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rfc:rfc989

Network Working Group John Linn (BBNCC) Request for Comments: 989 IAB Privacy Task Force

                                                          February 1987
         Privacy Enhancement for Internet Electronic Mail:
     Part I: Message Encipherment and Authentication Procedures

STATUS OF THIS MEMO

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

ACKNOWLEDGMENT

 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, Matt Bishop, Danny Cohen, Tom Daniel, Charles Fox, Morrie
 Gasser, Steve Kent (chairman), John Laws, Steve Lipner, Dan Nessett,
 Mike Padlipsky, Rob Shirey, Miles Smid, Steve Walker, and Steve
 Wilbur.

1 Executive Summary

 This RFC defines message encipherment and authentication procedures,
 as the initial phase of an effort to provide privacy enhancement
 services for electronic mail transfer in the Internet.  Detailed key
 management mechanisms to support these procedures will be defined in
 a subsequent RFC.  As a goal of this initial phase, it is intended
 that the procedures defined here be compatible with a wide range of
 key management approaches, including both conventional (symmetric)
 and public-key (asymmetric) approaches for encryption of data
 encrypting keys.  Use of conventional cryptography for message text
 encryption and/or authentication is anticipated.
 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.

Linn, Privacy Task Force [Page 1] RFC 989 February 1987

2 Terminology

 For descriptive purposes, this RFC uses some terms defined in the OSI
 X.400 Message Handling System Model.  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 User
 Agent.  A User Agent (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's goal is to define 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 points.  Two distinct privacy
 enhancement service options are supported:
    1.  an option providing sender authentication and integrity
        verification
    2.  an option providing sender authentication and integrity
        verification in addition to confidentiality service through
        encryption
 No facility for confidentiality service in the absence of
 authentication is provided.  Encryption and authentication facilities
 may be applied selectively to portions of a message's contents; this
 allows less sensitive portions of messages (e.g., descriptive fields)

Linn, Privacy Task Force [Page 2] RFC 989 February 1987

 to be processed by a recipient's delegate in the absence of the
 recipient's personal cryptographic keys.
 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 offer compatibility with non-
           enhanced Internet components.  Privacy enhancements will be
           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 transformations.
      3.   The defined mechanisms offer compatibility 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
           environments.
      4.   The defined mechanisms offer compatibility 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 an electronic mail
           privacy enhancement be available to the broadest possible
           user community, it is desirable that the selected mechanism
           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

Linn, Privacy Task Force [Page 3] RFC 989 February 1987

           each UA with which enhanced privacy 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.
 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 actions.
        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
 provided:
  1. - disclosure protection,

Linn, Privacy Task Force [Page 4] RFC 989 February 1987

  1. - sender authenticity, and
  1. - message integrity measures,
 but the following privacy-relevant concerns are not addressed:
  1. - access control,
  1. - traffic flow security,
  1. - address list accuracy,
  1. - routing control,
  1. - issues relating to the serial reuse of PCs by multiple users,
  1. - assurance of message receipt and non-deniability of receipt, and
  1. - automatic association of acknowledgments with the messages to

which they refer

 An important goal is that privacy enhancement mechanisms impose a
 minimum of burden on the users they serve.  In particular, this goal
 suggests eventual automation of the key management mechanisms
 supporting message encryption and authentication.  In order to
 facilitate deployment and testing of pilot privacy enhancement
 implementations in the near term, however, compatibility with out-
 of-band (e.g., manual) key distribution must also be supported.
 A message's sender will determine whether privacy enhancements are to
 be performed on a particular message.  This will necessitate
 mechanisms by which a sender can 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.
 A two-level keying hierarchy is used to support privacy-enhanced
 message transmission:
   1.   Data Encrypting Keys (DEKs) are used for encryption of message

Linn, Privacy Task Force [Page 5] RFC 989 February 1987

        text and for computation of message authentication codes
        (MACs).  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.  An IK may either be a single symmetric
        cryptographic key or, where asymmetric (public-key)
        cryptography is used for DEK encryption, the composition of a
        public component used by an originator and a secret component
        used by a recipient.  Ordinarily, the same IK will be used for
        all messages sent between a given originator-recipient pair
        over a period of time.  Each transmitted message includes a
        representation of the DEK(s) used for message encryption
        and/or authentication, encrypted under an individual IK per
        named recipient.  This representation is accompanied by an
        identifier (IK ID) to enable the recipient to determine which
        IK was used, and so to decrypt the representation yielding the
        DEK required for message text decryption and/or MAC
        verification.
 An encoding procedure is employed in order to represent encrypted
 message text in a universally transmissible form and to enable
 messages encrypted on one type of system to be decrypted on a
 different type.  Four phases are involved in this process.  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.  The canonical
 representation is padded to an integral multiple of eight octets, as
 required by the encryption algorithm.  MAC computation is performed,
 and (if disclosure protection is required), the padded canonical
 representation is encrypted.  The output of this step 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 (to be defined in Section 4.8) 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 MAC verification and
 decryption on the received message text.  First, the printable
 encoding is converted to a bitstring.  If the transmitted message was
 encrypted, it is decrypted into the canonical representation.  If the
 message was not encrypted, decoding from the printable form produces
 the canonical representation directly.  The MAC is verified, and the

Linn, Privacy Task Force [Page 6] RFC 989 February 1987

 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 ISO draft international standard DIS 8227 [1] shall be used for
 encryption of message text and for computation of authentication
 codes on messages.  The DEA-1 is equivalent to the Data Encryption
 Standard (DES), as defined in FIPS PUB 46 [2].  When used for these
 purposes, the DEA-1 shall be used in the Cipher Block Chaining (CBC)
 mode, as defined in ISO DIS 8372 [3].  The CBC mode definition in DIS
 8372 is equivalent to that provided in FIPS PUB 81 [4].  A unique
 initializing vector (IV) will be generated for and transmitted with
 each encrypted electronic mail message.
 An algorithm other than DEA-1 may be employed, provided that it
 satisfies the following requirements:
     1.  it must be a 64-bit block cipher, enciphering and deciphering
         in 8 octet blocks
     2.  it is usable in the ECB and CBC modes defined in DIS8372
     3.  it is able to be keyed using the procedures and parameters
         defined in this RFC
     4.  it is appropriate for MAC computation
     5.  cryptographic key field lengths are limited to 16 octets
         in length
 Certain operations require that one key be encrypted under another
 key (interchange key) for purposes of transmission.  For purposes of
 this RFC, such encryption will be performed using DEA-1 in Electronic
 Codebook (ECB) mode.  An optional facility is available to an
 interchange key provider to indicate that an associated key is to be
 used for encryption in another mode (e.g., the Encrypt-Decrypt-
 Encrypt (EDE) mode used for key encryption and decryption with pairs
 of 64-bit keys, as described [5] by ASC X3T1).
 Future support of public key algorithms for key encryption is under
 consideration, and it is intended that the procedures defined in this
 RFC be appropriate to allow such usage.  Support of key encryption
 modes other than ECB is optional for implementations, however.
 Therefore, in support of universal interoperability, interchange key
 providers should not specify other modes in the absence of a priori
 information indicating that recipients are equipped to perform key
 encryption in other modes.

Linn, Privacy Task Force [Page 7] RFC 989 February 1987

4.3 Canonical Encoding

 Any encryption scheme 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
 transport facilities.  SMTP, designed primarily for interpersonal
 messages and anticipating systems and transport media which may be
 restricted to a 7-bit character set, can transmit any 7-bit
 characters (but not arbitrary 8-bit binary data) in message text.
 SMTP introduces other transparency constraints related to line
 lengths and message delimiters.  Message text may not contain the
 string "<CR><LF>.<CR><LF>" in sequence before the end of a message,
 and must contain the string "<CR><LF>" at least every 1000
 characters.  Another important SMTP transparency issue must be noted.
 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(tm) 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 [6] 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 using 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 MAC 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 the disadvantage that it would
 imply internal file (e.g., mailbox) formats which would be
 incompatible with the systems on which they reside, an untenable
 prospect.  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.
 Our approach to this problem selects a canonical character set,
 uniformly representable across privacy-enhanced UAs regardless of
 their systems' native character sets, to transport encrypted mail
 text (but not electronic mail transport headers!) between endpoints.

Linn, Privacy Task Force [Page 8] RFC 989 February 1987

 In this approach, an outbound privacy-enhanced message is transformed
 between four forms, in sequence:
   1.   (Local_Form) The message text is created (e.g., via an editor)
        in the system's native character set, with lines delimited in
        accordance with local convention.
   2.   (Canonicalize) The message text is converted to the universal
        canonical form, equivalent to the inter-SMTP representation as
        defined in RFC822 [7] (ASCII character set, <CR><LF> line
        delimiters).  (The processing required to perform this
        conversion is relatively small, at least on systems whose
        native character set is ASCII.)
   3.   (Encipher/Authenticate) A padded version of the canonical
        plaintext representation is created by appending zero-valued
        octets to the end of the representation until the length is an
        integral multiple of 8 octets, as is required to perform
        encryption in the DEA-1 CBC mode.  No padding is applied if
        the canonical plaintext representation's length is already a
        multiple of 8 octets.  This padded representation is used as
        the input to the encryption function and to the MAC
        computation function.
   4.   (Encode to Printable Form) The bits resulting from the
        encryption operation are 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).  Use
        of a 64-character subset of International Alphabet IA5 is
        proposed, 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 encoding function's output is delimited into text lines
        (using local conventions), with each line containing 64
        printable characters.  The encoding process is performed as
        follows, transforming strings of 3 arbitrary (8-bit)
        characters to strings of 4 encoded characters:
        4a.  Proceeding from left to right across the input characters
             (considered as a contiguous bitstring), each group of 6
             bits 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 1, are selected so as to be
             universally representable, and the set excludes
             characters with particular significance to SMTP e.g.,

Linn, Privacy Task Force [Page 9] RFC 989 February 1987

             ".", "<CR>", "<LF>").
        4b.  If fewer than 3 input characters are available in a final
             quantum, 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 a 65th reserved, universally
             representable character ("=").  Use of a reserved
             character for padding allows compensatory processing to
             be performed by a recipient, allowing the decoded message
             text's length to be precisely the same as the input
             message's length.  A final 3-octet input quantum will be
             represented as a 4 octet encoding with no terminal "=", a
             2-octet input quantum will be represented as 3 octets
             followed by one terminal "=", and a 1-octet input quantum
             will be represented as 2 octets followed by two
             occurrences of "=".
 A sender may exclude one or more portions of a message from
 encryption/authentication processing.  Explicit action is required to
 exclude a portion of a message from such processing; by default,
 encryption/authentication 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/authentication processing.  An
 excluded area is represented in the inter-SMTP transmission form
 (universal across communicating systems) by bracketing with the
 reserved delimiter "*".  Cryptographic state is preserved
 transparently across an excluded area and continued after the end of
 the excluded area.  A printable encoding quantum (per step 4b) is
 completed before the delimiter "*" is output to initiate or terminate
 the representation of an excluded block.  Note that the
 canonicalizing transformation (step 2 above) and the encoding to
 printable form (step 4 above) are applied to all portions of message
 text, even those excluded from encryption and authentication.
 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)))
 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.

Linn, Privacy Task Force [Page 10] RFC 989 February 1987

      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 delimit portions of an
      encoded message to which encryption/authentication
      processing has not been applied.
                       Printable Encoding Characters
                                  Table 1

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.  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
 portion.
 The encapsulation mechanism to be used for privacy-enhanced mail is
 derived from that described in RFC934 [8] 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.  Note that, while encryption and/or authentication
 processing of transmitted mail may depend on information contained in
 the enclosing header (e.g., "To:"), 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

Linn, Privacy Task Force [Page 11] RFC 989 February 1987

 processing can be applied recursively.
 Sensitive data should be 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.  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.  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 the "Subject:" field, it is recommended that the
 enclosing header contain a "Subject:" field indicating that
 "Encrypted Mail Follows".
 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.

4.5 Processing for Authentication Without Confidentiality

 When a message is to be authenticated without confidentiality
 service, a DEK is generated [9] for use in MAC computation, and a MAC
 is computed using that DEK.  For each individually identified
 recipient, an IK is selected and identified with an "X-IK-ID:" field.
 Each "X-IK-ID:" field is followed by an "X-Key-Info:" field which
 transfers the key under which MAC computation was performed,
 encrypted under the IK identified by the preceding "X-IK-ID:" field,
 along with a representation of the MAC encrypted under the same IK.
 The encapsulated text portion following the encapsulated header is
 canonically encoded, and coded into printable characters for
 transmission, but not encrypted.

Linn, Privacy Task Force [Page 12] RFC 989 February 1987

 Enclosing Header Portion
        (Contains header fields per RFC-822)
 Blank Line
        (Separates Enclosing Header from Encapsulated Message)
 Encapsulated Message
     Pre-Encapsulation Boundary (Pre-EB)
  1. —-PRIVACY-ENHANCED MESSAGE BOUNDARY—–
     Encapsulated Header Portion
         (Contains encryption control fields inserted in plaintext.
         Examples include "X-IV:", "X-IK-ID:", "X-Key-Info:",
         and "X-Pad-Count:".  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 "Subject:", etc.)
     Post-Encapsulation Boundary (Post-EB)
  1. —-PRIVACY-ENHANCED MESSAGE BOUNDARY—–
                         Message Encapsulation
                               Figure 1

4.6 Processing for Authentication and Confidentiality

 When a message is to be authenticated with confidentiality service, a
 DEK is generated for use in MAC computation and a variant of the DEK
 is formed for use in message encryption.  For each individually
 identified recipient, an IK is selected and identified with an "X-
 IK-ID:" field.  Each "X-IK-ID:" field is followed by an "X-Key-Info:"
 field, which transfers the DEK and computed MAC, each encrypted under
 the IK identified in the preceding "X-IK-ID:" field.  The
 encapsulated text portion following the encapsulated header is
 canonically encoded, encrypted, and coded into printable characters

Linn, Privacy Task Force [Page 13] RFC 989 February 1987

 for transmission.

4.7 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.  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 MAC 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 MAC 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.

4.8 Summary of Added Header and Control Fields

 This section summarizes the syntax and semantics of the new header
 and control fields to be added to messages in the course of privacy
 enhancement processing, indicating whether a particular field occurs
 in a message's encapsulated header portion or its encapsulated text
 portion.  Figure 2 shows the appearance of a small example
 encapsulated message using these fields.  In all cases, hexadecimal
 quantities are represented as contiguous strings of digits, where
 each digit is represented by a character from the ranges "0"-"9" or
 upper case "A"-"F".  Unless otherwise specified, all arguments are to
 be processed in a case-sensitive fashion.
 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

Linn, Privacy Task Force [Page 14] RFC 989 February 1987

 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.
 The "X-IK-ID" and "X-Key-Info" fields are the only encapsulated
 header fields with lengths which can vary beyond a size conveniently
 printable on a line.  Whitespace may be used between the subfields of
 these fields to fold them in the manner of RFC-822; such whitespace
 is not to be interpreted as a part of a subfield.
  1. —-PRIVACY-ENHANCED MESSAGE BOUNDARY—–

X-Proc-Type: 1,E

    X-Pad-Count: 1
    X-IV: F8143EDE5960C597
    X-IK-ID: JL:3:ECB
    X-Key-Info: 9FD3AAD2F2691B9A,B70665BB9BF7CBCD
    X-IK-ID: JL:1:ECB
    X-Key-Info: 161A3F75DC82EF26,E2EF532C65CBCFF7
    LLrHB0eJzyhP+/fSStdW8okeEnv47jxe7SJ/iN72ohNcUk2jHEUSoH1nvNSIWL9M
    8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIkjHUlBLpvXR0UrUzYbkNpk0agV2IzUpk
    J6UiRRGcDSvzrsoK+oNvqu6z7Xs5Xfz5rDqUcMlK1Z6720dcBWGGsDLpTpSCnpot
    dXd/H5LMDWnonNvPCwQUHt==
    -----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
                       Example Encapsulated Message
                                 Figure 2
   X-IK-ID:      This field is placed in the encapsulated header
                 portion of a message to identify the Interchange Key
                 used for encryption of an associated Data Encrypting
                 Key or keys (used for message text encryption and/or
                 MAC computation).  This field is used for messages
                 authenticated without confidentiality service and for
                 messages authenticated with confidentiality service.
                 The field contains (in order) an Issuing Authority
                 subfield and an IK Qualifier subfield, and may also
                 contain an optional IK Use Indicator subfield.  The
                 subfields are delimited by the colon character (":"),
                 optionally followed by whitespace.  Section 5.1.2,
                 Interchange Keys, discusses the semantics of these
                 subfields and specifies the alphabet from which they
                 are chosen.  Note that multiple X-IK-ID fields may
                 occur within a single encapsulated header.  Each X-
                 IK-ID field is associated with an immediately
                 subsequent X-Key-Info field.
   X-IV:         This field is placed in the encapsulated header
                 portion of a message to carry the Initializing Vector

Linn, Privacy Task Force [Page 15] RFC 989 February 1987

                 used for message encryption.  It is used only for
                 messages where confidentiality service is applied.
                 Following the field name, and one or more delimiting
                 whitespace characters, a 64-bit Initializing Vector
                 is represented as a contiguous string of 16
                 hexadecimal digits.
   X-Key-Info:   This field is placed in a message's encapsulated
                 header portion to transfer two items: a DEK and a
                 MAC.  Both items are encrypted under the IK
                 identified by a preceding X-IK-ID field; they are
                 represented as two strings of contiguous hexadecimal
                 digits, separated by a comma.  For DEA-1, 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 algorithms.
                 The MAC is a 64-bit quantity, represented as 16
                 hexadecimal digits.  The MAC is computed under an
                 unmodified version of the DEK.  Message encryption is
                 performed using a variant of the DEK, formed by
                 modulo-2 addition of the hexadecimal quantity
                 F0F0F0F0F0F0F0F0 to the DEK.
   X-Pad-Count:  This field is placed in the encapsulated header
                 portion of a message to indicate the number of zero-
                 valued octets which were added to pad the input
                 stream to the encryption function to an integral
                 multiple of eight octets, as required by the DEA-1
                 CBC encryption mode.  A decimal number in the range
                 0-7 follows the field name, and one or more
                 delimiting whitespace characters.  Inclusion of this
                 field allows disambiguation between terminal zero-
                 valued octets in message text (admittedly, a
                 relatively unlikely prospect) and zero-valued octets
                 inserted for padding purposes.
   X-Proc-Type:  This field is placed in the encapsulated header
                 portion of a message to identify the type of
                 processing performed on the transmitted message.  The
                 first subfield is a decimal version number, which
                 will be used if future developments make it necessary
                 to redefine the interpretation of encapsulated header
                 fields.  At present, this field may assume only the
                 value "1".  The second subfield, delimited by a
                 comma, assumes one of two single-character alphabetic
                 values: "A" and "E", to signify, respectively, (1)
                 authentication processing only and (2) the
                 combination of authentication and confidentiality
                 service through encryption.

Linn, Privacy Task Force [Page 16] RFC 989 February 1987

5 Key Management

5.1 Types of Keys

5.1.1 Data Encrypting Keys (DEKs)

 Data Encrypting Keys (DEKs) are used for encryption of message text
 and for computation of message authentication codes (MACs).  It is
 strongly recommended that DEKs be generated and used on a one-time
 basis.  A transmitted message will incorporate a representation of
 the DEK encrypted under an interchange key (IK) known to the
 authorized recipient.
 DEK generation can be performed either centrally by key distribution
 centers (KDCs) or by endpoint systems.  One advantage of centralized
 KDC-based generation is that DEKs can be returned to endpoints
 already encrypted under the IKs of message recipients.  This reduces
 IK exposure and simplifies endpoint key management requirements.
 Further, dedicated KDC systems may be able to implement better
 algorithms for random key 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 by
 endpoints reduces the frequency with which senders must make real-
 time queries of (potentially unique) servers in order to send mail,
 enhancing communications availability.

5.1.2 Interchange Keys (IKs)

 Interchange Keys (IKs) are used to encrypt Data Encrypting Keys.  In
 general, the granularity of IK usage 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 share a common interchange key.  When asymmetric
 cryptography is used, an originator and recipient must possess
 appropriate public and secret components which, in composition,
 constitute an interchange key.
 The means by which interchange keys are provided to appropriate
 parties are outside the scope of this RFC, but may be centralized
 (e.g., via key management servers) or decentralized (e.g., via direct
 distribution among users).  In any case, a given IK is associated
 with a responsible Issuing Authority (IA).  When an IA generates and
 distributes an IK, associated control information must be provided to
 direct how that IK is to be used.  In order to select the appropriate
 IK to use in message encryption, a sender must retain a
 correspondence between IKs 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

Linn, Privacy Task Force [Page 17] RFC 989 February 1987

 appropriate.
 When a privacy-enhanced message is transmitted, an indication of the
 IK (or IKs, in the case of a message sent to multiple recipients)
 used for DEK encryption must be included.  To this end, the IK ID
 construct is defined to provide the following data:
      1.   Identification of the relevant Issuing Authority (IA
           subfield)
      2.   Qualifier string to distinguish the particular IK within
           the set of IKs distributed by the IA (IK qualifier
           subfield)
      3.   (Optional) Indicator of IK usage mode (IK use indicator
           subfield)
 The subfields of an IK ID are delimited with the colon character
 (":").  The IA and IK qualifier 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):
 IAorIKQual   :=      1*ia-char
 ia-char      :=      DIGIT / ALPHA / "'" / "+" / "(" / ")" /
                      "," / "." / "/" / "=" / "?" / "-" / "@" /
                      "%" / "!" / '"' / "_" / "<" / ">"
 The IK use indicator subfield assumes a value from a small set of
 reserved strings, described later in this section.
 IA identifiers must be assigned in a manner which assures uniqueness.
 This can be done on a centralized or hierarchic basis.
 The IK qualifier string format may vary among different IAs, but must
 satisfy certain functional constraints.  An IA's IK qualifiers must
 be sufficient to distinguish among the set of IKs issued by that IA.
 Since a message may be sent with multiple IK IDs, corresponding to
 multiple intended recipients, each recipient must be able to
 determine which IK is intended for it.  Moreover, if no corresponding
 IK is available in the recipient's database when a message arrives,
 the recipient must be able to determine which IK to request and to
 identify that IK'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 an IK shared
 between A and B, but the second would use an IK shared among list
 members.

Linn, Privacy Task Force [Page 18] RFC 989 February 1987

 While use of a monotonically increasing number as an IK qualifier is
 sufficient to distinguish among the set of IKs distributed by an IA,
 it offers no facility for a recipient lacking a matching IK to
 determine the appropriate IK to request.  This suggests that sender
 and recipient name information should be incorporated into an IK
 qualifier, along with a number to distinguish among multiple IKs used
 between a sender/recipient pair.  In order to support universal
 interoperability, it is necessary to assume a universal form for the
 naming information.  General definition of such a form requires
 further study; issues and possible approaches will be noted in
 Section 6.  As an interim measure, the following IK qualifier format
 is suggested:
            <sender-name>/<recipient-name>/<numid>
 where <sender-name> and <recipient-name> are in the following form:
            <user>@<domain-qualified-host>
 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.  The
 <numid> is a contiguous string of decimal digits.
 The IK use indicator subfield is an optional facility, provided to
 identify the encryption mode in which the IK is to be used.
 Currently, this subfield may assume the following reserved string
 values: "ECB" and "EDE"; the default value is ECB.
 An example IK ID adhering to this recommendation is as follows:
        ptf-kmc:linn@CCY.BBN.COM/privacy-tf@C.ISI.EDU/2:ECB
 This IK ID would indicate that IA "ptf-kmc" has issued an IK for use
 on messages sent from "linn@CCY.BBN.COM" to "privacy-tf@C.ISI.EDU",
 that the IA has associated number 2 with that IK, and that the IK is
 to be used in ECB mode.
 IKs will remain valid for a period which will be longer than a single
 message and will be identified by an expiration time distributed
 along with the IK; IK cryptoperiod is dictated in part by a tradeoff
 between key management overhead and revocation responsiveness.  It
 would be undesirable to delete an IK permanently before receipt of a
 message encrypted using that IK, as this would render the message
 permanently undecipherable.  Access to an expired IK 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 IKs 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 IKs for indefinite

Linn, Privacy Task Force [Page 19] RFC 989 February 1987

 periods.

6 User Naming

 Unique naming of electronic mail users, as is needed in order to
 select corresponding keys correctly, is an important topic and one
 requiring significant study.  A logical association exists between
 key distribution and name/directory server functions; their
 relationship is a topic deserving further consideration.  These
 issues have not been fully resolved at this writing.  The interim
 architecture relies on association of IKs with user names represented
 in a universal form, which has the following properties:
        1.   The universal form must be specifiable by an IA as it
             distributes IKs and known to a UA as it processes
             received IKs and IK IDs.  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 representation.
        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 global form used
 for Internet mail transmission may cause property 2 to be violated.
 The use of flat (non-hierarchic) electronic mail user identifiers,
 which are unrelated to the hosts on which the users reside, appears
 useful.  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 Domain
 Name 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

Linn, Privacy Task Force [Page 20] RFC 989 February 1987

 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 [10]
 which is invoked by a user, and lies above the existing UA sublayer.
 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, a cache of IKs is maintained in a
 local file, with entries managed manually based on pairwise
 agreements between originators and recipients.  This cache is,
 effectively, a simple database.  IKs are selected for transmitted
 messages based on recipient names, and corresponding IK IDs are
 placed into the message's encapsulated header.  When a message is
 received, the IK ID is used as a basis for a lookup in the database,
 yielding the appropriate IK entry.  DEKs and IVs are generated
 dynamically within the program.
 Options (e.g., authentication only vs. authentication with
 confidentiality service) are selected by command line arguments to
 the standalone program.  Destination addresses are specified in the
 same fashion.  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 for IK ID fields.

8 Areas For Further Study

 The procedures defined in this RFC are sufficient to support pilot
 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

Linn, Privacy Task Force [Page 21] RFC 989 February 1987

        functions
   2.   Standardization of Issuing Authority functions, including
        protocols for communications among IAs and between User Agents
        and IAs
   3.   Use of public key encryption algorithms to encrypt data
        encrypting keys
   4.   Interoperability with X.400 mail
 We anticipate generation of subsequent RFCs which will address these
 topics.

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

NOTES:

   [1]  Information Processing Systems: Data Encipherment: Block
        Cipher Algorithm DEA 1.
   [2]  Federal Information Processing Standards Publication 46, Data
        Encryption Standard, 15 January 1977.
   [3]  Information Processing Systems: Data Encipherment: Modes of
        Operation of a 64-bit Block Cipher
   [4]  Federal Information Processing Standards Publication 81, DES
        Modes of Operation, 2 December 1980.

Linn, Privacy Task Force [Page 22] RFC 989 February 1987

   [5]  Addendum to the Transport Layer Protocol Definition for
        Providing Connection Oriented End to End Cryptographic Data
        Protection Using a 64-Bit Block Cipher, X3T1-85-50.3, draft of
        19 December 1985, Gaithersburg, MD, p. 15.
   [6]  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.
   [7]  Crocker, D. Standard for the Format of ARPA Internet Text
        Messages (RFC822), August 1982.
   [8]  Rose, M. T., and Stefferud, E. A., Proposed Standard for
        Message Encapsulation, January 1985.
   [9]  Key generation for authentication 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 5.1.1 identifies possible advantages of
        a centralized server approach.
   [10] Note that in the UNIX(tm) 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.

Linn, Privacy Task Force [Page 23]

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