Internet DRAFT - draft-fenton-identified-mail


Network Working Group                                          J. Fenton
Internet-Draft                                                 M. Thomas
Expires: November 14, 2005                           Cisco Systems, Inc.
                                                            May 13, 2005

                        Identified Internet Mail

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes extensions to the format of electronic mail
   messages and a public-key infrastructure to permit verification of
   the source of messages by either mail transport agents (MTAs) or mail
   user agents (MUAs).  This may be used to implement a policy which,
   for example, favors the delivery of identified messages over messages
   lacking signatures or having incorrect signatures.  Mechanisms by
   which signing of messages and verification of signatures can be
   performed by trusted MTAs, in order to minimize impact on end users,

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   are also presented.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  5
   3.  Identified Internet Mail Concepts  . . . . . . . . . . . . . .  6
     3.1   Application of Signatures  . . . . . . . . . . . . . . . .  8
     3.2   Verification of Signatures . . . . . . . . . . . . . . . .  8
   4.  Message Format . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1   Header and Verification Syntax . . . . . . . . . . . . . . 10
     4.2   Relationship to MIME . . . . . . . . . . . . . . . . . . . 11
   5.  The Signature Header . . . . . . . . . . . . . . . . . . . . . 12
     5.1   IIM Signature Calculation  . . . . . . . . . . . . . . . . 12
       5.1.1   Canonicalization . . . . . . . . . . . . . . . . . . . 13
     5.2   IIM-Sig Tag Values . . . . . . . . . . . . . . . . . . . . 13
     5.3   The Verification Header  . . . . . . . . . . . . . . . . . 16
     5.4   Use of From header . . . . . . . . . . . . . . . . . . . . 17
   6.  Key Management . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.1   Key Registration . . . . . . . . . . . . . . . . . . . . . 18
       6.1.1   Key Authorization via KRS  . . . . . . . . . . . . . . 19
       6.1.2   Key Authorization via DNS  . . . . . . . . . . . . . . 22
       6.1.3   Null Key Checks  . . . . . . . . . . . . . . . . . . . 23
       6.1.4   Key Authorization Results  . . . . . . . . . . . . . . 23
     6.2   Policy Options . . . . . . . . . . . . . . . . . . . . . . 25
   7.  Usage examples . . . . . . . . . . . . . . . . . . . . . . . . 27
     7.1   Simple message transfer  . . . . . . . . . . . . . . . . . 27
     7.2   Outsourced business functions  . . . . . . . . . . . . . . 27
     7.3   PDAs and Similar Devices . . . . . . . . . . . . . . . . . 27
     7.4   Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 27
     7.5   Affinity Addresses . . . . . . . . . . . . . . . . . . . . 28
     7.6   Third-party Message Transmission . . . . . . . . . . . . . 29
   8.  DNS considerations . . . . . . . . . . . . . . . . . . . . . . 30
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 31
     9.1   Potential Attacks  . . . . . . . . . . . . . . . . . . . . 31
       9.1.1   Key Registration Server Denial-of-Service Attack . . . 31
       9.1.2   Key Registration Server Stall Tactic . . . . . . . . . 31
       9.1.3   Misappropriated private key  . . . . . . . . . . . . . 32
       9.1.4   Message Replay Attack  . . . . . . . . . . . . . . . . 32
     9.2   Other Considerations . . . . . . . . . . . . . . . . . . . 33
   10.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 34
   11.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
   12.   References . . . . . . . . . . . . . . . . . . . . . . . . . 36
     12.1  Normative References . . . . . . . . . . . . . . . . . . . 36
     12.2  Informative References . . . . . . . . . . . . . . . . . . 36
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 36
       Intellectual Property and Copyright Statements . . . . . . . . 38

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1.  Introduction

   Internet users have recently been subjected to a torrent of
   unsolicited email messages.  These generally take two forms:

   1.  Messages originated by "spammers" to send advertising or
       solicitation, or as part of a confidence scheme or other fraud

   2.  Messages sent automatically by worms and other malware attempting
       to infect additional systems

   In both cases, a large proportion of such messages attempt to
   disguise their true source, in order to frustrate attempts to shut
   down the spammer, to disguise the identity of the infected system
   sending the message, or to make it appear that the message came from
   an entity the recipient trusts (e.g., a bank).  Since SMTP permits
   senders to use any return address they wish, the addition of a
   signature to messages limits the opportunity of spammers or malware
   to forge return addresses, and thus provides some degree of
   accountability for the source of email messages.

   As currently used, message signatures such as those generated by (for
   example) PGP are generally used to indicate authorship of the message
   by a particular individual.  The intent here is somewhat different:
   we want to determine if the sender of the message has authorization
   (from the administration of the domain) for the use of an email
   address and associate the signature with the message itself.  In
   order to make the signature transparent to recipients who do not
   intend to verify it, the signature is encapsulated in the message
   differently than in a signed PGP message.  The public key used to
   sign the message is included in the header, and its binding with the
   From or Sender header in the message can be verified with the sending

   The email identification problem is characterized by extreme
   scalability requirements.  There are currently on the order of 30
   million domains and a much larger number of individual addresses.  It
   is important to preserve the positive aspects of the current email
   infrastructure, such as the ability for anyone to communicate with
   anyone else without introduction.  Due to the desire to retain this
   "any to any" characteristic of email and the (perhaps) lower standard
   of trust required to accept an email message as compared with, for
   example, processing a financial transaction, we present an
   alternative key management model here.

   What is presented here is not by itself a solution to the spam
   problem.  The intent is to give tools to the recipient to allow the
   classification and prioritization of desired mail.  Since much

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   undesirable mail is currently characterized by forged return
   addresses, the identification of such messages is a major focus of
   this effort.  Some degree of accountability for the source of email
   messages will also result, although spammers will still have the
   ability to operate their own domains and key infrastructure, create
   large numbers of bogus identities, and sign messages.  For this
   reason, identification of the sender is really a foundation on which
   accreditation and reputation services can be based.  It is hoped that
   through a combination of such mechanisms, spam, fraudulent, and
   malware-generated messages may eventually be marginalized to the
   point that they are not the serious problem they are today.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

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3.  Identified Internet Mail Concepts

   Identified Internet Mail (IIM) provides a means by which
   cryptographic signatures can be applied to email messages to
   demonstrate that the sender of the message was authorized to use a
   given email address.  Message recipients can verify the signature and
   consult the sender's domain to determine whether the key that was
   used to sign the message was authorized by that domain for that
   address.  This confirms that the message was sent by an party
   authorized to use the sender's email address.

   Just as an email administrator, by creating an email account, gives a
   user the ability to receive mail addressed to a given address, a
   means for authorizing the transmission of email messages is needed.
   The administrator can delegate the use of an address in several ways.
   The administrator could operate a mail transfer agent (MTA) or mail
   submission agent (MSA) for the domain that authenticates the sender
   when accepting a message.  This MTA or MSA would typically have a
   private key that is authorized to send mail for anyone in the domain.
   Alternatively, the administrator could register a public key for the
   sender with which, assuming the sender has an MTA or MUA with the
   appropriate software and the corresponding private key, the sender
   could sign his own outgoing messages.  In the latter case, the
   private key would typically be authorized for one or more specific
   addresses that the sender is authorized to use.

   One central principle used here is that an email address doesn't
   represent a user's identity.  Rather, an email address is something
   that the owner or administrator of a domain delegates to a user.  For
   example, John Doe may have the email address of, but
   only as long as he remains employed by's owner (or as
   long as he uses as an ISP).  When this relationship
   changes, John's identity doesn't change, but his authorization to use does.  For this reason the administrator of, and not John, must have control over the authorization
   to use the address.

   IIM permits keys to be authorized for specific email addresses
   ("user-level keys") and/or for all addresses in a domain ("domain-
   level keys").  User-level keying is needed to support some important
   use cases.  For example:

   o  Domains which want to authorize a partner, such as an advertising
      provider or other outsourced function, to use a specific address
      for a given duration.

   o  Domains which want to allow frequent travelers to send messages
      locally without the need to connect with a particular MSA.

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   o  "Affinity" domains (e.g., college alumni associations) which
      provide forwarding of incoming mail but which do not operate a
      mail submission agent for outgoing mail.

   It is expected that many domains will only authorize keys, probably
   held by outgoing MTAs, which are used for all addresses in the
   domain.  Other domains might allocate a small number of user-level
   keys, but perform most signing at the domain level.  A few domains,
   such as the affinity domains mentioned above, might do all message
   signing at the user level.

   A typical message flow for an IIM message is as follows:

          |           |
          |           |
     1    |           |
   ======>|   sMTA    |==========
          |           |          =             +-----------+
          |           |           =     2      |           |
          +-----------+            ===========>|           |
                                        3      |   rMTA    |    5
          +-----------+             ===========|           |==========>
          |           |            =           |           |
          |           |           =     =======|           |
          |   DNS     |          =     =       +-----------+
          |           |<=========     =
          |           |              =
          |           |             =
          +-----------+            =
                                  = 4
          +-----------+          =
          |           |         =
          |           |        =
          |   KRS     |<=======
          |           |
          |           |

   Note: This example flow illustrates signing and verification done at
   MTAs, though there is no requirement that it be placed in any
   particular element (MUA, MSA, MTA).

   1.  The sending MTA receives mail from a source that it determines is
       allowed to send mail using its domain's name.

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   2.  The sending MTA signs the mail and forwards it to the receiving

   3.  The receiving domain's MTA determines that there is a signature
       from the responsible domain which asserts that the public key can
       be verified via a KRS lookup.  The MTA performs a DNS lookup to
       get the address of the sending domain's KRS.  This is a key
       security property: that the sending domain has control of the
       contents of its DNS.

   4.  The receiving MTA formulates a query back to the sending domain's
       KRS with the fingerprint of the key that signed the message and
       the 2822 address of which it would like to determine the

   5.  The receiving MTA then decides the disposition of the mail based
       upon the result of the authorization query, typically replacing
       the IIM-VERIFY header with its opinion and forwarding it along to
       the receiving MUA.

3.1  Application of Signatures

   Identified Internet Mail (IIM) messages are characterized by the
   presence of a header, IIM-Sig, in the message.  This header contains
   all of the information required to verify the internal consistency of
   the message itself, including the signature, the public key
   corresponding to the private key used to sign the message, options
   such as the choice of canonicalization and cryptographic algorithms,
   and information on how to verify the authorization of the supplied
   public key.

   Messages may be signed either directly by the author's MUA, by an
   MSA, or by a subsequent MTA.  If the signature is applied by other
   than the author's MUA, other mechanisms (such as SMTP AUTH, or access
   control over the network from which messages will be signed) SHOULD
   be used to ensure that only authorized messages are signed.  This is
   needed to provide some assurance that an attacker will not be able to
   obtain message signing simply by choosing the right MTA through which
   to send.

3.2  Verification of Signatures

   An MTA at any point in the message transmission path or a recipient
   MUA may verify the authorization of the message.  This is done by an
   internal check of the consistency of the message to ensure that the
   message is properly signed using the included signature and public
   key.  A verifying MTA MUST also check the authorization of the public

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   key to be used with the given email address by consulting either a
   Key Registration Server (KRS) as described in Section 6.1.1 or DNS as
   described in Section 6.1.2 at the originating domain, as specified by
   the signature.  This authorization check MAY be performed in parallel
   with the message consistency check.  The message MUST be considered
   authorized only if the key is authorized and the message passes its
   consistency check.

   If no signature is present, or if a message signature fails its
   consistency check, a "null key" check SHOULD be performed with the
   originating domain to determine its preference for how the message
   should be handled.  If the null key check indicates that all
   legitimate messages are signed by the domain, the verifying MTA
   SHOULD discard unsigned messages.  It SHOULD NOT generate a "bounce"
   message when doing so.

   MUAs MAY frequently wish to avail themselves of the results of
   verification by MTAs within their trust domain, because such
   verifications are more likely to be performed in a timely manner,
   because the MUA doesn't implement message signature verification, or
   because the MUA is operating in an offline mode.  Verifying MTAs MAY
   attach an IIM-VERIFY header to the message with the results of
   message verification to be acted upon by MUAs.  MUAs using the
   information in this header SHOULD act only upon IIM-VERIFY headers
   applied by a known and trusted entity.  In order to avoid the
   possibility of an MUA acting on a spoofed header sent with the
   message, verifying MTAs MUST remove any IIM-VERIFY headers already
   present in messages they process.

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4.  Message Format

   An identified internet mail message is in the format of a
   conventional mail message as defined in RFC 2822 [5].  Two new
   headers, referred to as the signature and the verification header,
   are defined.

4.1  Header and Verification Syntax

   Identified Internet Mail uses a common encoding for the signature
   header (IIM-Sig), the verification header (IIM-Verify), as well as
   for key verification responses from a KRS or from DNS.  This consists
   of a set of tag-value pairs consisting of a tag delimited by a ":"
   followed by a single value within double quotes separated from the
   following tag by a ";".  Tags may be any number of alphanumeric
   characters, although at present only single-character tags are
   defined.  A receiver decoding an unknown tag MUST silently discard
   the tag and value (after incorporating the tag-value pair into the
   hash calculation for the signature, if applicable).

   Values are a series of double quoted strings containing either base64
   text, plain text, or quoted printable text, as defined in RFC 2045
   [1], section 6.7.  The context of the tag will determine the encoding
   of each value.  As of this writing, the only tag which uses the
   quoted printable format is the "c" (copy) tag.  The definition of
   plain text in this context is ASCII codes 32-126 decimal, with the
   exception of ASCII 34 (double quote).

   Each value SHOULD be split into multiple lines as a series of quoted
   strings to keep the line length less than 78 octets.  Decoders MUST
   interpret multiple quoted strings in a value as if they were all part
   of a single quoted line.  Encoders MUST NOT violate the maximum line
   length of any particular header line.  Decoders MUST ignore strings
   which span line breaks as the meaning is ambiguous given the way that
   mail header continuation lines are formed and often reformatted by

   With the exception of the double quote character (which is converted
   to =22) and the equals sign (which is converted to =3D), received
   headers compliant with RFC 2822 [5] will contain no characters which
   require quoted printable conversion because they will have been
   encoded as described in RFC 2047 [2].  The conversion of the quote
   and equals sign is required in order to unambiguously encapsulate the
   header in a quoted string.  For example, the subject line:

   Subject: Einstein's "E=mc**2"

   would be encoded as:

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   Subject: Einstein's =22E=3Dmc**2=22

   Headers with characters which require conversion (perhaps from legacy
   MTAs which are not RFC 2822 compliant) SHOULD be converted as
   described in RFC 2047 [2].  Otherwise, the copied header may be
   converted to quoted printable form as described in section 6.7 of RFC
   2045 [1], with the additional conversion of double quote and equals
   sign characters to =22 and =3D respectively.

   New tags MUST choose whether they are of type plain text, quoted
   printable, or base64.  In general, the plain text type may be used if
   it is not permissible to have 8 bit and/or syntactically problematic
   values.  Binary forms MUST be encoded as base64, and free form text
   (e.g., user supplied) MUST be typed as quoted printable.

   The syntax of tags is as follows:

   tagvaluepair = 1*ALPHA ":" DQUOTE 1*valuechar DQUOTE
             *(CRLF 1*WSP DQUOTE 1*valuechar DQUOTE)
   valuechar = %x20-21 / %x23-7E
                ;printing characters except double quote

4.2  Relationship to MIME

   With the exception of the canonicalization procedure described in
   Section 5.1.1, the Identified Internet Mail signing process treats
   the body of messages as simply a string of characters.  IIM messages
   may be either in plain-text or in MIME format; no special treatment
   is afforded to MIME content.  Message attachments in MIME format are
   included in the content which is signed, provided the chosen
   canonicalization (in particular the body length count, if specified)
   includes the portion of the message containing the attachment.

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5.  The Signature Header

   The Signature header MUST be included in a message in order for it to
   be considered an Identified Internet Mail message.  It has the

   signature = "IIM-Sig:" SP *([CRLF 1*WSP] tagvaluepair ";")
               tagvaluepair CRLF

   The signature-text contains a number of fields which represent the
   signature itself, a public key used to create the signature, and
   related information.  An example of a signature is as follows:

   IIM-SIG: v:"1"; h:""; d:""; z:"home";
      m:"krs"; t:"1094844765.338603"; x:"432000"; a:"rsa-sha1";
      b:"nofws:1192"; e:"Iw==";
      c:"From: John Doe <>";
      c:"Date: Fri, 10 Sep 2004 12:25:51 -0700 (PDT)";
      c:"Subject: RE: Lunch"

5.1  IIM Signature Calculation

   All tags and their values in the IIM-Sig header are included in the
   cryptographic hash with the sole exception of the s: (signature) tag
   and its value.  Tags that are not understood by the receiver are
   included.  The tags and their values are simply concatenated to each
   other when forming the cryptographic hash in the order they are
   present in the IIM-Sig line.  That is: "[...]"
   Syntactic markers are NOT included and the value used in the hash is
   before encoding and after decoding.  The final hash algorithm is as

        TRUNC (SHA1 (SIGTAGVALS, SHA1 (BODY)), 12)

   where SIGTAGVALS is the encoding described above for the header tags/
   values and BODY is the SHA-1 hash of the body of the email itself,
   including the leading CRLF.  Note that SHA-1 value of the body uses
   the full 16 bytes of the hash (i.e., not truncated).

   When calculating the hash on base64 and quoted printable values, the
   hash is taken after the encoding.  Likewise, the receiver MUST

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   incorporate the values into the hash before actually decoding the

5.1.1  Canonicalization

   In order to minimize the likelihood of signature mismatch due to
   innocuous message modification, a canonicalization algorithm MAY be
   specified by the signer of the message.  Two canonicalization
   algorithms are currently defined, "plain" and "nofws".  In addition,
   a body length count MAY be specified to limit the signature
   calculation to a subset of the body text.

   Plain canonicalization is effectively the null canonicalization.
   Bytes of the body are included in the hash without modification.  The
   "nofws" canonicalization removes all whitespace characters in the
   body and then strips the eighth bit of the data prior to calculating
   the hash.  In this context, whitespace characters are defined as:

   nofws-whitespace = 1*( SP / HTAB / CR / LF / %x0B / %x0C )
                      ; last 2 are vertical tab and form feed

   The body length count allows the signer of a message to permit data
   to be appended to the end of the body of a signed message.  This
   capability is provided because it is very common for mailing lists to
   add trailers to messages (e.g., instructions how to get off the
   list).  Until those messages are also signed, the body length count
   is a useful tool for the receiver since it MAY as a matter of policy
   accept messages having valid signatures with extraneous data,
   possibly highlighting the unsigned area.  Alternatively, it may strip
   the extraneous data or reject the message outright.  A signer MAY not
   want to permit appendages to messages and can set the length to "-1"
   to indicate that all bytes of the body are to be used to calculate
   the signature.  The body length count is made following the
   canonicalization algorithm; whitespace characters are not counted
   when using the "nofws" algorithm.

   Signers that wish to ensure that no modification of any sort may
   occur MUST specify the "plain" algorithm and a length of -1.

   Despite the measures described above, some messages, particularly
   those containing 8-bit data, may be subject to modification in
   transit invalidating the signature.  Messages containing 8-bit data
   SHOULD be converted to MIME format prior to signing, using a suitable
   content transfer-encoding such as quoted-printable or base-64.

5.2  IIM-Sig Tag Values

   The tags used in the signature are as follows.  The type of each tag

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   is shown in parentheses after its name.

   a: Algorithm (plain-text).  One-way hash and public key algorithm.
      Currently this MUST be rsa-sha1.  This tag MUST be present.

   b: Body canonicalization (plain-text).  This tag informs the receiver
      of the type of canonicalization used to calculate the signature
      and the number of bytes in the body of the email included in the
      cryptographic hash, starting from 0 at the CRLF preceding the
      body.  Its value is of the form "Canon-algorithm:Length".
      Recognized values of Canon-algorithm are "plain" and "nofws".  A
      length of -1 specifies that all bytes of the body are to be
      included in the signature calculation.  This tag is OPTIONAL and
      defaults to "plain:-1" if it is absent.

   c: Copied header (quoted-printable).  A copied header is a header
      which the sender would like to cryptographically sign.  No other
      headers are included into the cryptographic hash.  The From header
      MUST be included; the Sender header MUST also be included if the
      signature is on behalf of a Sender address which is different from
      the From address.  The copied headers SHOULD also include the
      Subject and Date headers and MAY include any other headers present
      at the time of signing at the discretion of the signer.

      In calculating the hash, the value MUST be encoded as the copied
      header followed by a colon (":"), followed by a single space,
      followed by the rest of the value of the copied header.

   d: Domain of signer (plain-text).  This tag denotes the signing
      domain.  It is used to inform the receiver of the appropriate
      level of address that is considered the authoritative domain in
      this context.  For example, if a message is received from, the d: tag might indicate that the domain is or  If this tag does not correspond
      to the hostname of the From or Sender address or to a parent
      domain, the signature MUST be ignored.  This tag MUST be present.

   e: Public exponent (base-64).  The RSA public exponent of the
      supplied signing key.  This tag MUST be present.

   h: Signing host (plain-text).  The hostname of the applier of the
      signature.  This is purely informational and is not in verifying
      the signature.  This tag is OPTIONAL.

   m: Method (plain-text).  This tag determines which method the signer
      desires the receiver to use to check the authorization of the
      supplied public key.  This MUST be either "krs" or "dns".  This
      tag is OPTIONAL and defaults to "krs" if it is absent.

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   n: Modulus (base-64).  The RSA public modulus of the supplied signing
      key.  Note that the key length is implicit with the number of
      decoded bits in the modulus.  Signers MUST support key lengths of
      1024 bits and SHOULD support 768 and 1536 bits.  Receivers MUST
      support key lengths of 768, 1024 and 1536 bits.  This tag MUST be

   s: Signature (base-64).  The RSA signature over the computed one-way
      hash.  The format of the signed message follows PKCS #1v2 with
      PKCS 1.5 padding.  That is, 02||PS||00||M, where PS is the
      padding, M is the signed hash.  If the length of M is greater than
      12, the first 12 octets MUST be used and the subsequent octets
      silently ignored.  Refer to PKCS #1v2.1 RFC 3447 [6] for the
      format of PS.  This tag MUST be present.

   t: Timestamp (plain-text).  The time that this signature was created.
      The format is the standard Unix seconds-since-1970 followed by a
      fractional number which MUST be guaranteed to be unique for this
      signing key.  The intent of the fraction is to guarantee the
      uniqueness of any given signature at any particular instance.  The
      value is expressed as an unsigned integer in decimal ASCII.  This
      tag MUST be present.

   v: Version (plain-text).  This MUST currently be set to "1".  This
      tag MUST be present, and MUST be the first tag in the IIM-SIG

   x: Signature TTL (plain-text).  Signature expiration in seconds-
      since-1970 format.  Signatures MUST NOT be considered valid if the
      current time at the receiver is past the expiration date.  The
      value is expressed as an unsigned integer in decimal ASCII.  This
      tag MUST be present.

   z: Signature semantics (plain-text).  This tag has two possible
      values: "home" and "routing".  When a signing entity (MTA, MSA, or
      MUA) wants to assert the origin of a message, it tags the
      signature with the "home" value.  An MTA applying a "home"
      signature SHOULD perform some sort of access control to filter out
      mail from outsiders trying to forge mail or SHOULD authenticate
      the submitter of the message.  If the entity merely wants to
      attest that the mail passed through it on its way to the
      destination, it uses the "routing" value.  Routing signatures are
      similar to signed Received headers and are of primarily forensic
      value.  This tag SHOULD be present.  A missing signature semantics
      tag should be interpreted as "home".

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5.3  The Verification Header

   The verification header is an optional header which MAY be used to
   convey the verification of a message from an MTA to an MUA or a
   subsequent MTA within the same trust domain.  It has the syntax:

   signature = "IIM-Verify:" SP *([CRLF 1*WSP] tagvaluepair ";")
               tagvaluepair CRLF

   The verification header indicates whether an MTA was able to
   successfully verify the message according to whatever policies it
   decides to use.  A recipient MUA or MTA MAY decide to rely on the
   presence of a verification header in applying policy to the message
   (e.g., moving an unverified message to a lower-priority folder), or
   it may do such verification locally.

   The verification header is not cryptographically protected, in order
   to avoid the need to manage keys for verifying MTAs.  The
   verification header SHOULD be deleted from the header when the
   message is sent via SMTP outside the trust domain of the sender, and
   it MUST be discarded if it received from an SMTP peer that is not
   trusted by the recipient (normally one that is within the recipient's
   administrative control).  There MUST be at most one verification
   header in a message; MTAs which perform message verification MUST
   ensure that they either agree with the contents of any existing
   verification header, or replace it with a new one.

   An example of a verification header is as follows:

   IIM-Verify: s:"y"; v:"y"; r:"68"; h:""

   The tags and values used by the verification header are as follows:

   s: Signature (plain-text).  The value is "y" if there is a signature
      line from the sending domain (i.e., the domain suffix of the
      Sender or From header).  Otherwise the value is "n".  This tag
      MUST be present.

   v: Verify (plain-text).  The value is "y" if the home domain's
      signature is both present and the public key operation verifies
      correctly.  This tag MUST be present.

   r: Rating (plain-text).  The value here is between -127 and 127 with
      negative values expressing an adverse rating, zero being neutral
      and positive values indicating a favorable rating.  The rating
      value is completely at the discretion of the entity supplying the
      IIM-Verify header and MAY take into account many different factors
      including the rating supplied by the home domain's KRS, local and

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      third party ratings, and any other factors the verifying entity
      considers relevant.  This tag SHOULD be present.

   h: Host (plain-text).  This is the fully qualified domain name of the
      verifying host.  It should be noted that since the IIM-Verify
      header is not cryptographically protected, users or subsequent
      MTAs which make use of the IIM-Verify header must independently
      ensure that it is not subject to tampering.

   c: Comment (plain-text).  This is a free form string intended to
      convey a human readable comment about the operation.  This is
      typically used to send diagnostic information for failed
      operations.  This tag MAY be present.

5.4  Use of From header

   Identified Internet Mail associates the key in the message with
   either the Sender or From address of the message.  This is done to
   allow mailing lists which re-originate messages and apply a Sender
   header (but retain the original From address) to sign the re-
   originated messages.  However, it is the From address that is most
   commonly seen by the recipient, and it is important that if the
   Sender address is used to verify the message, that it be made visible
   to the user.

   In order to retain the current semantics and visibility of the From
   header, verifying mail agents SHOULD take steps to ensure that the
   Sender address is prominently visible to the user if it is used to
   verify the message and is different from the From address.  If MUA
   implementations that highlight the signed address are not available,
   this MAY be done by rewriting the From address in a manner which
   remains compliant with RFC 2822 [5].  If performed, the rewriting
   SHOULD retrieve the original From header from one of the c: tags of
   the signature, and include the Sender address (also retrieved from
   one of the c: tags of the signature) in the display-name of the
   address.  For example:

   From: "John Q. User via <>" <>

   This sort of address inconsistency is expected for mailing lists, but
   might be otherwise used to mislead the recipient, for example if a
   message supposedly from had a Sender
   address of

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6.  Key Management

   In order for these signatures to be meaningful, a method for
   verifying the validity of the key used to sign the message needs to
   be available.  The PGP [7] approach to this issue of trust is through
   the use of trusted introducers, where individual keys are signed by
   others that may be trusted by the user of the public key.  However,
   such a model does not scale well to very large communities of users
   where several generations of trust would be required.

   Another approach to this problem would be through the use of X.509
   certificates.  While certificates are attractive for many types of
   transactions, many consider it undesirable to require that any domain
   wishing to send signed mail must obtain a certificate through a
   recognized certificate authority.  The ability to quickly and easily
   revoke the authorization for keys, especially in the case of user-
   level keys, is also a problem that is most easily solved by
   consulting the originating domain.

   In IIM the method of verifying the validity of the key is an online
   query sent to the signing domain, or to a server designated by that
   domain.  This process is referred to as a key registration or key
   authorization check.  A hash or fingerprint of the key being verified
   is used to lookup a record in a database operated by the signing
   domain.  Since the use of a fingerprint makes the size of the data
   required to verify key authorization independent of the length of the
   key (or certificate), PGP-style signed keys or X.509 certificates
   could be sent in messages as well, verified using the same
   mechanisms, and could perhaps be used for other purposes in addition
   to message signatures.  The format for sending such keys and
   certificates is beyond the scope of this document.

6.1  Key Registration

   In order to receive email messages, domains typically use one or more
   MX (mail exchanger) resource records to indicate to where mail for
   that domain should be directed.  Similarly, DNS resource records can
   be used directly or indirectly to verify the authorization of keys
   used to sign email messages, or to locate one or more hosts which may
   be considered authoritative to verify the association of keys with
   email addresses in the domain.

   A new textual DNS resource record, referred to as a KR record, is
   defined for publishing and accessing information relating to key
   registration for a domain.  The value 1010 (decimal) is being used
   for the KR record type in experimental use; this will need to change
   if and when IANA assigns a record type.  An alternative method of
   publishing and accessing key registration information using TXT

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   records is described in Section 8.

   To accommodate different deployment needs, two methods of determining
   the authorization of public keys are defined.  The choice of
   authorization method to be used for a particular message is specified
   by the signer in the method (m:) tag of the signature.

   The first method uses HTTP to query a host, referred to as a Key
   Registration Server (KRS), which is located via a DNS record lookup.
   This method provides fine-grained control over key authorization;
   keys may be authorized for use with a list of specific email
   addresses, and multiple keys may be authorized for a given address.
   It also takes advantage of a great deal of existing infrastructure
   used to distribute web services, and can be hosted on an existing web
   server if desired.

   The second method of verifying the authorization of a public key is
   to store the authorization directly in DNS KR records.  This method
   is attractive for domains which have a relatively small number of
   domain-level keys, because there is no need to operate (or outsource
   the operation of) a KRS.  This method is suitable primarily for keys
   which are authorized for an entire domain (typically used by outgoing

   Because of limitations imposed by DNS wildcards and the potential
   privacy issues with storing user email addresses in DNS records, the
   KRS method SHOULD be used for user-level keys.  For domains with only
   domain-level keys, authorization via either KRS or DNS MAY be used at
   the option of the sending domain.  DNS may be easier for domains that
   have no externally-accessible Web server on which to run the KRS
   service; KRS may be easier for domains where the administration of
   mail services and name service is performed by different groups.
   Signing agents need only specify the form of key registration used by
   their domain.  Verifying agents MUST support both the KRS and DNS

   In all cases, the domain referenced for authorization in the
   signature MUST be either the same as or a parent domain of the
   address being verified.  For example, in order to verify the address, the domain referenced by the signature could be or, but not  Signatures
   violating this rule MUST be ignored.

6.1.1  Key Authorization via KRS

   The use of a Key Registration Server (KRS) provides maximum
   flexibility for domains which support user-level key authorization,
   and provides administrative separation between management of DNS

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   zones and email authorization.

   A key registration server confirms (or denies) the binding between
   the specified email address used by the message and the key used to
   sign the message.  It does so by receiving a query containing the key
   fingerprint (digest of the public key) and the email address being
   verified, which will be either the From or Sender address.  It
   returns a value based on the policy of the sending domain as to
   whether the key is authorized to be used in sending a message from
   the specified address.

   When the KRS method has been specified by the sender, the first step
   for the recipient is to consult its local cache of key
   authorizations, if any.  If a result from a previous key
   authorization check has been cached and is still valid according to
   the time-to-live in the cached request, the verifier SHOULD use this
   result to establish whether the key is authorized for the address.

   If the local cache check fails, the next step is to locate the
   address(es) of the domain's KRS(es).  This is done by doing a DNS
   lookup for a KR record for the domain specified in the d: tag of the
   signature.  In order to satisfy this request, the zone file for the
   domain would contain records such as the following: IN KR 10 10 378 "v:\"IIM1\"\;s:\"200\"\;
                                 k:\"\"\;" IN KR 10 10 378 "v:\"IIM1\"\;s:\"200\"\;

   Once a URI for the KRS query has been obtained, verification of
   public keys from key registration servers is accomplished via a
   properly-formatted HTTP GET request.  A sample request might be
   formatted as follows:

   The fields in the query are as follows:

   name: The address (From or Sender) being verified.

   keyfp: The public key fingerprint that was supplied in the IIM-Sig
      line.  The fingerprint is created as follows: create the binary
      representation of the RSA exponent (e) and modulus (n) and
      concatenate them as e|n.  Run this value through SHA1 over the
      full length and convert the first 12 bytes of the output of the
      SHA1 operation to base 64.  That is, base64 (TRUNC (SHA1 ((e|n)),

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   domain: The domain corresponding to the query to be performed.  This
      is used primarily to allow a single KRS to support multiple
      domains, with each domain database being independently maintained.
      This value corresponds to the d: value in the signature being
      checked; it MUST be the same as or a parent domain of the address
      associated with the signature.

   The following are some excerpts from a hypothetical KRS database:

   #Auth  TTL  Address             Service    Key Fingerprint
   pass 86400 SMTP 073FDD7DD6D6EF6D1413FD7B3C577EFC
                                       # Tom's usual address
   pass 86400     SMTP 073FDD7DD6D6EF6D1413FD7B3C577EFC
                                       # Rewriting of Tom's address
   pass 86400    SMTP 91881749E520D8F53B0B91BBD8963D0D
                                       # Dick's PC
   pass 86400    SMTP 549D8949351DDA4E7C961E0F58727795
                                       # Dick's PDA
   fail 864000   SMTP 8C8252070CA9ED401DD2EE2A7B31A8CF
                                       # Harry's stolen PC
   pass 86400   SMTP 17E64AC44DD5F8891560919D3FC6EA52
                                       # Harry's new PC
   pass 86400   SMTP 073FDD7DD6D6EF6D1413FD7B3C577EFC
                                       # Tom is Harry's administrative
                                       # assistant, so Harry allows Tom
                                       # to originate mail for him.
   pass 604800 *       SMTP 27985A61447CC8B514A82BFA4597174A
                                       # Outgoing MTA key.  MTA keys are
                                       # less likely to require rapid
                                       # revocation, hence the long TTL.
   pass 86400  SMTP *
                                       # Any key will work for this addr
                                       # NOT RECOMMENDED!

   The above example illustrates much of the motivation behind creation
   of a network element, the KRS, for key verification.  Support for
   multiple keys per address and multiple addresses per key would
   require, in general, wildcarding of both the key fingerprint and
   email address fields, something that is not possible in DNS.  Direct
   key authorization via DNS would also require that users' email
   addresses be contained in DNS records, which might raise privacy
   concerns as DNS information is not considered private.  Furthermore,
   the potentially large number and short time-to-live of user-level key
   authorization records may present loading issues for DNS.

   The ability to configure multiple key registration servers for a

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   given domain is intended to provide a degree of fault-tolerance and
   distribution of the key-verification load.  The availability
   requirement for key registration servers is somewhat higher than for
   mail exchangers (and probably more comparable to that of domain name
   servers) because real-time access to the key registration servers is
   often required at the time an email message is received or relayed.
   Accordingly, each domain defining key registration servers SHOULD
   define at least two, and they SHOULD be located on different

   The key registration servers for a domain need to be kept in as close
   synchronization as possible.  In particular, any key revocations that
   take place MUST be reflected immediately in all key registration
   servers for the domain.

   This key management approach requires that only legitimate key-to-
   address bindings be registered on the key registration servers.  Key
   registration servers MUST use a mechanism that ensures that only
   authorized users are able to deposit key fingerprints on the server
   and revoke them.  This may involve a mechanism such as an
   authenticated HTTP exchange that requires the user's password in
   order to register a public key fingerprint for that user on the

   In order to prevent harvesting of email addresses, KRSes MUST NOT
   respond with any email address other than that presented in the query
   or a more general address (for example, when the key fingerprint
   corresponds to a domain MTA).

6.1.2  Key Authorization via DNS

   In order to accommodate domains with a relatively small number of
   infrequently changing keys, a domain MAY choose to advertise the
   authorization of its keys via DNS.  In the DNS model, caching of key
   authorization is provided by DNS itself, rather by a cache locally
   maintained by the verifier.

   To check key authorization via DNS, the verifier forms a query for a
   KR record of the form <keyfp>.<domain>, where <keyfp> is the
   fingerprint of the public key, calculated as described above,
   expressed in base 64 format.  A sample record is as follows: KR
     "v:\"IIM1\"; s:\"200\"; r:\"100\"; t:\"3600\"; m:\"*\""

   Unlike the KRS query method, key authorization via DNS is based only
   on the key fingerprint itself; the email address is not included in
   the query.  This is done to simplify the query and because DNS does

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   not provide the wildcard functionality to support multiple specific
   email addresses being authorized for a particular key, as is possible
   with the KRS method.

6.1.3  Null Key Checks

   In the absence of an IIM signature on a message, it is desirable to
   provide a means by which the originating domain can express its
   policy on the signing of messages, and by implication its preference
   on how unsigned messages SHOULD be handled.  This policy is
   determined through a process known as a Null Key Check.

   When an unsigned message is received by a verifying MTA or MUA, a
   null key check SHOULD be performed.  This check is performed by
   forming a DNS query for a KR record in the name of the "domain" in
   the message.  Since there is no IIM-Sig containing a d: tag
   indicating the responsible domain, the null key check must be
   performed on the entire right-hand side of the email address of the
   From header, or in the case of a From header containing multiple
   addresses, the right-hand side of the email address in the Sender
   header.  A sample DNS record is as follows: KR  "v:\"IIM1\"; s:\"200\"; a:\"fail\";
                     t:\"864000\"; m:\"*\""

   The results are formatted as shown in Section 6.1.4.  Generally, only
   two of the three possible status values make sense: either the
   sending domain asserts that the message is to be presumed to be
   unauthorized, or that the message is of unknown authorization.

   With only the above null key record in DNS, it might be possible for
   an attacker to avoid the null key check by using an address in an
   unknown subdomain of a legitimate domain (e.g.,
   in the above example).  For this reason, domains publishing a null
   key policy SHOULD publish both a record for their domain and a
   wildcard record covering subdomains.  For example: KR  "v:\"IIM1\"; s:\"200\"; a:\"fail\";
                     t:\"864000\"; m:\"*\""
   * KR "v:\"IIM1\"; s:\"200\"; a:\"fail\";
                      t:\"864000\"; m:\"*\""

6.1.4  Key Authorization Results

   The KRS and DNS query methods share a common format for the query
   result with the exception that tagged values from DNS are quoted with
   single quotes rather than double quotes in order to make them easier

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   to incorporate in typical zone files.  Tags and their meanings are as

   v: Version of the response.  Currently this MUST be set to "IIM1".
      This tag MUST be present, and MUST be the first tag in the
      response.  Responses not beginning with v:"IIM1" MUST be

   s: Status.  Follows the general convention of SMTP/HTTP status values
      (i.e., 200, 300, 400, 500 semantics) with the following values

      200: the lookup succeeded.
      201: the lookup succeeded, but the keyfp/name combination
           was not found
      500: any permanent failure.

   t: Time to live.  Responses SHOULD be cached by the verifier so as to
      reduce the query/response load back to the KRS.  Time to live is
      expressed in seconds from when the query was sent.  This value is
      used only for KRS responses and is ignored if present in DNS
      responses.  This value is OPTIONAL, and if absent, the response is
      not cached by the verifier.

   a: Authorization.  This tag contains the authorization status of the
      given name and key fingerprint association.  A value of "pass"
      indicates that the KRS/DNS approves of this key fingerprint/name
      combination.  A value of "fail" indicates the KRS/DNS doesn't
      approve, because the key is unknown, unapproved for this name,
      revoked, or for any other reason.  A value of "Unknown" indicates
      that the KRS/DNS doesn't have any specific information one way or
      the other.  For signed messages "unknown" doesn't make much sense,
      but in the case of an unsigned message where the domain cannot
      ensure that all of its outgoing mail is signed, "unknown" status
      is probably appropriate.  The verification in this case SHOULD
      treat the mail as if were unsigned.

   r: Rating.  Like rating in the IIM-Verify, an integer between -127
      and 127 which is at the sole discretion of the entity producing
      the rating.  Normally, revoked keys from the home KRS would be
      given a (very) negative rating.  This tag is REQUIRED unless the
      a: tag is present.

   m: Matches.  Some key fingerprints may in fact sign for more than the
      single address that is present in the query.  In order to cut down
      trips to the KRS, the Matches field describes with normal Unix
      wildcard syntax what address patterns match this key fingerprint.
      This tag is REQUIRED.  For example, m:"*" would inform

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      the cache logic of the requester that future queries from with this key fingerprint be given the same rating.

   k: KRS.  Specifies the URL for a KRS to be used to complete the
      request.  This tag allows the address of a KRS to be specified in
      a DNS record, which is referenced as described in Section 6.1.1.
      If present, all other tags with the exception of v: and s: will be
      ignored.  This tag is OPTIONAL and MUST NOT appear in a response
      from a KRS.

   c: Comment.  This is a free form string intended to convey a human
      readable comment about the operation.  This is typically used to
      send diagnostic information for failed operations, etc.  This tag
      is OPTIONAL.

   Note that while the syntax of the matching pattern uses normal unix
   wildcard syntax, the semantics of the wildcarding are actually
   constrained to be a "longest prefix match" algorithm where the prefix
   components are allowed to be either the left hand side of the email
   address, or the successive subdomain components.  In all cases, the
   scope of a Matches value MUST NOT exceed the domain of the KRS or DNS
   lookup used to retrieve the authorization.  That is, an entity from cannot say that it matches *@*.com since it is not
   authorized to sign for all .com domains.

6.2  Policy Options

   Identified Internet Mail by itself introduces no new policy with
   respect to handling email.  However, the benefit of using IIM lies
   with the widespread deployment of policy which encourages the signing
   of email and eventually marginalizes unsigned messages.

   One place where policy MAY be implemented is at the receiving user.
   The user MAY verify the signatures of messages as they are received,
   and place unsigned messages in a "bulk mail" or similar folder to be
   read (if at all) on a lower-priority basis.  This would typically be
   done through an enhancement to the mail user agent, probably at the
   time messages are downloaded via a protocol such as POP.  Since the
   user must verify the authorization of all keys not in the user's key
   cache, this could lengthen message downloading times, and may present
   a problem for transitory users such as those on dial-up lines.

   A recipient or intermediate MTA MAY verify the message signatures and
   add a verification header to incoming messages.  This considerably
   simplifies things for the user, who can now use an existing mail user
   agent.  Most MUAs have the ability to filter messages based on
   message headers or content; these filters would be used to implement
   whatever policy the user wishes with respect to unsigned mail.

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   A verifying MTA MAY implement a policy with respect to unsigned mail,
   regardless of whether or not it applies the verification header to
   signed messages.  Separate policies MAY be defined for unsigned
   messages, messages with incorrect signatures, and in cases where the
   signature cannot be verified, for example if all the key registration
   servers are unreachable.  Treatment of unsigned messages SHOULD be
   based on the results of the null key check described in
   Section 6.1.3.

   If the verifying MTA is able to verify the public key of the sender
   and check the signature on the message as the message is received,
   the MTA MAY reject the message with an error such as:

   5yx Unsigned messages not accepted
   5uv Message signature incorrect

   If it is not possible to verify the authorization of the public key
   in the message, perhaps because the key registration server is not
   available, a temporary failure message could be generated, such as:

   4yx Unable to verify signature - key registration server unavailable

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7.  Usage examples

7.1  Simple message transfer

   The above sections largely describe the process of signing and
   verifying a message which goes directly from one user to another.
   One special case is where the recipient has requested forwarding of
   the email message from the original address to another, through the
   use of a Unix .forward file or equivalent.  In this case the message
   is typically forwarded without modification, except for the addition
   of a Received header to the message and a change in the Envelope-to
   address.  In this case, the eventual recipient should be able to
   verify the original signature since the signed content has not
   changed, and attribute the message correctly.

7.2  Outsourced business functions

   Outsourced business functions represent a use case that motivates the
   need for user-level keying.  Examples of outsourced business
   functions are legitimate email marketing providers and corporate
   benefits providers.  In either case, the outsourced function would
   like to be able to send messages using the email domain of the client
   company.  At the same time, the client may be reluctant to register a
   key for the provider that grants the ability to send messages for any
   address in the domain.

   With user-level keying, the outsourcing company can generate a
   keypair and the client company can register the public key for a
   specific address such as  This would enable
   the provider to send messages using that specific address and have
   them verify properly.  The client company retains control over the
   email address because it retains the authority to revoke the key
   registration at any time.

7.3  PDAs and Similar Devices

   PDAs are one example of the use of multiple keys per user.  Suppose
   that John Doe wanted to be able to send messages using his corporate
   email address,, and the device did not have the
   ability to make a VPN connection to the corporate network.  If the
   device was equipped with a private key registered for by the administrator of that domain, and appropriate
   software to sign messages, John could send IIM messages through the
   outgoing network of the PDA service provider.

7.4  Mailing Lists

   There is a wide range of behavior in forwarders and mailing lists

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   (collectively called "forwarders" below), ranging from those which
   make no modification to the message itself (other than to add a
   Received header and change the envelope information) to those which
   may add headers, change the Subject header, add content to the body
   (typically at the end), or reformat the body in some manner.

   Forwarders which do not modify the body or signed headers of a
   message with a valid signature MAY re-sign the message as described
   below.  Forwarders which make any modification to a message that
   could result in its signature becoming invalid SHOULD re-sign using
   an appropriate identification (e.g.,,
   but normally they SHOULD do so only for messages which were received
   with valid signatures or other indications that the messages being
   signed are not spoofed.

   Forwarders which wish to re-sign a message MUST apply a Sender header
   to the message to identify the address being used to sign the message
   and MUST remove any pre-existing Sender header as required by RFC
   2822 [5].  The forwarder applies a new IIM-Sig header with the
   signature, public key, and related information of the forwarder.
   Previously existing IIM-Sig headers SHOULD NOT be removed.

7.5  Affinity Addresses

   "Affinity addresses" are email addresses users employ to have an
   email address that is independent of any changes in email service
   provider they may choose to make.  They are typically associated with
   college alumni associations, professional organizations, and
   recreational organizations with which they expect to have a long-term
   relationship.  These domains usually provide forwarding of incoming
   email, but (currently) usually depend on the user to send outgoing
   messages through their own service provider's MTA.  They usually have
   an associated Web application which authenticates the user and allows
   the forwarding address to be changed.

   With Identified Internet Mail, affinity domains could use the Web
   application to allow users to register their own public keys to be
   used to sign messages on behalf of their affinity address.  This is
   another application that takes advantage of user-level keying, and
   domains used for affinity addresses would typically have a very large
   number of user-level keys.  Alternatively, the affinity domain could
   decide to start handling outgoing mail, and could operate a mail
   submission agent that authenticates users before accepting and
   signing messages for them.  This is of course dependent on user's
   service provider not blocking the relevant TCP ports used for mail

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7.6  Third-party Message Transmission

   Third-party message transmission refers to the authorized sending of
   mail by an Internet application on behalf of a user.  For example, a
   website providing news may allow the reader to forward a copy of the
   message to a friend; this is typically done using the reader's email
   address.  This is sometimes referred to as the "Evite problem", named
   after the website of the same name that allows a user to send
   invitations to friends.

   One way this can be handled is to continue to put the reader's email
   address in the From field of the message, but put an address owned by
   the site into the Sender field, and sign the message on behalf of the
   Sender.  A verifying MTA SHOULD accept this and rewrite the From
   field to indicate the address that was verified, i.e., From: John Doe
   via <>.

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8.  DNS considerations

   Any discussion of key management rooted in DNS would be incomplete
   without addressing the choice of DNS records for that task.  The
   architecturally preferred method is to use a new resource record
   type, but in practice some resolvers, DNS servers, and firewalls
   cannot accommodate a new resource record type.  The common workaround
   for that problem is the use of an existing record such as TXT,
   perhaps with a distinguishing application-dependent prefix in order
   to avoid collisions with other uses of TXT records.

   As discussed in Section 6.1.3, wildcard DNS records are required to
   close a possible attack against the null key check.  The use of a
   prefix would interfere with the with the required wildcard
   functionality.  For that reason, null key records cannot use a
   distinguishing prefix.  This does cause extra collisions with other
   uses of the domain's root TXT record space, which could increase the
   likelihood that a TXT record lookup for the domain will exceed the
   maximum UDP response size.

   A compromise using both a new resource record (RR), known as a KR
   record, and TXT records is proposed.

   Wherever a KR record including a key fingerprint is published, an
   identical TXT record SHOULD be published.  The _krs prefix is used in
   all records with the exception of null key records.  This prefix is
   used in both KR and TXT records to provide greater consistency
   between the corresponding resource records.

   The use of the _krs prefix for all other key authorization data other
   than the null key record is intended to minimize collisions with
   other TXT records and to allow the _krs prefix to be delegated to the
   mail administrator, in situations where mail and DNS administration
   are done by different organizations.

   Verifying agents SHOULD use the KR record if it is available in their
   environment.  If the KR lookup fails, a lookup for the corresponding
   TXT record SHOULD be performed.  The intent is to encourage the use
   of KR records.  However, no migration strategy to eliminate the use
   of TXT records has been defined.

   For brevity, examples show the use of the KR record only, but
   publication and lookup of the corresponding TXT records SHOULD be
   performed as well.

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9.  Security Considerations

   Fundamentally, the addition of signatures to email messages is all
   about security, although not in the usual way of ensuring the secrecy
   of data.

9.1  Potential Attacks

   It has been observed that any mechanism that is introduced which
   attempts to stem the flow of spam is subject to intensive attack.
   Identified Internet Mail needs to be carefully scrutinized to
   identify potential attack vectors and the vulnerability to each.
   Some of the attacks that have been considered are described in the
   following sections.

9.1.1  Key Registration Server Denial-of-Service Attack

   Since the key registration servers are distributed (potentially
   separate for each domain), the number of servers that would need to
   be attacked to defeat this mechanism on an Internet-wide basis is
   very large.  Nevertheless, key registration servers for individual
   domains could be attacked, impeding the verification of messages from
   that domain.  This is not significantly different from the ability of
   an attacker to deny service to the mail exchangers for a given
   domain, although it affects outgoing, not incoming, mail.

   A variation on this attack is that if a very large amount of mail
   were to be sent using spoofed addresses from a given domain, the key
   registration servers for that domain could be overwhelmed with
   requests.  However, given the low overhead of HTTP requests to the
   KRSes compared with handling of the email message itself, such an
   attack would be difficult to mount.

9.1.2  Key Registration Server Stall Tactic

   An attacker trying to "jam" the signature mechanism might set up a
   key registration server for a domain they control that responds very
   slowly or perhaps not at all.  They then send a large number of
   messages from that domain, in an attempt to bring the signature
   verification mechanism to a crawl and get domains to turn it off.
   This could be mitigated by the use of appropriate timeouts on key
   lookups and possibly by adapting these timeouts to message load.
   Note that it is considerably easier to mitigate this attack when the
   signature check is done by the terminating MTA than the MUA because
   of the MTA's ability to return a temporary failure when the key can't
   be retrieved.

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9.1.3  Misappropriated private key

   If the private key for a user is resident on their computer and is
   not protected by an appropriately secure passphrase, it is possible
   for malware to send mail as that user.  The malware would, however,
   not be able to generate signed spoofs of other senders' addresses,
   which would aid in identification of the infected user and would
   limit the possibilities for certain types of attacks involving
   socially-engineered messages.

   A larger problem occurs if malware on many users' computers obtains
   the private keys for those users and transmits them via a covert
   channel to a site where they may be shared.  The compromised users
   would likely not know of the misappropriation until they receive
   "bounce" messages from messages they are supposed to have sent.  Many
   users may not understand the significance of these bounce messages
   and would not take action.

   One countermeasure is to use a passphrase, although users tend to
   choose weak passphrases.  Nevertheless, the decoded private key might
   be briefly available to compromise by malware when it is entered, or
   might be discovered via keystroke logging.  The added complexity of
   entering a passphrase each time one sends a message would also tend
   to discourage the use of a secure passphrase.

   A somewhat more effective countermeasure is to send messages through
   an outgoing MTA that can authenticate the sender and will sign the
   message using its key which is normally authorized for all addresses
   in the domain.  Such an MTA can also apply controls on the volume of
   outgoing mail each user is permitted to originate in order to further
   limit the ability of malware to generate bulk email.

9.1.4  Message Replay Attack

   In this attack, a spammer sends a message to be spammed to an
   accomplice, which results in the message being signed by the
   originating MTA (presuming that the sender doesn't have a valid
   individual key for the domain).  The accomplice resends the message,
   including the original signature, to a large number of recipients,
   possibly by sending the message to many compromised machines that act
   as MTAs.  The messages, not having been modified by the accomplice,
   have valid signatures.

   Several techniques for dealing with this type of attack have been
   considered.  One is to include in the request to the KRS not only the
   fingerprint of the signing key but also a fingerprint of the
   signature which would allow the KRS to detect, and flag as
   unauthorized, the use of the same signature a very large number of

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   times.  This would require the KRS to maintain a cache of signature
   fingerprints, and would make caching of key registration data by
   verifying MTAs and MUAs impossible.  Both of these factors are very
   undesirable from a performance standpoint.  In addition, some
   checking of message date/time fields would need to be introduced in
   order to allow aging of the signature cache, where currently there is
   no assumption that the sender have a valid real-time clock.

   A similar approach is for verifying MTAs and MUAs to cache the
   signatures themselves and detect duplications.  However, only large
   recipient MTAs are likely to process enough of the spam messages in
   order to detect the duplications.  Furthermore, there are legitimate
   use cases involving mail forwarding where the same message might take
   different paths to the same MTA, so this can only be applied in cases
   where unexpectedly large numbers of duplicate signatures are

   Other partial solutions to this problem involve the use of reputation
   services to convey the fact that the specific email address is being
   used for spam, and that messages from that sender are likely to be
   spam.  This requires a real-time detection mechanism (such as
   detection by the KRS as described above) in order to react quickly
   enough.  However, such measures might be prone to abuse, if for
   example an attacker resent a large number of messages received from a
   victim in order to make them appear to be a spammer.

9.2  Other Considerations

   At present, a message can be forwarded giving the sender no
   indication as to the recipient's actual location (IP address, domain,
   or eventual email address) on the Internet.  A sender monitoring
   queries to its KRS might be able to infer some of this when the
   recipient's MTA, or even the actual recipient, checks the
   identification of incoming messages.  In cases where this may be
   sensitive, trusted proxies SHOULD be employed by the recipient and/or
   their domain.

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10.  IANA Considerations

   Use of the _krs prefix in TXT records will require registration by
   IANA.  IANA will also need to allocate a permanent DNS resource
   record number for the newly-defined KR record type.

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11.  Acknowledgements

   Dave Rossetti provided much of the original motivation to address
   this problem.  In addition, thanks to Fred Baker, Mark Baugher,
   Patrik Faltstrom, Don Johnsen, Dave Oran, Shamim Pirzada, Sanjay Pol,
   Christian Renaud, and Dan Wing for their suggestions and much helpful
   discussion around this issue.

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12.  References

12.1  Normative References

   [1]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

   [2]  Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
        Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
        November 1996.

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

   [4]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [5]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.

   [6]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards
        (PKCS) #1: RSA Cryptography Specifications Version 2.1",
        RFC 3447, February 2003.

12.2  Informative References

   [7]  Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
        Exchange Formats", RFC 1991, August 1996.

Authors' Addresses

   Jim Fenton
   Cisco Systems, Inc.
   MS SJ-24/2
   170 W. Tasman Drive
   San Jose, CA  95134-1706

   Phone: +1 408 526 5914

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   Michael Thomas
   Cisco Systems, Inc.
   MS SJ-9/2
   170 W. Tasman Drive
   San Jose, CA  95134-1706

   Phone: +1 408 525 5386

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   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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