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RFC 1113 - Privacy enhancement for Internet electronic mail: Par

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Network Working Group                                            J. Linn
Request for Comments:  1113                                          DEC
Obsoletes RFCs: 989, 1040                         IAB Privacy Task Force
                                                             August 1989

           Privacy Enhancement for Internet Electronic Mail:
      Part I -- Message Encipherment and Authentication Procedures


   This RFC suggests a draft standard elective protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.


   This RFC is the outgrowth of a series of IAB Privacy Task Force
   meetings and of internal working papers distributed for those
   meetings.  I would like to thank the following Privacy Task Force
   members and meeting guests for their comments and contributions at
   the meetings which led to the preparation of this RFC: David
   Balenson, Curt Barker, Jim Bidzos, Matt Bishop, Danny Cohen, Tom
   Daniel, Charles Fox, Morrie Gasser, Russ Housley, Steve Kent
   (chairman), John Laws, Steve Lipner, Dan Nessett, Mike Padlipsky, Rob
   Shirey, Miles Smid, Steve Walker, and Steve Wilbur.

Table of Contents

   1.  Executive Summary                                               2
   2.  Terminology                                                     3
   3.  Services, Constraints, and Implications                         3
   4.  Processing of Messages                                          7
   4.1  Message Processing Overview                                    7
   4.1.1  Types of Keys                                                7
   4.1.2  Processing Procedures                                        8
   4.2  Encryption Algorithms and Modes                                9
   4.3  Privacy Enhancement Message Transformations                   10
   4.3.1  Constraints                                                 10
   4.3.2  Approach                                                    11  Step 1: Local Form                                        12  Step 2: Canonical Form                                    12  Step 3: Authentication and Encipherment                   12  Step 4: Printable Encoding                                13  Summary of Transformations                                15
   4.4  Encapsulation Mechanism                                       15
   4.5  Mail for Mailing Lists                                        17
   4.6  Summary of Encapsulated Header Fields                         18

   4.6.1  Per-Message Encapsulated Header Fields                      20  X-Proc-Type Field                                         20  X-DEK-Info Field                                          21
   4.6.2  Encapsulated Header Fields Normally Per-Message             21  X-Sender-ID Field                                         22  X-Certificate Field                                       22  X-MIC-Info Field                                          23
   4.6.3  Encapsulated Header Fields with Variable Occurrences        23  X-Issuer-Certificate Field                                23
   4.6.4  Per-Recipient Encapsulated Header Fields                    24  X-Recipient-ID Field                                      24  X-Key-Info Field                                          24  Symmetric Key Management                                24  Asymmetric Key Management                               25
   5.  Key Management                                                 26
   5.1  Data Encrypting Keys (DEKs)                                   26
   5.2  Interchange Keys (IKs)                                        26
   5.2.1  Subfield Definitions                                        28  Entity Identifier Subfield                                28  Issuing Authority Subfield                                29  Version/Expiration Subfield                               29
   5.2.2  IK Cryptoperiod Issues                                      29
   6.  User Naming                                                    29
   6.1  Current Approach                                              29
   6.2  Issues for Consideration                                      30
   7.  Example User Interface and Implementation                      30
   8.  Areas For Further Study                                        31
   9.  References                                                     32
   NOTES                                                              32

1.  Executive Summary

   This RFC defines message encipherment and authentication procedures,
   in order to provide privacy enhancement services for electronic mail
   transfer in the Internet.  It is one member of a related set of four
   RFCs.  The procedures defined in the current RFC are intended to be
   compatible with a wide range of key management approaches, including
   both symmetric (secret-key) and asymmetric (public-key) approaches
   for encryption of data encrypting keys.  Use of symmetric
   cryptography for message text encryption and/or integrity check
   computation is anticipated.  RFC-1114 specifies supporting key
   management mechanisms based on the use of public-key certificates.
   RFC-1115 specifies algorithm and related information relevant to the
   current RFC and to RFC-1114.  A subsequent RFC will provide details
   of paper and electronic formats and procedures for the key management
   infrastructure being established in support of these services.

   Privacy enhancement services (confidentiality, authentication, and

   message integrity assurance) are offered through the use of end-to-
   end cryptography between originator and recipient User Agent
   processes, with no special processing requirements imposed on the
   Message Transfer System at endpoints or at intermediate relay sites.
   This approach allows privacy enhancement facilities to be
   incorporated on a site-by-site or user-by-user basis without impact
   on other Internet entities.  Interoperability among heterogeneous
   components and mail transport facilities is supported.

2.  Terminology

   For descriptive purposes, this RFC uses some terms defined in the OSI
   X.400 Message Handling System Model per the 1984 CCITT
   Recommendations.  This section replicates a portion of X.400's
   Section 2.2.1, "Description of the MHS Model: Overview" in order to
   make the terminology clear to readers who may not be familiar with
   the OSI MHS Model.

   In the MHS model, a user is a person or a computer application.  A
   user is referred to as either an originator (when sending a message)
   or a recipient (when receiving one).  MH Service elements define the
   set of message types and the capabilities that enable an originator
   to transfer messages of those types to one or more recipients.

   An originator prepares messages with the assistance of his or her
   User Agent (UA).  A UA is an application process that interacts with
   the Message Transfer System (MTS) to submit messages.  The MTS
   delivers to one or more recipient UAs the messages submitted to it.
   Functions performed solely by the UA and not standardized as part of
   the MH Service elements are called local UA functions.

   The MTS is composed of a number of Message Transfer Agents (MTAs).
   Operating together, the MTAs relay messages and deliver them to the
   intended recipient UAs, which then make the messages available to the
   intended recipients.

   The collection of UAs and MTAs is called the Message Handling System
   (MHS).  The MHS and all of its users are collectively referred to as
   the Message Handling Environment.

3.  Services, Constraints, and Implications

   This RFC defines mechanisms to enhance privacy for electronic mail
   transferred in the Internet.  The facilities discussed in this RFC
   provide privacy enhancement services on an end-to-end basis between
   sender and recipient UAs.  No privacy enhancements are offered for
   message fields which are added or transformed by intermediate relay

   Authentication and integrity facilities are always applied to the
   entirety of a message's text.  No facility for confidentiality
   without authentication is provided.  Encryption facilities may be
   applied selectively to portions of a message's contents; this allows
   less sensitive portions of messages (e.g., descriptive fields) to be
   processed by a recipient's delegate in the absence of the recipient's
   personal cryptographic keys.  In the limiting case, where the
   entirety of message text is excluded from encryption, this feature
   can be used to yield the effective combination of authentication and
   integrity services without confidentiality.

   In keeping with the Internet's heterogeneous constituencies and usage
   modes, the measures defined here are applicable to a broad range of
   Internet hosts and usage paradigms.  In particular, it is worth
   noting the following attributes:

      1.  The mechanisms defined in this RFC are not restricted to a
          particular host or operating system, but rather allow
          interoperability among a broad range of systems.  All
          privacy enhancements are implemented at the application
          layer, and are not dependent on any privacy features at
          lower protocol layers.

      2.  The defined mechanisms are compatible with non-enhanced
          Internet components.  Privacy enhancements are implemented
          in an end-to-end fashion which does not impact mail
          processing by intermediate relay hosts which do not
          incorporate privacy enhancement facilities.  It is
          necessary, however, for a message's sender to be cognizant
          of whether a message's intended recipient implements privacy
          enhancements, in order that encoding and possible
          encipherment will not be performed on a message whose
          destination is not equipped to perform corresponding inverse

      3.  The defined mechanisms are compatible with a range of mail
          transport facilities (MTAs).  Within the Internet,
          electronic mail transport is effected by a variety of SMTP
          implementations.  Certain sites, accessible via SMTP,
          forward mail into other mail processing environments (e.g.,
          USENET, CSNET, BITNET).  The privacy enhancements must be
          able to operate across the SMTP realm; it is desirable that
          they also be compatible with protection of electronic mail
          sent between the SMTP environment and other connected

      4.  The defined mechanisms are compatible with a broad range of
          electronic mail user agents (UAs).  A large variety of

          electronic mail user agent programs, with a corresponding
          broad range of user interface paradigms, is used in the
          Internet.  In order that electronic mail privacy
          enhancements be available to the broadest possible user
          community, selected mechanisms should be usable with the
          widest possible variety of existing UA programs.  For
          purposes of pilot implementation, it is desirable that
          privacy enhancement processing be incorporable into a
          separate program, applicable to a range of UAs, rather than
          requiring internal modifications to each UA with which
          privacy-enhanced services are to be provided.

      5.  The defined mechanisms allow electronic mail privacy
          enhancement processing to be performed on personal computers
          (PCs) separate from the systems on which UA functions are
          implemented.  Given the expanding use of PCs and the limited
          degree of trust which can be placed in UA implementations on
          many multi-user systems, this attribute can allow many users
          to process privacy-enhanced mail with a higher assurance
          level than a strictly UA-based approach would allow.

      6.  The defined mechanisms support privacy protection of
          electronic mail addressed to mailing lists (distribution
          lists, in ISO parlance).

      7.  The mechanisms defined within this RFC are compatible with a
          variety of supporting key management approaches, including
          (but not limited to) manual pre-distribution, centralized
          key distribution based on symmetric cryptography, and the
          use of public-key certificates.  Different key management
          mechanisms may be used for different recipients of a
          multicast message.  While support for a particular key
          management mechanism is not a minimum essential requirement
          for compatibility with this RFC, adoption of the public-key
          certificate approach defined in companion RFC-1114 is
          strongly recommended.

   In order to achieve applicability to the broadest possible range of
   Internet hosts and mail systems, and to facilitate pilot
   implementation and testing without the need for prior modifications
   throughout the Internet, three basic restrictions are imposed on the
   set of measures to be considered in this RFC:

      1.  Measures will be restricted to implementation at endpoints
          and will be amenable to integration at the user agent (UA)
          level or above, rather than necessitating integration into
          the message transport system (e.g., SMTP servers).

      2.  The set of supported measures enhances rather than restricts
          user capabilities.  Trusted implementations, incorporating
          integrity features protecting software from subversion by
          local users, cannot be assumed in general.  In the absence
          of such features, it appears more feasible to provide
          facilities which enhance user services (e.g., by protecting
          and authenticating inter-user traffic) than to enforce
          restrictions (e.g., inter-user access control) on user

      3.  The set of supported measures focuses on a set of functional
          capabilities selected to provide significant and tangible
          benefits to a broad user community.  By concentrating on the
          most critical set of services, we aim to maximize the added
          privacy value that can be provided with a modest level of
          implementation effort.

   As a result of these restrictions, the following facilities can be

      1.  disclosure protection,

      2.  sender authenticity,

      3.  message integrity measures, and

      4.  (if asymmetric key management is used) non-repudiation of

   but the following privacy-relevant concerns are not addressed:

      1.  access control,

      2.  traffic flow confidentiality,

      3.  address list accuracy,

      4.  routing control,

      5.  issues relating to the casual serial reuse of PCs by
          multiple users,

      6.  assurance of message receipt and non-deniability of receipt,

      7.  automatic association of acknowledgments with the messages
          to which they refer, and

      8.  message duplicate detection, replay prevention, or other

          stream-oriented services.

   A message's sender will determine whether privacy enhancements are to
   be performed on a particular message.  Therefore, a sender must be
   able to determine whether particular recipients are equipped to
   process privacy-enhanced mail.  In a general architecture, these
   mechanisms will be based on server queries; thus, the query function
   could be integrated into a UA to avoid imposing burdens or
   inconvenience on electronic mail users.

4.  Processing of Messages

4.1  Message Processing Overview

   This subsection provides a high-level overview of the components and
   processing steps involved in electronic mail privacy enhancement
   processing.  Subsequent subsections will define the procedures in
   more detail.

4.1.1  Types of Keys

   A two-level keying hierarchy is used to support privacy-enhanced
   message transmission:

      1.  Data Encrypting Keys (DEKs) are used for encryption of
          message text and (with certain choices among a set of
          alternative algorithms) for computation of message integrity
          check (MIC) quantities.  DEKs are generated individually for
          each transmitted message; no predistribution of DEKs is
          needed to support privacy-enhanced message transmission.

      2.  Interchange Keys (IKs) are used to encrypt DEKs for
          transmission within messages.  Ordinarily, the same IK will
          be used for all messages sent from a given originator to a
          given recipient over a period of time.  Each transmitted
          message includes a representation of the DEK(s) used for
          message encryption and/or MIC computation, encrypted under
          an individual IK per named recipient.  The representation is
          associated with "X-Sender-ID:" and "X-Recipient-ID:" fields,
          which allow each individual recipient to identify the IK
          used to encrypt DEKs and/or MICs for that recipient's use.
          Given an appropriate IK, a recipient can decrypt the
          corresponding transmitted DEK representation, yielding the
          DEK required for message text decryption and/or MIC
          verification.  The definition of an IK differs depending on
          whether symmetric or asymmetric cryptography is used for DEK

         2a. When symmetric cryptography is used for DEK
             encryption, an IK is a single symmetric key shared
             between an originator and a recipient.  In this
             case, the same IK is used to encrypt MICs as well
             as DEKs for transmission.  Version/expiration
             information and IA identification associated with
             the originator and with the recipient must be
             concatenated in order to fully qualify a symmetric

         2b. When asymmetric cryptography is used, the IK
             component used for DEK encryption is the public
             component of the recipient.  The IK component used
             for MIC encryption is the private component of the
             originator, and therefore only one encrypted MIC
             representation need be included per message, rather than
             one per recipient.  Each of these IK
             components can be fully qualified in an
             "X-Recipient-ID:" or "X-Sender-ID:" field,

4.1.2  Processing Procedures

   When privacy enhancement processing is to be performed on an outgoing
   message, a DEK is generated [1] for use in message encryption and (if
   a chosen MIC algorithm requires a key) a variant of the DEK is formed
   for use in MIC computation.  DEK generation can be omitted for the
   case of a message in which all contents are excluded from encryption,
   unless a chosen MIC computation algorithm requires a DEK.

   An "X-Sender-ID:" field is included in the header to provide one
   identification component for the IK(s) used for message processing.
   IK components are selected for each individually named recipient; a
   corresponding "X-Recipient-ID:" field, interpreted in the context of
   a prior "X-Sender-ID:" field, serves to identify each IK.  Each "X-
   Recipient-ID:" field is followed by an "X-Key-Info:" field, which
   transfers a DEK encrypted under the IK appropriate for the specified
   recipient.  When symmetric key management is used for a given
   recipient, the "X-Key-Info:" field also transfers the message's
   computed MIC, encrypted under the recipient's IK.  When asymmetric
   key management is used, a prior "X-MIC-Info:" field carries the
   message's MIC encrypted under the private component of the sender.

   A four-phase transformation procedure is employed in order to
   represent encrypted message text in a universally transmissible form
   and to enable messages encrypted on one type of host computer to be
   decrypted on a different type of host computer.  A plaintext message
   is accepted in local form, using the host's native character set and

   line representation.  The local form is converted to a canonical
   message text representation, defined as equivalent to the inter-SMTP
   representation of message text.  This canonical representation forms
   the input to the MIC computation and encryption processes.

   For encryption purposes, the canonical representation is padded as
   required by the encryption algorithm.  The padded canonical
   representation is encrypted (except for any regions which are
   explicitly excluded from encryption).  The encrypted text (along with
   the canonical representation of regions which were excluded from
   encryption) is encoded into a printable form.  The printable form is
   composed of a restricted character set which is chosen to be
   universally representable across sites, and which will not be
   disrupted by processing within and between MTS entities.

   The output of the encoding procedure is combined with a set of header
   fields carrying cryptographic control information.  The result is
   passed to the electronic mail system to be encapsulated as the text
   portion of a transmitted message.

   When a privacy-enhanced message is received, the cryptographic
   control fields within its text portion provide the information
   required for the authorized recipient to perform MIC verification and
   decryption of the received message text.  First, the printable
   encoding is converted to a bitstring.  Encrypted portions of the
   transmitted message are decrypted.  The MIC is verified.  The
   canonical representation is converted to the recipient's local form,
   which need not be the same as the sender's local form.

4.2  Encryption Algorithms and Modes

   For purposes of this RFC, the Block Cipher Algorithm DEA-1, defined
   in ANSI X3.92-1981 [2] shall be used for encryption of message text.
   The DEA-1 is equivalent to the Data Encryption Standard (DES), as
   defined in FIPS PUB 46 [3].  When used for encryption of text, the
   DEA-1 shall be used in the Cipher Block Chaining (CBC) mode, as
   defined in ISO IS 8372 [4].  The identifier string "DES-CBC", defined
   in RFC-1115, signifies this algorithm/mode combination.  The CBC mode
   definition in IS 8372 is equivalent to that provided in FIPS PUB 81
   [5] and in ANSI X3.106-1983 [16].  Use of other algorithms and/or
   modes for message text processing will require case-by-case study to
   determine applicability and constraints.  Additional algorithms and
   modes approved for use in this context will be specified in
   successors to RFC-1115.

   It is an originator's responsibility to generate a new pseudorandom
   initializing vector (IV) for each privacy-enhanced electronic mail
   message unless the entirety of the message is excluded from

   encryption.  Section 4.3.1 of [17] provides rationale for this
   requirement, even in a context where individual DEKs are generated
   for individual messages.  The IV will be transmitted with the

   Certain operations require that one key be encrypted under an
   interchange key (IK) for purposes of transmission.  A header facility
   indicates the mode in which the IK is used for encryption.  RFC-1115
   specifies encryption algorithm/mode identifiers, including DES-ECB,
   DES-EDE, and RSA.  All implementations using symmetric key management
   should support DES-ECB IK use, and all implementations using
   asymmetric key management should support RSA IK use.

   RFC-1114, released concurrently with this RFC, specifies asymmetric,
   certificate-based key management procedures to support the message
   processing procedures defined in this document.  The message
   processing procedures can also be used with symmetric key management,
   given prior distribution of suitable symmetric IKs through out-of-
   band means.  Support for the asymmetric approach defined in RFC-1114
   is strongly recommended.

4.3  Privacy Enhancement Message Transformations

4.3.1  Constraints

   An electronic mail encryption mechanism must be compatible with the
   transparency constraints of its underlying electronic mail
   facilities.  These constraints are generally established based on
   expected user requirements and on the characteristics of anticipated
   endpoint and transport facilities.  An encryption mechanism must also
   be compatible with the local conventions of the computer systems
   which it interconnects.  In our approach, a canonicalization step is
   performed to abstract out local conventions and a subsequent encoding
   step is performed to conform to the characteristics of the underlying
   mail transport medium (SMTP).  The encoding conforms to SMTP
   constraints, established to support interpersonal messaging.  SMTP's
   rules are also used independently in the canonicalization process.
   RFC-821's [7] Section 4.5 details SMTP's transparency constraints.

   To prepare a message for SMTP transmission, the following
   requirements must be met:

      1.  All characters must be members of the 7-bit ASCII character

      2.  Text lines, delimited by the character pair <CR><LF>, must
          be no more than 1000 characters long.

      3.  Since the string <CR><LF>.<CR><LF> indicates the end of a
          message, it must not occur in text prior to the end of a

   Although SMTP specifies a standard representation for line delimiters
   (ASCII <CR><LF>), numerous systems use a different native
   representation to delimit lines.  For example, the <CR><LF> sequences
   delimiting lines in mail inbound to UNIX systems are transformed to
   single <LF>s as mail is written into local mailbox files.  Lines in
   mail incoming to record-oriented systems (such as VAX VMS) may be
   converted to appropriate records by the destination SMTP [8] server.
   As a result, if the encryption process generated <CR>s or <LF>s,
   those characters might not be accessible to a recipient UA program at
   a destination which uses different line delimiting conventions.  It
   is also possible that conversion between tabs and spaces may be
   performed in the course of mapping between inter-SMTP and local
   format; this is a matter of local option.  If such transformations
   changed the form of transmitted ciphertext, decryption would fail to
   regenerate the transmitted plaintext, and a transmitted MIC would
   fail to compare with that computed at the destination.

   The conversion performed by an SMTP server at a system with EBCDIC as
   a native character set has even more severe impact, since the
   conversion from EBCDIC into ASCII is an information-losing
   transformation.  In principle, the transformation function mapping
   between inter-SMTP canonical ASCII message representation and local
   format could be moved from the SMTP server up to the UA, given a
   means to direct that the SMTP server should no longer perform that
   transformation.  This approach has a major disadvantage: internal
   file (e.g., mailbox) formats would be incompatible with the native
   forms used on the systems where they reside.  Further, it would
   require modification to SMTP servers, as mail would be passed to SMTP
   in a different representation than it is passed at present.

4.3.2  Approach

   Our approach to supporting privacy-enhanced mail across an
   environment in which intermediate conversions may occur encodes mail
   in a fashion which is uniformly representable across the set of
   privacy-enhanced UAs regardless of their systems' native character
   sets.  This encoded form is used to represent mail text from sender
   to recipient, but the encoding is not applied to enclosing mail
   transport headers or to encapsulated headers inserted to carry
   control information between privacy-enhanced UAs.  The encoding's
   characteristics are such that the transformations anticipated between
   sender and recipient UAs will not prevent an encoded message from
   being decoded properly at its destination.

   A sender may exclude one or more portions of a message from
   encryption processing, but authentication processing is always
   applied to the entirety of message text.  Explicit action is required
   to exclude a portion of a message from encryption processing; by
   default, encryption is applied to the entirety of message text.  The
   user-level delimiter which specifies such exclusion is a local
   matter, and hence may vary between sender and recipient, but all
   systems should provide a means for unambiguous identification of
   areas excluded from encryption processing.

   An outbound privacy-enhanced message undergoes four transformation
   steps, described in the following four subsections.  Step 1: Local Form

   The message text is created in the system's native character set,
   with lines delimited in accordance with local convention.  Step 2: Canonical Form

   The entire message text, including both those portions subject to
   encipherment processing and those portions excluded from such
   processing, is converted to a universal canonical form, analogous to
   the inter-SMTP representation [9] as defined in RFC-821 and RFC-822
   [10] (ASCII character set, <CR><LF> line delimiters).  The processing
   required to perform this conversion is minimal on systems whose
   native character set is ASCII.  (Note: Since the output of the
   canonical encoding process will never be submitted directly to SMTP,
   but only to subsequent steps of the privacy enhancement encoding
   process, the dot-stuffing transformation discussed in RFC-821,
   section 4.5.2, is not required.)  Since a message is converted to a
   standard character set and representation before encryption, it can
   be decrypted and its MIC can be verified at any type of destination
   host computer.  The decryption and MIC verification is performed
   before any conversions which may be necessary to transform the
   message into a destination-specific local form.  Step 3: Authentication and Encipherment

   The canonical form is input to the selected MIC computation algorithm
   in order to compute an integrity check quantity for the message.  No
   padding is added to the canonical form before submission to the MIC
   computation algorithm, although certain MIC algorithms will apply
   their own padding in the course of computing a MIC.

   Padding is applied to the canonical form as needed to perform
   encryption in the DEA-1 CBC mode, as follows: The number of octets to
   be encrypted is determined by subtracting the number of octets

   excluded from encryption from the total length of the canonically
   encoded text.  Octets with the hexadecimal value FF (all ones) are
   appended to the canonical form as needed so that the text octets to
   be encrypted, along with the added padding octets, fill an integral
   number of 8-octet encryption quanta.  No padding is applied if the
   number of octets to be encrypted is already an integral multiple of
   8.  The use of hexadecimal FF (a value outside the 7-bit ASCII set)
   as a padding value allows padding octets to be distinguished from
   valid data without inclusion of an explicit padding count indicator.

   The regions of the message which have not been excluded from
   encryption are encrypted.  To support selective encipherment
   processing, an implementation must retain internal indications of the
   positions of excluded areas excluded from encryption with relation to
   non-excluded areas, so that those areas can be properly delimited in
   the encoding procedure defined in step 4.  If a region excluded from
   encryption intervenes between encrypted regions, cryptographic state
   (e.g., IVs and accumulation of octets into encryption quanta) is
   preserved and continued after the excluded region.  Step 4: Printable Encoding

   Proceeding from left to right, the bit string resulting from step 3
   is encoded into characters which are universally representable at all
   sites, though not necessarily with the same bit patterns (e.g.,
   although the character "E" is represented in an ASCII-based system as
   hexadecimal 45 and as hexadecimal C5 in an EBCDIC-based system, the
   local significance of the two representations is equivalent).  This
   encoding step is performed for all privacy-enhanced messages, even if
   an entire message is excluded from encryption.

   A 64-character subset of International Alphabet IA5 is used, enabling
   6 bits to be represented per printable character.  (The proposed
   subset of characters is represented identically in IA5 and ASCII.)
   Two additional characters, "=" and "*", are used to signify special
   processing functions.  The character "=" is used for padding within
   the printable encoding procedure.  The character "*" is used to
   delimit the beginning and end of a region which has been excluded
   from encipherment processing.  The encoding function's output is
   delimited into text lines (using local conventions), with each line
   except the last containing exactly 64 printable characters and the
   final line containing 64 or fewer printable characters.  (This line
   length is easily printable and is guaranteed to satisfy SMTP's 1000-
   character transmitted line length limit.)

   The encoding process represents 24-bit groups of input bits as output
   strings of 4 encoded characters. Proceeding from left to right across
   a 24-bit input group extracted from the output of step 3, each 6-bit

   group is used as an index into an array of 64 printable characters.
   The character referenced by the index is placed in the output string.
   These characters, identified in Table 0, are selected so as to be
   universally representable, and the set excludes characters with
   particular significance to SMTP (e.g., ".", "<CR>", "<LF>").

   Special processing is performed if fewer than 24 bits are available
   in an input group, either at the end of a message or (when the
   selective encryption facility is invoked) at the end of an encrypted
   region or an excluded region.  A full encoding quantum is always
   completed at the end of a message and before the delimiter "*" is
   output to initiate or terminate the representation of a block
   excluded from encryption.  When fewer than 24 input bits are
   available in an input group, zero bits are added (on the right) to
   form an integral number of 6-bit groups.  Output character positions
   which are not required to represent actual input data are set to the
   character "=".  Since all canonically encoded output is an integral
   number of octets, only the following cases can arise: (1) the final
   quantum of encoding input is an integral multiple of 24 bits; here,
   the final unit of encoded output will be an integral multiple of 4
   characters with no "=" padding, (2) the final quantum of encoding
   input is exactly 8 bits; here, the final unit of encoded output will
   be two characters followed by two "=" padding characters, or (3) the
   final quantum of encoding input is exactly 16 bits; here, the final
   unit of encoded output will be three characters followed by one "="
   padding character.  Summary of Transformations

   In summary, the outbound message is subjected to the following
   composition of transformations:

        Transmit_Form = Encode(Encipher(Canonicalize(Local_Form)))

   The inverse transformations are performed, in reverse order, to
   process inbound privacy-enhanced mail:

       Local_Form = DeCanonicalize(Decipher(Decode(Transmit_Form)))

   Value Encoding  Value Encoding  Value Encoding  Value Encoding
       0 A            17 R            34 i            51 z
       1 B            18 S            35 j            52 0
       2 C            19 T            36 k            53 1
       3 D            20 U            37 l            54 2
       4 E            21 V            38 m            55 3
       5 F            22 W            39 n            56 4
       6 G            23 X            40 o            57 5
       7 H            24 Y            41 p            58 6
       8 I            25 Z            42 q            59 7
       9 J            26 a            43 r            60 8
      10 K            27 b            44 s            61 9
      11 L            28 c            45 t            62 +
      12 M            29 d            46 u            63 /
      13 N            30 e            47 v
      14 O            31 f            48 w         (pad) =
      15 P            32 g            49 x
      16 Q            33 h            50 y           (1) *

   (1) The character "*" is used to enclose portions of an
   encoded message to which encryption processing has not
   been applied.

                       Printable Encoding Characters
                                  Table 1

   Note that the local form and the functions to transform messages to
   and from canonical form may vary between the sender and recipient
   systems without loss of information.

4.4  Encapsulation Mechanism

   Encapsulation of privacy-enhanced messages within an enclosing layer

   of headers interpreted by the electronic mail transport system offers
   a number of advantages in comparison to a flat approach in which
   certain fields within a single header are encrypted and/or carry
   cryptographic control information.  Encapsulation provides generality
   and segregates fields with user-to-user significance from those
   transformed in transit.  All fields inserted in the course of
   encryption/authentication processing are placed in the encapsulated
   header.  This facilitates compatibility with mail handling programs
   which accept only text, not header fields, from input files or from
   other programs.  Further, privacy enhancement processing can be
   applied recursively.  As far as the MTS is concerned, information
   incorporated into cryptographic authentication or encryption
   processing will reside in a message's text portion, not its header

   The encapsulation mechanism to be used for privacy-enhanced mail is
   derived from that described in RFC-934 [11] which is, in turn, based
   on precedents in the processing of message digests in the Internet
   community.  To prepare a user message for encrypted or authenticated
   transmission, it will be transformed into the representation shown in
   Figure 1.

   As a general design principle, sensitive data is protected by
   incorporating the data within the encapsulated text rather than by
   applying measures selectively to fields in the enclosing header.
   Examples of potentially sensitive header information may include
   fields such as "Subject:", with contents which are significant on an
   end-to-end, inter-user basis.  The (possibly empty) set of headers to
   which protection is to be applied is a user option.  It is strongly
   recommended, however, that all implementations should replicate
   copies of "X-Sender-ID:" and "X-Recipient-ID:" fields within the
   encapsulated text.

   If a user wishes disclosure protection for header fields, they must
   occur only in the encapsulated text and not in the enclosing or
   encapsulated header.  If disclosure protection is desired for a
   message's subject indication, it is recommended that the enclosing
   header contain a "Subject:" field indicating that "Encrypted Mail

   If an authenticated version of header information is desired, that
   data can be replicated within the encapsulated text portion in
   addition to its inclusion in the enclosing header.  For example, a
   sender wishing to provide recipients with a protected indication of a
   message's position in a series of messages could include a copy of a
   timestamp or message counter field within the encapsulated text.

   A specific point regarding the integration of privacy-enhanced mail

   facilities with the message encapsulation mechanism is worthy of
   note.  The subset of IA5 selected for transmission encoding
   intentionally excludes the character "-", so encapsulated text can be
   distinguished unambiguously from a message's closing encapsulation
   boundary (Post-EB) without recourse to character stuffing.

   Enclosing Header Portion
           (Contains header fields per RFC-822)

   Blank Line
           (Separates Enclosing Header from Encapsulated Message)

   Encapsulated Message

       Pre-Encapsulation Boundary (Pre-EB)

       Encapsulated Header Portion
           (Contains encryption control fields inserted in plaintext.
           Examples include "X-DEK-Info:", "X-Sender-ID:", and
           Note that, although these control fields have line-oriented
           representations similar to RFC-822 header fields, the set
           of fields valid in this context is disjoint from those used
           in RFC-822 processing.)

       Blank Line
           (Separates Encapsulated Header from subsequent encoded
           Encapsulated Text Portion)

       Encapsulated Text Portion
           (Contains message data encoded as specified in Section 4.3;
           may incorporate protected copies of enclosing and
           encapsulated header fields such as "Subject:", etc.)

       Post-Encapsulation Boundary (Post-EB)

                           Message Encapsulation
                                 Figure 1

4.5  Mail for Mailing Lists

   When mail is addressed to mailing lists, two different methods of
   processing can be applicable: the IK-per-list method and the IK-per-
   recipient method.  The choice depends on the information available to

   the sender and on the sender's preference.

   If a message's sender addresses a message to a list name or alias,
   use of an IK associated with that name or alias as a entity (IK-per-
   list), rather than resolution of the name or alias to its constituent
   destinations, is implied.  Such an IK must, therefore, be available
   to all list members.  For the case of asymmetric key management, the
   list's private component must be available to all list members.  This
   alternative will be the normal case for messages sent via remote
   exploder sites, as a sender to such lists may not be cognizant of the
   set of individual recipients.  Unfortunately, it implies an
   undesirable level of exposure for the shared IK, and makes its
   revocation difficult.  Moreover, use of the IK-per-list method allows
   any holder of the list's IK to masquerade as another sender to the
   list for authentication purposes.

   If, in contrast, a message's sender is equipped to expand the
   destination mailing list into its individual constituents and elects
   to do so (IK-per-recipient), the message's DEK (and, in the symmetric
   key management case, MIC) will be encrypted under each per-recipient
   IK and all such encrypted representations will be incorporated into
   the transmitted message.  Note that per-recipient encryption is
   required only for the relatively small DEK and MIC quantities carried
   in the "X-Key-Info:" field, not for the message text which is, in
   general, much larger.  Although more IKs are involved in processing
   under the IK-per-recipient method, the pairwise IKs can be
   individually revoked and possession of one IK does not enable a
   successful masquerade of another user on the list.

4.6  Summary of Encapsulated Header Fields

   This section summarizes the syntax and semantics of the encapsulated
   header fields to be added to messages in the course of privacy
   enhancement processing.  The fields are presented in three groups.
   Normally, the groups will appear in encapsulated headers in the order
   in which they are shown, though not all fields in each group will
   appear in all messages. In certain indicated cases, it is recommended
   that the fields be replicated within the encapsulated text portion as
   well as being included within the encapsulated header.  Figures 2 and
   3 show the appearance of small example encapsulated messages.  Figure
   2 assumes the use of symmetric cryptography for key management.
   Figure 3 illustrates an example encapsulated message in which
   asymmetric key management is used.

   Unless otherwise specified, all field arguments are processed in a
   case-sensitive fashion.  In most cases, numeric quantities are
   represented in header fields as contiguous strings of hexadecimal
   digits, where each digit is represented by a character from the

   ranges "0"-"9" or upper case "A"-"F".  Since public-key certificates
   and quantities encrypted using asymmetric algorithms are large in
   size, use of a more space-efficient encoding technique is appropriate
   for such quantities, and the encoding mechanism defined in Section of this RFC, representing 6 bits per printed character, is
   adopted.  The example shown in Figure 3 shows asymmetrically
   encrypted quantities (e.g., "X-MIC-Info:", "X-Key-Info:") with 64-
   character printed representations, corresponding to 384 bits.  The
   fields carrying asymmetrically encrypted quantities also illustrate
   the use of folding as defined in RFC-822, section 3.1.1.

   X-Proc-Type: 3,ENCRYPTED
   X-DEK-Info: DES-CBC,F8143EDE5960C597
   X-Sender-ID: linn@ccy.bbn.com::
   X-Recipient-ID: linn@ccy.bbn.com:ptf-kmc:3
   X-Key-Info: DES-ECB,RSA-MD2,9FD3AAD2F2691B9A,B70665BB9BF7CBCD,
   X-Recipient-ID: privacy-tf@venera.isi.edu:ptf-kmc:4
   X-Key-Info: DES-ECB,RSA-MD2,161A3F75DC82EF26,E2EF532C65CBCFF7,


               Example Encapsulated Message (Symmetric Case)
                                 Figure 2

   X-Proc-Type: 3,ENCRYPTED
   X-DEK-Info: DES-CBC,F8143EDE5960C597
   X-Sender-ID: linn@ccy.bbn.com::
   X-MIC-Info: RSA-MD2,RSA,
   X-Recipient-ID: linn@ccy.bbn.com:RSADSI:3

   X-Key-Info: RSA,
   X-Recipient-ID: privacy-tf@venera.isi.edu:RSADSI:4
   X-Key-Info: RSA,


              Example Encapsulated Message (Asymmetric Case)
                                 Figure 3

   Although the encapsulated header fields resemble RFC-822 header
   fields, they are a disjoint set and will not in general be processed
   by the same parser which operates on enclosing header fields.  The
   complexity of lexical analysis needed and appropriate for
   encapsulated header field processing is significantly less than that
   appropriate to RFC-822 header processing.  For example, many
   characters with special significance to RFC-822 at the syntactic
   level have no such significance within encapsulated header fields.

   When the length of an encapsulated header field is longer than the
   size conveniently printable on a line, whitespace may be used to fold
   the field in the manner of RFC-822, section 3.1.1.  Any such inserted
   whitespace is not to be interpreted as a part of a subfield.  As a
   particular example, due to the length of public-key certificates and
   of quantities encrypted using asymmetric algorithms, such quantities
   may often need to be folded across multiple printed lines.  In order
   to facilitate such folding in a uniform manner, the bits representing
   such a quantity are to be divided into an ordered set (with leftmost
   bits first) of zero or more 384-bit groups (corresponding to 64-
   character printed representations), followed by a final group of bits
   which may be any length up to 384 bits.

4.6.1  Per-Message Encapsulated Header Fields

   This group of encapsulated header fields contains fields which occur
   no more than once in a privacy-enhanced message, generally preceding
   all other encapsulated header fields.  X-Proc-Type Field

   The "X-Proc-Type:" encapsulated header field, required for all
   privacy-enhanced messages, identifies the type of processing

   performed on the transmitted message.  Only one "X-Proc-Type:" field
   occurs in a message; the "X-Proc-Type:" field must be the first
   encapsulated header field in the message.

   The "X-Proc-Type:" field has two subfields, separated by a comma.
   The first subfield is a decimal number which is used to distinguish
   among incompatible encapsulated header field interpretations which
   may arise as changes are made to this standard.  Messages processed
   according to this RFC will carry the subfield value "3" to
   distinguish them from messages processed in accordance with prior
   RFCs 989 and 1040.

   The second subfield may assume one of two string values: "ENCRYPTED"
   or "MIC-ONLY".  Unless all of a message's encapsulated text is
   excluded from encryption, the "X-Proc-Type:" field's second subfield
   must specify "ENCRYPTED".  Specification of "MIC-ONLY", when applied
   in conjunction with certain combinations of key management and MIC
   algorithm options, permits certain fields which are superfluous in
   the absence of encryption to be omitted from the encapsulated header.
   In particular, "X-Recipient-ID:" and "X-Key-Info:" fields can be
   omitted for recipients for whom asymmetric cryptography is used,
   assuming concurrent use of a keyless MIC computation algorithm.  The
   "X-DEK-Info:" field can be omitted for all "MIC-ONLY" messages.  X-DEK-Info Field

   The "X-DEK-Info:" encapsulated header field identifies the message
   text encryption algorithm and mode, and also carries the Initializing
   Vector used for message encryption.  No more than one "X-DEK-Info:"
   field occurs in a message; the field is required except for messages
   specified as "MIC-ONLY" in the "X-Proc-Type:" field.

   The "X-DEK-Info:" field carries two arguments, separated by a comma.
   For purposes of this RFC, the first argument must be the string
   "DES-CBC", signifying (as defined in RFC-1115) use of the DES
   algorithm in the CBC mode.  The second argument represents a 64-bit
   Initializing Vector (IV) as a contiguous string of 16 hexadecimal
   digits.  Subsequent revisions of RFC-1115 will specify any additional
   values which may appear as the first argument of this field.

4.6.2  Encapsulated Header Fields Normally Per-Message

   This group of encapsulated header fields contains fields which
   ordinarily occur no more than once per message.  Depending on the key
   management option(s) employed, some of these fields may be absent
   from some messages.  The "X-Sender-ID" field may occur more than once
   in a message if different sender-oriented IK components (perhaps
   corresponding to different versions) must be used for different

   recipients. In this case later occurrences override prior
   occurrences.  If a mixture of symmetric and asymmetric key
   distribution is used within a single message, the recipients for each
   type of key distribution technology should be grouped together to
   simplify parsing.  X-Sender-ID Field

   The "X-Sender-ID:" encapsulated header field, required for all
   privacy-enhanced messages, identifies a message's sender and provides
   the sender's IK identification component.  It should be replicated
   within the encapsulated text.  The IK identification component
   carried in an "X-Sender-ID:" field is used in conjunction with all
   subsequent "X-Recipient-ID:" fields until another "X-Sender-ID:"
   field occurs; the ordinary case will be that only a single "X-
   Sender-ID:" field will occur, prior to any "X-Recipient-ID:" fields.

   The "X-Sender-ID:" field contains (in order) an Entity Identifier
   subfield, an (optional) Issuing Authority subfield, and an (optional)
   Version/Expiration subfield.  The optional subfields are omitted if
   their use is rendered redundant by information carried in subsequent
   "X-Recipient-ID:" fields; this will ordinarily be the case where
   symmetric cryptography is used for key management.  The subfields are
   delimited by the colon character (":"), optionally followed by

   Section 5.2, Interchange Keys, discusses the semantics of these
   subfields and specifies the alphabet from which they are chosen.
   Note that multiple "X-Sender-ID:" fields may occur within a single
   encapsulated header.  All "X-Recipient-ID:" fields are interpreted in
   the context of the most recent preceding "X-Sender-ID:" field; it is
   illegal for an "X-Recipient-ID:" field to occur in a header before an
   "X-Sender-ID:" has been provided.  X-Certificate Field

   The "X-Certificate:" encapsulated header field is used only when
   asymmetric key management is employed for one or more of a message's
   recipients.  To facilitate processing by recipients (at least in
   advance of general directory server availability), inclusion of this
   field in all messages is strongly recommended.  The field transfers a
   sender's certificate as a numeric quantity, represented with the
   encoding mechanism defined in Section of this RFC.  The
   semantics of a certificate are discussed in RFC-1114.  The
   certificate carried in an "X-Certificate:" field is used in
   conjunction with "X-Sender-ID:" and "X-Recipient-ID:" fields for
   which asymmetric key management is employed.  X-MIC-Info Field

   The "X-MIC-Info:" encapsulated header field, used only when
   asymmetric key management is employed for at least one recipient of a
   message, carries three arguments, separated by commas.  The first
   argument identifies the algorithm under which the accompanying MIC is
   computed; RFC-1115 specifies the acceptable set of MIC algorithm
   identifiers.  The second argument identifies the algorithm under
   which the accompanying MIC is encrypted; for purposes of this RFC,
   the string "RSA" as described in RFC-1115  must occur, identifying
   use of the RSA algorithm.  The third argument is a MIC,
   asymmetrically encrypted using the originator's private component.
   As discussed earlier in this section, the asymmetrically encrypted
   MIC is represented using the technique described in Section
   of this RFC.

   The "X-MIC-Info:" field will occur immediately following the
   message's "X-Sender-ID:" field and any "X-Certificate:" or "X-
   Issuer-Certificate:" fields.  Analogous to the "X-Sender-ID:" field,
   an "X-MIC-Info:" field applies to all subsequent recipients for whom
   asymmetric key management is used.

4.6.3  Encapsulated Header Fields with Variable Occurrences

   This group of encapsulated header fields contains fields which will
   normally occur variable numbers of times within a message, with
   numbers of occurrences ranging from zero to non-zero values which are
   independent of the number of recipients.  X-Issuer-Certificate Field

   The "X-Issuer-Certificate:" encapsulated header field is meaningful
   only when asymmetric key management is used for at least one of a
   message's recipients.  A typical "X-Issuer-Certificate:" field would
   contain the certificate containing the public component used to sign
   the certificate carried in the message's "X-Certificate:" field, for
   recipients' use in chaining through that certificate's certification
   path.  Other "X-Issuer-Certificate:" fields, typically representing
   higher points in a certification path, also may be included by a
   sender.  The order in which "X-Issuer-Certificate:" fields are
   included need not correspond to the order of the certification path;
   the order of that path may in general differ from the viewpoint of
   different recipients.  More information on certification paths can be
   found in RFC-1114.

   The certificate is represented in the same manner as defined for the
   "X-Certificate:" field, and any "X-Issuer-Certificate:" fields will
   ordinarily follow the "X-Certificate:" field directly.  Use of the

   "X-Issuer-Certificate:" field is optional even when asymmetric key
   management is employed, although its incorporation is strongly
   recommended in the absence of alternate directory server facilities
   from which recipients can access issuers' certificates.

4.6.4  Per-Recipient Encapsulated Header Fields

   This group of encapsulated header fields normally appears once for
   each of a message's named recipients.  As a special case, these
   fields may be omitted in the case of a "MIC-ONLY" message to
   recipients for whom asymmetric key management is employed, given that
   the chosen MIC algorithm is keyless.  X-Recipient-ID Field

   The "X-Recipient-ID:" encapsulated header field identifies a
   recipient and provides the recipient's IK identification component.
   One "X-Recipient-ID:" field is included for each of a message's named
   recipients. It should be replicated within the encapsulated text.
   The field contains (in order) an Entity Identifier subfield, an
   Issuing Authority subfield, and a Version/Expiration subfield.  The
   subfields are delimited by the colon character (":"), optionally
   followed by whitespace.

   Section 5.2, Interchange Keys, discusses the semantics of the
   subfields and specifies the alphabet from which they are chosen.  All
   "X-Recipient-ID:" fields are interpreted in the context of the most
   recent preceding "X-Sender-ID:" field; it is illegal for an "X-
   Recipient-ID:" field to occur in a header before an "X-Sender-ID:"
   has been provided.  X-Key-Info Field

   One "X-Key-Info:" field is included for each of a message's named
   recipients.  Each "X-Key-Info:" field is interpreted in the context
   of the most recent preceding "X-Recipient-ID:" field; normally, an
   "X-Key-Info:" field will immediately follow its associated "X-
   Recipient-ID:" field.  The field's argument(s) differ depending on
   whether symmetric or asymmetric key management is used for a
   particular recipient.  Symmetric Key Management

   When symmetric key management is employed for a given recipient, the
   "X-Key-Info:" encapsulated header field transfers four items,
   separated by commas: an IK Use Indicator, a MIC Algorithm Indicator,
   a DEK and a MIC.  The IK Use Indicator identifies the algorithm and
   mode in which the identified IK was used for DEK encryption for a

   particular recipient.  For recipients for whom symmetric key
   management is used, it may assume the reserved string values "DES-
   ECB" or "DES-EDE", as defined in RFC-1115.

   The MIC Algorithm Indicator identifies the MIC computation algorithm
   used for a particular recipient; values for this subfield are defined
   in RFC-1115.  The DEK and MIC are encrypted under the IK identified
   by a preceding "X-Recipient-ID:" field and prior "X-Sender-ID:"
   field; they are represented as two strings of contiguous hexadecimal
   digits, separated by a comma.

   When DEA-1 is used for message text encryption, the DEK
   representation will be 16 hexadecimal digits (corresponding to a 64-
   bit key); this subfield can be extended to 32 hexadecimal digits
   (corresponding to a 128-bit key) if required to support other

   Symmetric encryption of MICs is always performed in the same
   encryption mode used to encrypt the message's DEK.  Encrypted MICs,
   like encrypted DEKs, are represented as contiguous strings of
   hexadecimal digits.  The size of a MIC is dependent on the choice of
   MIC algorithm as specified in the MIC Algorithm Indicator subfield.  Asymmetric Key Management

   When asymmetric key management is employed for a given recipient, the
   "X-Key-Info:" field transfers two quantities, separated by commas.
   The first argument is an IK Use Indicator identifying the algorithm
   (and mode, if applicable) in which the DEK is encrypted; for purposes
   of this RFC, the IK Use Indicator subfield will always assume the
   reserved string value "RSA" (as defined in RFC-1115) for recipients
   for whom asymmetric key management is employed, signifying use of the
   RSA algorithm.  The second argument is a DEK, encrypted (using
   asymmetric encryption) under the recipient's public component.

   Throughout this RFC we have adopted the terms "private component" and
   "public component" to refer to the quantities which are,
   respectively, kept secret and made publically available in asymmetric
   cryptosystems.  This convention is adopted to avoid possible
   confusion arising from use of the term "secret key" to refer to
   either the former quantity or to a key in a symmetric cryptosystem.

   As discussed earlier in this section, the asymmetrically encrypted
   DEK is represented using the technique described in Section
   of this RFC.

5.  Key Management

   Several cryptographic constructs are involved in supporting the
   privacy-enhanced message processing procedure.  A set of fundamental
   elements is assumed.  Data Encrypting Keys (DEKs) are used to encrypt
   message text and (for some MIC computation algorithms) in the message
   integrity check (MIC) computation process.  Interchange Keys (IKs)
   are used to encrypt DEKs and MICs for transmission with messages.  In
   a certificate-based asymmetric key management architecture,
   certificates are used as a means to provide entities' public
   components and other information in a fashion which is securely bound
   by a central authority.  The remainder of this section provides more
   information about these constructs.

5.1  Data Encrypting Keys (DEKs)

   Data Encrypting Keys (DEKs) are used for encryption of message text
   and (with some MIC computation algorithms) for computation of message
   integrity check quantities (MICs).  It is strongly recommended that
   DEKs be generated and used on a one-time, per-message, basis.  A
   transmitted message will incorporate a representation of the DEK
   encrypted under an appropriate interchange key (IK) for each of the
   named recipients.

   DEK generation can be performed either centrally by key distribution
   centers (KDCs) or  by endpoint systems.  Dedicated KDC systems may be
   able to  implement stronger algorithms for random DEK generation than
   can be supported in endpoint systems.  On the other hand,
   decentralization allows endpoints to be relatively self-sufficient,
   reducing the level of trust which must be placed in components other
   than a message's originator and recipient.  Moreover, decentralized
   DEK generation at endpoints reduces the frequency with which senders
   must make real-time queries of (potentially unique) servers in order
   to send mail, enhancing communications availability.

   When symmetric cryptography is used, one advantage of centralized
   KDC-based generation is that DEKs can be returned to endpoints
   already encrypted under the IKs of message recipients rather than
   providing the IKs to the senders.  This reduces IK exposure and
   simplifies endpoint key management requirements.  This approach has
   less value if asymmetric cryptography is used for key management,
   since per-recipient public IK components are assumed to be generally
   available and per-sender private IK components need not necessarily
   be shared with a KDC.

5.2  Interchange Keys (IKs)

   Interchange Key (IK) components are used to encrypt DEKs and MICs.

   In general, IK granularity is at the pairwise per-user level except
   for mail sent to address lists comprising multiple users.  In order
   for two principals to engage in a useful exchange of privacy-enhanced
   electronic mail using conventional cryptography, they must first
   possess common IK components (when symmetric key management is used)
   or complementary IK components (when asymmetric key management is
   used).  When symmetric cryptography is used, the IK consists of a
   single component, used to encrypt both DEKs and MICs.  When
   asymmetric cryptography is used, a recipient's public component is
   used as an IK to encrypt DEKs (a transformation invertible only by a
   recipient possessing the corresponding private component), and the
   originator's private component is used to encrypt MICs (a
   transformation invertible by all recipients, since the originator's
   certificate provides the necessary public component of the

   While this RFC does not prescribe the means by which interchange keys
   are provided to appropriate parties, it is useful to note that such
   means may be centralized (e.g., via key management servers) or
   decentralized (e.g., via pairwise agreement and direct distribution
   among users).  In any case, any given IK component is associated with
   a responsible Issuing Authority (IA).  When certificate-based
   asymmetric key management, as discussed in RFC-1114, is employed, the
   IA function is performed by a Certification Authority (CA).

   When an IA generates and distributes an IK component, associated
   control information is provided to direct how it is to be used.  In
   order to select the appropriate IK(s) to use in message encryption, a
   sender must retain a correspondence between IK components and the
   recipients with which they are associated.  Expiration date
   information must also be retained, in order that cached entries may
   be invalidated and replaced as appropriate.

   Since a message may be sent with multiple IK components identified,
   corresponding to multiple intended recipients, each recipient's UA
   must be able to determine that recipient's intended IK component.
   Moreover, if no corresponding IK component is available in the
   recipient's database when a message arrives, the recipient must be
   able to identify the required IK component and identify that IK
   component's associated IA.  Note that different IKs may be used for
   different messages between a pair of communicants.  Consider, for
   example, one message sent from A to B and another message sent (using
   the IK-per-list method) from A to a mailing list of which B is a
   member.  The first message would use IK components associated
   individually with A and B, but the second would use an IK component
   shared among list members.

   When a privacy-enhanced message is transmitted, an indication of the

   IK components used for DEK and MIC encryption must be included.  To
   this end, the "X-Sender-ID:" and "X-Recipient-ID:" encapsulated
   header fields provide the following data:

      1. Identification of the relevant Issuing Authority (IA subfield)

      2.  Identification of an entity with which a particular IK
          component is associated (Entity Identifier or EI subfield)

      3.  Version/Expiration subfield

   The colon character (":") is used to delimit the subfields within an
   "X-Sender-ID:" or "X-Recipient-ID:".  The IA, EI, and
   version/expiration subfields are generated from a restricted
   character set, as prescribed by the following BNF (using notation as
   defined in RFC-822, sections 2 and 3.3):

   IKsubfld       :=       1*ia-char

   ia-char        :=       DIGIT / ALPHA / "'" / "+" / "(" / ")" /
                           "," / "." / "/" / "=" / "?" / "-" / "@" /
                           "%" / "!" / '"' / "_" / "<" / ">"
   An example "X-Recipient-ID:" field is as follows:

      X-Recipient-ID: linn@ccy.bbn.com:ptf-kmc:2

   This example field indicates that IA "ptf-kmc" has issued an IK
   component for use on messages sent to "linn@ccy.bbn.com", and that
   the IA has provided the number 2 as a version indicator for that IK

5.2.1  Subfield Definitions

   The following subsections define the subfields of "X-Sender-ID:" and
   "X-Recipient-ID:" fields.  Entity Identifier Subfield

   An entity identifier is constructed as an IKsubfld.  More
   restrictively, an entity identifier subfield assumes the following


   In order to support universal interoperability, it is necessary to
   assume a universal form for the naming information.  For the case of
   installations which transform local host names before transmission
   into the broader Internet, it is strongly recommended that the host

   name as presented to the Internet be employed.  Issuing Authority Subfield

   An IA identifier subfield is constructed as an IKsubfld.  IA
   identifiers must be assigned in a manner which assures uniqueness.
   This can be done on a centralized or hierarchic basis.  Version/Expiration Subfield

   A version/expiration subfield is constructed as an IKsubfld.  The
   version/expiration subfield format may vary among different IAs, but
   must satisfy certain functional constraints.  An IA's
   version/expiration subfields must be sufficient to distinguish among
   the set of IK components issued by that IA for a given identified
   entity.  Use of a monotonically increasing number is sufficient to
   distinguish among the IK components provided for an entity by an IA;
   use of a timestamp additionally allows an expiration time or date to
   be prescribed for an IK component.

5.2.2  IK Cryptoperiod Issues

   An IK component's cryptoperiod is dictated in part by a tradeoff
   between key management overhead and revocation responsiveness.  It
   would be undesirable to delete an IK component permanently before
   receipt of a message encrypted using that IK component, as this would
   render the message permanently undecipherable.  Access to an expired
   IK component would be needed, for example, to process mail received
   by a user (or system) which had been inactive for an extended period
   of time.  In order to enable very old IK components to be deleted, a
   message's recipient desiring encrypted local long term storage should
   transform the DEK used for message text encryption via re-encryption
   under a locally maintained IK, rather than relying on IA maintenance
   of old IK components for indefinite periods.

6.  User Naming

6.1  Current Approach

   Unique naming of electronic mail users, as is needed in order to
   select corresponding keys correctly, is an important topic and one
   which has received significant study.  Our current architecture
   associates IK components with user names represented in a universal
   form ("user@domain-qualified-host"), relying on the following

      1.  The universal form must be specifiable by an IA as it
          distributes IK components and known to a UA as it processes

          received IK components and IK component identifiers.  If a
          UA or IA uses addresses in a local form which is different
          from the universal form, it must be able to perform an
          unambiguous mapping from the universal form into the local

      2.  The universal form, when processed by a sender UA, must have
          a recognizable correspondence with the form of a recipient
          address as specified by a user (perhaps following local
          transformation from an alias into a universal form).

   It is difficult to ensure these properties throughout the Internet.
   For example, an MTS which transforms address representations between
   the local form used within an organization and the universal form as
   used for Internet mail transmission may cause property 2 to be

6.2  Issues for Consideration

   The use of flat (non-hierarchic) electronic mail user identifiers,
   which are unrelated to the hosts on which the users reside, may offer
   value.  As directory servers become more widespread, it may become
   appropriate for would-be senders to search for desired recipients
   based on such attributes.  Personal characteristics, like social
   security numbers, might be considered.  Individually-selected
   identifiers could be registered with a central authority, but a means
   to resolve name conflicts would be necessary.

   A point of particular note is the desire to accommodate multiple
   names for a single individual, in order to represent and allow
   delegation of various roles in which that individual may act.  A
   naming mechanism that binds user roles to keys is needed.  Bindings
   cannot be immutable since roles sometimes change (e.g., the
   comptroller of a corporation is fired).

   It may be appropriate to examine the prospect of extending the
   DARPA/DoD domain system and its associated name servers to resolve
   user names to unique user IDs.  An additional issue arises with
   regard to mailing list support: name servers do not currently perform
   (potentially recursive) expansion of lists into users.  ISO and CSNet
   are working on user-level directory service mechanisms, which may
   also bear consideration.

7.  Example User Interface and Implementation

   In order to place the mechanisms and approaches discussed in this RFC
   into context, this section presents an overview of a prototype
   implementation. This implementation is a standalone program which is

   invoked by a user, and lies above the existing UA sublayer.  In the
   UNIX system, and possibly in other environments as well, such a
   program can be invoked as a "filter" within an electronic mail UA or
   a text editor, simplifying the sequence of operations which must be
   performed by the user.  This form of integration offers the advantage
   that the program can be used in conjunction with a range of UA
   programs, rather than being compatible only with a particular UA.

   When a user wishes to apply privacy enhancements to an outgoing
   message, the user prepares the message's text and invokes the
   standalone program (interacting with the program in order to provide
   address information and other data required to perform privacy
   enhancement processing), which in turn generates output suitable for
   transmission via the UA.  When a user receives a privacy-enhanced
   message, the UA delivers the message in encrypted form, suitable for
   decryption and associated processing by the standalone program.

   In this prototype implementation (based on symmetric key management),
   a cache of IK components is maintained in a local file, with entries
   managed manually based on information provided by originators and
   recipients.  This cache is, effectively, a simple database.  IK
   components are selected for transmitted messages based on the
   sender's identity and on recipient names, and corresponding "X-
   Sender-ID:" and "X-Recipient-ID:" fields are placed into the
   message's encapsulated header.  When a message is received, these
   fields are used as a basis for a lookup in the database, yielding the
   appropriate IK component entries.  DEKs and IVs are generated
   dynamically within the program.

   Options and destination addresses are selected by command line
   arguments to the standalone program.  The function of specifying
   destination addresses to the privacy enhancement program is logically
   distinct from the function of specifying the corresponding addresses
   to the UA for use by the MTS.  This separation results from the fact
   that, in many cases, the local form of an address as specified to a
   UA differs from the Internet global form as used in "X-Sender-ID:"
   and "X-Recipient-ID:" fields.

8.  Areas For Further Study

   The procedures defined in this RFC are sufficient to support
   implementation of privacy-enhanced electronic mail transmission among
   cooperating parties in the Internet.  Further effort will be needed,
   however, to enhance robustness, generality, and interoperability.  In
   particular, further work is needed in the following areas:

      1.  User naming techniques, and their relationship to the domain
          system, name servers, directory services, and key management


      2.  Detailed standardization of Issuing Authority and directory
          service functions and interactions.

      3.  Privacy-enhanced interoperability with X.400 mail.

   We anticipate generation of subsequent RFCs which will address these

9.  References

   This section identifies background references which may be useful to
   those contemplating use of the mechanisms defined in this RFC.

       ISO 7498/Part 2 - Security Architecture, prepared by ISO/TC97/SC
       21/WG 1 Ad hoc group on Security, extends the OSI Basic Reference
       Model to cover security aspects which are general architectural
       elements of communications protocols, and provides an annex with
       tutorial and background information.

       US Federal Information Processing Standards Publication (FIPS
       PUB) 46, Data Encryption Standard, 15 January 1977, defines the
       encipherment algorithm used for message text encryption and
       Message Authentication Code (MAC) computation.

       FIPS PUB 81, DES Modes of Operation, 2 December 1980, defines
       specific modes in which the Data Encryption Standard algorithm
       may to be used to perform encryption.

       FIPS PUB 113, Computer Data Authentication, May 1985, defines a
       specific procedure for use of the Data Encryption Standard
       algorithm to compute a MAC.


  [1]  Key generation for MIC computation and message text encryption
       may either be performed by the sending host or by a centralized
       server.  This RFC does not constrain this design alternative.
       Section 5.1 identifies possible advantages of a centralized
       server approach if symmetric key management is employed.

  [2]  American National Standard Data Encryption Algorithm (ANSI
       X3.92-1981), American National Standards Institute, Approved 30
       December 1980.

  [3]  Federal Information Processing Standards Publication 46, Data
       Encryption Standard, 15 January 1977.

  [4]  Information Processing Systems: Data Encipherment: Modes of
       Operation of a 64-bit Block Cipher.

  [5]  Federal Information Processing Standards Publication 81, DES
       Modes of Operation, 2 December 1980.

  [6]  ANSI X9.17-1985, American National Standard, Financial
       Institution Key Management (Wholesale), American Bankers
       Association, April 4, 1985, Section 7.2.

  [7]  Postel, J., "Simple Mail Transfer Protocol" RFC-821,
       USC/Information Sciences Institute, August 1982.

  [8]  This transformation should occur only at an SMTP endpoint, not at
       an intervening relay, but may take place at a gateway system
       linking the SMTP realm with other environments.

  [9]  Use of the SMTP canonicalization procedure at this stage was
       selected since it is widely used and implemented in the Internet
       community, not because SMTP interoperability with this
       intermediate result is required; no privacy-enhanced message will
       be passed to SMTP for transmission directly from this step in the
       four-phase transformation procedure.

 [10]  Crocker, D., "Standard for the Format of ARPA Internet Text
       Messages", RFC-822, August 1982.

 [11]  Rose, M. and E. Stefferud, "Proposed Standard for Message
       Encapsulation", RFC-934, January 1985.

 [12]  CCITT Recommendation X.411 (1988), "Message Handling Systems:
       Message Transfer System: Abstract Service Definition and

 [13]  CCITT Recommendation X.509 (1988), "The Directory -
       Authentication Framework".

 [14]  Kille, S., "Mapping between X.400 and RFC-822", RFC-987, June

 [15]  Federal Information Processing Standards Publication 113,
       Computer Data Authentication, May 1985.

 [16]  American National Standard for Information Systems - Data
       Encryption Algorithm - Modes of Operation (ANSI X3.106-1983),
       American National Standards Institute - Approved 16 May 1983.

 [17]  Voydock, V. and S. Kent, "Security Mechanisms in High-Level

       Network Protocols", ACM Computing Surveys, Vol. 15, No. 2, Pages
       135-171, June 1983.

Author's Address

       John Linn
       Secure Systems
       Digital Equipment Corporation
       85 Swanson Road, BXB1-2/D04
       Boxborough, MA  01719-1326

       Phone: 508-264-5491

       EMail: Linn@ultra.enet.dec.com


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