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Copyright © The Internet Society (2006).
DomainKeys Identified Mail [DKIM] (Allman, E., “DomainKeys Identified Mail (DKIM) Signatures,” July 2006.) [I‑D.ietf‑dkim‑base] provides a cryptographic mechanism for domains to assert responsibility for the messages they sign. A related mechanism would allow an administrator to publish various statements about their email accountability practices. This draft defines the requirement for this additional mechanism.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
4. Use Scenarios
4.1. Scenario 1: Bigbank.example.com
4.2. Scenario 2: DKIM Signing Complete State
4.3. Scenario 3: Outsourced First Party Signing
4.4. Scenario 4: Resent Original Mail
4.5. Scenario 5: Incremental Deployment of Signing
5.1. Discovery Requirements
5.2. Transport requirements
5.3. Practice and Expectation Requirements
5.3.1. Negative Commentary
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 [I‑D.ietf‑dkim‑base] (Allman, E., “DomainKeys Identified Mail (DKIM) Signatures,” July 2006.). In addition there are two other documents [I‑D.ietf‑dkim‑overview] (Hansen, T., “DomainKeys Identified Mail (DKIM) Service Overview,” June 2006.) and [I‑D.ietf‑dkim‑threats] (Fenton, J., “Analysis of Threats Motivating DomainKeys Identified Mail (DKIM),” May 2006.) 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 there is generally no a priori knowledge of a sending domain's practices. If a protocol could be developed for a domain to publish its DKIM signing practices, a message verifier could take that into account when it receives a unsigned piece of email.
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 to 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] (Resnick, P., “Internet Message Format,” April 2001.).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 give better guarantees that mail sent in their name is with the consent of the domain holder. The first step is, of course, to use DKIM-base to sign all of the outgoing mail so that a receiver can make a 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 occasional 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 delegate to other entities the ability to sign for the domain holder with either a first party signature or the equivalent. 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 may be able to 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 constrain the third party to only be able to produce valid signatures in the benefits.example.com 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. Another concern is that not all DNS providers give the ability to specify 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.
Resent mail is a common occurrence in many scenarios in the email world of today. For example, Alice sends a DKIM signed message with a published practice of signing all messages sent from Alice's domain to Bob's mailing list in another domain. Bob, being a good net citizen, wants to be able to take his part of the responsibility of the message in question, so he DKIM signs the message.
Note that this scenario is completely orthogonal to whether Alice's signature survived Bob's mailing list: Bob merely wants to assert his part in the chain of accountability for the benefit of the ultimate receivers. It would be useful for this practice to be encouraged as it gives a more accurate view of who handled the message, and the trustworthiness of the handlers. It also has the side benefit that remailers that are not friendly to DKIM first party signatures (ie, break them) can still be potentially assessed by the receiver based on the receiver's opinion of the signing domains that actually survived.
As a practical matter, it may be difficult for a domain to roll out DKIM signing such that they can publish the DKIM Signing Complete practice for its given the complexities of the user population, outsourced vendors sending on its behalf, etc. This leaves open an exploit that high-value mail must be classified to the least common denominator of the published practices. It would be desirable to allow a domain holder to publish a list of exceptions which would have a stronger practices statement.
In this situation, bigbank.example.com might be ready to say that firstname.lastname@example.org is always signed, but the rest of the domain, say, is not. Another situation is that the practices of some address local parts in a given domain are not the same as practices of other local parts. Using the same example of email@example.com being a transactional kind of email which would like to publish very strong practices, mixed in with the rest of the user population local parts which may go through mailing lists, etc, for which a less strong statement is appropriate. It should be said that DKIM, through the use of subdomains, can already support this kind of differentiation. That is, in order to publish a strong practice, one only has to segregate those cases into different subdomains. For example: *@accounts.bigbank.example.com would publish a strong practice while *@bigbank.example.com could publish a more permissive practice.
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 [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).From 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]
The attack is essentially identical: it only requires that mailinglist.com spoof the From: address to whatever other customer of ISP.com. That is, mailinglist.com can be any example.com with bad intentions. Worse, is that the ISP will ordinarily have no clue as to whether its customers are running mailing lists or not, so it would not have the ability "legitimate" From: spoofing (ie, a real mailing list) from illegitimate spoofing.
A legitimate flow follows:
- An ISP signs for both a.com and mailinglist.com
- mailinglist.com sends a piece of mail From: firstname.lastname@example.org, Sender: email@example.com
- ISP signs the message with d=ISP.com
- The receiver at this point has no idea who ISP.com was signing on behalf of because they are both legitimately signed by ISP.com
[INFORMATIVE NOTE: this requirement seems to have rather weak support. It's mainly been added so that it can be issue-tracked. /mat]
[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]
[INFORMATIVE NOTE: the main thrust of this requirement is that practices should only be published for that which the publisher has control, and should not restate what is ultimately the local policy of the receiver.]
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.
still free to good home
|[I-D.ietf-dkim-base]||Allman, E., “DomainKeys Identified Mail (DKIM) Signatures,” draft-ietf-dkim-base-04 (work in progress), July 2006.|
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|[RFC2822]||Resnick, P., “Internet Message Format,” RFC 2822, April 2001.|
|[I-D.ietf-dkim-overview]||Hansen, T., “DomainKeys Identified Mail (DKIM) Service Overview,” draft-ietf-dkim-overview-01 (work in progress), June 2006.|
|[I-D.ietf-dkim-threats]||Fenton, J., “Analysis of Threats Motivating DomainKeys Identified Mail (DKIM),” draft-ietf-dkim-threats-03 (work in progress), May 2006.|
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