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Network Working Group J. Fenton Request for Comments: 4686 Cisco Systems, Inc. Category: Informational September 2006

  Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).


 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

Fenton Informational [Page 1] RFC 4686 DKIM Threat Analysis September 2006

Table of Contents

 1. Introduction ....................................................3
    1.1. Terminology and Model ......................................3
    1.2. Document Structure .........................................5
 2. The Bad Actors ..................................................6
    2.1. Characteristics ............................................6
    2.2. Capabilities ...............................................6
    2.3. Location ...................................................8
         2.3.1. Externally-Located Bad Actors .......................8
         2.3.2. Within Claimed Originator's Administrative Unit .....8
         2.3.3. Within Recipient's Administrative Unit ..............9
 3. Representative Bad Acts .........................................9
    3.1. Use of Arbitrary Identities ................................9
    3.2. Use of Specific Identities ................................10
         3.2.1. Exploitation of Social Relationships ...............10
         3.2.2. Identity-Related Fraud .............................11
         3.2.3. Reputation Attacks .................................11
         3.2.4. Reflection Attacks .................................11
 4. Attacks on Message Signing .....................................12
    4.1. Attacks against Message Signatures ........................12
         4.1.1. Theft of Private Key for Domain ....................13
         4.1.2. Theft of Delegated Private Key .....................13
         4.1.3. Private Key Recovery via Side Channel Attack .......14
         4.1.4. Chosen Message Replay ..............................14
         4.1.5. Signed Message Replay ..............................16
         4.1.6. Denial-of-Service Attack against Verifier ..........16
         4.1.7. Denial-of-Service Attack against Key Service .......17
         4.1.8. Canonicalization Abuse .............................17
         4.1.9. Body Length Limit Abuse ............................17
         4.1.10. Use of Revoked Key ................................18
         4.1.11. Compromise of Key Server ..........................18
         4.1.12. Falsification of Key Service Replies ..............19
         4.1.13. Publication of Malformed Key Records
                 and/or Signatures .................................19
         4.1.14. Cryptographic Weaknesses in Signature Generation ..20
         4.1.15. Display Name Abuse ................................21
         4.1.16. Compromised System within Originator's Network ....21
         4.1.17. Verification Probe Attack .........................21
         4.1.18. Key Publication by Higher-Level Domain ............22
    4.2. Attacks against Message Signing Practices .................23
         4.2.1. Look-Alike Domain Names ............................23
         4.2.2. Internationalized Domain Name Abuse ................23
         4.2.3. Denial-of-Service Attack against Signing
                Practices ..........................................24
         4.2.4. Use of Multiple From Addresses .....................24
         4.2.5. Abuse of Third-Party Signatures ....................24
         4.2.6. Falsification of Sender Signing Practices Replies ..25

Fenton Informational [Page 2] RFC 4686 DKIM Threat Analysis September 2006

    4.3. Other Attacks .............................................25
         4.3.1. Packet Amplification Attacks via DNS ...............25
 5. Derived Requirements ...........................................26
 6. Security Considerations ........................................26
 7. Informative References .........................................27
 Appendix A. Acknowledgements ......................................28

1. Introduction

 The DomainKeys Identified Mail (DKIM) protocol is being specified by
 the IETF DKIM Working Group.  The DKIM protocol defines a 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.  This document
 addresses threats relative to two works in progress by the DKIM
 Working Group, the DKIM signature specification [DKIM-BASE] and DKIM
 Sender Signing Practices [DKIM-SSP].
 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 the IETF DKIM Working Group
 effort.  By applying a signature, a good player enables a verifier 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

 An administrative unit (AU) is the portion of the path of an email
 message 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.
 The origin address is 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 Mail User Agent
 (MUA) as the source of the message.

Fenton Informational [Page 3] RFC 4686 DKIM Threat Analysis September 2006

 The following diagram illustrates a typical usage flowchart for DKIM:
                    |       SIGNATURE CREATION        |
                    |  (Originating or Relaying AU)   |
                    |                                 |
                    |   Sign (Message, Domain, Key)   |
                    |                                 |
                                     | - Message (Domain, Key)
   +-----------+    |     SIGNATURE VERIFICATION      |
   |           |    |  (Relaying or Delivering AU)    |
   |    KEY    |    |                                 |
   |   QUERY   +--->|  Verify (Message, Domain, Key)  |
   |           |    |                                 |
   +-----------+    +----------------+----------------+
                                     |  - Verified Domain
   +-----------+                     V  - [Report]
   |  SENDER   |    +----------------+----------------+
   |  SIGNING  |    |                                 |
   | PRACTICES +--->|        SIGNER EVALUATION        |
   |   QUERY   |    |                                 |
   |           |    +---------------------------------+
 DKIM operates entirely on the content (body and selected header
 fields) of the message, as defined in RFC 2822 [RFC2822].  The
 transmission of messages via SMTP, defined in RFC 2821 [RFC2821], and
 such elements as the envelope-from and envelope-to addresses and the
 HELO domain are not relevant to DKIM verification.  This is an
 intentional decision made to allow verification of messages via
 protocols other than SMTP, such as POP [RFC1939] and IMAP [RFC3501]
 which an MUA acting as a verifier might use.
 The Sender Signing Practices Query referred to in the diagram above
 is a means by which the verifier can query the alleged author's
 domain to determine their practices for signing messages, which in
 turn may influence their evaluation of the message.  If, for example,
 a message arrives without any valid signatures, and the alleged
 author's domain advertises that they sign all messages, the verifier
 might handle that message differently than if a signature was not
 necessarily to be expected.

Fenton Informational [Page 4] RFC 4686 DKIM Threat Analysis September 2006

1.2. Document Structure

 The remainder of this document describes the problems that DKIM might
 be expected to address, and the extent to which it may be successful
 in so doing.  These are described in terms of the potential bad
 actors, their capabilities and location in the network, and the bad
 acts that they might wish to commit.
 This is followed by a description of postulated attacks on DKIM
 message signing and on the use of Sender Signing Practices to assist
 in the treatment of unsigned messages.  A list of derived
 requirements is also presented, which is intended to guide the DKIM
 design and review process.
 The sections dealing with attacks on DKIM each begin with a table
 summarizing the postulated attacks in each category along with their
 expected impact and likelihood.  The following definitions were used
 as rough criteria for scoring the attacks:
    High:  Affects the verification of messages from an entire domain
       or multiple domains
    Medium:  Affects the verification of messages from specific users,
       Mail Transfer Agents (MTAs), and/or bounded time periods
    Low:  Affects the verification of isolated individual messages
    High:  All email users should expect this attack on a frequent
    Medium:  Email users 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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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, that
 the recipient does not want to receive.  These tools typically allow
 one to falsify the origin address of messages, and may, in the
 future, be capable of generating message signatures as well.
 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 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, including DNS cache-poisoning attacks
 and IP routing attacks.

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
     associated with the domain
 and the ability to do at least some of the following:
 1.  Submit messages to MTAs and Message Submission Agents (MSAs) at
     multiple locations in the Internet

Fenton Informational [Page 6] RFC 4686 DKIM Threat Analysis September 2006

 2.  Construct arbitrary message header fields, including those
     claiming to be mailing lists, resenders, and other mail agents
 3.  Sign messages on behalf of domains under their control
 4.  Generate substantial numbers of either unsigned or apparently-
     signed messages that might be used to attempt a denial-of-service
 5.  Resend messages that may have been previously signed by the
 6.  Transmit messages using any envelope information desired
 7.  Act as an authorized submitter for messages from a compromised
 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 falsify advertisements of DNS-based keys or signing
 3.  Access to significant computing resources, for example, through
     the conscription of worm-infected "zombie" computers.  This could
     allow the bad actor to perform various types of brute-force
 4.  Ability to eavesdrop on existing traffic, perhaps from a wireless
 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 author and
 recipient, if that attack is useful.

Fenton Informational [Page 7] RFC 4686 DKIM Threat Analysis September 2006

2.3. Location

 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 from multiple locations (a
 "distributed bad actor").
 It should also be noted that with the use of "zombies" and other
 proxies, externally-located bad actors may gain some of the
 capabilities of being located within the claimed originator's or
 recipient's administrative unit.  This emphasizes the importance of
 appropriate security measures, such as authenticated submission of
 messages, even within administrative units.

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
 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 that 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 that
 legitimately send or resend 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 Message Submission Agents
 that are configured to authenticate the author.

Fenton Informational [Page 8] RFC 4686 DKIM Threat Analysis September 2006

 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 that 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 spoof a header field
 indicating the results of verification.  This header field would
 normally be added by the verifier, which would also detect spoofed
 header fields on messages it was attempting to verify.  This could be
 used to falsely indicate that the message was authenticated
 As in the originator case, these bad actors can be dealt with by
 controlling the submission of messages within the administrative
 unit.  Since DKIM permits verification to occur anywhere within the
 recipient's administrative unit, these threats can also be minimized
 by moving verification closer to the recipient, such as at the Mail
 Delivery Agent (MDA), or on the recipient's MUA itself.

3. Representative Bad Acts

 One of the most fundamental bad acts being attempted is the delivery
 of messages that are not intended to have been sent 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 that aim to
 obscure the identity of the actual author.  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
 Particularly when coupled with sender signing practices that indicate
 the domain owner signs all messages, DKIM can be effective in
 mitigating against the abuse of addresses not controlled by bad

Fenton Informational [Page 9] RFC 4686 DKIM Threat Analysis September 2006

 actors.  DKIM is not effective against the use of addresses
 controlled by bad actors.  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
 DKIM does not attempt to identify the signer individually, but rather
 identifies the domain that they control.  Accreditation and
 reputation systems and locally-maintained whitelists and blacklists
 can be used to enhance the accountability of DKIM-verified addresses
 and/or the likelihood that signed messages are desirable.

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 from  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 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.
 DKIM is effective against the use of specific identities only when
 there is an expectation that such messages will, in fact, be signed.
 The primary means for establishing this is the use of Sender Signing
 Practices (SSP), which will be specified by the IETF DKIM Working

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
 that mail themselves to addresses in the infected host's address
 book.  In this case, however, the author's address may not be
 falsified, so DKIM would not be effective in defending against this

Fenton Informational [Page 10] RFC 4686 DKIM Threat Analysis September 2006

 It is also possible for address books to be harvested and used by an
 attacker to post messages from elsewhere.  DKIM could 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 author.
 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.
 When signatures are verified by or for the recipient, DKIM is
 effective in defending against the fraudulent use of origin addresses
 on signed messages.  When the published sender signing practices of
 the origin address indicate that all messages from that address
 should be signed, DKIM further mitigates against the attempted
 fraudulent use of the origin address on unsigned messages.

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

3.2.4. Reflection Attacks

 A commonly-used tactic by some bad actors is the indirect
 transmission of messages by intentionally mis-addressing the message
 and causing it to be "bounced", or sent to the return address (RFC
 2821 envelope-from address) on the message.  In this case, the
 specific identity asserted in the email is that of the actual target
 of the message, to whom the message is "returned".
 DKIM does not, in general, attempt to validate the RFC2821.mailfrom
 return address on messages, either directly (noting that the mailfrom

Fenton Informational [Page 11] RFC 4686 DKIM Threat Analysis September 2006

 address is an element of the SMTP protocol, and not the message
 content on which DKIM operates), or via the optional Return-Path
 header field.  Furthermore, as is noted in Section 4.4 of RFC 2821
 [RFC2821], it is common and useful practice for a message's return
 path not to correspond to the origin address.  For these reasons,
 DKIM is not effective against reflection attacks.

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 (referred to
 here as Sender Signing Practices or SSP) can be expected to be the
 target of attacks.

4.1. Attacks against Message Signatures

 The following is a summary of postulated attacks against DKIM
 | Attack Name                                 | Impact | Likelihood |
 | Theft of private key for domain             |  High  |     Low    |
 | Theft of delegated private key              | Medium |   Medium   |
 | Private key recovery via side channel 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 service|  High  |   Medium   |
 | 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      |  High  |   Medium   |
 |  network                                    |        |            |
 | Verification probe attack                   | Medium |   Medium   |
 | Key publication by higher-level domain      |  High  |     Low    |

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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, such as 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 that are valid for all addresses in a domain typically reside in
 MTAs that 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.  Other mechanisms, such as the use of
 dedicated hardware devices that contain the private key and perform
 the cryptographic signature operation, would be very effective in
 denying export of the private key to those without physical access to
 the device.  Such devices would almost certainly make the theft of
 the key visible, so that appropriate action (revocation of the
 corresponding public key) can be taken should that happen.

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.
 A related threat is the exploitation of weaknesses in the delegation
 process itself.  This threat can be mitigated through the use of
 customary precautions against the theft of private keys and the
 falsification of public keys in transit.  For example, the exposure
 to theft can be minimized if the delegate generates the keypair to be
 used, and sends the public key to the domain owner.  The exposure to
 falsification (substitution of a different public key) can be reduced
 if this transmission is signed by the delegate and verified by the
 domain owner.

Fenton Informational [Page 13] RFC 4686 DKIM Threat Analysis September 2006

4.1.3. Private Key Recovery via Side Channel Attack

 All popular digital signature algorithms are subject to a variety of
 side channel attacks.  The most well-known of these are timing
 channels [Kocher96], power analysis [Kocher99], and cache timing
 analysis [Bernstein04].  Most of these attacks require either
 physical access to the machine or the ability to run processes
 directly on the target machine.  Defending against these attacks is
 out of scope for DKIM.
 However, remote timing analysis (at least on local area networks) is
 known to be feasible [Boneh03], particularly in server-type platforms
 where the attacker can inject traffic that will immediately be
 subject to the cryptographic operation in question.  With enough
 samples, these techniques can be used to extract private keys even in
 the face of modest amounts of noise in the timing measurements.
 The three commonly proposed countermeasures against timing analysis
 1.  Make the operation run in constant time.  This turns out in
     practice to be rather difficult.
 2.  Make the time independent of the input data.  This can be
     difficult, but see [Boneh03] for more details.
 3.  Use blinding.  This is generally considered the best current
     practice countermeasure, and while not proved generally secure is
     a countermeasure against known timing attacks.  It adds about
     2-10% to the cost of the operation and is implemented in many
     common cryptographic libraries.  Unfortunately, Digital Signature
     Algorithm (DSA) and Elliptic Curve DSA (ECDSA) do not have
     standard methods though some defenses may exist.
 Note that adding random delays to the operation is only a partial
 countermeasure.  Because the noise is generally uniformly
 distributed, a large enough number of samples can be used to average
 it out and extract an accurate timing signal.

4.1.4. Chosen Message Replay

 Chosen message replay 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
 himself/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.

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 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 solving 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
 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
 A variation on this attack involves the attacker sending a message
 with the intent of obtaining a signed reply containing their original
 message.  The reply might come from an innocent user or might be an
 automatic response such as a "user unknown" bounce message.  In some
 cases, this signed reply message might accomplish the attacker's
 objectives if replayed.  This variation on chosen message replay can
 be mitigated by limiting the extent to which the original content is
 quoted in automatic replies, and by the use of complementary
 mechanisms such as egress content filtering.
 Revocation of the signature or the associated key 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 is likely to result in a great deal of collateral
 damage (in the form of signature verification failures) should a key
 be revoked suddenly.
 Signature revocation addresses the collateral damage problem at the
 expense of significant scaling requirements.  At the extreme,
 verifiers could be required to check for revocation of each signature
 verified, which would result in very significant transaction rates.
 An alternative, "revocation identifiers", has been proposed, which
 would permit revocation on an intermediate level of granularity,
 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 a revocation database.

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4.1.5. Signed Message Replay

 Signed message replay refers to the retransmission of already-signed
 messages to additional recipients beyond those intended by the author
 or the original poster of the message.  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 author's reputation or that of the author's domain.
 A larger number of domains are potential victims of signed message
 replay than chosen message replay 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 that do not modify the content
 of the message and signed header fields and hence do not invalidate
 the signature, engage in a form of signed message replay.  The use of
 body length limits and other mechanisms to enhance the survivability
 of messages effectively enhances the ability to do so.  The only
 things that distinguish this case from undesirable forms of signed
 message replay is the intent of the replayer, which cannot be
 determined by the network.

4.1.6. Denial-of-Service Attack against Verifier

 While it takes some computing resources to sign and verify a
 signature, it takes negligible computing 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 messages.
 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.  The verifier could also maintain state
 representing the current signature verification failure rate and
 adopt a defensive posture when attacks may be under way.

Fenton Informational [Page 16] RFC 4686 DKIM Threat Analysis September 2006

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 that 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 that 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 that would have
 permitted attackers, in some cases, to alter the meaning of a
 Message signatures that 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
 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 of 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 that add a
 trailer, perhaps identifying the list, at the end of messages.

Fenton Informational [Page 17] RFC 4686 DKIM Threat Analysis September 2006

 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.  Receiving MUAs can mitigate this threat by, at a
 minimum, identifying 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 that 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 (TTL) for keys not under the domain
 administrator's direct control (assuming, of course, that control of
 the TTL 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 theft and
 the effort 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.

Fenton Informational [Page 18] RFC 4686 DKIM Threat Analysis September 2006

 In order to limit the ability to sign a message to entities
 authorized by the owner of a signing domain, a relationship must be
 established between the signing address and the location from which a
 public key is obtained to verify the message.  DKIM does this by
 publishing either the public key or a reference to it within the DNS
 hierarchy of the signing domain.  The verifier derives the location
 from which to retrieve the public key from the signing address or
 domain.  The security of the verification process is therefore
 dependent on the security of the DNS hierarchy for the signing
 An attacker might successfully compromise the host that is the
 primary key server for the signing domain, such as the domain's DNS
 master server.  Another approach might be to compromise a higher-
 level DNS server and change the delegation of name servers for the
 signing domain to others under the control of the attacker.
 This attack can be mitigated somewhat by independent monitoring to
 audit the key service.  Such auditing of the key service should occur
 by means of zone transfers rather than queries to the zone's primary
 server, so that the addition of records to the zone can be detected.

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.  In the case of a
 cache poisoning attack, the vulnerabilities created by this attack
 are both localized and of limited duration, although records with
 relatively long TTL may persist beyond the attack itself.

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.

Fenton Informational [Page 19] RFC 4686 DKIM Threat Analysis September 2006

4.1.14. Cryptographic Weaknesses in Signature Generation

 The cryptographic algorithms used to generate mail signatures,
 specifically the hash algorithm and digital signature generation and
 verification 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 that
 has considerably lowered the time required to create two messages
 with the same hash value.  This trend can be expected to continue.
 One consequence of a weakness in the hash algorithm is a hash
 collision attack.  Hash collision attacks in message signing systems
 involve the same person creating two different messages that have the
 same hash value, where only one of the two messages would normally be
 signed.  The attack is based on the second message inheriting the
 signature of the first.  For DKIM, this means that a sender might
 create a "good" message and a "bad" message, where some filter at the
 signing party's site would sign the good message but not the bad
 message.  The attacker gets the good message signed, and then
 incorporates that signature in the bad message.  This scenario is not
 common, but could happen, for example, at a site that does content
 analysis on messages before signing them.
 Current known attacks against SHA-1 make this attack extremely
 difficult to mount, but as attacks improve and computing power
 becomes more readily available, such an attack could become
 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, and not only in the
 signature itself, to ensure that an attacker is not able to create
 signatures using algorithms weaker than the domain wishes to permit.
 Because 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 or (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.

Fenton Informational [Page 20] RFC 4686 DKIM Threat Analysis September 2006

4.1.15. Display Name Abuse

 Message signatures only relate to the address-specification portion
 of an email address, while 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 a 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 hold the attacker accountable for using another's display name.
 This is an attack that must be dealt with in the recipient's MUA.
 One approach is to require that the signer's address specification
 (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
 that 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.

4.1.17. Verification Probe Attack

 As noted above, bad actors (attackers) can sign messages on behalf of
 domains they control.  Since they may also control the key service
 (e.g., the authoritative DNS name servers for the _domainkey
 subdomain), it is possible for them to observe public key lookups,
 and their source, when messages are verified.

Fenton Informational [Page 21] RFC 4686 DKIM Threat Analysis September 2006

 One such attack, which we will refer to as a "verification probe", is
 to send a message with a DKIM signature to each of many addresses in
 a mailing list.  The messages need not contain valid signatures, and
 each instance of the message would typically use a different
 selector.  The attacker could then monitor key service requests and
 determine which selectors had been accessed, and correspondingly
 which addressees used DKIM verification.  This could be used to
 target future mailings at recipients who do not use DKIM
 verification, on the premise that these addressees are more likely to
 act on the message contents.

4.1.18. Key Publication by Higher-Level Domain

 In order to support the ability of a domain to sign for subdomains
 under its administrative control, DKIM permits the domain of a
 signature (d= tag) to be any higher-level domain than the signature's
 address (i= or equivalent).  However, since there is no mechanism for
 determining common administrative control of a subdomain, it is
 possible for a parent to publish keys that are valid for any domain
 below them in the DNS hierarchy.  In other words, mail from the
 domain could be signed using keys published by,, or us, in addition to the domain itself.
 Operation of a domain always requires a trust relationship with
 higher-level domains.  Higher-level domains already have ultimate
 power over their subdomains:  they could change the name server
 delegation for the domain or disenfranchise it entirely.  So it is
 unlikely that a higher-level domain would intentionally compromise a
 subdomain in this manner.  However, if higher-level domains send mail
 on their own behalf, they may wish to publish keys at their own
 level.  Higher-level domains must employ special care in the
 delegation of keys they publish to ensure that any of their
 subdomains are not compromised by misuse of such keys.

Fenton Informational [Page 22] RFC 4686 DKIM Threat Analysis September 2006

4.2. Attacks against Message Signing Practices

 The following is a summary of postulated attacks against signing
 | Attack Name                                 | Impact | Likelihood |
 | Look-alike domain names                     |  High  |    High    |
 | Internationalized domain name abuse         |  High  |    High    |
 | Denial-of-service attack against signing    | Medium |   Medium   |
 | practices                                   |        |            |
 | Use of multiple From addresses              |   Low  |   Medium   |
 | Abuse of third-party signatures             | Medium |    High    |
 | Falsification of Sender Signing Practices   | Medium |   Medium   |
 | replies                                     |        |            |

4.2.1. Look-Alike Domain Names

 Attackers may attempt to circumvent signing practices of a domain by
 using a domain name that is close to, but not the same as, the domain
 with signing practices.  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.
 A related attack is possible when the MUA does not render the domain
 name in an easily recognizable format.  If, for example, a Chinese
 domain name is rendered in "punycode" as, the
 unfamiliarity of that representation may enable other domains to more
 easily be mis-recognized as the expected domain.
 Users that are unfamiliar with internet naming conventions may also
 mis-recognize certain names.  For example, users may confuse with, the latter of which may
 have been registered by an attacker.

4.2.2. Internationalized Domain Name Abuse

 Internationalized domain names present a special case of the look-
 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 that are
 visually indistinguishable from other, possibly high-value domains.
 This is discussed in detail in Unicode Technical Report 36 [UTR36].

Fenton Informational [Page 23] RFC 4686 DKIM Threat Analysis September 2006

 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 practices of other domains.

4.2.3. Denial-of-Service Attack against Signing Practices

 Just as the publication of public keys by a domain can be impacted by
 an attacker, so can the publication of Sender Signing Practices (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
 practices are "we don't sign everything", which means that SSP
 failures result in the verifier's failure to heed more stringent
 signing practices.
 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 practices should rarely change, so TTL values
 can be relatively large.

4.2.4. Use of Multiple From Addresses

 Although this usage is never seen by most recipients, RFC 2822
 [RFC2822] permits the From address to contain multiple address
 specifications.  The lookup of Sender Signing Practices is based on
 the From address, so if addresses from multiple domains are in the
 From address, the question arises which signing practices 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

4.2.5. Abuse of Third-Party Signatures

 In a number of situations, including mailing lists, event
 invitations, and "send this article to a friend" services, the DKIM
 signature on a message may not come from the originating address
 domain.  For this reason, "third-party" signatures, those attached by
 the mailing list, invitation service, or news service, frequently
 need to be regarded as having some validity.  Since this effectively
 makes it possible for any domain to sign any message, a sending

Fenton Informational [Page 24] RFC 4686 DKIM Threat Analysis September 2006

 domain may publish sender signing practices stating that it does not
 use such services, and accordingly that verifiers should view such
 signatures with suspicion.
 However, the restrictions placed on a domain by publishing "no
 third-party" signing practices effectively disallows many existing
 uses of email.  For the majority of domains that are unable to adopt
 these practices, an attacker may with some degree of success sign
 messages purporting to come from the domain.  For this reason,
 accreditation and reputation services, as well as locally-maintained
 whitelists and blacklists, will need to play a significant role in
 evaluating messages that have been signed by third parties.

4.2.6. Falsification of Sender Signing Practices Replies

 In an analogous manner to the falsification of key service replies
 described in Section 4.1.12, replies to sender signing practices
 queries can also be falsified.  One such attack would be to weaken
 the signing practices to make unsigned messages allegedly from a
 given domain appear less suspicious.  Another attack on a victim
 domain that is not signing messages could attempt to make the
 domain's messages look more suspicious, in order to interfere with
 the victim's ability to send mail.
 As with the falsification of key service replies, DNSSEC is the
 preferred means of mitigating this attack.  Even in the absence of
 DNSSEC, vulnerabilities due to cache poisoning are localized.

4.3. Other Attacks

 This section describes attacks against other Internet infrastructure
 that are enabled by deployment of DKIM.  A summary of these
 postulated attacks is as follows:
    | Attack Name                          | Impact | Likelihood |
    | Packet amplification attacks via DNS |   N/A  |   Medium   |

4.3.1. Packet Amplification Attacks via DNS

 Recently, there has been an increase in denial-of-service attacks
 involving the transmission of spoofed UDP DNS requests to openly-
 accessible domain name servers [US-CERT-DNS].  To the extent that the
 response from the name server is larger than the request, the name
 server functions as an amplifier for such an attack.

Fenton Informational [Page 25] RFC 4686 DKIM Threat Analysis September 2006

 DKIM contributes indirectly to this attack by requiring the
 publication of fairly large DNS records for distributing public keys.
 The names of these records are also well known, since the record
 names can be determined by examining properly-signed messages.  This
 attack does not have an impact on DKIM itself.  DKIM, however, is not
 the only application that uses large DNS records, and a DNS-based
 solution to this problem will likely be required.

5. Derived Requirements

 This section lists requirements for DKIM not explicitly stated in the
 above discussion.  These requirements include:
    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 signature algorithm identifier in the message must be one of
    the ones listed in a key record for the identified domain.
    The algorithm(s) used for message signatures need to be secure
    against expected cryptographic developments several years in the

6. Security Considerations

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

Fenton Informational [Page 26] RFC 4686 DKIM Threat Analysis September 2006

7. Informative References

 [Bernstein04]  Bernstein, D., "Cache Timing Attacks on AES",
                April 2004.
 [Boneh03]      Boneh, D. and D. Brumley, "Remote Timing Attacks are
                Practical", Proc. 12th USENIX Security Symposium,
 [DKIM-BASE]    Allman, E., "DomainKeys Identified Mail (DKIM)
                Signatures", Work in Progress, August 2006.
 [DKIM-SSP]     Allman, E., "DKIM Sender Signing Practices", Work in
                Progress, August 2006.
 [Kocher96]     Kocher, P., "Timing Attacks on Implementations of
                Diffie-Hellman, RSA, and other Cryptosystems",
                Advances in Cryptology, pages 104-113, 1996.
 [Kocher99]     Kocher, P., Joffe, J., and B. Yun, "Differential Power
                Analysis: Leaking Secrets", Crypto '99, pages 388-397,
 [RFC1939]      Myers, J. and M. Rose, "Post Office Protocol - Version
                3", STD 53, RFC 1939, May 1996.
 [RFC2821]      Klensin, J., "Simple Mail Transfer Protocol",
                RFC 2821, April 2001.
 [RFC2822]      Resnick, P., "Internet Message Format", RFC 2822,
                April 2001.
                VERSION 4rev1", RFC 3501, March 2003.
 [RFC4033]      Arends, R., Austein, R., Larson, M., Massey, D., and
                S. Rose, "DNS Security Introduction and Requirements",
                RFC 4033, March 2005.
 [US-CERT-DNS]  US-CERT, "The Continuing Denial of Service Threat
                Posed by DNS Recursion".
 [UTR36]        Davis, M. and M. Suignard, "Unicode Technical Report
                #36: Unicode Security Considerations", UTR 36,
                July 2005.

Fenton Informational [Page 27] RFC 4686 DKIM Threat Analysis September 2006

Appendix A. Acknowledgements

 The author wishes to thank Phillip Hallam-Baker, Eliot Lear, Tony
 Finch, Dave Crocker, Barry Leiba, Arvel Hathcock, Eric Allman, Jon
 Callas, Stephen Farrell, Doug Otis, Frank Ellermann, Eric Rescorla,
 Paul Hoffman, Hector Santos, and numerous others on the ietf-dkim
 mailing list for valuable suggestions and constructive criticism of
 earlier versions of this document.

Author's Address

 Jim Fenton
 Cisco Systems, Inc.
 MS SJ-9/2
 170 W. Tasman Drive
 San Jose, CA  95134-1706
 Phone:  +1 408 526 5914

Fenton Informational [Page 28] RFC 4686 DKIM Threat Analysis September 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Fenton Informational [Page 29]

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