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DomainKeys Identified Mail                                     T. Hansen
Internet-Draft                                         AT&T Laboratories
Intended status: Informational                                D. Crocker
Expires: August 28, 2008                     Brandenburg InternetWorking
                                                         P. Hallam-Baker
                                                           VeriSign Inc.
                                                       February 25, 2008


           DomainKeys Identified Mail (DKIM) Service Overview
                      draft-ietf-dkim-overview-09

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document provides an overview of the DomainKeys Identified Mail
   (DKIM) service and describes how it can fit into a messaging service.
   It also describes how DKIM relates to other IETF message signature
   technologies.  It is intended for those who are adopting, developing,
   or deploying DKIM.  DKIM allows an organization to take



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   responsibility for transmitting a message, in a way that can be
   validated by a recipient.  The organization can be the author's, the
   originating sending site, an intermediary, or one of their agents.
   An organization may use one or more domain names to accomplish this.
   DKIM defines a domain-level digital signature authentication
   framework for email, using public-key cryptography and key server
   technology [RFC4871].  This permits verification of a message source,
   an intermediary, or one of their agents, as well as the integrity of
   its contents.  DKIM will also provide a mechanism that permits
   potential email signers to publish information about their email
   signing practices; this will permit email receivers to make
   additional assessments about messages.  Such protection of email
   identity can assist in the global control of "spam" and "phishing".


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  DKIM's Scope . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Prior Work . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Internet Mail Background . . . . . . . . . . . . . . . . .  6
     1.4.  Discussion Venue . . . . . . . . . . . . . . . . . . . . .  6
   2.  The DKIM Value Proposition . . . . . . . . . . . . . . . . . .  6
     2.1.  Identity Verification  . . . . . . . . . . . . . . . . . .  7
     2.2.  Enabling Trust Assessments . . . . . . . . . . . . . . . .  7
   3.  DKIM Goals . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Functional Goals . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Operational Goals  . . . . . . . . . . . . . . . . . . . .  9
   4.  DKIM Function  . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  The Basic Signing Service  . . . . . . . . . . . . . . . . 11
     4.2.  Characteristics of a DKIM signature  . . . . . . . . . . . 11
     4.3.  The Selector construct . . . . . . . . . . . . . . . . . . 11
     4.4.  Verification . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Service Architecture . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Administration and Maintenance . . . . . . . . . . . . . . 15
     5.2.  Signing  . . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.3.  Verifying  . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.4.  Unverified or Unsigned Mail  . . . . . . . . . . . . . . . 16
     5.5.  Assessing  . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.6.  DKIM Placement within an ADMD  . . . . . . . . . . . . . . 17
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Internet Mail Background  . . . . . . . . . . . . . . 19
     A.1.  Administrative Management Domain (ADMD)  . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
   Intellectual Property and Copyright Statements . . . . . . . . . . 23



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1.  Introduction

   This document provides a description of the architecture and
   functionality for DomainKeys Identified Mail (DKIM).  It is intended
   for those who are adopting, developing, or deploying DKIM.  It will
   also be helpful for those who are considering extending DKIM, either
   into other areas of use or to support additional features.  This
   overview does not provide information on threats to DKIM or email, or
   details on the protocol specifics, which can be found in [RFC4686]
   and [RFC4871], respectively.  The document assumes a background in
   basic email and network security technology and services.

   DKIM allows an organization to take responsibility for a message, in
   a way that can be validated by a recipient.  The organization can be
   the author's, the originating sending site, an intermediary, or one
   of their agents.  DKIM defines a domain-level digital signature
   authentication framework for email through the use of public-key
   cryptography and key server technology.  [RFC4871] It permits
   verification of the signer of a message, as well as the integrity of
   its contents.  DKIM will also provide a mechanism that permits
   potential email signers to publish information about their email
   signing practices; this will permit email receivers to make
   additional assessments of unsigned messages.  Such protection of
   email identity can assist in the global control of "spam" and
   "phishing".

   Neither this document nor DKIM attempts to provide solutions to the
   world's problems with spam, phishing, virii, worms, joe jobs, etc.
   DKIM provides one basic tool, in what needs to be a large arsenal,
   for improving basic trust in the Internet mail service.  However by
   itself, DKIM is not sufficient to that task and this overview does
   not pursue the issues of integrating DKIM into these larger efforts,
   beyond a simple reference within a system diagram.  Rather, it is a
   basic introduction to the technology and its use.

1.1.  DKIM's Scope

   DKIM signatures can be created by a direct handler of a message,
   either as its author or as an intermediary.  It can also be created
   by an independent service that is providing assistance to a handler
   of the message.  Whoever does the signing chooses the domain name to
   be used as the basis for later assessments.  Hence, the reputation
   associated with that domain name is an additional basis for
   evaluating whether to trust the message for delivery.  The owner of
   the domain name being used for a DKIM signature is declaring that
   they accept responsibility for the message and may thus be held
   accountable for it.




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   DKIM is intended as a value-added feature for email.  Mail that is
   not signed by DKIM is handled in the same way as it was before DKIM
   was defined.  The message will be evaluated by established analysis
   and filtering techniques.  (A signing policy may provide additional
   information for that analysis and filtering.)  Over time, widespread
   DKIM adoption could permit more strict handling of messages that are
   not signed.  However early benefits do not require this and probably
   do not warrant this.

   DKIM's capabilities have a narrow scope.  It is an enabling
   technology, intended for use in the larger context of determining
   message legitimacy.  This larger context is complex, so it is easy to
   assume that a component like DKIM, which actually provides only a
   limited service, instead satisfies the broader set of requirements.

   By itself, a DKIM signature:

   o  Does not offer any assertions about the behaviors of the identity
      doing the signing.

   o  Does not prescribe any specific actions for receivers to take upon
      successful signature verification.

   o  Does not provide protection after signature verification.

   o  Does not protect against re-sending (replay of) a message that
      already has a verified signature; therefore a transit intermediary
      or a recipient can re-post the message in such a way that the
      signature would remain verifiable, although the new recipient(s)
      would not have been specified by the author.

1.2.  Prior Work

   Historically, email delivery assessment decisions have been based on
   an identity that used the IP Address of the system that directly sent
   the message (that is, the previous email "hop"), [RFC4408] or on the
   message content (e.g.  [RFC4406] and [RFC4407]).  The IP Address is
   obtained via underlying Internet information mechanisms and is
   therefore trusted to be accurate.  Besides having some known security
   weaknesses, the use of addresses presents a number of functional and
   operational problems.  Consequently there is a widespread desire to
   use an identifier that has better correspondence to organizational
   boundaries.  Domain names are viewed as often satisfying this need.

   There have been four previous IETF efforts at standardizing an
   Internet email signature scheme.  Their goals have differed from
   those of DKIM.




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   o  Privacy Enhanced Mail (PEM) was first published in 1987.
      [RFC0989]

   o  PEM eventually transformed into MIME Object Security Services
      (MOSS) in 1995.  [RFC1848] Today, these two are only of historical
      interest.

   o  Pretty Good Privacy (PGP) was developed by Phil Zimmermann and
      first released in 1991.  [RFC1991] A later version was
      standardized as OpenPGP.  [RFC2440] [RFC3156] [RFC4880]

   o  RSA Security independently developed Secure MIME (S/MIME) to
      transport a PKCS #7 data object.  [RFC3851]

   Development of both S/MIME and OpenPGP has continued.  While each has
   achieved a significant user base, neither one has achieved ubiquity
   in deployment or use.

   To the extent that other message-signing services might have been
   adapted to do the job that DKIM is designed to perform, it was felt
   that re-purposing any of those would be more problematic than
   creating a separate service.  That said, DKIM uses security algorithm
   components that have a long history, including use within some of
   those other messaging security services.

   DKIM has a distinctive approach for distributing and vouching for
   keys.  It uses a key-centric Public Key Infrastructure (PKI) rather
   than the more typical approaches based on a certificate in the styles
   of Kohnfelder (X.509) [Kohnfelder] or Zimmermann (web of trust).  For
   DKIM, the owner of a domain name asserts the validity of a key,
   rather than relying on the key having a broader semantic implication
   of the assertion, such as a quality assessment of the key's owner.
   DKIM treats quality assessment as an independent, value-added
   service, beyond the initial work of deploying a verifying signature
   service.

   Further, DKIM's PKI is provided by adding information records to the
   existing Domain Name System (DNS) [RFC1034], rather than requiring
   deployment of a new query infrastructure.  This approach has
   significant operational advantages.  First, it avoids the
   considerable barrier of creating a new global infrastructure; hence
   it leverages a global base of administrative experience and highly
   reliable distributed operation.  Second, the technical aspect of the
   DNS is already known to be efficient.  Any new service would have to
   undergo a period of gradual maturation, with potentially problematic
   early-stage behaviors.  By (re-)using the DNS, DKIM avoids these
   growing pains.




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1.3.  Internet Mail Background

   The basic Internet Email service has evolved extensively over its
   several decades of continuous operation.  Its modern architecture
   comprises a number of specialized components.  A discussion about
   Mail User Agents (MUA), Mail Handling Services (MHS), Mail Transfer
   Agents (MTA), Mail Submission Agents (MSA), Mail Delivery Agents
   (MDA), Mail Service Providers (MSP), Administrative Management
   Domains (ADMDs), and their relationships can be found in Appendix A.

1.4.  Discussion Venue

   NOTE TO RFC EDITOR:   This "Discussion Venue" section is to be
      removed prior to publication.

   This document is being discussed on the DKIM mailing list,
   ietf-dkim@mipassoc.org.


2.  The DKIM Value Proposition

   The nature and origins of a message are often falsely stated.  Such
   misrepresentations may (but not necessarily) be employed in order to
   perpetrate abuse.  DKIM provides a foundation for distinguishing
   legitimate mail, and thus a means of associating a verifiable
   identifier with a message.  Given the presence of that identifier, a
   receiver can make decisions about further handling of the message,
   based upon assessments of the identity that is associated with the
   identifier.

   Receivers who successfully verify a signature can use information
   about the signer as part of a program to limit spam, spoofing,
   phishing, or other undesirable behavior.  DKIM does not, itself,
   prescribe any specific actions by the recipient; rather it is an
   enabling technology for services that do.

   These services will typically:

   1.  Determine a verified identity, if possible.

   2.  Determine whether a known identity is trusted.

   The role of DKIM is to perform the first of these; DKIM is an enabler
   for the second.







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2.1.  Identity Verification

   Consider an attack made against an organization or against customers
   of an organization.  The name of the organization is linked to
   particular Internet domain names (identifiers).  One point of
   leverage for attackers is either to use a legitimate domain name,
   without authorization, or to use a "cousin" name that is similar to
   one that is legitimate, but is not controlled by the target
   organization.  An assessment service that uses DKIM can differentiate
   between domains used by known organizations and domains used by
   others.  As such, DKIM performs the positive step of identifying
   messages associated with verifiable identities, rather than the
   negative step of identifying messages with problematic use of
   identities.  Whether a verified identity belongs to a Good Actor or a
   Bad Actor becomes a later step of assessment.

2.2.  Enabling Trust Assessments

   Email receiving services are faced with a basic decision: Should they
   deliver a newly-arrived message to the indicated recipient?  That is,
   does the receiving service trust that the message is sufficiently
   "safe" to be viewed?  For the modern Internet, most receiving
   services have an elaborate engine that formulates this quality
   assessment.  These engines take a variety of information as input to
   the decision, such as from reputation lists and accreditation
   services.  As the engine processes information, it raises or lowers
   its trust assessment for the message.

   DKIM provides additional information to this process by declaring a
   valid "responsible" identity about which the engine can make quality
   assessments.  By itself, a valid DKIM signature neither lowers nor
   raises the level of trust associated with the message, but it enables
   other mechanisms to be used for doing so.

   An organization might build upon its use of DKIM by publishing
   information about its Signing Practices (SP).  This could permit
   detecting some messages that purport to be associated with a domain,
   but which are not.  As such, an SP can cause the trust assessment to
   be reduced, or leave it unchanged.


3.  DKIM Goals

   DKIM adds an end-to-end authentication mechanism to the existing
   email transfer infrastructure.  This motivates functional goals about
   the authentication itself and operational goals about its integration
   with the rest of the Internet email service.




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3.1.  Functional Goals

3.1.1.  Use Domain-level granularity for assurance

   DKIM seeks accountability at the coarse granularity of an
   organization or, perhaps, a department.  An existing Internet service
   construct that enables this granularity is the Domain Name [RFC1034].
   DKIM binds the signing key record to the Domain Name.  Further
   benefits of using domain names include simplifying key management,
   enabling signing by the infrastructure as opposed to the MUA, and
   potential privacy issues.

   Contrast this with OpenPGP and S/MIME, which provide end-to-end
   validation in terms of individual authors, notably using full email
   addresses.

3.1.2.  Implementation Locality

   Any party, anywhere along the transit path can implement DKIM
   signing.  Its use is not confined to the end systems or only in a
   boundary MTA.

3.1.3.  Allow delegation of signing to independent parties

   Different parties have different roles in the process of email
   exchange.  Some are easily visible to end users and others are
   primarily visible to operators of the service.  DKIM was designed to
   support signing by any of these different parties and to permit them
   to sign with any domain name that they deem appropriate (and for
   which they hold authorized signing keys.)  As an example an
   organization that creates email content often delegates portions of
   its processing or transmission to an outsourced group.  DKIM supports
   this mode of activity, in a manner that is not normally visible to
   end users.

3.1.4.  Distinguish the core authentication mechanism from its
        derivative uses

   An authenticated identity can be subject to a variety of processing
   policies, either ad hoc or standardized.  The only semantics inherent
   to a DKIM signature is that the signer is asserting (some)
   responsibility for the message.  All other mechanisms and meanings
   are built on this core service.  One such mechanism might assert a
   relationship between the signing identity and the author, as
   specified in the From: header field's domain identity[RFC2822].
   Another might specify how to treat an unsigned message with that
   From: field domain.




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3.1.5.  Retain ability to have anonymous email

   The ability to send a message that does not identify its author is
   considered to be a valuable quality of the current email service that
   needs to be retained.  DKIM is compatible with this goal since it
   permits authentication of the email system operator, rather than the
   content author.  If it is possible to obtain effectively anonymous
   accounts at example.com, knowing that a message definitely came from
   example.com does not threaten the anonymity of the user who authored
   it.

3.2.  Operational Goals

3.2.1.  Treat verification failure the same as no signature present

   As a sub-goal to the requirement for transparency, a DKIM signature
   verifier is to treat messages with signatures that fail as if they
   were unsigned.  Hence the message will revert to normal handling,
   through the receiver's existing filtering mechanisms.  Thus, DKIM
   specifies that an assessing site is not to take a message that has a
   broken signature and treat it any differently than if the signature
   weren't there.

   Contrast this with OpenPGP and S/MIME, which were designed for strong
   cryptographic protection.  This included treating verification
   failure as message failure.

3.2.2.  Make signatures transparent to non-supporting recipients

   In order to facilitate incremental adoption, DKIM is designed to be
   transparent to recipients that do not support it.  A DKIM signature
   does not "get in the way" for such recipients.

   Contrast this with S/MIME and OpenPGP, which modify the message body.
   Hence, their presence is potentially visible to email recipients,
   whose user software needs to process the associated constructs.

3.2.3.  Permit incremental adoption for incremental benefit

   DKIM can immediately provide benefits between any two organizations
   that exchange email and implement DKIM.  In the usual manner of
   "network effects", the benefits of DKIM increase dramatically as its
   adoption increases.

   Although it is envisioned that this mechanism will call upon
   independent services to aid in the assessment of DKIM results, they
   are not essential in order to obtain initial benefit.  For example
   DKIM allows (possibly large) pair-wise sets of email providers and



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   spam filtering companies to distinguish mail that is associated with
   a known organization from mail that might deceptively purport to have
   the affiliation.  This in turn allows the development of "whitelist"
   schemes whereby authenticated mail from a known source with good
   reputation is allowed to bypass some anti-abuse filters.

   In effect the email receiver is using their set of known
   relationships to generate their own reputation data.  This works
   particularly well for traffic between large sending providers and
   large receiving providers.  However it also works well for any
   operator, public or private, that has mail traffic dominated by
   exchanges among a stable set of organizations.

   Management of email deliverability problems currently represents a
   significant pain point for email administrators at every point on the
   mail transit path.  Administrators who have deployed DKIM
   verification have an incentive to evangelize the use of DKIM
   signatures to senders who may subsequently complain that their email
   is not being delivered.

3.2.4.  Minimize the amount of required infrastructure

   A new service, or an enhancement to an existing service, requires
   adoption in a critical mass of system components, before it can be
   useful.  The greater the number of required adopters, the higher the
   adoption barrier.  This becomes particularly serious when adoption is
   required by independent, intermediary -- that is, infrastructure --
   service providers.  In order to allow early adopters to gain early
   benefit, DKIM makes no changes to the core Internet Mail service and,
   instead, can provide a useful benefit for any individual pair of
   signers and verifiers who are exchanging mail.  Similarly, DKIM's
   reliance on the Domain Name System greatly reduces the amount of new
   administrative infrastructure that is needed across the open
   Internet.

3.2.5.  Permit wide range of deployment choices

   DKIM can be deployed at a variety of places within an organization's
   email service.  This permits the organization to choose how much or
   how little they want DKIM to be part of their service, rather than
   part of a more localized operation.


4.  DKIM Function

   DKIM has a very constrained set of capabilities, primarily targeting
   email while it is in transit from an author to a set of recipients.
   It creates the ability to associate verifiable information with a



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   message, especially a responsible identity.  When a message does not
   have a valid signature associated with the author, DKIM SP will
   permit the domain name of the author to be used for obtaining
   information about their signing practices.

4.1.  The Basic Signing Service

   With the DKIM signature mechanism, a signer chooses a signing
   identity based on their domain name, performs digital signing on the
   message, and records signature information in a DKIM header field.  A
   verifier obtains the domain name and the "selector" from the DKIM
   header field, queries for a public key associated with the name, and
   verifies the signature.

   DKIM permits any domain name to be used for signing, and supports
   extensible choices for various algorithms.  As is typical for
   Internet standards, there is a core set of algorithms that all
   implementations are required to support, in order to guarantee basic
   interoperability.

   DKIM permits restricting the use of a signature key (by using s=) to
   signing messages for particular types of services, such as only for
   email.  This is intended to be helpful when delegating signing
   authority, such as to a particular department or to a third-party
   outsourcing service.

   With DKIM the signer explicitly lists the headers that are signed,
   such as From:, Date: and Subject:.  By choosing the minimal set of
   headers needed, the signature is likely to be considerably more
   robust against the handling vagaries of intermediary MTAs.

4.2.  Characteristics of a DKIM signature

   A DKIM signature covers the message body and selected header fields.
   The signer computes a hash of the selected header fields and another
   hash of the body.  The signer then uses a private key to
   cryptographically encode this information, along with other signing
   parameters.  Signature information is placed into the DKIM-Signature
   header field, a new [RFC2822] header field of the message.

4.3.  The Selector construct

   The key for a signature is associated with a domain name, as
   specified in the d= parameter of the DKIM-Signature header.  That
   domain name, or the domain name or address in the i= parameter,
   provide the complete identity used for making assessments about the
   signer.  (The DKIM specification does not give any guidance on how to
   do an assessment.)  However this name is not sufficient for making a



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   DNS query to obtain the key needed to verify the signature.

   A single domain can use multiple signing keys and/or multiple
   potential signers.  To support this, DKIM identifies a particular
   signature as a combination of the domain name and an added field,
   called the "selector", specified in separate DKIM-Signature header
   field parameters.

   NOTE:   The semantics of the selector (if any) are strictly reserved
      to the signer and should be treated as an opaque string by all
      other parties.  If verifiers were to employ the selector as part
      of a name assessment mechanism, then there would be no remaining
      mechanism for making a transition from an old, or compromised, key
      to a new one.

   Signers often need to support multiple assessments about their
   organization, such as to distinguish one type of message from
   another, or one portion of the organization from another.  To permit
   assessments that are independent, one method is for an organization
   to use different sub-domains in the "d=" parameter, such as
   "transaction.example.com" versus "newsletter.example.com", or
   "productA.example.com" versus "productB.example.com".

4.4.  Verification

   After a message has been signed, any agent in the message transit
   path can verify the signature to determine that the signing identity
   took responsibility for the message.  Message recipients can verify
   the signature by querying the DNS for the signer's domain directly,
   to retrieve the appropriate public key, and thereby confirm that the
   message was attested to by a party in possession of the private key
   for the signing domain.  Typically, verification will be done by an
   agent in the Administrative Management Domain (ADMD) of the message
   recipient.

















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5.  Service Architecture

   The DKIM service is divided into components that are performed using
   different, external services, such as for key retrieval and relaying
   email.  The basic DKIM signing specification defines an initial set
   of these services (using DNS and SMTP), in order to ensure a basic
   level of interoperability.












































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                              |
                              |- RFC2822 Message
                              V
   +--------+  +------------------------------------+
   | Private|  | ORIGINATING OR RELAYING ADMD (MSA) |
   | Key    |.>| Sign Message                       |
   | Store  |  +--------------+---------------------+
   +--------+                 |
    (paired)                  |
   +--------+                 |                 +-----------+
   | Public |                 |                 | Remote    |
   | Key    |             [Internet]            | Sender    |
   | Store  |                 |                 | Practices |
   +----+---+                 |                 +-----+-----+
        .                     V                       .
        .   +-----------------------------------+     .
        .   | RELAYING OR DELIVERING ADMD (MDA) |     .
        .   | Message Signed?                   |     .
        .   +--------+---------------+----------+     .
        .            |yes            |no              .
        .            V               |                .
        .      +------------+        |                .
        +.....>| Verify     +----+   |                .
               | Signatures |    |   |                .
               +-----+------+    |   |                .
                 pass|       fail|   |                .
                     V           |   |                .
                 +--------+      |   |                .
        +.......>| Assess |      |   |                .
        .        | Signer |      V   V                .
        .        +---+----+    +-------+              .
        .            |        / Check   \<............+
        .            +------>/  Signing  \
        .            |      /   Practices \<..........+
        .            |     +-------+-------+          .
        .            |             |                  .
        .            |             V                  .
    +---+---------+  |       +-----------+     +------+-----+
    |Reputation/  |  |       | Message   |     | Local Info |
    |Accreditation|  +------>| Filtering |     | on Sender  |
    |Info         |          | Engine    |     | Practices  |
    +-------------+          +-----------+     +------------+

                    Figure 1: DKIM Service Architecture

   As shown in Figure 1, basic message processing is divided between the
   MSA and the MDA.




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   The MSA  The MSA signs the message, using private information from
      the Key Store.

   The MDA  The MDA verifies the signature or determines whether a
      signature was required.  Verifying the signature uses public
      information from the Key Store.  If the signature passes,
      reputation information is used to asses the signer and that
      information is passed to the message filtering system.  If the
      signature fails or there is no signature, information about the
      related signing practices is retrieved remotely and/or locally,
      and that information is passed to the message filtering system.

   Note:  Figure 1 does not show the effects on the message handling
      when multiple signatures or non-author signatures are present.

5.1.  Administration and Maintenance

   A number of tables and services are used to provide external
   information.  Each of these introduces administration and maintenance
   requirements.

   Key Store  DKIM uses public/private (asymmetric) key cryptography.
      The signer users a private key and the validator uses the
      corresponding public key.  The current DKIM signing specification
      provides for querying the Domain Names Service (DNS), to permit a
      validator to obtain the public key.  The signing organization
      therefore must have a means of adding a key to the DNS, for every
      selector/domain-name combination.  Further, the signing
      organization needs policies for distributing and revising keys.

   Reputation/Accreditation  If a message contains a valid signature,
      then the verifier can evaluate the associated domain name's
      reputation.  Quality-assessment information, which is associated
      with a domain name, comes in many forms and from many sources.
      DKIM does not define assessment services.  It's relevance to them
      is to provide a validated domain name, upon which assessments can
      be made.

   Signing Practices (SP)  Separate from determining the validity of a
      signature, and separate from assessing the reputation of the
      organization that is associated with the signed identity, there is
      an the opportunity to determine any organizational practices
      concerning a domain name.  Practices can range widely.  They can
      be published by the owner of the domain or they can be maintained
      by the evaluating site.  They can pertain to the use of the domain
      name, such as whether it is used for signing messages, whether all
      mail having that domain name in the author From: header field is
      signed, or whether such mail is to be discarded in the absence of



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      an appropriate signature.  The statements of practice are made at
      the level of a domain name, and are distinct from assessments made
      about particular messages, as occur in a Message Filtering Engine.
      Such assessments of practices can provide useful input for the
      Message Filtering Engine's determination of message handling.  As
      practices are defined, each domain name owner needs to consider
      what information to publish.  The nature and degree of checking
      practices, if any is performed, is optional to the evaluating site
      and is strictly a matter of local policy.

5.2.  Signing

   Signing can be performed by a component of the ADMD that creates the
   message, and/or within any ADMD along the relay path.  The signer
   uses the appropriate private key.

5.3.  Verifying

   Verification can be performed by any functional component along the
   relay and delivery path.  Verifiers retrieve the public key based
   upon the parameters stored in the message.

5.4.  Unverified or Unsigned Mail

   Note that a failed signature causes the message to be treated in the
   same manner as one that is unsigned.  Messages lacking a valid author
   signature (a signature associated with the author of the message as
   opposed to a signature associated with an intermediary) can prompt a
   query for any published "signing practices" information, as an aid in
   determining whether the author information has been used without
   authorization.

5.5.  Assessing

   Figure 1 shows the verified identity as being used to assess an
   associated reputation, but it could be applied for other tasks, such
   as management tracking of mail.  A popular use of reputation
   information is as input to a filtering engine that decides whether to
   deliver -- and possibly whether to specially mark -- a message.
   Filtering engines have become complex and sophisticated.  Their
   details are outside of the scope of DKIM, other than the expectation
   that the validated identity produced by DKIM will be added to the
   varied soup of rules used by the engines.  The rules can cover signed
   messages and can deal with unsigned messages from a domain, if the
   domain has published information about its practices.






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5.6.  DKIM Placement within an ADMD

   It is expected that the most common venue for a DKIM implementation
   will be within the infrastructures of the authoring organization's
   outbound service and the receiving organization's inbound service,
   such as a department or a boundary MTA.  DKIM can be implemented in
   an author's or recipient MUA, but this is expected to be less
   typical, since it has higher administration and support costs.

   A Mediator, such as a mailing list, often can re-post a message
   without breaking the DKIM signature.  Furthermore it can add its own
   signature.  This can be added by the Mediator software itself, or by
   any outbound component in the Mediator's ADMD.


6.  Security Considerations

   The security considerations of the DKIM protocol are described in the
   DKIM base specification [RFC4871].


7.  IANA Considerations

   There are no actions for IANA.

   NOTE TO RFC EDITOR:   This section may be removed prior to
      publication.


8.  Acknowledgements

   Many people contributed to the development of the DomainKeys
   Identified Mail and the efforts of the DKIM Working Group is
   gratefully acknowledged.  In particular, we would like to thank Jim
   Fenton for his extensive feedback diligently provided on every
   version of this document.


9.  Informative References

   [I-D.kucherawy-sender-auth-header]
              Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status",
              draft-kucherawy-sender-auth-header-11 (work in progress),
              February 2008.

   [Kohnfelder]
              Kohnfelder, L., "Towards a Practical Public-key



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              Cryptosystem", May 1978.

   [RFC0989]  Linn, J. and IAB Privacy Task Force, "Privacy enhancement
              for Internet electronic mail: Part I: Message encipherment
              and authentication procedures", RFC 989, February 1987.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1848]  Crocker, S., Galvin, J., Murphy, S., and N. Freed, "MIME
              Object Security Services", RFC 1848, October 1995.

   [RFC1991]  Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
              Exchange Formats", RFC 1991, August 1996.

   [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
              "OpenPGP Message Format", RFC 2440, November 1998.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC3156]  Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
              "MIME Security with OpenPGP", RFC 3156, August 2001.

   [RFC3164]  Lonvick, C., "The BSD Syslog Protocol", RFC 3164,
              August 2001.

   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 2004.

   [RFC4406]  Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
              RFC 4406, April 2006.

   [RFC4407]  Lyon, J., "Purported Responsible Address in E-Mail
              Messages", RFC 4407, April 2006.

   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

   [RFC4686]  Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", RFC 4686, September 2006.

   [RFC4870]  Delany, M., "Domain-Based Email Authentication Using



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              Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
              May 2007.

   [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.


Appendix A.  Internet Mail Background

   Internet Mail is split between the user world, in the form of Mail
   User Agents (MUA), and the transmission world, in the form of the
   Mail Handling Service (MHS) composed of Mail Transfer Agents (MTA).
   The MHS is responsible for accepting a message from one user, the
   author, and delivering it to one or more other users, the recipients.
   This creates a virtual MUA-to-MUA exchange environment.  The first
   component of the MHS is called the Mail Submission Agent (MSA) and
   the last is called the Mail Delivery Agent (MDA).

   An email Mediator is both an inbound MDA and outbound MSA.  It takes
   delivery of a message and re-posts it for further distribution,
   retaining the original From: header field.  A mailing list is a
   common example of a Mediator.

   The modern Internet Mail service is marked by many independent
   operators, many different components for providing users with service
   and many other components for performing message transfer.
   Consequently, it is necessary to distinguish administrative
   boundaries that surround sets of functional components, which are
   subject to coherent operational policies.

   As elaborated on below, every MSA is a candidate for signing using
   DKIM, and every MDA is a candidate for doing DKIM verification.

A.1.  Administrative Management Domain (ADMD)

   Operation of Internet Mail services is apportioned to different
   providers (or operators).  Each can be composed of an independent
   ADministrative Management Domain (ADMD).  An ADMD operates with an
   independent set of policies and interacts with other ADMDs according
   to differing types and amounts of trust.  Examples include: an end-
   user operating their desktop client that connects to an independent
   email service, a department operating a submission agent or a local
   Relay, an organization's IT group that operates enterprise Relays,
   and an ISP operating a public shared email service.



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   Each of these can be configured into many combinations of
   administrative and operational relationships, with each ADMD
   potentially having a complex arrangement of functional components.
   Figure 2 depicts the relationships among ADMDs.  Perhaps the most
   salient aspect of an ADMD is the differential trust that determines
   its policies for activities within the ADMD, versus those involving
   interactions with other ADMDs.

   Basic types of ADMDs include:



      Edge:   Independent transfer services, in networks at the edge of
         the Internet Mail service.

      User:   End-user services.  These might be subsumed under an Edge
         service, such as is common for web-based email access.

      Transit:   These are Mail Service Providers (MSP) offering value-
         added capabilities for Edge ADMDs, such as aggregation and
         filtering.

   Note that Transit services are quite different from packet-level
   transit operation.  Whereas end-to-end packet transfers usually go
   through intermediate routers, email exchange across the open Internet
   is often directly between the Edge ADMDs, at the email level.

   +--------+                            +--------+    +--------+
   | ADMD#1 |                            | ADMD#3 |    | ADMD#4 |
   | ------ |                            | ------ |    | ------ |
   |        |   +----------------------->|        |    |        |
   | User   |   |                        |--Edge--+--->|--User  |
   |  |     |   |                   +--->|        |    |        |
   |  V     |   |                   |    +--------+    +--------+
   | Edge---+---+                   |
   |        |   |    +----------+   |
   +--------+   |    |  ADMD#2  |   |
                |    |  ------  |   |
                |    |          |   |
                +--->|-Transit--+---+
                     |          |
                     +----------+

        Figure 2: ADministrative Management Domains (ADMD) Example

   In Figure 2, ADMD numbers 1 and 2 are candidates for doing DKIM
   signing, and ADMD numbers 2, 3 and 4 are candidates for doing DKIM
   verification.



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   The distinction between Transit network and Edge network transfer
   services is primarily significant because it highlights the need for
   concern over interaction and protection between independent
   administrations.  The interactions between functional components
   within a single ADMD are subject to the policies of that domain.
   Although any pair of ADMDs can arrange for whatever policies they
   wish, Internet Mail is designed to permit inter-operation without
   prior arrangement.

   Common ADMD examples are:



         Enterprise Service Providers:

            Operators of an organization's internal data and/or mail
            services.

         Internet Service Providers:

            Operators of underlying data communication services that, in
            turn, are used by one or more Relays and Users.  It is not
            necessarily their job to perform email functions, but they
            can, instead, provide an environment in which those
            functions can be performed.

         Mail Service Providers:

            Operators of email services, such as for end-users, or
            mailing lists.


Authors' Addresses

   Tony Hansen
   AT&T Laboratories
   200 Laurel Ave.
   Middletown, NJ  07748
   USA

   Email: tony+dkimov@maillennium.att.com










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   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale, CA  94086
   USA

   Email: dcrocker@bbiw.net


   Phillip Hallam-Baker
   VeriSign Inc.

   Email: pbaker@verisign.com






































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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
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