(s). This term is not meant
to imply that a root CA is necessarily at the top of any hierarchy,
simply that the CA in question is trusted directly.
A "subordinate CA" is one that is not a root CA for the end entity in
question. Often, a subordinate CA will not be a root CA for any
entity, but this is not mandatory.
3.1.1.3. Registration Authority
In addition to end-entities and CAs, many environments call for the
existence of a Registration Authority (RA) separate from the
Certification Authority. The functions that the registration
authority may carry out will vary from case to case but MAY include
personal authentication, token distribution, revocation reporting,
name assignment, key generation, archival of key pairs, et cetera.
This document views the RA as an OPTIONAL component: when it is not
present, the CA is assumed to be able to carry out the RA's functions
so that the PKI management protocols are the same from the end-
entity's point of view.
Again, we distinguish, where necessary, between the RA and the tools
used (the "RA equipment").
Note that an RA is itself an end entity. We further assume that all
RAs are in fact certified end entities and that RAs have private keys
that are usable for signing. How a particular CA equipment
identifies some end entities as RAs is an implementation issue (i.e.,
this document specifies no special RA certification operation). We
do not mandate that the RA is certified by the CA with which it is
interacting at the moment (so one RA may work with more than one CA
whilst only being certified once).
In some circumstances, end entities will communicate directly with a
CA even where an RA is present. For example, for initial
registration and/or certification, the subject may use its RA, but
communicate directly with the CA in order to refresh its certificate.
3.1.2. PKI Management Requirements
The protocols given here meet the following requirements on PKI
management
1. PKI management must conform to the ISO/IEC 9594-8/ITU-T X.509
standards.
2. It must be possible to regularly update any key pair without
affecting any other key pair.
3. The use of confidentiality in PKI management protocols must be
kept to a minimum in order to ease acceptance in environments
where strong confidentiality might cause regulatory problems.
4. PKI management protocols must allow the use of different
industry-standard cryptographic algorithms (specifically
including RSA, DSA, MD5, and SHA-1). This means that any given
CA, RA, or end entity may, in principle, use whichever
algorithms suit it for its own key pair(s).
5. PKI management protocols must not preclude the generation of key
pairs by the end-entity concerned, by an RA, or by a CA. Key
generation may also occur elsewhere, but for the purposes of PKI
management we can regard key generation as occurring wherever
the key is first present at an end entity, RA, or CA.
6. PKI management protocols must support the publication of
certificates by the end-entity concerned, by an RA, or by a CA.
Different implementations and different environments may choose
any of the above approaches.
7. PKI management protocols must support the production of
Certificate Revocation Lists (CRLs) by allowing certified end
entities to make requests for the revocation of certificates.
This must be done in such a way that the denial-of-service
attacks, which are possible, are not made simpler.
8. PKI management protocols must be usable over a variety of
"transport" mechanisms, specifically including mail, http,
TCP/IP and ftp.
9. Final authority for certification creation rests with the CA.
No RA or end-entity equipment can assume that any certificate
issued by a CA will contain what was requested; a CA may alter
certificate field values or may add, delete, or alter extensions
according to its operating policy. In other words, all PKI
entities (end-entities, RAs, and CAs) must be capable of
handling responses to requests for certificates in which the
actual certificate issued is different from that requested (for
example, a CA may shorten the validity period requested). Note
that policy may dictate that the CA must not publish or
otherwise distribute the certificate until the requesting entity
has reviewed and accepted the newly-created certificate
(typically through use of the certConf message).
10. A graceful, scheduled change-over from one non-compromised CA
key pair to the next (CA key update) must be supported (note
that if the CA key is compromised, re-initialization must be
performed for all entities in the domain of that CA). An end
entity whose PSE contains the new CA public key (following a CA
key update) must also be able to verify certificates verifiable
using the old public key. End entities who directly trust the
old CA key pair must also be able to verify certificates signed
using the new CA private key (required for situations where the
old CA public key is "hardwired" into the end entity's
cryptographic equipment).
11. The functions of an RA may, in some implementations or
environments, be carried out by the CA itself. The protocols
must be designed so that end entities will use the same protocol
regardless of whether the communication is with an RA or CA.
Naturally, the end entity must use the correct RA of CA public
key to protect the communication.
12. Where an end entity requests a certificate containing a given
public key value, the end entity must be ready to demonstrate
possession of the corresponding private key value. This may be
accomplished in various ways, depending on the type of
certification request. See Section 4.3 for details of the in-
band methods defined for the PKIX-CMP (i.e., Certificate
Management Protocol) messages.
3.1.3. PKI Management Operations
The following diagram shows the relationship between the entities
defined above in terms of the PKI management operations. The letters
in the diagram indicate "protocols" in the sense that a defined set
of PKI management messages can be sent along each of the lettered
lines.
+---+ cert. publish +------------+ j
| | <--------------------- | End Entity | <-------
| C | g +------------+ "out-of-band"
| e | | ^ loading
| r | | | initial
| t | a | | b registration/
| | | | certification
| / | | | key pair recovery
| | | | key pair update
| C | | | certificate update
| R | PKI "USERS" V | revocation request
| L | -------------------+-+-----+-+------+-+-------------------
| | PKI MANAGEMENT | ^ | ^
| | ENTITIES a | | b a | | b
| R | V | | |
| e | g +------+ d | |
| p | <------------ | RA | <-----+ | |
| o | cert. | | ----+ | | |
| s | publish +------+ c | | | |
| i | | | | |
| t | V | V |
| o | g +------------+ i
| r | <------------------------| CA |------->
| y | h +------------+ "out-of-band"
| | cert. publish | ^ publication
| | CRL publish | |
+---+ | | cross-certification
e | | f cross-certificate
| | update
| |
V |
+------+
| CA-2 |
+------+
Figure 1 - PKI Entities
At a high level, the set of operations for which management
messages are defined can be grouped as follows.
1. CA establishment: When establishing a new CA, certain steps are
required (e.g., production of initial CRLs, export of CA public
key).
2. End entity initialization: this includes importing a root CA
public key and requesting information about the options supported
by a PKI management entity.
3. Certification: various operations result in the creation of new
certificates:
1. initial registration/certification: This is the process
whereby an end entity first makes itself known to a CA or RA,
prior to the CA issuing a certificate or certificates for
that end entity. The end result of this process (when it is
successful) is that a CA issues a certificate for an end
entity's public key, and returns that certificate to the end
entity and/or posts that certificate in a public repository.
This process may, and typically will, involve multiple
"steps", possibly including an initialization of the end
entity's equipment. For example, the end entity's equipment
must be securely initialized with the public key of a CA, to
be used in validating certificate paths. Furthermore, an end
entity typically needs to be initialized with its own key
pair(s).
2. key pair update: Every key pair needs to be updated regularly
(i.e., replaced with a new key pair), and a new certificate
needs to be issued.
3. certificate update: As certificates expire, they may be
"refreshed" if nothing relevant in the environment has
changed.
4. CA key pair update: As with end entities, CA key pairs need
to be updated regularly; however, different mechanisms are
required.
5. cross-certification request: One CA requests issuance of a
cross-certificate from another CA. For the purposes of this
standard, the following terms are defined. A "cross-
certificate" is a certificate in which the subject CA and the
issuer CA are distinct and SubjectPublicKeyInfo contains a
verification key (i.e., the certificate has been issued for
the subject CA's signing key pair). When it is necessary to
distinguish more finely, the following terms may be used: a
cross-certificate is called an "inter-domain cross-
certificate" if the subject and issuer CAs belong to
different administrative domains; it is called an "intra-
domain cross-certificate" otherwise.
1. Note 1. The above definition of "cross-certificate"
aligns with the defined term "CA-certificate" in X.509.
Note that this term is not to be confused with the X.500
"cACertificate" attribute type, which is unrelated.
2. Note 2. In many environments, the term "cross-
certificate", unless further qualified, will be
understood to be synonymous with "inter-domain cross-
certificate" as defined above.
3. Note 3. Issuance of cross-certificates may be, but is
not necessarily, mutual; that is, two CAs may issue
cross-certificates for each other.
6. cross-certificate update: Similar to a normal certificate
update, but involving a cross-certificate.
4. Certificate/CRL discovery operations: some PKI management
operations result in the publication of certificates or CRLs:
1. certificate publication: Having gone to the trouble of
producing a certificate, some means for publishing it is
needed. The "means" defined in PKIX MAY involve the messages
specified in Sections 5.3.13 to 5.3.16, or MAY involve other
methods (LDAP, for example) as described in [RFC2559],
[RFC2585] (the "Operational Protocols" documents of the PKIX
series of specifications).
2. CRL publication: As for certificate publication.
5. Recovery operations: some PKI management operations are used when
an end entity has "lost" its PSE:
1. key pair recovery: As an option, user client key materials
(e.g., a user's private key used for decryption purposes) MAY
be backed up by a CA, an RA, or a key backup system
associated with a CA or RA. If an entity needs to recover
these backed up key materials (e.g., as a result of a
forgotten password or a lost key chain file), a protocol
exchange may be needed to support such recovery.
6. Revocation operations: some PKI operations result in the creation
of new CRL entries and/or new CRLs:
1. revocation request: An authorized person advises a CA of an
abnormal situation requiring certificate revocation.
7. PSE operations: whilst the definition of PSE operations (e.g.,
moving a PSE, changing a PIN, etc.) are beyond the scope of this
specification, we do define a PKIMessage (CertRepMessage) that
can form the basis of such operations.
Note that on-line protocols are not the only way of implementing the
above operations. For all operations, there are off-line methods of
achieving the same result, and this specification does not mandate
use of on-line protocols. For example, when hardware tokens are
used, many of the operations MAY be achieved as part of the physical
token delivery.
Later sections define a set of standard messages supporting the above
operations. Transport protocols for conveying these exchanges in
different environments (file-based, on-line, E-mail, and WWW) are
beyond the scope of this document and are specified separately.
4. Assumptions and Restrictions
4.1. End Entity Initialization
The first step for an end entity in dealing with PKI management
entities is to request information about the PKI functions supported
and to securely acquire a copy of the relevant root CA public key(s).
4.2. Initial Registration/Certification
There are many schemes that can be used to achieve initial
registration and certification of end entities. No one method is
suitable for all situations due to the range of policies that a CA
may implement and the variation in the types of end entity which can
occur.
However, we can classify the initial registration/certification
schemes that are supported by this specification. Note that the word
"initial", above, is crucial: we are dealing with the situation where
the end entity in question has had no previous contact with the PKI.
Where the end entity already possesses certified keys, then some
simplifications/alternatives are possible.
Having classified the schemes that are supported by this
specification we can then specify some as mandatory and some as
optional. The goal is that the mandatory schemes cover a sufficient
number of the cases that will arise in real use, whilst the optional
schemes are available for special cases that arise less frequently.
In this way, we achieve a balance between flexibility and ease of
implementation.
We will now describe the classification of initial
registration/certification schemes.
4.2.1. Criteria Used
4.2.1.1. Initiation of Registration/Certification
In terms of the PKI messages that are produced, we can regard the
initiation of the initial registration/certification exchanges as
occurring wherever the first PKI message relating to the end entity
is produced. Note that the real-world initiation of the
registration/certification procedure may occur elsewhere (e.g., a
personnel department may telephone an RA operator).
The possible locations are at the end entity, an RA, or a CA.
4.2.1.2. End Entity Message Origin Authentication
The on-line messages produced by the end entity that requires a
certificate may be authenticated or not. The requirement here is to
authenticate the origin of any messages from the end entity to the
PKI (CA/RA).
In this specification, such authentication is achieved by the PKI
(CA/RA) issuing the end entity with a secret value (initial
authentication key) and reference value (used to identify the secret
value) via some out-of-band means. The initial authentication key
can then be used to protect relevant PKI messages.
Thus, we can classify the initial registration/certification scheme
according to whether or not the on-line end entity -> PKI messages
are authenticated or not.
Note 1: We do not discuss the authentication of the PKI -> end entity
messages here, as this is always REQUIRED. In any case, it can be
achieved simply once the root-CA public key has been installed at the
end entity's equipment or it can be based on the initial
authentication key.
Note 2: An initial registration/certification procedure can be secure
where the messages from the end entity are authenticated via some
out-of-band means (e.g., a subsequent visit).
4.2.1.3. Location of Key Generation
In this specification, "key generation" is regarded as occurring
wherever either the public or private component of a key pair first
occurs in a PKIMessage. Note that this does not preclude a
centralized key generation service; the actual key pair MAY have been
generated elsewhere and transported to the end entity, RA, or CA
using a (proprietary or standardized) key generation request/response
protocol (outside the scope of this specification).
Thus, there are three possibilities for the location of "key
generation": the end entity, an RA, or a CA.
4.2.1.4. Confirmation of Successful Certification
Following the creation of an initial certificate for an end entity,
additional assurance can be gained by having the end entity
explicitly confirm successful receipt of the message containing (or
indicating the creation of) the certificate. Naturally, this
confirmation message must be protected (based on the initial
authentication key or other means).
This gives two further possibilities: confirmed or not.
4.2.2. Mandatory Schemes
The criteria above allow for a large number of initial
registration/certification schemes. This specification mandates that
conforming CA equipment, RA equipment, and EE equipment MUST support
the second scheme listed below (Section 4.2.2.2). Any entity MAY
additionally support other schemes, if desired.
4.2.2.1. Centralized Scheme
In terms of the classification above, this scheme is, in some ways,
the simplest possible, where:
o initiation occurs at the certifying CA;
o no on-line message authentication is required;
o "key generation" occurs at the certifying CA (see Section
4.2.1.3);
o no confirmation message is required.
In terms of message flow, this scheme means that the only message
required is sent from the CA to the end entity. The message must
contain the entire PSE for the end entity. Some out-of-band means
must be provided to allow the end entity to authenticate the message
received and to decrypt any encrypted values.
4.2.2.2. Basic Authenticated Scheme
In terms of the classification above, this scheme is where:
o initiation occurs at the end entity;
o message authentication is REQUIRED;
o "key generation" occurs at the end entity (see Section 4.2.1.3);
o a confirmation message is REQUIRED.
In terms of message flow, the basic authenticated scheme is as
follows:
End entity RA/CA
========== =============
out-of-band distribution of Initial Authentication
Key (IAK) and reference value (RA/CA -> EE)
Key generation
Creation of certification request
Protect request with IAK
-->>-- certification request -->>--
verify request
process request
create response
--<<-- certification response --<<--
handle response
create confirmation
-->>-- cert conf message -->>--
verify confirmation
create response
--<<-- conf ack (optional) --<<--
handle response
(Where verification of the cert confirmation message fails, the RA/CA
MUST revoke the newly issued certificate if it has been published or
otherwise made available.)
4.3. Proof-of-Possession (POP) of Private Key
In order to prevent certain attacks and to allow a CA/RA to properly
check the validity of the binding between an end entity and a key
pair, the PKI management operations specified here make it possible
for an end entity to prove that it has possession of (i.e., is able
to use) the private key corresponding to the public key for which a
certificate is requested. A given CA/RA is free to choose how to
enforce POP (e.g., out-of-band procedural means versus PKIX-CMP
in-band messages) in its certification exchanges (i.e., this may be a
policy issue). However, it is REQUIRED that CAs/RAs MUST enforce POP
by some means because there are currently many non-PKIX operational
protocols in use (various electronic mail protocols are one example)
that do not explicitly check the binding between the end entity and
the private key. Until operational protocols that do verify the
binding (for signature, encryption, and key agreement key pairs)
exist, and are ubiquitous, this binding can only be assumed to have
been verified by the CA/RA. Therefore, if the binding is not
verified by the CA/RA, certificates in the Internet Public-Key
Infrastructure end up being somewhat less meaningful.
POP is accomplished in different ways depending upon the type of key
for which a certificate is requested. If a key can be used for
multiple purposes (e.g., an RSA key) then any appropriate method MAY
be used (e.g., a key that may be used for signing, as well as other
purposes, SHOULD NOT be sent to the CA/RA in order to prove
possession).
This specification explicitly allows for cases where an end entity
supplies the relevant proof to an RA and the RA subsequently attests
to the CA that the required proof has been received (and validated!).
For example, an end entity wishing to have a signing key certified
could send the appropriate signature to the RA, which then simply
notifies the relevant CA that the end entity has supplied the
required proof. Of course, such a situation may be disallowed by
some policies (e.g., CAs may be the only entities permitted to verify
POP during certification).
4.3.1. Signature Keys
For signature keys, the end entity can sign a value to prove
possession of the private key.
4.3.2. Encryption Keys
For encryption keys, the end entity can provide the private key to
the CA/RA, or can be required to decrypt a value in order to prove
possession of the private key (see Section 5.2.8). Decrypting a
value can be achieved either directly or indirectly.
The direct method is for the RA/CA to issue a random challenge to
which an immediate response by the EE is required.
The indirect method is to issue a certificate that is encrypted for
the end entity (and have the end entity demonstrate its ability to
decrypt this certificate in the confirmation message). This allows a
CA to issue a certificate in a form that can only be used by the
intended end entity.
This specification encourages use of the indirect method because it
requires no extra messages to be sent (i.e., the proof can be
demonstrated using the {request, response, confirmation} triple of
messages).
4.3.3. Key Agreement Keys
For key agreement keys, the end entity and the PKI management entity
(i.e., CA or RA) must establish a shared secret key in order to prove
that the end entity has possession of the private key.
Note that this need not impose any restrictions on the keys that can
be certified by a given CA. In particular, for Diffie-Hellman keys
the end entity may freely choose its algorithm parameters provided
that the CA can generate a short-term (or one-time) key pair with the
appropriate parameters when necessary.
4.4. Root CA Key Update
This discussion only applies to CAs that are directly trusted by some
end entities. Self-signed CAs SHALL be considered as directly
trusted CAs. Recognizing whether a non-self-signed CA is supposed to
be directly trusted for some end entities is a matter of CA policy
and is thus beyond the scope of this document.
The basis of the procedure described here is that the CA protects its
new public key using its previous private key and vice versa. Thus,
when a CA updates its key pair it must generate two extra
cACertificate attribute values if certificates are made available
using an X.500 directory (for a total of four: OldWithOld,
OldWithNew, NewWithOld, and NewWithNew).
When a CA changes its key pair, those entities who have acquired the
old CA public key via "out-of-band" means are most affected. It is
these end entities who will need access to the new CA public key
protected with the old CA private key. However, they will only
require this for a limited period (until they have acquired the new
CA public key via the "out-of-band" mechanism). This will typically
be easily achieved when these end entities' certificates expire.
The data structure used to protect the new and old CA public keys is
a standard certificate (which may also contain extensions). There
are no new data structures required.
Note 1. This scheme does not make use of any of the X.509 v3
extensions as it must be able to work even for version 1
certificates. The presence of the KeyIdentifier extension would make
for efficiency improvements.
Note 2. While the scheme could be generalized to cover cases where
the CA updates its key pair more than once during the validity period
of one of its end entities' certificates, this generalization seems
of dubious value. Not having this generalization simply means that
the validity periods of certificates issued with the old CA key pair
cannot exceed the end of the OldWithNew validity period.
Note 3. This scheme ensures that end entities will acquire the new
CA public key, at the latest by the expiry of the last certificate
they owned that was signed with the old CA private key (via the
"out-of-band" means). Certificate and/or key update operations
occurring at other times do not necessarily require this (depending
on the end entity's equipment).
4.4.1. CA Operator Actions
To change the key of the CA, the CA operator does the following:
1. Generate a new key pair;
2. Create a certificate containing the old CA public key signed with
the new private key (the "old with new" certificate);
3. Create a certificate containing the new CA public key signed with
the old private key (the "new with old" certificate);
4. Create a certificate containing the new CA public key signed with
the new private key (the "new with new" certificate);
5. Publish these new certificates via the repository and/or other
means (perhaps using a CAKeyUpdAnn message);
6. Export the new CA public key so that end entities may acquire it
using the "out-of-band" mechanism (if required).
The old CA private key is then no longer required. However, the old
CA public key will remain in use for some time. The old CA public
key is no longer required (other than for non-repudiation) when all
end entities of this CA have securely acquired the new CA public key.
The "old with new" certificate must have a validity period starting
at the generation time of the old key pair and ending at the expiry
date of the old public key.
The "new with old" certificate must have a validity period starting
at the generation time of the new key pair and ending at the time by
which all end entities of this CA will securely possess the new CA
public key (at the latest, the expiry date of the old public key).
The "new with new" certificate must have a validity period starting
at the generation time of the new key pair and ending at or before
the time by which the CA will next update its key pair.
4.4.2. Verifying Certificates
Normally when verifying a signature, the verifier verifies (among
other things) the certificate containing the public key of the
signer. However, once a CA is allowed to update its key there are a
range of new possibilities. These are shown in the table below.
Repository contains NEW Repository contains only OLD
and OLD public keys public key (due to, e.g.,
delay in publication)
PSE PSE Contains PSE Contains PSE Contains
Contains OLD public NEW public OLD public
NEW public key key key
key
Signer's Case 1: Case 3: Case 5: Case 7:
certifi- This is In this case Although the In this case
cate is the the verifier CA operator the CA
protected standard must access has not operator has
using NEW case where the updated the not updated
public the repository in repository the the repository
key verifier order to get verifier can and so the
can the value of verify the verification
directly the NEW certificate will FAIL
verify the public key directly -
certificate this is thus
without the same as
using the case 1.
repository
Signer's Case 2: Case 4: Case 6: Case 8:
certifi- In this In this case The verifier Although the
cate is case the the verifier thinks this CA operator
protected verifier can directly is the has not
using OLD must verify the situation of updated the
public access the certificate case 2 and repository the
key repository without will access verifier can
in order using the the verify the
to get the repository repository; certificate
value of however, the directly -
the OLD verification this is thus
public key will FAIL the same as
case 4.
4.4.2.1. Verification in Cases 1, 4, 5, and 8
In these cases, the verifier has a local copy of the CA public key
that can be used to verify the certificate directly. This is the
same as the situation where no key change has occurred.
Note that case 8 may arise between the time when the CA operator has
generated the new key pair and the time when the CA operator stores
the updated attributes in the repository. Case 5 can only arise if
the CA operator has issued both the signer's and verifier's
certificates during this "gap" (the CA operator SHOULD avoid this as
it leads to the failure cases described below)
4.4.2.2. Verification in Case 2
In case 2, the verifier must get access to the old public key of the
CA. The verifier does the following:
1. Look up the caCertificate attribute in the repository and pick
the OldWithNew certificate (determined based on validity periods;
note that the subject and issuer fields must match);
2. Verify that this is correct using the new CA key (which the
verifier has locally);
3. If correct, check the signer's certificate using the old CA key.
Case 2 will arise when the CA operator has issued the signer's
certificate, then changed the key, and then issued the verifier's
certificate; so it is quite a typical case.
4.4.2.3. Verification in Case 3
In case 3, the verifier must get access to the new public key of the
CA. The verifier does the following:
1. Look up the CACertificate attribute in the repository and pick
the NewWithOld certificate (determined based on validity periods;
note that the subject and issuer fields must match);
2. Verify that this is correct using the old CA key (which the
verifier has stored locally);
3. If correct, check the signer's certificate using the new CA key.
Case 3 will arise when the CA operator has issued the verifier's
certificate, then changed the key, and then issued the signer's
certificate; so it is also quite a typical case.
4.4.2.4. Failure of Verification in Case 6
In this case, the CA has issued the verifier's PSE, which contains
the new key, without updating the repository attributes. This means
that the verifier has no means to get a trustworthy version of the
CA's old key and so verification fails.
Note that the failure is the CA operator's fault.
4.4.2.5. Failure of Verification in Case 7
In this case, the CA has issued the signer's certificate protected
with the new key without updating the repository attributes. This
means that the verifier has no means to get a trustworthy version of
the CA's new key and so verification fails.
Note that the failure is again the CA operator's fault.
4.4.3. Revocation - Change of CA Key
As we saw above, the verification of a certificate becomes more
complex once the CA is allowed to change its key. This is also true
for revocation checks as the CA may have signed the CRL using a newer
private key than the one within the user's PSE.
The analysis of the alternatives is the same as for certificate
verification.
5. Data Structures
This section contains descriptions of the data structures required
for PKI management messages. Section 6 describes constraints on
their values and the sequence of events for each of the various PKI
management operations.
5.1. Overall PKI Message
All of the messages used in this specification for the purposes of
PKI management use the following structure:
PKIMessage ::= SEQUENCE {
header PKIHeader,
body PKIBody,
protection [0] PKIProtection OPTIONAL,
extraCerts [1] SEQUENCE SIZE (1..MAX) OF CMPCertificate
OPTIONAL
}
PKIMessages ::= SEQUENCE SIZE (1..MAX) OF PKIMessage
The PKIHeader contains information that is common to many PKI
messages.
The PKIBody contains message-specific information.
The PKIProtection, when used, contains bits that protect the PKI
message.
The extraCerts field can contain certificates that may be useful to
the recipient. For example, this can be used by a CA or RA to
present an end entity with certificates that it needs to verify its
own new certificate (if, for example, the CA that issued the end
entity's certificate is not a root CA for the end entity). Note that
this field does not necessarily contain a certification path; the
recipient may have to sort, select from, or otherwise process the
extra certificates in order to use them.
5.1.1. PKI Message Header
All PKI messages require some header information for addressing and
transaction identification. Some of this information will also be
present in a transport-specific envelope. However, if the PKI
message is protected, then this information is also protected (i.e.,
we make no assumption about secure transport).
The following data structure is used to contain this information:
PKIHeader ::= SEQUENCE {
pvno INTEGER { cmp1999(1), cmp2000(2) },
sender GeneralName,
recipient GeneralName,
messageTime [0] GeneralizedTime OPTIONAL,
protectionAlg [1] AlgorithmIdentifier OPTIONAL,
senderKID [2] KeyIdentifier OPTIONAL,
recipKID [3] KeyIdentifier OPTIONAL,
transactionID [4] OCTET STRING OPTIONAL,
senderNonce [5] OCTET STRING OPTIONAL,
recipNonce [6] OCTET STRING OPTIONAL,
freeText [7] PKIFreeText OPTIONAL,
generalInfo [8] SEQUENCE SIZE (1..MAX) OF
InfoTypeAndValue OPTIONAL
}
PKIFreeText ::= SEQUENCE SIZE (1..MAX) OF UTF8String
The pvno field is fixed (at 2) for this version of this
specification.
The sender field contains the name of the sender of the PKIMessage.
This name (in conjunction with senderKID, if supplied) should be
sufficient to indicate the key to use to verify the protection on the
message. If nothing about the sender is known to the sending entity
(e.g., in the init. req. message, where the end entity may not know
its own Distinguished Name (DN), e-mail name, IP address, etc.), then
the "sender" field MUST contain a "NULL" value; that is, the SEQUENCE
OF relative distinguished names is of zero length. In such a case,
the senderKID field MUST hold an identifier (i.e., a reference
number) that indicates to the receiver the appropriate shared secret
information to use to verify the message.
The recipient field contains the name of the recipient of the
PKIMessage. This name (in conjunction with recipKID, if supplied)
should be usable to verify the protection on the message.
The protectionAlg field specifies the algorithm used to protect the
message. If no protection bits are supplied (note that PKIProtection
is OPTIONAL) then this field MUST be omitted; if protection bits are
supplied, then this field MUST be supplied.
senderKID and recipKID are usable to indicate which keys have been
used to protect the message (recipKID will normally only be required
where protection of the message uses Diffie-Hellman (DH) keys).
These fields MUST be used if required to uniquely identify a key
(e.g., if more than one key is associated with a given sender name)
and SHOULD be omitted otherwise.
The transactionID field within the message header is to be used to
allow the recipient of a message to correlate this with an ongoing
transaction. This is needed for all transactions that consist of
more than just a single request/response pair. For transactions that
consist of a single request/response pair, the rules are as follows.
A client MAY populate the transactionID field of the request. If a
server receives such a request that has the transactionID field set,
then it MUST set the transactionID field of the response to the same
value. If a server receives such request with a missing
transactionID field, then it MAY set transactionID field of the
response.
For transactions that consist of more than just a single
request/response pair, the rules are as follows. Clients SHOULD
generate a transactionID for the first request. If a server receives
such a request that has the transactionID field set, then it MUST set
the transactionID field of the response to the same value. If a
server receives such request with a missing transactionID field, then
it MUST populate the transactionID field of the response with a
server-generated ID. Subsequent requests and responses MUST all set
the transactionID field to the thus established value. In all cases
where a transactionID is being used, a given client MUST NOT have
more than one transaction with the same transactionID in progress at
any time (to a given server). Servers are free to require uniqueness
of the transactionID or not, as long as they are able to correctly
associate messages with the corresponding transaction. Typically,
this means that a server will require the {client, transactionID}
tuple to be unique, or even the transactionID alone to be unique, if
it cannot distinguish clients based on transport-level information.
A server receiving the first message of a transaction (which requires
more than a single request/response pair) that contains a
transactionID that does not allow it to meet the above constraints
(typically because the transactionID is already in use) MUST send
back an ErrorMsgContent with a PKIFailureInfo of transactionIdInUse.
It is RECOMMENDED that the clients fill the transactionID field with
128 bits of (pseudo-) random data for the start of a transaction to
reduce the probability of having the transactionID in use at the
server.
The senderNonce and recipNonce fields protect the PKIMessage against
replay attacks. The senderNonce will typically be 128 bits of
(pseudo-) random data generated by the sender, whereas the recipNonce
is copied from the senderNonce of the previous message in the
transaction.
The messageTime field contains the time at which the sender created
the message. This may be useful to allow end entities to
correct/check their local time for consistency with the time on a
central system.
The freeText field may be used to send a human-readable message to
the recipient (in any number of languages). The first language used
in this sequence indicates the desired language for replies.
The generalInfo field may be used to send machine-processable
additional data to the recipient. The following generalInfo
extensions are defined and MAY be supported.
5.1.1.1. ImplicitConfirm
This is used by the EE to inform the CA that it does not wish to send
a certificate confirmation for issued certificates.
implicitConfirm OBJECT IDENTIFIER ::= {id-it 13}
ImplicitConfirmValue ::= NULL
If the CA grants the request to the EE, it MUST put the same
extension in the PKIHeader of the response. If the EE does not find
the extension in the response, it MUST send the certificate
confirmation.
5.1.1.2. ConfirmWaitTime
This is used by the CA to inform the EE how long it intends to wait
for the certificate confirmation before revoking the certificate and
deleting the transaction.
confirmWaitTime OBJECT IDENTIFIER ::= {id-it 14}
ConfirmWaitTimeValue ::= GeneralizedTime
5.1.2. PKI Message Body
PKIBody ::= CHOICE {
ir [0] CertReqMessages, --Initialization Req
ip [1] CertRepMessage, --Initialization Resp
cr [2] CertReqMessages, --Certification Req
cp [3] CertRepMessage, --Certification Resp
p10cr [4] CertificationRequest, --PKCS #10 Cert. Req.
popdecc [5] POPODecKeyChallContent --pop Challenge
popdecr [6] POPODecKeyRespContent, --pop Response
kur [7] CertReqMessages, --Key Update Request
kup [8] CertRepMessage, --Key Update Response
krr [9] CertReqMessages, --Key Recovery Req
krp [10] KeyRecRepContent, --Key Recovery Resp
rr [11] RevReqContent, --Revocation Request
rp [12] RevRepContent, --Revocation Response
ccr [13] CertReqMessages, --Cross-Cert. Request
ccp [14] CertRepMessage, --Cross-Cert. Resp
ckuann [15] CAKeyUpdAnnContent, --CA Key Update Ann.
cann [16] CertAnnContent, --Certificate Ann.
rann [17] RevAnnContent, --Revocation Ann.
crlann [18] CRLAnnContent, --CRL Announcement
pkiconf [19] PKIConfirmContent, --Confirmation
nested [20] NestedMessageContent, --Nested Message
genm [21] GenMsgContent, --General Message
genp [22] GenRepContent, --General Response
error [23] ErrorMsgContent, --Error Message
certConf [24] CertConfirmContent, --Certificate confirm
pollReq [25] PollReqContent, --Polling request
pollRep [26] PollRepContent --Polling response
}
The specific types are described in Section 5.3 below.
5.1.3. PKI Message Protection
Some PKI messages will be protected for integrity. (Note that if an
asymmetric algorithm is used to protect a message and the relevant
public component has been certified already, then the origin of the
message can also be authenticated. On the other hand, if the public
component is uncertified, then the message origin cannot be
automatically authenticated, but may be authenticated via out-of-band
means.)
When protection is applied, the following structure is used:
PKIProtection ::= BIT STRING
The input to the calculation of PKIProtection is the DER encoding of
the following data structure:
ProtectedPart ::= SEQUENCE {
header PKIHeader,
body PKIBody
}
There MAY be cases in which the PKIProtection BIT STRING is
deliberately not used to protect a message (i.e., this OPTIONAL field
is omitted) because other protection, external to PKIX, will be
applied instead. Such a choice is explicitly allowed in this
specification. Examples of such external protection include PKCS #7
[PKCS7] and Security Multiparts [RFC1847] encapsulation of the
PKIMessage (or simply the PKIBody (omitting the CHOICE tag), if the
relevant PKIHeader information is securely carried in the external
mechanism). It is noted, however, that many such external mechanisms
require that the end entity already possesses a public-key
certificate, and/or a unique Distinguished Name, and/or other such
infrastructure-related information. Thus, they may not be
appropriate for initial registration, key-recovery, or any other
process with "boot-strapping" characteristics. For those cases it
may be necessary that the PKIProtection parameter be used. In the
future, if/when external mechanisms are modified to accommodate
boot-strapping scenarios, the use of PKIProtection may become rare or
non-existent.
Depending on the circumstances, the PKIProtection bits may contain a
Message Authentication Code (MAC) or signature. Only the following
cases can occur:
5.1.3.1. Shared Secret Information
In this case, the sender and recipient share secret information
(established via out-of-band means or from a previous PKI management
operation). PKIProtection will contain a MAC value and the
protectionAlg will be the following (see also Appendix D.2):
id-PasswordBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 13}
PBMParameter ::= SEQUENCE {
salt OCTET STRING,
owf AlgorithmIdentifier,
iterationCount INTEGER,
mac AlgorithmIdentifier
}
In the above protectionAlg, the salt value is appended to the shared
secret input. The OWF is then applied iterationCount times, where
the salted secret is the input to the first iteration and, for each
successive iteration, the input is set to be the output of the
previous iteration. The output of the final iteration (called
"BASEKEY" for ease of reference, with a size of "H") is what is used
to form the symmetric key. If the MAC algorithm requires a K-bit key
and K <= H, then the most significant K bits of BASEKEY are used. If
K > H, then all of BASEKEY is used for the most significant H bits of
the key, OWF("1" || BASEKEY) is used for the next most significant H
bits of the key, OWF("2" || BASEKEY) is used for the next most
significant H bits of the key, and so on, until all K bits have been
derived. [Here "N" is the ASCII byte encoding the number N and "||"
represents concatenation.]
Note: it is RECOMMENDED that the fields of PBMParameter remain
constant throughout the messages of a single transaction (e.g.,
ir/ip/certConf/pkiConf) in order to reduce the overhead associated
with PasswordBasedMac computation).
5.1.3.2. DH Key Pairs
Where the sender and receiver possess Diffie-Hellman certificates
with compatible DH parameters, in order to protect the message the
end entity must generate a symmetric key based on its private DH key
value and the DH public key of the recipient of the PKI message.
PKIProtection will contain a MAC value keyed with this derived
symmetric key and the protectionAlg will be the following:
id-DHBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 30}
DHBMParameter ::= SEQUENCE {
owf AlgorithmIdentifier,
-- AlgId for a One-Way Function (SHA-1 recommended)
mac AlgorithmIdentifier
-- the MAC AlgId (e.g., DES-MAC, Triple-DES-MAC [PKCS11],
} -- or HMAC [RFC2104, RFC2202])
In the above protectionAlg, OWF is applied to the result of the
Diffie-Hellman computation. The OWF output (called "BASEKEY" for
ease of reference, with a size of "H") is what is used to form the
symmetric key. If the MAC algorithm requires a K-bit key and K <= H,
then the most significant K bits of BASEKEY are used. If K > H, then
all of BASEKEY is used for the most significant H bits of the key,
OWF("1" || BASEKEY) is used for the next most significant H bits of
the key, OWF("2" || BASEKEY) is used for the next most significant H
bits of the key, and so on, until all K bits have been derived.
[Here "N" is the ASCII byte encoding the number N and "||" represents
concatenation.]
5.1.3.3. Signature
In this case, the sender possesses a signature key pair and simply
signs the PKI message. PKIProtection will contain the signature
value and the protectionAlg will be an AlgorithmIdentifier for a
digital signature (e.g., md5WithRSAEncryption or dsaWithSha-1).
5.1.3.4. Multiple Protection
In cases where an end entity sends a protected PKI message to an RA,
the RA MAY forward that message to a CA, attaching its own protection
(which MAY be a MAC or a signature, depending on the information and
certificates shared between the RA and the CA). This is accomplished
by nesting the entire message sent by the end entity within a new PKI
message. The structure used is as follows.
NestedMessageContent ::= PKIMessages
(The use of PKIMessages, a SEQUENCE OF PKIMessage, lets the RA batch
the requests of several EEs in a single new message. For simplicity,
all messages in the batch MUST be of the same type (e.g., ir).) If
the RA wishes to modify the message(s) in some way (e.g., add
particular field values or new extensions), then it MAY create its
own desired PKIBody. The original PKIMessage from the EE MAY be
included in the generalInfo field of PKIHeader (to accommodate, for
example, cases in which the CA wishes to check POP or other
information on the original EE message). The infoType to be used in
this situation is {id-it 15} (see Section 5.3.19 for the value of
id-it) and the infoValue is PKIMessages (contents MUST be in the same
order as the requests in PKIBody).
5.2. Common Data Structures
Before specifying the specific types that may be placed in a PKIBody,
we define some data structures that are used in more than one case.
5.2.1. Requested Certificate Contents
Various PKI management messages require that the originator of the
message indicate some of the fields that are required to be present
in a certificate. The CertTemplate structure allows an end entity or
RA to specify as much as it wishes about the certificate it requires.
CertTemplate is identical to a Certificate, but with all fields
optional.
Note that even if the originator completely specifies the contents of
a certificate it requires, a CA is free to modify fields within the
certificate actually issued. If the modified certificate is
unacceptable to the requester, the requester MUST send back a
certConf message that either does not include this certificate (via a
CertHash), or does include this certificate (via a CertHash) along
with a status of "rejected". See Section 5.3.18 for the definition
and use of CertHash and the certConf message.
See Appendix C and [CRMF] for CertTemplate syntax.
5.2.2. Encrypted Values
Where encrypted values (restricted, in this specification, to be
either private keys or certificates) are sent in PKI messages, the
EncryptedValue data structure is used.
See [CRMF] for EncryptedValue syntax.
Use of this data structure requires that the creator and intended
recipient be able to encrypt and decrypt, respectively. Typically,
this will mean that the sender and recipient have, or are able to
generate, a shared secret key.
If the recipient of the PKIMessage already possesses a private key
usable for decryption, then the encSymmKey field MAY contain a
session key encrypted using the recipient's public key.
5.2.3. Status codes and Failure Information for PKI Messages
All response messages will include some status information. The
following values are defined.
PKIStatus ::= INTEGER {
accepted (0),
grantedWithMods (1),
rejection (2),
waiting (3),
revocationWarning (4),
revocationNotification (5),
keyUpdateWarning (6)
}
Responders may use the following syntax to provide more information
about failure cases.
PKIFailureInfo ::= BIT STRING {
badAlg (0),
badMessageCheck (1),
badRequest (2),
badTime (3),
badCertId (4),
badDataFormat (5),
wrongAuthority (6),
incorrectData (7),
missingTimeStamp (8),
badPOP (9),
certRevoked (10),
certConfirmed (11),
wrongIntegrity (12),
badRecipientNonce (13),
timeNotAvailable (14),
unacceptedPolicy (15),
unacceptedExtension (16),
addInfoNotAvailable (17),
badSenderNonce (18),
badCertTemplate (19),
signerNotTrusted (20),
transactionIdInUse (21),
unsupportedVersion (22),
notAuthorized (23),
systemUnavail (24),
systemFailure (25),
duplicateCertReq (26)
}
PKIStatusInfo ::= SEQUENCE {
status PKIStatus,
statusString PKIFreeText OPTIONAL,
failInfo PKIFailureInfo OPTIONAL
}
5.2.4. Certificate Identification
In order to identify particular certificates, the CertId data
structure is used.
See [CRMF] for CertId syntax.
5.2.5. Out-of-band root CA Public Key
Each root CA must be able to publish its current public key via some
"out-of-band" means. While such mechanisms are beyond the scope of
this document, we define data structures that can support such
mechanisms.
There are generally two methods available: either the CA directly
publishes its self-signed certificate, or this information is
available via the Directory (or equivalent) and the CA publishes a
hash of this value to allow verification of its integrity before use.
OOBCert ::= Certificate
The fields within this certificate are restricted as follows:
o The certificate MUST be self-signed (i.e., the signature must be
verifiable using the SubjectPublicKeyInfo field);
o The subject and issuer fields MUST be identical;
o If the subject field is NULL, then both subjectAltNames and
issuerAltNames extensions MUST be present and have exactly the
same value;
o The values of all other extensions must be suitable for a self-
signed certificate (e.g., key identifiers for subject and issuer
must be the same).
OOBCertHash ::= SEQUENCE {
hashAlg [0] AlgorithmIdentifier OPTIONAL,
certId [1] CertId OPTIONAL,
hashVal BIT STRING
}
The intention of the hash value is that anyone who has securely
received the hash value (via the out-of-band means) can verify a
self-signed certificate for that CA.
5.2.6. Archive Options
Requesters may indicate that they wish the PKI to archive a private
key value using the PKIArchiveOptions structure.
See [CRMF] for PKIArchiveOptions syntax.
5.2.7. Publication Information
Requesters may indicate that they wish the PKI to publish a
certificate using the PKIPublicationInfo structure.
See [CRMF] for PKIPublicationInfo syntax.
5.2.8. Proof-of-Possession Structures
If the certification request is for a signing key pair (i.e., a
request for a verification certificate), then the proof-of-possession
of the private signing key is demonstrated through use of the
POPOSigningKey structure.
See Appendix C and [CRMF] for POPOSigningKey syntax, but note that
POPOSigningKeyInput has the following semantic stipulations in this
specification.
POPOSigningKeyInput ::= SEQUENCE {
authInfo CHOICE {
sender [0] GeneralName,
publicKeyMAC PKMACValue
},
publicKey SubjectPublicKeyInfo
}
On the other hand, if the certification request is for an encryption
key pair (i.e., a request for an encryption certificate), then the
proof-of-possession of the private decryption key may be demonstrated
in one of three ways.
5.2.8.1. Inclusion of the Private Key
By the inclusion of the private key (encrypted) in the CertRequest
(in the thisMessage field of POPOPrivKey (see Appendix C) or in the
PKIArchiveOptions control structure, depending upon whether or not
archival of the private key is also desired).
5.2.8.2. Indirect Method
By having the CA return not the certificate, but an encrypted
certificate (i.e., the certificate encrypted under a randomly-
generated symmetric key, and the symmetric key encrypted under the
public key for which the certification request is being made) -- this
is the "indirect" method mentioned previously in Section 4.3.2. The
end entity proves knowledge of the private decryption key to the CA
by providing the correct CertHash for this certificate in the
certConf message. This demonstrates POP because the EE can only
compute the correct CertHash if it is able to recover the
certificate, and it can only recover the certificate if it is able to
decrypt the symmetric key using the required private key. Clearly,
for this to work, the CA MUST NOT publish the certificate until the
certConf message arrives (when certHash is to be used to demonstrate
POP). See Section 5.3.18 for further details.
5.2.8.3. Challenge-Response Protocol
By having the end entity engage in a challenge-response protocol
(using the messages POPODecKeyChall and POPODecKeyResp; see below)
between CertReqMessages and CertRepMessage -- this is the "direct"
method mentioned previously in Section 4.3.2. (This method would
typically be used in an environment in which an RA verifies POP and
then makes a certification request to the CA on behalf of the end
entity. In such a scenario, the CA trusts the RA to have done POP
correctly before the RA requests a certificate for the end entity.)
The complete protocol then looks as follows (note that req' does not
necessarily encapsulate req as a nested message):
EE RA CA
---- req ---->
<--- chall ---
---- resp --->
---- req' --->
<--- rep -----
---- conf --->
<--- ack -----
<--- rep -----
---- conf --->
<--- ack -----
This protocol is obviously much longer than the 3-way exchange given
in choice (2) above, but allows a local Registration Authority to be
involved and has the property that the certificate itself is not
actually created until the proof-of-possession is complete. In some
environments, a different order of the above messages may be
required, such as the following (this may be determined by policy):
EE RA CA
---- req ---->
<--- chall ---
---- resp --->
---- req' --->
<--- rep -----
<--- rep -----
---- conf --->
---- conf --->
<--- ack -----
<--- ack -----
If the cert. request is for a key agreement key (KAK) pair, then the
POP can use any of the 3 ways described above for enc. key pairs,
with the following changes: (1) the parenthetical text of bullet 2)
is replaced with "(i.e., the certificate encrypted under the
symmetric key derived from the CA's private KAK and the public key
for which the certification request is being made)"; (2) the first
parenthetical text of the challenge field of "Challenge" below is
replaced with "(using PreferredSymmAlg (see Section 5.3.19.4 and
Appendix E.5) and a symmetric key derived from the CA's private KAK
and the public key for which the certification request is being
made)". Alternatively, the POP can use the POPOSigningKey structure
given in [CRMF] (where the alg field is DHBasedMAC and the signature
field is the MAC) as a fourth alternative for demonstrating POP if
the CA already has a D-H certificate that is known to the EE.
The challenge-response messages for proof-of-possession of a private
decryption key are specified as follows (see [MvOV97], p.404 for
details). Note that this challenge-response exchange is associated
with the preceding cert. request message (and subsequent cert.
response and confirmation messages) by the transactionID used in the
PKIHeader and by the protection (MACing or signing) applied to the
PKIMessage.
POPODecKeyChallContent ::= SEQUENCE OF Challenge
Challenge ::= SEQUENCE {
owf AlgorithmIdentifier OPTIONAL,
witness OCTET STRING,
challenge OCTET STRING
}
Note that the size of Rand needs to be appropriate for encryption
under the public key of the requester. Given that "int" will
typically not be longer than 64 bits, this leaves well over 100 bytes
of room for the "sender" field when the modulus is 1024 bits. If, in
some environment, names are so long that they cannot fit (e.g., very
long DNs), then whatever portion will fit should be used (as long as
it includes at least the common name, and as long as the receiver is
able to deal meaningfully with the abbreviation).
POPODecKeyRespContent ::= SEQUENCE OF INTEGER
5.2.8.4. Summary of PoP Options
The text in this section provides several options with respect to POP
techniques. Using "SK" for "signing key", "EK" for "encryption key",
and "KAK" for "key agreement key", the techniques may be summarized
as follows:
RAVerified;
SKPOP;
EKPOPThisMessage;
KAKPOPThisMessage;
KAKPOPThisMessageDHMAC;
EKPOPEncryptedCert;
KAKPOPEncryptedCert;
EKPOPChallengeResp; and
KAKPOPChallengeResp.
Given this array of options, it is natural to ask how an end entity
can know what is supported by the CA/RA (i.e., which options it may
use when requesting certificates). The following guidelines should
clarify this situation for EE implementers.
RAVerified. This is not an EE decision; the RA uses this if and only
if it has verified POP before forwarding the request on to the CA, so
it is not possible for the EE to choose this technique.
SKPOP. If the EE has a signing key pair, this is the only POP method
specified for use in the request for a corresponding certificate.
EKPOPThisMessage and KAKPOPThisMessage. Whether or not to give up
its private key to the CA/RA is an EE decision. If the EE decides to
reveal its key, then these are the only POP methods available in this
specification to achieve this (and the key pair type will determine
which of these two methods to use).
KAKPOPThisMessageDHMAC. The EE can only use this method if (1) the
CA has a DH certificate available for this purpose, and (2) the EE
already has a copy of this certificate. If both these conditions
hold, then this technique is clearly supported and may be used by the
EE, if desired.
EKPOPEncryptedCert, KAKPOPEncryptedCert, EKPOPChallengeResp,
KAKPOPChallengeResp. The EE picks one of these (in the
subsequentMessage field) in the request message, depending upon
preference and key pair type. The EE is not doing POP at this point;
it is simply indicating which method it wants to use. Therefore, if
the CA/RA replies with a "badPOP" error, the EE can re-request using
the other POP method chosen in subsequentMessage. Note, however,
that this specification encourages the use of the EncryptedCert
choice and, furthermore, says that the challenge-response would
typically be used when an RA is involved and doing POP verification.
Thus, the EE should be able to make an intelligent decision regarding
which of these POP methods to choose in the request message.
5.3. Operation-Specific Data Structures
5.3.1. Initialization Request
An Initialization request message contains as the PKIBody a
CertReqMessages data structure, which specifies the requested
certificate(s). Typically, SubjectPublicKeyInfo, KeyId, and Validity
are the template fields which may be supplied for each certificate
requested (see Appendix D profiles for further information). This
message is intended to be used for entities when first initializing
into the PKI.
See Appendix C and [CRMF] for CertReqMessages syntax.
5.3.2. Initialization Response
An Initialization response message contains as the PKIBody an
CertRepMessage data structure, which has for each certificate
requested a PKIStatusInfo field, a subject certificate, and possibly
a private key (normally encrypted with a session key, which is itself
encrypted with the protocolEncrKey).
See Section 5.3.4 for CertRepMessage syntax. Note that if the PKI
Message Protection is "shared secret information" (see Section
5.1.3), then any certificate transported in the caPubs field may be
directly trusted as a root CA certificate by the initiator.
5.3.3. Certification Request
A Certification request message contains as the PKIBody a
CertReqMessages data structure, which specifies the requested
certificates. This message is intended to be used for existing PKI
entities who wish to obtain additional certificates.
See Appendix C and [CRMF] for CertReqMessages syntax.
Alternatively, the PKIBody MAY be a CertificationRequest (this
structure is fully specified by the ASN.1 structure
CertificationRequest given in [PKCS10]). This structure may be
required for certificate requests for signing key pairs when
interoperation with legacy systems is desired, but its use is
strongly discouraged whenever not absolutely necessary.
5.3.4. Certification Response
A Certification response message contains as the PKIBody a
CertRepMessage data structure, which has a status value for each
certificate requested, and optionally has a CA public key, failure
information, a subject certificate, and an encrypted private key.
CertRepMessage ::= SEQUENCE {
caPubs [1] SEQUENCE SIZE (1..MAX) OF Certificate
OPTIONAL,
response SEQUENCE OF CertResponse
}
CertResponse ::= SEQUENCE {
certReqId INTEGER,
status PKIStatusInfo,
certifiedKeyPair CertifiedKeyPair OPTIONAL,
rspInfo OCTET STRING OPTIONAL
-- analogous to the id-regInfo-utf8Pairs string defined
-- for regInfo in CertReqMsg [CRMF]
}
CertifiedKeyPair ::= SEQUENCE {
certOrEncCert CertOrEncCert,
privateKey [0] EncryptedValue OPTIONAL,
-- see [CRMF] for comment on encoding
publicationInfo [1] PKIPublicationInfo OPTIONAL
}
CertOrEncCert ::= CHOICE {
certificate [0] Certificate,
encryptedCert [1] EncryptedValue
}
Only one of the failInfo (in PKIStatusInfo) and certificate (in
CertifiedKeyPair) fields can be present in each CertResponse
(depending on the status). For some status values (e.g., waiting),
neither of the optional fields will be present.
Given an EncryptedCert and the relevant decryption key, the
certificate may be obtained. The purpose of this is to allow a CA to
return the value of a certificate, but with the constraint that only
the intended recipient can obtain the actual certificate. The
benefit of this approach is that a CA may reply with a certificate
even in the absence of a proof that the requester is the end entity
that can use the relevant private key (note that the proof is not
obtained until the certConf message is received by the CA). Thus,
the CA will not have to revoke that certificate in the event that
something goes wrong with the proof-of-possession (but MAY do so
anyway, depending upon policy).
5.3.5. Key Update Request Content
For key update requests the CertReqMessages syntax is used.
Typically, SubjectPublicKeyInfo, KeyId, and Validity are the template
fields that may be supplied for each key to be updated. This message
is intended to be used to request updates to existing (non-revoked
and non-expired) certificates (therefore, it is sometimes referred to
as a "Certificate Update" operation). An update is a replacement
certificate containing either a new subject public key or the current
subject public key (although the latter practice may not be
appropriate for some environments).
See Appendix C and [CRMF] for CertReqMessages syntax.
5.3.6. Key Update Response Content
For key update responses, the CertRepMessage syntax is used. The
response is identical to the initialization response.
See Section 5.3.4 for CertRepMessage syntax.
5.3.7. Key Recovery Request Content
For key recovery requests the syntax used is identical to the
initialization request CertReqMessages. Typically,
SubjectPublicKeyInfo and KeyId are the template fields that may be
used to supply a signature public key for which a certificate is
required (see Appendix D profiles for further information).
See Appendix C and [CRMF] for CertReqMessages syntax. Note that if a
key history is required, the requester must supply a Protocol
Encryption Key control in the request message.
5.3.8. Key Recovery Response Content
For key recovery responses, the following syntax is used. For some
status values (e.g., waiting) none of the optional fields will be
present.
KeyRecRepContent ::= SEQUENCE {
status PKIStatusInfo,
newSigCert [0] Certificate OPTIONAL,
caCerts [1] SEQUENCE SIZE (1..MAX) OF
Certificate OPTIONAL,
keyPairHist [2] SEQUENCE SIZE (1..MAX) OF
CertifiedKeyPair OPTIONAL
}
5.3.9. Revocation Request Content
When requesting revocation of a certificate (or several
certificates), the following data structure is used. The name of the
requester is present in the PKIHeader structure.
RevReqContent ::= SEQUENCE OF RevDetails
RevDetails ::= SEQUENCE {
certDetails CertTemplate,
crlEntryDetails Extensions OPTIONAL
}
5.3.10. Revocation Response Content
The revocation response is the response to the above message. If
produced, this is sent to the requester of the revocation. (A
separate revocation announcement message MAY be sent to the subject
of the certificate for which revocation was requested.)
RevRepContent ::= SEQUENCE {
status SEQUENCE SIZE (1..MAX) OF PKIStatusInfo,
revCerts [0] SEQUENCE SIZE (1..MAX) OF CertId OPTIONAL,
crls [1] SEQUENCE SIZE (1..MAX) OF CertificateList
OPTIONAL
}
5.3.11. Cross Certification Request Content
Cross certification requests use the same syntax (CertReqMessages) as
normal certification requests, with the restriction that the key pair
MUST have been generated by the requesting CA and the private key
MUST NOT be sent to the responding CA. This request MAY also be used
by subordinate CAs to get their certificates signed by the parent CA.
See Appendix C and [CRMF] for CertReqMessages syntax.
5.3.12. Cross Certification Response Content
Cross certification responses use the same syntax (CertRepMessage) as
normal certification responses, with the restriction that no
encrypted private key can be sent.
See Section 5.3.4 for CertRepMessage syntax.
5.3.13. CA Key Update Announcement Content
When a CA updates its own key pair, the following data structure MAY
be used to announce this event.
CAKeyUpdAnnContent ::= SEQUENCE {
oldWithNew Certificate,
newWithOld Certificate,
newWithNew Certificate
}
5.3.14. Certificate Announcement
This structure MAY be used to announce the existence of certificates.
Note that this message is intended to be used for those cases (if
any) where there is no pre-existing method for publication of
certificates; it is not intended to be used where, for example, X.500
is the method for publication of certificates.
CertAnnContent ::= Certificate
5.3.15. Revocation Announcement
When a CA has revoked, or is about to revoke, a particular
certificate, it MAY issue an announcement of this (possibly upcoming)
event.
RevAnnContent ::= SEQUENCE {
status PKIStatus,
certId CertId,
willBeRevokedAt GeneralizedTime,
badSinceDate GeneralizedTime,
crlDetails Extensions OPTIONAL
}
A CA MAY use such an announcement to warn (or notify) a subject that
its certificate is about to be (or has been) revoked. This would
typically be used where the request for revocation did not come from
the subject concerned.
The willBeRevokedAt field contains the time at which a new entry will
be added to the relevant CRLs.
5.3.16. CRL Announcement
When a CA issues a new CRL (or set of CRLs) the following data
structure MAY be used to announce this event.
CRLAnnContent ::= SEQUENCE OF CertificateList
5.3.17. PKI Confirmation Content
This data structure is used in the protocol exchange as the final
PKIMessage. Its content is the same in all cases -- actually there
is no content since the PKIHeader carries all the required
information.
PKIConfirmContent ::= NULL
Use of this message for certificate confirmation is NOT RECOMMENDED;
certConf SHOULD be used instead. Upon receiving a PKIConfirm for a
certificate response, the recipient MAY treat it as a certConf with
all certificates being accepted.
5.3.18. Certificate Confirmation Content
This data structure is used by the client to send a confirmation to
the CA/RA to accept or reject certificates.
CertConfirmContent ::= SEQUENCE OF CertStatus
CertStatus ::= SEQUENCE {
certHash OCTET STRING,
certReqId INTEGER,
statusInfo PKIStatusInfo OPTIONAL
}
For any particular CertStatus, omission of the statusInfo field
indicates ACCEPTANCE of the specified certificate. Alternatively,
explicit status details (with respect to acceptance or rejection) MAY
be provided in the statusInfo field, perhaps for auditing purposes at
the CA/RA.
Within CertConfirmContent, omission of a CertStatus structure
corresponding to a certificate supplied in the previous response
message indicates REJECTION of the certificate. Thus, an empty
CertConfirmContent (a zero-length SEQUENCE) MAY be used to indicate
rejection of all supplied certificates. See Section 5.2.8, item (2),
for a discussion of the certHash field with respect to proof-of-
possession.
5.3.19. PKI General Message Content
InfoTypeAndValue ::= SEQUENCE {
infoType OBJECT IDENTIFIER,
infoValue ANY DEFINED BY infoType OPTIONAL
}
-- where {id-it} = {id-pkix 4} = {1 3 6 1 5 5 7 4}
GenMsgContent ::= SEQUENCE OF InfoTypeAndValue
5.3.19.1. CA Protocol Encryption Certificate
This MAY be used by the EE to get a certificate from the CA to use to
protect sensitive information during the protocol.
GenMsg: {id-it 1}, < absent >
GenRep: {id-it 1}, Certificate | < absent >
EEs MUST ensure that the correct certificate is used for this
purpose.
5.3.19.2. Signing Key Pair Types
This MAY be used by the EE to get the list of signature algorithms
(e.g., RSA, DSA) whose subject public key values the CA is willing to
certify. Note that for the purposes of this exchange, rsaEncryption
and rsaWithSHA1, for example, are considered to be equivalent; the
question being asked is, "Is the CA willing to certify an RSA public
key?"
GenMsg: {id-it 2}, < absent >
GenRep: {id-it 2}, SEQUENCE SIZE (1..MAX) OF
AlgorithmIdentifier
5.3.19.3. Encryption/Key Agreement Key Pair Types
This MAY be used by the client to get the list of encryption/key
agreement algorithms whose subject public key values the CA is
willing to certify.
GenMsg: {id-it 3}, < absent >
GenRep: {id-it 3}, SEQUENCE SIZE (1..MAX) OF
AlgorithmIdentifier
5.3.19.4. Preferred Symmetric Algorithm
This MAY be used by the client to get the CA-preferred symmetric
encryption algorithm for any confidential information that needs to
be exchanged between the EE and the CA (for example, if the EE wants
to send its private decryption key to the CA for archival purposes).
GenMsg: {id-it 4}, < absent >
GenRep: {id-it 4}, AlgorithmIdentifier
5.3.19.5. Updated CA Key Pair
This MAY be used by the CA to announce a CA key update event.
GenMsg: {id-it 5}, CAKeyUpdAnnContent
5.3.19.6. CRL
This MAY be used by the client to get a copy of the latest CRL.
GenMsg: {id-it 6}, < absent >
GenRep: {id-it 6}, CertificateList
5.3.19.7. Unsupported Object Identifiers
This is used by the server to return a list of object identifiers
that it does not recognize or support from the list submitted by the
client.
GenRep: {id-it 7}, SEQUENCE SIZE (1..MAX) OF OBJECT IDENTIFIER
5.3.19.8. Key Pair Parameters
This MAY be used by the EE to request the domain parameters to use
for generating the key pair for certain public-key algorithms. It
can be used, for example, to request the appropriate P, Q, and G to
generate the DH/DSA key, or to request a set of well-known elliptic
curves.
GenMsg: {id-it 10}, OBJECT IDENTIFIER -- (Algorithm object-id)
GenRep: {id-it 11}, AlgorithmIdentifier | < absent >
An absent infoValue in the GenRep indicates that the algorithm
specified in GenMsg is not supported.
EEs MUST ensure that the parameters are acceptable to it and that the
GenRep message is authenticated (to avoid substitution attacks).
5.3.19.9. Revocation Passphrase
This MAY be used by the EE to send a passphrase to a CA/RA for the
purpose of authenticating a later revocation request (in the case
that the appropriate signing private key is no longer available to
authenticate the request). See Appendix B for further details on the
use of this mechanism.
GenMsg: {id-it 12}, EncryptedValue
GenRep: {id-it 12}, < absent >
5.3.19.10. ImplicitConfirm
See Section 5.1.1.1 for the definition and use of {id-it 13}.
5.3.19.11. ConfirmWaitTime
See Section 5.1.1.2 for the definition and use of {id-it 14}.
5.3.19.12 Original PKIMessage
See Section 5.1.3 for the definition and use of {id-it 15}.
5.3.19.13. Supported Language Tags
This MAY be used to determine the appropriate language tag to use in
subsequent messages. The sender sends its list of supported
languages (in order, most preferred to least); the receiver returns
the one it wishes to use. (Note: each UTF8String MUST include a
language tag.) If none of the offered tags are supported, an error
MUST be returned.
GenMsg: {id-it 16}, SEQUENCE SIZE (1..MAX) OF UTF8String
GenRep: {id-it 16}, SEQUENCE SIZE (1) OF UTF8String
5.3.20. PKI General Response Content
GenRepContent ::= SEQUENCE OF InfoTypeAndValue
Examples of GenReps that MAY be supported include those listed in the
subsections of Section 5.3.19.
5.3.21. Error Message Content
This data structure MAY be used by EE, CA, or RA to convey error
info.
ErrorMsgContent ::= SEQUENCE {
pKIStatusInfo PKIStatusInfo,
errorCode INTEGER OPTIONAL,
errorDetails PKIFreeText OPTIONAL
}
This message MAY be generated at any time during a PKI transaction.
If the client sends this request, the server MUST respond with a
PKIConfirm response, or another ErrorMsg if any part of the header is
not valid. Both sides MUST treat this message as the end of the
transaction (if a transaction is in progress).
If protection is desired on the message, the client MUST protect it
using the same technique (i.e., signature or MAC) as the starting
message of the transaction. The CA MUST always sign it with a
signature key.
5.3.22. Polling Request and Response
This pair of messages is intended to handle scenarios in which the
client needs to poll the server in order to determine the status of
an outstanding ir, cr, or kur transaction (i.e., when the "waiting"
PKIStatus has been received).
PollReqContent ::= SEQUENCE OF SEQUENCE {
certReqId INTEGER }
PollRepContent ::= SEQUENCE OF SEQUENCE {
certReqId INTEGER,
checkAfter INTEGER, -- time in seconds
reason PKIFreeText OPTIONAL }
The following clauses describe when polling messages are used, and
how they are used. It is assumed that multiple certConf messages can
be sent during transactions. There will be one sent in response to
each ip, cp, or kup that contains a CertStatus for an issued
certificate.
1. In response to an ip, cp, or kup message, an EE will send a
certConf for all issued certificates and, following the ack, a
pollReq for all pending certificates.
2. In response to a pollReq, a CA/RA will return an ip, cp, or kup
if one or more of the pending certificates is ready; otherwise,
it will return a pollRep.
3. If the EE receives a pollRep, it will wait for at least as long
as the checkAfter value before sending another pollReq.
4. If an ip, cp, or kup is received in response to a pollReq, then
it will be treated in the same way as the initial response.
START
|
v
Send ir
| ip
v
Check status
of returned <------------------------+
certs |
| |
+------------------------>|<------------------+ |
| | | |
| (issued) v (waiting) | |
Add to <----------- Check CertResponse ------> Add to |
conf list for each certificate pending list |
/ |
/ |
(conf list) / (empty conf list) |
/ ip |
/ +----------------+
(empty pending list) / | pRep
END <---- Send certConf Send pReq------------>Wait
| ^ ^ |
| | | |
+-----------------+ +---------------+
(pending list)
In the following exchange, the end entity is enrolling for two
certificates in one request.
Step End Entity PKI
--------------------------------------------------------------------
1 Format ir
2 -> ir ->
3 Handle ir
4 Manual intervention is
required for both certs.
5 <- ip <-
6 Process ip
7 Format pReq
8 -> pReq ->
9 Check status of cert requests
10 Certificates not ready
11 Format pRep
12 <- pRep <-
13 Wait
14 Format pReq
15 -> pReq ->
16 Check status of cert requests
17 One certificate is ready
18 Format ip
19 <- ip <-
20 Handle ip
21 Format certConf
22 -> certConf ->
23 Handle certConf
24 Format ack
25 <- pkiConf <-
26 Format pReq
27 -> pReq ->
28 Check status of certificate
29 Certificate is ready
30 Format ip
31 <- ip <-
31 Handle ip
32 Format certConf
33 -> certConf ->
34 Handle certConf
35 Format ack
36 <- pkiConf <-
6. Mandatory PKI Management Functions
Some of the PKI management functions outlined in Section 3.1 above
are described in this section.
This section deals with functions that are "mandatory" in the sense
that all end entity and CA/RA implementations MUST be able to provide
the functionality described. This part is effectively the profile of
the PKI management functionality that MUST be supported. Note,
however, that the management functions described in this section do
not need to be accomplished using the PKI messages defined in Section
5 if alternate means are suitable for a given environment (see
Appendix D for profiles of the PKIMessages that MUST be supported).
6.1. Root CA Initialization
[See Section 3.1.1.2 for this document's definition of "root CA".]
A newly created root CA must produce a "self-certificate", which is a
Certificate structure with the profile defined for the "newWithNew"
certificate issued following a root CA key update.
In order to make the CA's self certificate useful to end entities
that do not acquire the self certificate via "out-of-band" means, the
CA must also produce a fingerprint for its certificate. End entities
that acquire this fingerprint securely via some "out-of-band" means
can then verify the CA's self-certificate and, hence, the other
attributes contained therein.
The data structure used to carry the fingerprint is the OOBCertHash.
6.2. Root CA Key Update
CA keys (as all other keys) have a finite lifetime and will have to
be updated on a periodic basis. The certificates NewWithNew,
NewWithOld, and OldWithNew (see Section 4.4.1) MAY be issued by the
CA to aid existing end entities who hold the current self-signed CA
certificate (OldWithOld) to transition securely to the new self-
signed CA certificate (NewWithNew), and to aid new end entities who
will hold NewWithNew to acquire OldWithOld securely for verification
of existing data.
6.3. Subordinate CA Initialization
[See Section 3.1.1.2 for this document's definition of "subordinate
CA".]
From the perspective of PKI management protocols, the initialization
of a subordinate CA is the same as the initialization of an end
entity. The only difference is that the subordinate CA must also
produce an initial revocation list.
6.4. CRL production
Before issuing any certificates, a newly established CA (which issues
CRLs) must produce "empty" versions of each CRL which are to be
periodically produced.
6.5. PKI Information Request
When a PKI entity (CA, RA, or EE) wishes to acquire information about
the current status of a CA, it MAY send that CA a request for such
information.
The CA MUST respond to the request by providing (at least) all of the
information requested by the requester. If some of the information
cannot be provided, then an error must be conveyed to the requester.
If PKIMessages are used to request and supply this PKI information,
then the request MUST be the GenMsg message, the response MUST be the
GenRep message, and the error MUST be the Error message. These
messages are protected using a MAC based on shared secret information
(i.e., PasswordBasedMAC) or using any other authenticated means (if
the end entity has an existing certificate).
6.6. Cross Certification
The requester CA is the CA that will become the subject of the
cross-certificate; the responder CA will become the issuer of the
cross-certificate.
The requester CA must be "up and running" before initiating the
cross-certification operation.
6.6.1. One-Way Request-Response Scheme:
The cross-certification scheme is essentially a one way operation;
that is, when successful, this operation results in the creation of
one new cross-certificate. If the requirement is that cross-
certificates be created in "both directions", then each CA, in turn,
must initiate a cross-certification operation (or use another
scheme).
This scheme is suitable where the two CAs in question can already
verify each other's signatures (they have some common points of
trust) or where there is an out-of-band verification of the origin of
the certification request.
Detailed Description:
Cross certification is initiated at one CA known as the responder.
The CA administrator for the responder identifies the CA it wants to
cross certify and the responder CA equipment generates an
authorization code. The responder CA administrator passes this
authorization code by out-of-band means to the requester CA
administrator. The requester CA administrator enters the
authorization code at the requester CA in order to initiate the on-
line exchange.
The authorization code is used for authentication and integrity
purposes. This is done by generating a symmetric key based on the
authorization code and using the symmetric key for generating Message
Authentication Codes (MACs) on all messages exchanged.
(Authentication may alternatively be done using signatures instead of
MACs, if the CAs are able to retrieve and validate the required
public keys by some means, such as an out-of-band hash comparison.)
The requester CA initiates the exchange by generating a cross-
certification request (ccr) with a fresh random number (requester
random number). The requester CA then sends the ccr message to the
responder CA. The fields in this message are protected from
modification with a MAC based on the authorization code.
Upon receipt of the ccr message, the responder CA validates the
message and the MAC, saves the requester random number, and generates
its own random number (responder random number). It then generates
(and archives, if desired) a new requester certificate that contains
the requester CA public key and is signed with the responder CA
signature private key. The responder CA responds with the cross
certification response (ccp) message. The fields in this message are
protected from modification with a MAC based on the authorization
code.
Upon receipt of the ccp message, the requester CA validates the
message (including the received random numbers) and the MAC. The
requester CA responds with the certConf message. The fields in this
message are protected from modification with a MAC based on the
authorization code. The requester CA MAY write the requester
certificate to the Repository as an aid to later certificate path
construction.
Upon receipt of the certConf message, the responder CA validates the
message and the MAC, and sends back an acknowledgement using the
PKIConfirm message. It MAY also publish the requester certificate as
an aid to later path construction.
Notes:
1. The ccr message must contain a "complete" certification request;
that is, all fields except the serial number (including, e.g., a
BasicConstraints extension) must be specified by the requester
CA.
2. The ccp message SHOULD contain the verification certificate of
the responder CA; if present, the requester CA must then verify
this certificate (for example, via the "out-of-band" mechanism).
(A simpler, non-interactive model of cross-certification may also be
envisioned, in which the issuing CA acquires the subject CA's public
key from some repository, verifies it via some out-of-band mechanism,
and creates and publishes the cross-certificate without the subject
CA's explicit involvement. This model may be perfectly legitimate
for many environments, but since it does not require any protocol
message exchanges, its detailed description is outside the scope of
this specification.)
6.7. End Entity Initialization
As with CAs, end entities must be initialized. Initialization of end
entities requires at least two steps:
o acquisition of PKI information
o out-of-band verification of one root-CA public key
(other possible steps include the retrieval of trust condition
information and/or out-of-band verification of other CA public keys).
6.7.1. Acquisition of PKI Information
The information REQUIRED is:
o the current root-CA public key
o (if the certifying CA is not a root-CA) the certification path
from the root CA to the certifying CA together with appropriate
revocation lists
o the algorithms and algorithm parameters that the certifying CA
supports for each relevant usage
Additional information could be required (e.g., supported extensions
or CA policy information) in order to produce a certification request
that will be successful. However, for simplicity we do not mandate
that the end entity acquires this information via the PKI messages.
The end result is simply that some certification requests may fail
(e.g., if the end entity wants to generate its own encryption key,
but the CA doesn't allow that).
The required information MAY be acquired as described in Section 6.5.
6.7.2. Out-of-Band Verification of Root-CA Key
An end entity must securely possess the public key of its root CA.
One method to achieve this is to provide the end entity with the CA's
self-certificate fingerprint via some secure "out-of-band" means.
The end entity can then securely use the CA's self-certificate.
See Section 6.1 for further details.
6.8. Certificate Request
An initialized end entity MAY request an additional certificate at
any time (for any purpose). This request will be made using the
certification request (cr) message. If the end entity already
possesses a signing key pair (with a corresponding verification
certificate), then this cr message will typically be protected by the
entity's digital signature. The CA returns the new certificate (if
the request is successful) in a CertRepMessage.
6.9. Key Update
When a key pair is due to expire, the relevant end entity MAY request
a key update; that is, it MAY request that the CA issue a new
certificate for a new key pair (or, in certain circumstances, a new
certificate for the same key pair). The request is made using a key
update request (kur) message (referred to, in some environments, as a
"Certificate Update" operation). If the end entity already possesses
a signing key pair (with a corresponding verification certificate),
then this message will typically be protected by the entity's digital
signature. The CA returns the new certificate (if the request is
successful) in a key update response (kup) message, which is
syntactically identical to a CertRepMessage.
7. Version Negotiation
This section defines the version negotiation used to support older
protocols between client and servers.
If a client knows the protocol version(s) supported by the server
(e.g., from a previous PKIMessage exchange or via some out-of-band
means), then it MUST send a PKIMessage with the highest version
supported by both it and the server. If a client does not know what
version(s) the server supports, then it MUST send a PKIMessage using
the highest version it supports.
If a server receives a message with a version that it supports, then
the version of the response message MUST be the same as the received
version. If a server receives a message with a version higher or
lower than it supports, then it MUST send back an ErrorMsg with the
unsupportedVersion bit set (in the failureInfo field of the
pKIStatusInfo). If the received version is higher than the highest
supported version, then the version in the error message MUST be the
highest version the server supports; if the received version is lower
than the lowest supported version then the version in the error
message MUST be the lowest version the server supports.
If a client gets back an ErrorMsgContent with the unsupportedVersion
bit set and a version it supports, then it MAY retry the request with
that version.
7.1. Supporting RFC 2510 Implementations
RFC 2510 did not specify the behaviour of implementations receiving
versions they did not understand since there was only one version in
existence. With the introduction of the present revision of the
specification, the following versioning behaviour is recommended.
7.1.1. Clients Talking to RFC 2510 Servers
If, after sending a cmp2000 message, a client receives an
ErrorMsgContent with a version of cmp1999, then it MUST abort the
current transaction. It MAY subsequently retry the transaction using
version cmp1999 messages.
If a client receives a non-error PKIMessage with a version of
cmp1999, then it MAY decide to continue the transaction (if the
transaction hasn't finished) using RFC 2510 semantics. If it does
not choose to do so and the transaction is not finished, then it MUST
abort the transaction and send an ErrorMsgContent with a version of
cmp1999.
7.1.2. Servers Receiving Version cmp1999 PKIMessages
If a server receives a version cmp1999 message it MAY revert to RFC
2510 behaviour and respond with version cmp1999 messages. If it does
not choose to do so, then it MUST send back an ErrorMsgContent as
described above in Section 7.
8. Security Considerations
8.1. Proof-Of-Possession with a Decryption Key
Some cryptographic considerations are worth explicitly spelling out.
In the protocols specified above, when an end entity is required to
prove possession of a decryption key, it is effectively challenged to
decrypt something (its own certificate). This scheme (and many
others!) could be vulnerable to an attack if the possessor of the
decryption key in question could be fooled into decrypting an
arbitrary challenge and returning the cleartext to an attacker.
Although in this specification a number of other failures in security
are required in order for this attack to succeed, it is conceivable
that some future services (e.g., notary, trusted time) could
potentially be vulnerable to such attacks. For this reason, we re-
iterate the general rule that implementations should be very careful
about decrypting arbitrary "ciphertext" and revealing recovered
"plaintext" since such a practice can lead to serious security
vulnerabilities.
8.2. Proof-Of-Possession by Exposing the Private Key
Note also that exposing a private key to the CA/RA as a proof-of-
possession technique can carry some security risks (depending upon
whether or not the CA/RA can be trusted to handle such material
appropriately). Implementers are advised to:
Exercise caution in selecting and using this particular POP
mechanism
When appropriate, have the user of the application explicitly
state that they are willing to trust the CA/RA to have a copy of
their private key before proceeding to reveal the private key.
8.3. Attack Against Diffie-Hellman Key Exchange
A small subgroup attack during a Diffie-Hellman key exchange may be
carried out as follows. A malicious end entity may deliberately
choose D-H parameters that enable him/her to derive (a significant
number of bits of) the D-H private key of the CA during a key
archival or key recovery operation. Armed with this knowledge, the
EE would then be able to retrieve the decryption private key of
another unsuspecting end entity, EE2, during EE2's legitimate key
archival or key recovery operation with that CA. In order to avoid
the possibility of such an attack, two courses of action are
available. (1) The CA may generate a fresh D-H key pair to be used
as a protocol encryption key pair for each EE with which it
interacts. (2) The CA may enter into a key validation protocol (not
specified in this document) with each requesting end entity to ensure
that the EE's protocol encryption key pair will not facilitate this
attack. Option (1) is clearly simpler (requiring no extra protocol
exchanges from either party) and is therefore RECOMMENDED.
9. IANA Considerations
The PKI General Message types are identified by object identifiers
(OIDs). The OIDs for the PKI General Message types defined in this
document were assigned from an arc delegated by the IANA to the PKIX
Working Group.
The cryptographic algorithms referred to in this document are
identified by object identifiers (OIDs). The OIDs for cryptographic
algorithms were assigned from several arcs owned by various
organizations, including RSA Security, Entrust Technologies, IANA and
IETF.
Should additional encryption algorithms be introduced, the advocates
for such algorithms are expected to assign the necessary OIDs from
their own arcs.
No further action by the IANA is necessary for this document or any
anticipated updates.
Normative References
[X509] International Organization for Standardization and
International Telecommunications Union, "Information
technology - Open Systems Interconnection - The
Directory: Public-key and attribute certificate
frameworks", ISO Standard 9594-8:2001, ITU-T
Recommendation X.509, March 2000.
[MvOV97] Menezes, A., van Oorschot, P. and S. Vanstone, "Handbook
of Applied Cryptography", CRC Press ISBN 0-8493-8523-7,
1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2202] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and
HMAC-SHA-1", RFC 2202, September 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC2482] Whistler, K. and G. Adams, "Language Tagging in Unicode
Plain Text", RFC 2482, January 1999.
[CRMF] Schaad, J., "Internet X.509 Public Key Infrastructure
Certificate Request Message Format (CRMF)", RFC 4211,
September 2005.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
Informative References
[CMPtrans] Kapoor, A., Tschalar, R. and T. Kause, "Internet X.509
Public Key Infrastructure -- Transport Protocols for
CMP", Work in Progress. 2004.
[PKCS7] RSA Laboratories, "The Public-Key Cryptography Standards
- Cryptographic Message Syntax Standard