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Copyright © The Internet Society (2006).
DomainKeys Identified Mail [DKIM] provides a cryptographic mechanism for domains to assert responsibility for the messages they sign. A related mechanism would allow an administrator ot publish various statements about their email accountability practices. This draft defines the requirement for this additional mechanism.
4. Use Scenarios
4.1. Scenario 1: Bigbank.example.com
4.2. Scenario 2: DKIM Signing Complete
4.3. Scenario 3: Outsourced First Party Signing
5.1. Discovery Requirements
5.2. Transport requirements
5.3. Practice and Expectation Requirements
5.4. Extensibility and Forward Compatibilty Requirements
6. Security Requirements
7. IANA Considerations
8. Security Considerations
10.1. Normative References
10.2. Informative References
§ Author's Address
§ Intellectual Property and Copyright Statements
The purpose of this draft is get out into the open a range of issues related to the perceived need for a signing practices information service primarily focused on DKIM. This document is intended to document well-agreed upon problems and requirements, in addition to less well-agreed upon requirements in an attempt to capture the issue as well as generalize the requirement as much as possible. These latter requirements will be noted as "[PROVISIONAL]" to indicate that there is not yet solid consensus, or that the problem is not well understood. A winnowing process is envisioned where the more difficult and/or speculative problems/requirement will be eliminated unless concrete problems with proven constituencies can be demonstrated, along with reasonable plausibility that they do not contradict more well agreed upon requirements.
The DomainKeys Identified Mail working group is chartered to create a base signing mechanism for email. This work is contained in draft-ietf-dkim-base-04.txt. In addition there are two other documents draft-ietf-dkim-overview-00.txt and draft-ietf-dkim-threats-03.txt which give an overview and a threat analysis of the chartered work. This draft reflects the requirements for the last part of the chartered work to define a protocol to publish DKIM signing practices.
While the base signing document defines a mechanism for signing and verifying DKIM signatures, there has been a great deal of interest in a signing practices protocol. The most pressing case seems to be the bid down attack inherent with almost all systems that allow optional authentication: how does a receiver know whether or not it should expect a message to contain authentication information? For email this is an especially difficult problem since generally there is no a priori knowledge of other domains so the safe assumption is the lowest common denominator which is no authentication at all. Thus a protocol needs to be developed which can allow a DKIM message verifier to determine the DKIM posture of the domain for messages it receives which arrive without a valid DKIM signature.
This draft is organized into two main sections: a Usage Scenario section which attempts to describe some common usage scenarios that DKIM is likely to be deployed in and the problems that are not solved by DKIM alone. The second is the Requirements that arise because of those usage scenarios, in addition more basic protocol requirements.
The email world is a diverse world with many deployment scenarios. This
section tries to outline some usage scenarios that it is expected
that DKIM signing/verifying will take place in, and how a new protocol
might be helpful to clarify the relevance of DKIM signed mail.
There seems to be a class of mail -- mostly transactional mail from high value domains -- that are the target of phishing attacks. In particular, the phishing scams forge the RFC2822.From address in addition to spoofing much of the content to trick unsuspecting users into revealing sensitive information. Domain holders sending this kind of mail would like the ability to guarantee that their mail is always from them. The first step is, of course, to use DKIM-base to sign all of their outgoing mail so that a receiver can make a positive determination that the mail is from the domain holder in question.
The problem with this scenario is that a receiver in the general case doesn't know what the practices are for a given domain, or what their expectations are for unsigned mail. An information service which allowed a receiver to query for those practices and expectations could be useful to close the gap where an attacker merely sends unsigned mail to exploit a bid down attack. It is assumed that receivers would use this information to treat such questionable mail with prejudice.
Note that for the foreseeable future, DKIM signature breakage for unrestricted use patterns (ie with users and especially where users are members of mailing lists) will likely suffer occassional damage in transit. While probably not a large percentage of total traffic, the kind (quality) of breakage may be significant for certain usage patterns. As such, this scenario defines a more limited situation where the risk of a legitimate piece of mail being mislabeled as unsigned outweights the risk of illegitimate mail being delivered in the eyes of the sender.
After auditing their outgoing mail and deploying DKIM signing for all of their legitimate outgoing mail, a domain could be said to be DKIM signing complete. That is, the domain has to the best of its ability insured that all mail legitimately purporting to have come from that domain contained a valid DKIM signature. Given the likelihood of signature damage in the current mail infrastructure as noted above, a domain can fit the DKIM signing complete scenario without wanting to take the risks associated with the more narrow scope of use in the previous scenario. A receiver, on the other hand, may be able to take advantage of the knowledge the domain's practice of signing all mail in order to use it to bias filters against the unexpected arrival of a piece of unsigned or damaged in transit mail.
Many domains do not run their own mail infrastructure, or may outsource parts of it to third parties. It is desirable for a domain holder to have the ability to be able to enumerate a list of domains that should be treated as equivalent to a first party signature from the domain holder itself. One obvious use scenario is a domain holder for a small domain that needs to have the ability for their outgoing ISP to sign all of their mail on behalf of the domain holder. Other use scenarios include outsourced bulk mail for marketing campaigns, as well as outsourcing various business functions such as insurance benefits, etc.
That said, DKIM uses DNS to store selectors. Thus there is always the ability for a domain holder to delegate all or parts of the _domainkey subdomain to a third party of the domain holder's choosing. That is, the domain holder can always set a NS record for _domainkey.example.com to, say, an email provider who manages that namespace. There is also the ability for the domain holder to partition its namespace into subdomains to further constrain how third parties. For example, a domain holder could delegate only _domainkey.benefits.example.com to a third party to further constrain the third party to only be able to sign messages on behalf of the benefits subdomain.
There have been concerns expressed about how well this would scale when the third party is, say, a large ISP that signs for thousands of domains. There has been concern about how well this would work for multiple delegations. Lastly, using NS delegations requires that the signer actively cooperate with the domain for whom it is signing. That is, it requires that the signer actively manage the _domainkey delegation for the domain holder. A domain holder would not, for example, be able to make a statement that ISP.com signing on its behalf was acceptable without ISP.com's cooperation. This by extension also applies to other third parties that a domain might like to effectively "whitelist" such as mailing lists that re-sign mail that the domain holder holds in esteem.
This section defines the requirements for The Protocol. As with most requirements drafts, these requirements define the MINIMUM requirements that a candidate protocol must provide. It should also be noted that The Protocol must fulfill all of the requirements.
[Informative Note: it's not clear to the author that all of the provisional requirements can fulfill the harder requirements. If this is determined to be true, the provisional requirement should either be dropped or the harder requirements revised]
[Informative Note: this, for all intents and purposes is a prohibition on anything that might produce loops; also though "deterministic" doesn't specify how many exchanges, the expectation is "few".]
In this section, a Practice is defined as a true statement according to the domain holder of its intended externally viewable behavior. An Expectation combines with a Practice to convey what the domain holder considers the likely outcome of the survivability of the Practice at a receiver. For example, a Practice that X is true when it leaves the domain, and an Expectation that it will|will-not|may|may-not remain true for some/all receivers.
[Informative Note: the DKIM Signing Complete Practice seems to be a pre-requisite for this Expectation]
[INFORMATIVE NOTE: there seems to be widespread consensus that a "neutral" or "I sign some mail" practice is useless to receivers. However, a null practice may help to cut short the policy lookup mechanism if it's published, and if that the case it seems worthwhile. Also, a null policy may have some forensic utility, such as gaging the number of domains considering/using DKIM for example.]
[INFORMATIVE NOTE: this is essentially saying that the protocol doesn't have to concern itself with being a blacklist repository.]
[INFORMATIVE NOTE: this is to counter a bid down attack; some comments indicated that this need only be done if the algorithm was considered suspect by the receiver; I'm not sure that I've captured that nuance correctly]
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an RFC.
This draft defines requirements for a new protocol and the security related requirements are defined above. There is an expectation that The Protocol will not always be required to have source authentication of the practices information which is noteworthy.
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