pre-workgroup                                                  J. Fenton
Internet-Draft                                       Cisco Systems, Inc.
Expires:  June 22, 2006                                December 19, 2005

    Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document provides an analysis of some threats against Internet
   mail that are intended to be addressed by signature-based mail
   authentication, in particular DomainKeys Identified Mail.  It
   discusses the nature and location of the bad actors, what their
   capabilities are, and what they intend to accomplish via their

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology and Model  . . . . . . . . . . . . . . . . . .  4
   2.  The Bad Actors . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Characteristics  . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Capabilities . . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Location . . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.3.1.  Externally-located Bad Actors  . . . . . . . . . . . .  7
       2.3.2.  Within Claimed Originator's Administrative Unit  . . .  8
       2.3.3.  Within Recipient's Administrative Unit . . . . . . . .  8
   3.  Representative Bad Acts  . . . . . . . . . . . . . . . . . . .  9
     3.1.  Use of Arbitrary Identities  . . . . . . . . . . . . . . .  9
     3.2.  Use of Specific Identities . . . . . . . . . . . . . . . .  9
       3.2.1.  Exploitation of Social Relationships . . . . . . . . . 10
       3.2.2.  Identity-Related Fraud . . . . . . . . . . . . . . . . 10
       3.2.3.  Reputation Attacks . . . . . . . . . . . . . . . . . . 10
   4.  Attacks on Message Signing . . . . . . . . . . . . . . . . . . 11
     4.1.  Attacks Against Message Signatures . . . . . . . . . . . . 12
       4.1.1.  Theft of Private Key for Domain  . . . . . . . . . . . 12
       4.1.2.  Theft of Delegated Private Key . . . . . . . . . . . . 13
       4.1.3.  Private Key Recovery via Timing Attack . . . . . . . . 13
       4.1.4.  Chosen Message Replay  . . . . . . . . . . . . . . . . 13
       4.1.5.  Signed Message Replay  . . . . . . . . . . . . . . . . 14
       4.1.6.  Denial-of-Service Attack Against Verifier  . . . . . . 15
       4.1.7.  Denial-of-Service Attack Against Key Service . . . . . 15
       4.1.8.  Canonicalization Abuse . . . . . . . . . . . . . . . . 15
       4.1.9.  Body Length Limit Abuse  . . . . . . . . . . . . . . . 16
       4.1.10. Use of Revoked Key . . . . . . . . . . . . . . . . . . 16
       4.1.11. Compromise of Key Server . . . . . . . . . . . . . . . 17
       4.1.12. Falsification of Key Service Replies . . . . . . . . . 17
       4.1.13. Publication of Malformed Key Records and/or
               Signatures . . . . . . . . . . . . . . . . . . . . . . 17
       4.1.14. Cryptographic Weaknesses in Signature Generation . . . 18
       4.1.15. Display Name Abuse . . . . . . . . . . . . . . . . . . 18
       4.1.16. Compromised System Within Originator's Network . . . . 19
     4.2.  Attacks Against Message Signing Policy . . . . . . . . . . 19
       4.2.1.  Look-Alike Domain Names  . . . . . . . . . . . . . . . 19
       4.2.2.  Internationalized Domain Name Abuse  . . . . . . . . . 19
       4.2.3.  Denial-of-Service Attack Against Signing Policy  . . . 20
       4.2.4.  Use of Multiple From Addresses . . . . . . . . . . . . 20
   5.  Derived Requirements . . . . . . . . . . . . . . . . . . . . . 20
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   8.  Informative References . . . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Glossary  . . . . . . . . . . . . . . . . . . . . . . 22
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 22
   Appendix C.  Edit History  . . . . . . . . . . . . . . . . . . . . 22

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   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
   Intellectual Property and Copyright Statements . . . . . . . . . . 25

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1.  Introduction

   DomainKeys Identified Mail (DKIM) [I-D.allman-dkim-base] defines a
   simple, low cost, and effective mechanism by which email messages can
   be cryptographically signed, permitting a signing domain to claim
   responsibility for the use of a given email address.  Message
   recipients can verify the signature by querying 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.

   Once the attesting party or parties have been established, the
   recipient may evaluate the message in the context of additional
   information such as locally-maintained whitelists, shared reputation
   services, and/or third-party accreditation.  The description of these
   mechanisms is outside the scope of this effort.  By applying a
   signature, a good player will be able to associate a positive
   reputation with the message, in hopes that it will receive
   preferential treatment by the recipient.

   This effort is not intended to address threats associated with
   message confidentiality nor does it intend to provide a long-term
   archival signature.

1.1.  Terminology and Model

   The following diagram illustrates a typical usage flowchart for DKIM:

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                      |       SIGNATURE CREATION        |
                      |  (Originating or Relaying ADMD) |
                      |                                 |
                      |   Sign (Message, Domain, Key)   |
                      |                                 |
                                       | - Message (Domain, Key)
     +-----------+    |     SIGNATURE VERIFICATION      |
     |           |    |  (Relaying or Delivering ADMD)  |
     |    KEY    |    |                                 |
     |   QUERY   +...>|  Verify (Message, Domain, Key)  |
     |           |    |                                 |
     +-----------+    +----------------+----------------+
                                       |  - Verified Domain
     +-----------+                     V  - [Report]
     |           |    +----------------+----------------+
     |  SIGNER   |    |                                 |
     | PRACTICES +...>|        SIGNER EVALUATION        |
     |   QUERY   |    |                                 |
     |           |    +---------------------------------+

   Definitions of some terms used in this document may be found in
   Appendix A.

   Placeholder for some discussion of 2821 vs. 2822 solutions, etc.

2.  The Bad Actors

2.1.  Characteristics

   The problem space being addressed by DKIM is characterized by a wide
   range of attackers in terms of motivation, sophistication, and

   At the low end of the spectrum are bad actors who may simply send
   email, perhaps using one of many commercially available tools, which
   the recipient does not want to receive.  These tools may or may not
   falsify the origin address of messages, and may, in the future, be
   capable of generating message signatures as well.

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   At the next tier are what would be considered "professional" senders
   of unwanted email.  These attackers would deploy specific
   infrastructure, including Mail Transfer Agents (MTAs), registered
   domains and possibly networks of compromised computers ("zombies") to
   send messages, and in some cases to harvest addresses to which to
   send.  These senders often operate as commercial enterprises and send
   messages on behalf of third parties.

   The most sophisticated and financially-motivated senders of messages
   are those who stand to receive substantial financial benefit, such as
   from an email-based fraud scheme.  These attackers can be expected to
   employ all of the above mechanisms and additionally may attack the
   Internet infrastructure itself, e.g., DNS cache-poisoning attacks; IP
   routing attacks via compromised network routing elements.

2.2.  Capabilities

   In general, the bad actors described above should be expected to have
   access to the following:

   1.  An extensive corpus of messages from domains they might wish to

   2.  Knowledge of the business aims and model for domains they might
       wish to impersonate

   3.  Access to public keys and associated authorization records
       published by the domain

   and the ability to do at least some of the following:

   1.  Submit messages to MTAs at multiple locations in the Internet

   2.  Construct arbitrary message headers, including those claiming to
       be mailing lists, resenders, and other mail agents

   3.  Sign messages on behalf of potentially-untraceable domains under
       their control

   4.  Generate substantial numbers of either unsigned or apparently-
       signed messages which might be used to attempt a denial of
       service attack

   5.  Resend messages which may have been previously signed by the

   6.  Transmit messages using any envelope information desired

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   As noted above, certain classes of bad actors may have substantial
   financial motivation for their activities, and therefore should be
   expected to have more capabilities at their disposal.  These include:

   1.  Manipulation of IP routing.  This could be used to submit
       messages from specific IP addresses or difficult-to-trace
       addresses, or to cause diversion of messages to a specific

   2.  Limited influence over portions of DNS using mechanisms such as
       cache poisoning.  This might be used to influence message
       routing, or to cause falsification of DNS-based key or policy

   3.  Access to significant computing resources, perhaps through the
       conscription of worm-infected "zombie" computers.  This could
       allow the bad actor to perform various types of brute-force

   4.  Ability to "wiretap" some existing traffic, perhaps from a
       wireless network.

   Either of the first two of these mechanisms could be used to allow
   the bad actor to function as a man-in-the-middle between sender and
   recipient, if that attack is useful.

2.3.  Location

   In the following discussion, the term "administrative unit", taken
   from [I-D.crocker-email-arch], is used to refer to a portion of the
   email path that is under common administration.  The originator and
   recipient typically develop trust relationships with the
   administrative units that send and receive their email, respectively,
   to perform the signing and verification of their messages.

   Bad actors or their proxies can be located anywhere in the Internet.
   Certain attacks are possible primarily within the administrative unit
   of the claimed originator and/or recipient domain have capabilities
   beyond those elsewhere, as described in the below sections.  Bad
   actors can also collude by acting in multiple locations
   simultaneously (a "distributed bad actor").

2.3.1.  Externally-located Bad Actors

   DKIM focuses primarily on bad actors located outside of the
   administrative units of the claimed originator and the recipient.
   These administrative units frequently correspond to the protected
   portions of the network adjacent to the originator and recipient.  It

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   is in this area that the trust relationships required for
   authenticated message submission do not exist and do not scale
   adequately to be practical.  Conversely, within these administrative
   units, there are other mechanisms such as authenticated message
   submission that are easier to deploy and more likely to be used than

   External bad actors are usually attempting to exploit the "any to
   any" nature of email which motivates most recipient MTAs to accept
   messages from anywhere for delivery to their local domain.  They may
   generate messages without signatures, with incorrect signatures, or
   with correct signatures from domains with little traceability.  They
   may also pose as mailing lists, greeting cards, or other agents which
   legitimately send or re-send messages on behalf of others.

2.3.2.  Within Claimed Originator's Administrative Unit

   Bad actors in the form of rogue or unauthorized users or malware-
   infected computers can exist within the administrative unit
   corresponding to a message's origin address.  Since the submission of
   messages in this area generally occurs prior to the application of a
   message signature, DKIM is not directly effective against these bad
   actors.  Defense against these bad actors is dependent upon other
   means, such as proper use of firewalls, and mail submission agents
   that are configured to authenticate the sender.

   In the special case where the administrative unit is non-contiguous
   (e.g., a company that communicates between branches over the external
   Internet), DKIM signatures can be used to distinguish between
   legitimate externally-originated messages and attempts to spoof
   addresses in the local domain.

2.3.3.  Within Recipient's Administrative Unit

   Bad actors may also exist within the administrative unit of the
   message recipient.  These bad actors may attempt to exploit the trust
   relationships which exist within the unit.  Since messages will
   typically only have undergone DKIM verification at the administrative
   unit boundary, DKIM is not effective against messages submitted in
   this area.

   For example, the bad actor may attempt to apply a header such as
   Authentication-Results [I-D.kucherawy-sender-auth-header] which would
   normally be added (and spoofing of which would be detected) at the
   boundary of the administrative unit.  This could be used to falsely
   indicate that the message was authenticated successfully.

   As in the originator case, these bad actors are best dealt with by

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   controlling the submission of messages within the administrative
   unit.  Depending on the characteristics of the administrative unit,
   cryptographic methods may or may not be needed to accomplish this.

3.  Representative Bad Acts

   One of the most fundamental bad acts being attempted is the delivery
   of messages which are not authorized by the alleged originating
   domain.  As described above, these messages might merely be unwanted
   by the recipient, or might be part of a confidence scheme or a
   delivery vector for malware.

3.1.  Use of Arbitrary Identities

   This class of bad acts includes the sending of messages which aim to
   obscure the identity of the actual sender.  In some cases the actual
   sender might be the bad actor, or in other cases might be a third-
   party under the control of the bad actor (e.g., a compromised

   DKIM is effective in mitigating against the use of addresses not
   controlled by bad actors, but is not effective against the use of
   addresses they control.  In other words, the presence of a valid DKIM
   signature does not guarantee that the signer is not a bad actor.  It
   also does not guarantee the accountability of the signer, since that
   is limited by the extent to which domain registration requires
   accountability for its registrants.  However, accreditation and
   reputation systems can be used to enhance the accountability of DKIM-
   verified addresses and/or the likelihood that signed messages are

3.2.  Use of Specific Identities

   A second major class of bad acts involves the assertion of specific
   identities in email.

   Note that some bad acts involving specific identities can sometimes
   be accomplished, although perhaps less effectively, with similar
   looking identities that mislead some recipients.  For example, if the
   bad actor is able to control the domain "" (note the "one"
   between the p and e), they might be able to convince some recipients
   that a message from is really
   Similar types of attacks using internationalized domain names have
   been hypothesized where it could be very difficult to see character
   differences in popular typefaces.  Similarly, if was
   controlled by a bad actor, the bad actor could sign messages from which might also mislead some recipients.  To

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   the extent that these domains are controlled by bad actors, DKIM is
   not effective against these attacks, although it could support the
   ability of reputation and/or accreditation systems to aid the user in
   identifying them.

3.2.1.  Exploitation of Social Relationships

   One reason for asserting a specific origin address is to encourage a
   recipient to read and act on particular email messages by appearing
   to be an acquaintance or previous correspondent that the recipient
   might trust.  This tactic has been used by email-propagated malware
   which mail themselves to addresses in the infected host's address
   book.  In this case, however, the sender's address may not be
   falsified, so DKIM would not be effective in defending against this

   It is also possible for address books to be harvested and used by an
   attacker to send messages from elsewhere.  DKIM would be effective in
   mitigating these acts by limiting the scope of origin addresses for
   which a valid signature can be obtained when sending the messages
   from other locations.

3.2.2.  Identity-Related Fraud

   Bad acts related to email-based fraud often, but not always, involve
   the transmission of messages using specific origin addresses of other
   entities as part of the fraud scheme.  The use of a specific address
   of origin sometimes contributes to the success of the fraud by
   helping convince the recipient that the message was actually sent by
   the alleged sender.

   To the extent that the success of the fraud depends on or is enhanced
   by the use of a specific origin address, the bad actor may have
   significant financial motivation and resources to circumvent any
   measures taken to protect specific addresses from unauthorized use.

3.2.3.  Reputation Attacks

   Another motivation for using a specific origin address in a message
   is to harm the reputation of another, commonly referred to as a "joe-
   job".  For example, a commercial entity might wish to harm the
   reputation of a competitor, perhaps by sending unsolicited bulk email
   on behalf of that competitor.  It is for this reason that reputation
   systems must be based on an identity that is, in practice, fairly

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4.  Attacks on Message Signing

   Bad actors can be expected to exploit all of the limitations of
   message authentication systems.  They are also likely to be motivated
   to degrade the usefulness of message authentication systems in order
   to hinder their deployment.  Both the signature mechanism itself and
   declarations made regarding use of message signatures (often referred
   to as Sender Signing Policy, Sender Signing Practices or SSP) can be
   expected to be the target of attacks.

   The sections below begin with a table summarizing the postulated
   attacks in each category along with their expected impact and
   likelihood.  The following criteria were used in scoring the attacks
   against these criteria:


   High: Affects the verification of messages by an entire domain or
      multiple domains

   Medium: Affects the verification of messages by specific users, MTAs,
      and/or bounded time periods

   Low: Affects the verification of isolated individual messages only


   High: All users of DKIM should expect this attack on a frequent basis

   Medium: Users of DKIM should expect this attack occasionally;
      frequently for a few users

   Low: Attack is expected to be rare and/or very infrequent

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4.1.  Attacks Against Message Signatures

          Summary of postulated attacks against DKIM signatures:

   | Attack Name                                 | Impact | Likelihood |
   | Theft of private key for domain             |  High  |     Low    |
   | Theft of delegated private key              | Medium |   Medium   |
   | Private key recovery via timing attack      |  High  |     Low    |
   | Chosen message replay                       |   Low  |     M/H    |
   | Signed message replay                       |   Low  |    High    |
   | Denial-of-service attack against verifier   |  High  |   Medium   |
   | Denial-of-service attack against key        |  High  |   Medium   |
   | service                                     |        |            |
   | Canonicalization abuse                      |   Low  |   Medium   |
   | Body length limit abuse                     | Medium |   Medium   |
   | Use of revoked key                          | Medium |     Low    |
   | Compromise of key server                    |  High  |     Low    |
   | Falsification of key service replies        | Medium |   Medium   |
   | Publication of malformed key records and/or |  High  |     Low    |
   | signatures                                  |        |            |
   | Cryptographic weaknesses in signature       |  High  |     Low    |
   | generation                                  |        |            |
   | Display name abuse                          | Medium |    High    |
   | Compromised system within originator's      | Medium |   Medium   |
   | network                                     |        |            |

4.1.1.  Theft of Private Key for Domain

   Message signing technologies such as DKIM are vulnerable to theft of
   the private keys used to sign messages.  This includes "out-of-band"
   means for this theft, including burglary, bribery, extortion, and the
   like, as well as electronic means for such theft, such as a
   compromise of network and host security around the place where a
   private key is stored.

   Keys which are valid for all addresses in a domain typically reside
   in MTAs which should be located in well-protected sites, such as data
   centers.  Various means should be employed for minimizing access to
   private keys, such as non-existence of commands for displaying their
   value, although ultimately memory dumps and the like will probably
   contain the keys.  Due to the unattended nature of MTAs, some
   countermeasures, such as the use of a pass phrase to "unlock" a key,
   are not practical to use.

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4.1.2.  Theft of Delegated Private Key

   There are several circumstances where a domain owner will want to
   delegate the ability to sign messages for the domain to an individual
   user or a third-party associated with an outsourced activity such as
   a corporate benefits administrator or a marketing campaign.  Since
   these keys may exist on less well-protected devices than the domain's
   own MTAs, they will in many cases be more susceptible to compromise.

   In order to mitigate this exposure, keys used to sign such messages
   can be restricted by the domain owner to be valid for signing
   messages only on behalf of specific addresses in the domain.  This
   maintains protection for the majority of addresses in the domain.

4.1.3.  Private Key Recovery via Timing Attack

   Timing attacks are a technique whereby the private key is recovered
   by observing the time required to sign a series of messages.  It
   requires both the ability to submit messages for signing as well as
   the ability to accurately measure the time required to compute the

   In most cases, an MTA has are enough variables (system load, clock
   resolution, queuing delays, etc.) to prevent the signing time from
   being measured accurately enough to be useful for a timing attack.
   Furthermore, while some domains, e.g., consumer ISPs, would allow an
   attacker to submit messages for signature, with many other domains
   this is difficult.  Other mechanisms, such as mailing lists hosted by
   the domain, might be paths by which an attacker might submit messages
   for signature, and should also be considered as possible vectors for
   timing attacks.

4.1.4.  Chosen Message Replay

   Chosen Message Replay (CMR) refers to the scenario where the attacker
   creates a message and obtains a signature for it by sending it
   through an MTA authorized by the originating domain to him/herself or
   an accomplice.  They then "replay" the signed message by sending it,
   using different envelope addresses, to a (typically large) number of
   other recipients.

   Due to the requirement to get an attacker-generated message signed,
   Chosen Message Replay would most commonly be experienced by consumer
   ISPs or others offering email accounts to clients, particularly where
   there is little or no accountability to the account holder (the
   attacker in this case).  One approach to this problem is for the
   domain to only sign email for clients that have passed a vetting
   process to provide traceability to the message originator in the

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   event of abuse.  At present, the low cost of email accounts (zero)
   does not make it practical for any vetting to occur.  It remains to
   be seen whether this will be the model with signed mail as well, or
   whether a higher level of trust will be required to obtain an email

   Revocation of the signature is a potential countermeasure.  However,
   the rapid pace at which the message might be replayed (especially
   with an army of "zombie" computers), compared with the time required
   to detect the attack and implement the revocation, is likely to be
   problematic.  A related problem is the likelihood that domains will
   use a small number of signing keys for a large number of customers,
   which is beneficial from a caching standpoint but presents a problem
   revoking some signatures and not others.  To this end, "revocation
   identifiers" have been proposed which would permit more fine-grained
   revocation, perhaps on a per-account basis.  Messages containing
   these identifiers would result in a query to a revocation database,
   which might be represented in DNS.  Further study is needed to
   determine if the benefits from revocation (given the potential speed
   of a replay attack) outweigh the transactional cost of querying the
   revocation database.

4.1.5.  Signed Message Replay

   Signed Message Replay (SMR) refers to the retransmission of already-
   signed messages to additional recipients beyond those intended by the
   sender.  The attacker arranges to receive a message from the victim,
   and then retransmits it intact but with different envelope addresses.
   This might be done, for example, to make it look like a legitimate
   sender of messages is sending a large amount of spam.  When
   reputation services are deployed, this could damage the originator's

   A larger number of domains are potential victims of SMR than of CMR,
   because the former does not require the ability for the attacker to
   send messages from the victim domain.  However, the capabilities of
   the attacker are lower.  Unless coupled with another attack such as
   body length limit abuse, it isn't possible for the attacker to use
   this, for example, for advertising.

   Many mailing lists, especially those which do not modify the content
   of the message and signed headers and hence do not invalidate the
   signature, engage in a form of SMR.  The only things that distinguish
   this case from undesirable forms of SMR is the intent of the
   replayer, which cannot be determined by the network.

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4.1.6.  Denial-of-Service Attack Against Verifier

   While it takes some compute resources to sign and verify a signature,
   it takes negligible compute resources to generate an invalid
   signature.  An attacker could therefore construct a "make work"
   attack against a verifier, by sending a large number of incorrectly-
   signed messages to a given verifier, perhaps with multiple signatures
   each.  The motivation might be to make it too expensive to verify

   While this attack is feasible, it can be greatly mitigated by the
   manner in which the verifier operates.  For example, it might decide
   to accept only a certain number of signatures per message, limit the
   maximum key size it will accept (to prevent outrageously large
   signatures from causing unneeded work), and verify signatures in a
   particular order.

4.1.7.  Denial-of-Service Attack Against Key Service

   An attacker might also attempt to degrade the availability of an
   originator's key service, in order to cause that originator's
   messages to be unverifiable.  One way to do this might be to quickly
   send a large number of messages with signatures which reference a
   particular key, thereby creating a heavy load on the key server.
   Other types of DoS attacks on the key server or the network
   infrastructure serving it are also possible.

   The best defense against this attack is to provide redundant key
   servers, preferably on geographically-separate parts of the Internet.
   Caching also helps a great deal, by decreasing the load on
   authoritative key servers when there are many simultaneous key
   requests.  The use of a key service protocol which minimizes the
   transactional cost of key lookups is also beneficial.  It is noted
   that the Domain Name System has all these characteristics.

4.1.8.  Canonicalization Abuse

   Canonicalization algorithms represent a tradeoff between the survival
   of the validity of a message signature and the desire not to allow
   the message to be altered inappropriately.  In the past,
   canonicalization algorithms have been proposed which would have
   permitted attackers, in some cases, to alter the meaning of a

   Message signatures which support multiple canonicalization algorithms
   give the signer the ability to decide the relative importance of
   signature survivability and immutability of the signed content.  If
   an unexpected vulnerability appears in a canonicalization algorithm

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   in general use, new algorithms can be deployed, although it will be a
   slow process because the signer can never be sure which algorithm(s)
   the verifier supports.  For this reason, canonicalization algorithms,
   like cryptographic algorithms, should undergo a wide and careful
   review process.

4.1.9.  Body Length Limit Abuse

   A body length limit is an optional indication from the signer how
   much content has been signed.  The verifier can either ignore the
   limit, verify the specified portion of the message, or truncate the
   message to the specified portion and verify it.  The motivation for
   this feature is the behavior of many mailing lists which add a
   trailer, perhaps identifying the list, at the end of messages.

   When body length limits are used, there is the potential for an
   attacker to add content to the message.  It has been shown that this
   content, although at the end, can cover desirable content, especially
   in the case of HTML messages.

   If the body length isn't specified, or if the verifier decides to
   ignore the limit, body length limits are moot.  If the verifier or
   recipient truncates the message at the signed content, there is no
   opportunity for the attacker to add anything.

   If the verifier observes body length limits when present, there is
   the potential that an attacker can make undesired content visible to
   the recipient.  The size of the appended content makes little
   difference, because it can simply be a URL reference pointing to the
   actual content.  Recipients need to use means to, at a minimum,
   identify the unsigned content in the message.

4.1.10.  Use of Revoked Key

   The benefits obtained by caching of key records opens the possibility
   that keys which have been revoked may be used for some period of time
   after their revocation.  The best examples of this occur when a
   holder of a key delegated by the domain administrator must be
   unexpectedly deauthorized from sending mail on behalf of one or more
   addresses in the domain.

   The caching of key records is normally short-lived, on the order of
   hours to days.  In many cases, this threat can be mitigated simply by
   setting a short time-to-live for keys not under the domain
   administrator's direct control (assuming, of course, that control of
   the time-to-live value may be specified for each record, as it can
   with DNS).  In some cases, such as the recovery following a stolen
   private key belonging to one of the domain's MTAs, the possibility of

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   theft and the time required to revoke the key authorization must be
   considered when choosing a TTL.  The chosen TTL must be long enough
   to mitigate denial-of-service attacks and provide reasonable
   transaction efficiency, and no longer.

4.1.11.  Compromise of Key Server

   Rather than by attempting to obtain a private key, an attacker might
   instead focus efforts on the server used to publish public keys for a
   domain.  As in the key theft case, the motive might be to allow the
   attacker to sign messages on behalf of the domain.  This attack
   provides the attacker with the additional capability to remove
   legitimate keys from publication, thereby denying the domain the
   ability for the signatures on its mail to verify correctly.

   The host which is the primary key server, such as a DNS master server
   for the domain, might be compromised.  Another approach might be to
   change the delegation of key servers at the next higher domain level.

   This attack can be mitigated somewhat by independent monitoring to
   audit the key service.  However, it may be difficult to detect the
   publication of additional keys by such means until the selector(s)
   added by the attackers are known.

4.1.12.  Falsification of Key Service Replies

   Replies from the key service may also be spoofed by a suitably
   positioned attacker.  For DNS, one such way to do this is "cache
   poisoning", in which the attacker provides unnecessary (and
   incorrect) additional information in DNS replies, which is cached.

   DNSSEC [RFC4033] is the preferred means of mitigating this threat,
   but the current uptake rate for DNSSEC is slow enough that one would
   not like to create a dependency on its deployment.  Fortunately, the
   vulnerabilities created by this attack are both localized and of
   limited duration, although records with relatively long TTL may be
   created with cache poisoning.

4.1.13.  Publication of Malformed Key Records and/or Signatures

   In this attack, the attacker publishes suitably crafted key records
   or sends mail with intentionally malformed signatures, in an attempt
   to confuse the verifier and perhaps disable verification altogether.
   This attack is really a characteristic of an implementation
   vulnerability, a buffer overflow or lack of bounds checking, for
   example, rather than a vulnerability of the signature mechanism
   itself.  This threat is best mitigated by careful implementation and
   creation of test suites that challenge the verification process.

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4.1.14.  Cryptographic Weaknesses in Signature Generation

   The cryptographic algorithms used to generate mail signatures,
   specifically the hash algorithm and the public-key encryption/
   decryption operations, may over time be subject to mathematical
   techniques that degrade their security.  At this writing, the SHA-1
   hash algorithm is the subject of extensive mathematical analysis
   which has considerably lowered the time required to create two
   messages with the same hash value.  This trend can be expected to

   The message signature system must be designed to support multiple
   signature and hash algorithms, and the signing domain must be able to
   specify which algorithms it uses to sign messages.  The choice of
   algorithms must be published in key records, rather than in the
   signature itself, to ensure that an attacker is not able to create
   signatures using algorithms weaker than the domain wishes to permit.

   Due to the fact that the signer and verifier of email do not, in
   general, communicate directly, negotiation of the algorithms used for
   signing cannot occur.  In other words, a signer has no way of knowing
   which algorithm(s) a verifier supports, nor (due to mail forwarding)
   where the verifier is.  For this reason, it is expected that once
   message signing is widely deployed, algorithm change will occur
   slowly, and legacy algorithms will need to be supported for a
   considerable period.  Algorithms used for message signatures
   therefore need to be secure against expected cryptographic
   developments several years into the future.

4.1.15.  Display Name Abuse

   Message signatures only relate to the address-specification portion
   of an email address, which some MUAs only display (or some recipients
   only pay attention to) the display name portion of the address.  This
   inconsistency leads to an attack where the attacker uses an From
   header field such as:

   From: "Dudley DoRight" <>

   In this example, the attacker,, can sign the
   message and still convince some recipients that the message is from
   Dudley DoRight, who is presumably a trusted individual.  Coupled with
   the use of a throw-away domain or email address, it may be difficult
   to bring the attacker to account for the use of another's display

   This is an attack which must be dealt with in the recipient's MUA.
   One approach is to require that the signer's address specification

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   (and not just the display name) be visible to the recipient.

4.1.16.  Compromised System Within Originator's Network

   In many cases, MTAs may be configured to accept, and sign, messages
   which originate within the topological boundaries of the originator's
   network (i.e., within a firewall).  The increasing use of compromised
   systems to send email presents a problem for such policies, because
   the attacker, using a compromised system as a proxy, can generate
   signed mail at will.

   Several approaches exist for mitigating this attack.  The use of
   authenticated submission, even within the network boundaries, can be
   used to limit the addresses for which the attacker may obtain a
   signature.  It may also help locate the compromised system that is
   the source of the messages more quickly.  Content analysis of
   outbound mail to identify undesirable and malicious content, as well
   as monitoring of the volume of messages being sent by users, may also
   prevent arbitrary messages from being signed and sent.

4.2.  Attacks Against Message Signing Policy

           Summary of postulated attacks against signing policy:

   | Attack Name                                 | Impact | Likelihood |
   | Look-alike domain names                     |  High  |    High    |
   | Internationalized domain name abuse         |  High  |   Medium   |
   | Denial-of-service attack against signing    | Medium |   Medium   |
   | policy                                      |        |            |
   | Use of multiple From addresses              |   Low  |   Medium   |

4.2.1.  Look-Alike Domain Names

   Attackers may attempt to circumvent signing policy of a domain by
   using a domain name which is close to, but not the same as the domain
   with a signing policy.  For instance, "" might be replaced
   by "".  If the message is not to be signed, DKIM does not
   require that the domain used actually exist (although other
   mechanisms may make this a requirement).  Services exist to monitor
   domain registrations to identify potential domain name abuse, but
   naturally do not identify the use of unregistered domain names.

4.2.2.  Internationalized Domain Name Abuse

   Internationalized domain names present a special case of the look-

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   alike domain name attack described above.  Due to similarities in the
   appearance of many Unicode characters, domains (particularly those
   drawing characters from different groups) may be created which are
   visually indistinguishable from other, possibly high-value domains.
   This is discussed in detail in Unicode TR 36 [UTR36].  Surveillance
   of domain registration records may point out some of these, but there
   are many such similarities.  As in the look-alike domain attack
   above, this technique may also be used to circumvent sender signing
   policy of other domains.

4.2.3.  Denial-of-Service Attack Against Signing Policy

   Just as the publication of public keys by a domain can be impacted by
   an attacker, so can the publication of Sender Signing Policy (SSP) by
   a domain.  In the case of SSP, the transmission of large amounts of
   unsigned mail purporting to come from the domain can result in a
   heavy transaction load requesting the SSP record.  More general DoS
   attacks against the servers providing the SSP records are possible as
   well.  This is of particular concern since the default signing policy
   is "we don't sign everything", which means that SSP, in effect, fails

   As with defense against DoS attacks for key servers, the best defense
   against this attack is to provide redundant servers, preferably on
   geographically-separate parts of the Internet.  Caching again helps a
   great deal, and signing policy should rarely change, so TTL values
   can be relatively large.

4.2.4.  Use of Multiple From Addresses

   Although this usage is rare, RFC 2822 [RFC2822] permits the From
   address to contain multiple address specifications.  The lookup of
   Sender Signing Policy is based on the From address, so if addresses
   from multiple domains are in the From address, the question arises
   which signing policy to use.  A rule (say, "use the first address")
   could be specified, but then an attacker could put a throwaway
   address prior to that of a high-value domain.  It is also possible
   for SSP to look at all addresses, and choose the most restrictive
   rule.  This is an area in need of further study.

5.  Derived Requirements

   This section, as yet incomplete, is an attempt to capture a set of
   requirements for DKIM from the above discussion.  These requirements

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      The store for key and SSP records must be capable of utilizing
      multiple geographically-dispersed servers.

      Key and SSP records must be cacheable, either by the verifier
      requesting them or by other infrastructure.

      The cache time-to-live for key records must be specifiable on a
      per-record basis.

      The algorithm(s) used by the signing domain associated with a
      given key must be specified independently of the signature itself.

6.  IANA Considerations

   This document defines no items requiring IANA assignment.

7.  Security Considerations

   This document describes the security threat environment in which
   DomainKeys Identified Mail (DKIM) is expected to provide some
   benefit, and presents a number of attacks relevant to its deployment.

8.  Informative References

              Allman, E., "DomainKeys Identified Mail (DKIM)",
              draft-allman-dkim-base-01 (work in progress),
              October 2005.

              Allman, E., "DKIM Sender Signing Policy",
              draft-allman-dkim-ssp-01 (work in progress), October 2005.

              Crocker, D., "Internet Mail Architecture",
              draft-crocker-email-arch-04 (work in progress),
              March 2005.

              Kucherawy, M., "Message Header for Indicating Sender
              Authentication Status",
              draft-kucherawy-sender-auth-header-02 (work in progress),
              May 2005.

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

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   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [UTR36]    Davis, M. and M. Suignard, "Unicode Security
              Considerations", UTR 36, July 2005.

Appendix A.  Glossary

   Origin address - The address on an email message, typically the RFC
   2822 From:  address, which is associated with the alleged author of
   the message and is displayed by the recipient's MUA as the source of
   the message.

   More definitions to be added.

Appendix B.  Acknowledgements

   The author wishes to thank Phillip Hallam-Baker, Eliot Lear, Tony
   Finch, Dave Crocker, Barry Leiba, Arvel Hathcock, Eric Allman, Jon
   Callas, and Stephen Farrell for valuable suggestions and constructive
   criticism of earlier versions of this draft.

Appendix C.  Edit History

   Changes since -00 draft:

   o  Changed beginning of introduction to make it consistent with -base

   o  Clarified reasons for focus on externally-located bad actors.

   o  Elaborated on reasons for effectiveness of address book attacks.

   o  Described attack time windows with respect to replay attacks.

   o  Added discussion of attacks using look-alike domains.

   o  Added section on key management attacks.

   Changes since -01 draft:

   o  Reorganized description of bad actors.

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   o  Greatly expanded description of attacks against DKIM and SSP.

   o  Added "derived requirements" section.

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Author's Address

   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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