RFC1040

Network Working Group J. Linn (BBNCC) Request for Comments: 1040 IAB Privacy Task Force Obsoletes RFCs: 989 January 1988

          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, Curt Barker, 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 integrity check computation 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

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  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 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 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
  points.

  Authentication and integrity facilities are always applied to the
  entirety of a message's text.  No facility for confidentiality
  service 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

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  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
          transformations.

      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
          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, the selected mechanism should be usable with the
          widest possible variety of existing UA programs.  For

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          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
          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.

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

          1.  disclosure protection,

          2.  sender authenticity, and

          3.  message integrity measures,

  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 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.

  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.  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.

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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 text and (with certain choices among a set of
          alternative algorithms) for computation of message integrity
          check quantities (MICs).  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.  An IK may be a single
          symmetric cryptographic key or, where asymmetric
          (public-key) cryptography is used to encrypt DEKs, 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 associated with sender and recipient
          identification header fields, which enable recipients to
          identify the IKs used.  With this information, the recipient
          can decrypt the transmitted DEK representation, yielding
          the DEK required for message text decryption and/or MIC
          verification.

  When privacy enhancement processing is to be performed on an outgoing
  message, a DEK is generated [1] for use in message encryption and a
  variant of the DEK is formed (if the chosen MIC algorithm requires a
  key) for use in MIC computation.  An "X-Sender-ID:" field is included
  in the header to provide one identification component for the IK(s)
  used for message processing.  An IK is selected for each individually
  identified 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 the DEK and computed MIC.  The
  DEK and MIC are encrypted for transmission under the appropriate IK.

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  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 system to be
  decrypted on a different type.  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 encryption and MIC computation 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 explicitly
  excluded from encryption).  The canonically encoded representation is
  encoded, after encryption, 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.  The MIC is verified.
  Encrypted portions of the transmitted message are decrypted, and 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 ISO draft international standard DIS 8227 [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 DIS 8372 [4].  The CBC mode
  definition in DIS 8372 is equivalent to that provided in FIPS PUB 81
  [5].  A unique initializing vector (IV) will be generated for and
  transmitted with each privacy-enhanced electronic mail message.

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  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 DIS
              8372.

          3.  It is able to be keyed using the procedures and
              parameters defined in this RFC.

          4.  It is appropriate for MIC computation, if the selected
              MIC computation algorithm is eCcryption-based.

          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.  This encryption
  may be performed using symmetric cryptography by using DEA-1 in
  Electronic Codebook (ECB) mode.  A header facility is available 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
  by ASC X3T1 [6], or public-key algorithms).

  Support of public key algorithms for key encryption is under active
  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.

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 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

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

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

          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
              message.

  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 [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.

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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.  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.

4.3.2.1 Step 1: Local Form

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

4.3.2.2 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 the universal canonical form,
  equivalent 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.  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 destination system before any conversion necessary to
  transform the message into a destination-specific local form is
  performed.

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4.3.2.3 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 encapsulated
  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.

4.3.2.4 Step 4: Printable Encoding

  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.

  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

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  from encipherment processing.  The encoding function's output is
  delimited into text lines (using local conventions), with each line
  containing 64 printable characters.

  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 1, 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.  In other words, 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.

  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.

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       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 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.  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
  portion.

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  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.

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

  Blank Line
           (Separates Enclosing Header from Encapsulated Message)

  Encapsulated Message

     Pre-Encapsulation Boundary (Pre-EB)
         -----PRIVACY-ENHANCED MESSAGE BOUNDARY-----

     Encapsulated Header Portion
         (Contains encryption control fields inserted in plaintext.
         Examples include "X-IV:", "X-Sender-ID:", and "X-Key-Info:".
         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)
         -----PRIVACY-ENHANCED MESSAGE BOUNDARY-----

                             Message Encapsulation
                                    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

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  copies of "X-Sender-ID:" and "X-Recipient-ID:" fields within the
  encapsulated text and include those replicated fields in encryption
  and MIC computations.

  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
  Follows".

  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.

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-
  perrecipient 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-
  perlist), 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 public-key
  cryptography, the secret component of the composite IK 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 or component, 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.

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  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 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-
  perrecipient 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 Added Header and Control Fields

  This section summarizes the syntax and semantics of the new
  encapsulated header fields to be added to messages in the course of
  privacy enhancement processing.  In certain indicated cases, it is
  recommended that the fields be replicated within the encapsulated
  text portion as well.  Figure 2 shows the appearance of a small
  example encapsulated message using these fields.  The example assumes
  the use of symmetric cryptography; no "X-Certificate:" field is
  carried.  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
  casesensitive 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
  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 between
  the subfields of these fields to fold them 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.

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  -----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
  X-Proc-Type: 2
  X-IV: F8143EDE5960C597
  X-Sender-ID: [email protected]:::
  X-Recipient-ID: [email protected]:ptf-kmc:3:BMAC:ECB
  X-Key-Info: 9FD3AAD2F2691B9A,B70665BB9BF7CBCD
  X-Recipient-ID: [email protected]:ptf-kmc:4:BMAC: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

4.6.1 X-Certificate Field

  The X-Certificate encapsulated header field is used only when
  public-key certificate key management is employed.  It transfers a
  sender's certificate as a string of hexadecimal digits.  The
  semantics of a certificate are discussed in Section 5.3,
  Certificates.  The certificate carried in an X-Certificate field is
  used in conjunction with all subsequent X-Sender-ID and X-RecipientID
  fields until another X-Certificate field occurs; the ordinary case
  will be that only a single X-Certificate field will occur, prior to
  any X-Sender-ID and X-Recipient-ID fields.

  Due to the length of a certificate, it may need to be folded across
  multiple printed lines.  In order to enable such folding to be
  performed, the hexadecimal digits representing the contents of a
  certificate are to be divided into an ordered set (with more
  significant digits first) of zero or more 64-digit groups, followed
  by a final digit group which may be any length up to 64-digits.  A
  single whitespace character is interposed between each pair of groups
  so that folding (per RFC-822, section 3.1.1) may take place; this
  whitespace is ignored in parsing the received digit string.

4.6.2 X-IV Field

  The X-IV encapsulated header field carries the Initializing Vector
  used for message encryption.  Only one X-IV field occurs in a
  message.  It appears in all messages, even if the entirety of message
  text is excluded from encryption.  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

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  digits.

4.6.3 X-Key-Info Field

  The X-Key-Info encapsulated header field transfers two items: a DEK
  and a MIC.  One X-Key-Info field is included for each of a message's
  named recipients.  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.  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.  MICs are also
  represented as contiguous strings of hexadecimal digits.  The size of
  a MIC is dependent on the choice of MIC algorithm as specified in the
  X-Recipient-ID field corresponding to a given recipient.

4.6.4 X-Proc-Type Field

  The X-Proc-Type encapsulated header field identifies the type of
  processing performed on the transmitted message.  Only one X-ProcType
  field occurs in a message.  It has one subfield, 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 "2".

4.6.5 X-Sender-ID Field

  The X-Sender-ID encapsulated header field provides the sender's
  interchange key identification component.  It should be replicated
  within the encapsulated text.  The interchange key 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, an (optional)
  Version/Expiration subfield, and an (optional) IK Use Indicator
  subfield.  The optional subfields are omitted if their use is
  rendered redundant by information carried in subsequent X-RecipientID
  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 whitespace.

  Section 5.2, Interchange Keys, discusses the semantics of these
  subfields and specifies the alphabet from which they are chosen.

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  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.

4.6.6 X-Recipient-ID Field

  The X-Recipient-ID encapsulated header field provides the recipient's
  interchange key 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,
  a Version/Expiration subfield, a MIC algorithm indicator subfield,
  and an IK Use Indicator subfield.  The subfields are delimited by the
  colon character (":"), optionally followed by whitespace.

  The MIC algorithm indicator is an ASCII string, selected from the
  values defined in Appendix A of this RFC.  Section 5.2, Interchange
  Keys, discusses the semantics of the other 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
  XSender-ID field; it is illegal for an X-Recipient-ID field to
  occur in a header before an X-Sender-ID has been provided.

5. Key Management

  Several cryptographic constructs are involved in supporting the
  privacy-enhanced message processing procedure.  While (as noted in
  the Executive Summary section of this RFC), key management mechanisms
  have not yet been fully defined, a set of fundamental elements are
  assumed.  Data Encrypting Keys (DEKs) are used to encrypt message
  text and in the message integrity check (MIC) computation process.
  Interchange Keys (IKs) are used to encrypt DEKs for transmission with
  messages.  In an asymmetric key management architecture, certificates
  are used as a means to provide entities' public key 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 for computation of message integrity check quantities (MICs).  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 appropriate interchange key (IK) for each
  the authorized recipient.

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  DEK generation can be performed either centrally by key distribution
  centers (KDCs) or by endpoint systems.  Dedicated KDC systems may be
  able to implement better 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 secret IK components need not necessarily be
  shared with a KDC.

5.2 Interchange Keys (IKs)

  Interchange Keys (IKs) are used to encrypt Data Encrypting Keys.  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
  share a common interchange key.  When symmetric cryptography is used,
  the interchange key consists of a single component.  When asymmetric
  cryptography is used, an originator and recipient must possess an
  asymmetric key's public and secret components, as appropriate.  This
  pair of components, when composed, constitute an interchange key.

  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 an IA generates and
  distributes an IK, associated control information is 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 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.

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  Since a message may be sent with multiple IK component
  representations, corresponding to multiple intended recipients, each
  recipient must be able to determine which IK component is intended
  for it.  Moreover, if no corresponding IK component is available in
  the recipient's database when a message arrives, the recipient must
  be able to determine which IK component to request and to 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 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.  Indicator of IK usage mode (IK use indicator subfield).

        4.  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: [email protected]:ptf-kmc:2:BMAC:ECB

  This example field indicates that IA "ptf-kmc" has issued an IK
  component for use on messages sent to "[email protected]", that the IA

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  has provided the number 2 as a version indicator for that IK
  component, that the BMAC MIC computation algorithm is to be used for
  the recipient, and that the IK component is to be used in ECB mode.

5.2.1 Subfield Definitions

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

5.2.1.1 Entity Identifier Subfield

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

                     <user>@<domain-qualified-host>

  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.

5.2.1.2 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.

5.2.1.3 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.

5.2.1.4 MIC Algorithm Identifier Subfield

  The MIC algorithm identifier, which occurs only within X-Recipient-ID
  fields, is used to identify the choice of message integrity check
  algorithm for a given recipient.  Appendix A of this RFC specifies
  the defined values for this subfield.

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5.2.1.5 IK Use Indicator Subfield

  The IK use indicator subfield is an optional facility, provided to
  identify the encryption mode in which an IK component is to be used.
  Currently, this subfield may assume the following reserved string
  values: "ECB", "EDE", "RSA256", "RSA512", and "RSA1024"; the default
  value is "ECB".

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.

5.3 Certificates

  In an asymmetric key management architecture, a certificate binds an
  entity's public key component to a representation of the entity's
  identity and other attributes of the entity.  A certificate's issuing
  authority signs the certificate, vouching for the correspondence
  between the entity's identity, attributes, and associated public key
  component.  Once signed, certificate copies may be posted on multiple
  servers in order to make recipients' certificates directly accessible
  to originators at dispersed locations.  This allows privacy-enhanced
  mail to be sent between an originator and a recipient without prior
  placement of a pairwise key at the originator and recipient, greatly
  enhancing mail system flexibility.  The properties of a certificate's
  authority-applied signature make it unnecessary to be concerned about
  the prospect that servers, or other entities, could undetectably
  modify certificate contents so as to associate a public key with an
  inappropriate entity.

  Per the 1988 CCITT Recommendations X.411 [12] and X.509 [13], a
  subject's certificate is defined to contain the following parameters:

          1.  A signature algorithm identifier, identifying the
              algorithm used by the certificate's issuer to compute the
              signature applied to the certificate.

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          2.  Issuer identification, identifying the certificate's
              issuer with an O/R name.

          3.  Validity information, providing date and time limits
              before and after which the certificate should not be
              used.

          4.  Subject identification, identifying the certificate's
              subject with an O/R name.

          5.  Subject's public key.

          6.  Algorithm identifier, identifying the algorithm with
              which the subject's public key is to be used.

          7.  Signature, an asymmetrically encrypted, hashed version of
              the above parameters, computed by the certificate's
              issuer.

  The Recommendations specify an ASN.1 encoding to define a
  certificate.  Pending further study, it is recommended that
  electronic mail privacy enhancement implementations using asymmetric
  cryptography for key management employ this encoding for
  certificates.  Section 4.2.3 of RFC-987 [14] specifies a procedure
  for mapping RFC-822 addresses into the O/R names used in X.411/X.509
  certificates.

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
  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 current
  architecture relies on association of IK components with user names
  represented in a universal form ("user@host"), relying on the
  following properties:

      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
          representation.

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      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
  violated.

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.  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(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.  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

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  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 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 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
          functions.

      2.  Standardization of Issuing Authority functions, including
          protocols for communications among IAs and between User
          Agents and IAs.

      3.  Specification of public key encryption algorithms to encrypt
          data encrypting keys.

      4.  Interoperability with X.400 mail.

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  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
     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.

A. Message Integrity Check Algorithms

  This appendix identifies the alternative algorithms which may be used
  to compute Message Integrity Check (MIC) values, and assigns them
  character string identifiers to be incorporated in "X-Recipient-ID:"
  fields to indicate the choice of algorithm employed for individual
  message recipients.

  MIC algorithms which utilize DEA-1 cryptography are computed using a
  key which is a variant of the DEK used for message text encryption.
  The variant is formed by modulo-2 addition of the hexadecimal
  quantity F0F0F0F0F0F0F0F0 to the encryption DEK.

A.1 Conventional MAC (MAC)

  A conventional MAC, denoted by the string "MAC", is computed using
  the DEA-1 algorithm in the fashion defined in FIPS PUB 113 [15].  Use
  of the conventional MAC is not recommended for multicast messages.
  The message's encapsulated text is padded at the end, per FIPS PUB
  113, with zero-valued octets as needed in order to form an integral
  number of 8-octet encryption quanta.  These padding octets are

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  inserted implicitly and are not transmitted with a message.  The
  result of a conventional MAC computation is a single 64-bit value.

A.2 Bidirectional MAC (BMAC)

  A bidirectional MAC, denoted by the string "BMAC", yields a result
  which is transferred as a single 128-bit value.  The BMAC is computed
  in the following manner:  First, the encapsulated text is padded at
  the end with zero-valued octets as needed in order to form an
  integral number of 8-octet encryption quanta.  These padding octets
  are inserted implicitly and are not transmitted with a message.  A
  conventional MAC is computed on the padded form, and the resulting
  64-bits form the high-order 64-bits of the BMAC result.

  The low-order 64-bits of the BMAC result are also formed by computing
  a conventional MAC, but the order of the 8-octet encryption quanta is
  reversed for purposes of computation. In other words, the first
  quantum entered into this computation is the last quantum in the
  encapsulated text, and includes any added padding.  The first quantum
  in the text is the last quantum processed as input to this
  computation.  The octets within each 8-octet quantum are not
  reordered.

NOTES:

    [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.

    [2]  Information Processing Systems: Data Encipherment: Block
         Cipher Algorithm DEA 1.

    [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]  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.

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    [7]  Postel, J., Simple Mail Transfer Protocol (RFC-821), 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. T. and Stefferud, E. A., 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 Procedures".

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

    [14] Kille, S. E., Mapping between X.400 and RFC-822 (RFC-987),
         June 1986.

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

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