Requirements for a DKIM Signing Practices Protocol
draft-ietf-dkim-ssp-requirements-03

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Copyright Notice

Copyright © The Internet Society (2007).

Abstract

DomainKeys Identified Mail (DKIM) provides a cryptographic mechanism for domains to assert responsibility for the messages they handle. A related mechanism will allow an administrator to publish various statements about their DKIM signing practices. This document defines requirements for this mechanism.

Requirements Language

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].

Table of Contents

1.  Introduction

2.  Definitions

3.  SSP Problem Scenarios
    3.1.  Problem Scenario 1: All Mail Signed with DKIM
    3.2.  Problem Scenario 2: Illegitimate Domain Name Use

4.  SSP Deployment Scenarios
    4.1.  Deployment Scenario 1: Outsourced Signing
    4.2.  Deployment Scenario 2: Determinism in Lookup Mechanism and Subdomain Coverage
    4.3.  Deployment Scenario 3: Resent Original Mail
    4.4.  Deployment Scenario 4: Incremental Deployment of Signing
    4.5.  Deployment Scenario 5: Transport Scenarios
    4.6.  Deployment Scenario 6: Human Legibility of Practices
    4.7.  Deployment Scenario 7: Cryptographic Downgrade Attacks
    4.8.  Deployment Scenario 8: Extensibility
    4.9.  Deployment Scenario 9: Security

5.  Requirements
    5.1.  Discovery Requirements
    5.2.  SSP Transport Requirements
    5.3.  Practice and Expectation Requirements
    5.4.  Extensibility and Forward Compatibility Requirements

6.  Security Requirements

7.  IANA Considerations

8.  Security Considerations

9.  Acknowledgments

10.  References
    10.1.  Normative References
    10.2.  Informative References

§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

DomainKeys Identified Mail [I‑D.ietf‑dkim‑base] (Allman, E., “DomainKeys Identified Mail (DKIM) Signatures,” July 2006.)defines a message level signing and verification mechanism for email. While a DKIM signed message speaks for itself, there is ambiguity if a message doesn't have a valid first party signature: is this to be expected or not?. For email this is an especially difficult problem since there is no expectation of a priori knowledge of a sending domain's practices. This ambiguity can be used to mount a bid down attack which is inherent with systems that allow optional authentication like email: if a receiver doesn't know otherwise, it should not assume that the lack of a valid signature is a priori invalid. Thus, an attacker can take advantage of the ambiguity and simply not sign messages. 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 an unsigned piece of email.

This document defines the requirements for a mechanism that permits the publication of Sender Signing Practices (SSP). The document is organized into two main sections: a Problem and Deployment Scenario section which describes the problems that SSP is intended to address as well as the deployment issues surrounding the base problems. The second section is the Requirements that arise because of those scenarios.

2.  Definitions

• Domain Holder: the entity that controls the contents of the DNS subtree starting at the domain, either directly or by delegation via NS records it controls.
• First Party Address: For DKIM, a first party address is defined to be the [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).From address in the message header; a first party address is also known as an Author address
• First Party Signature: a first party signature is a valid signature where the domain tag (d= or the more specific identity i= tag) matches the first party address. "Matches" in this context is defined in [I‑D.ietf‑dkim‑base] (Allman, E., “DomainKeys Identified Mail (DKIM) Signatures,” July 2006.)
• Third Party Signature: a third party signature is a valid signature that does not qualify as a First Party Signature. Note that a DKIM third party signature is not required to correspond to a third party address such as Sender or Listid, etc.
• Practice: a statement according to the [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).From domain holder of externally verifiable behavior in the email messages it sends. A practice should always be true when received by a topologically adjacent SMTP.
• Expectation: an Expectation combines with a Practice to convey what the domain holder considers the likely survivability of the Practice for a non-topologically adjacent receiver.
• DKIM Signing Complete: a Practice where the domain holder asserts that all legitimate mail will be sent with a valid First Party Signature.

3.  SSP Problem Scenarios

The email world is a diverse place 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.

3.1.  Problem Scenario 1: All Mail Signed with DKIM

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 contains a valid DKIM signature.

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. Thus the receiver is at a disadvantage in that it does not know if it should expect mail to be signed from a given domain or not. This knowledge gap leads to a trivially exploitable bid-down attack where the attacker merely sends unsigned mail; since the receiver doesn't know the practices of the signing domain, it cannot treat the message any more harshly for lack of a valid signature.

An information service which allowed a receiver to query for the practices and expectations of the first party domain when no valid first party signature is found could be useful in closing this gap. A receiver could use this information to treat such questionable mail with varying degrees of prejudice.

Note that for the foreseeable future, unrestricted use patterns of mail (eg where users may be members of mailing lists, etc) will likely suffer occasional non-malicious signature failure in transit. While probably not a large percentage of total traffic, the kind of breakage may be a significant concern for those usage patterns. This scenario defines where the sender cannot set any expectation as to whether an individual message will arrive intact.

Even without that expectation, a receiver may be able to take advantage of the knowledge that the domain's practice is to sign all mail and bias filters against unsigned or damaged in transit mail. This information should not be expected to be used in a binary yes/no fashion, but instead as a data point among others in a filtering system.

1. Mail with a [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).From A purportedly sends to B with a missing or broken DKIM first party signature from A
2. B would like to know whether that is an expected state of affairs.
3. A provides information that it signs all outgoing mail, but places no expectation on whether it will arrive with an intact first party signature.
4. B could use this information to bias its filters such that it looks somewhat more suspicious.

3.2.  Problem Scenario 2: Illegitimate Domain Name Use

A class of mail typified by transactional mail from high value domains is the target of phishing attacks. In particular, many 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 an enhanced guarantee that mail sent in their name should always arrive with the proof that the domain holder consented to its transmission. That is, the message should contain a valid first party signature as defined above.

From a receiver's standpoint, knowing that a domain not only signs all of its mail, but places a very high value on the receipt of a valid first party signature from that domain is helpful. Hence a receiver can know that the domain not only signs all of its mail, but also feels it essential that legitimate mail must have its first party signatures survive transit. A receiver with the knowledge of the sender's expectations in hand might choose to process messages not conforming to the published practices in a special manner.

[Informative Note: in terms of a receiving filter, one may choose to treat scenario 2 much more harshly than scenario 1; where scenario 1 looks odd, scenario 2 looks like something is very wrong]

1. Mail with a [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).From A purportedly sends to B with a missing or broken first party DKIM signature from A
2. B would like to know whether that is an expected state of affairs.
3. A provides information that it signs all outgoing mail, but places an expectation that it should arrive with an intact first party signature, and that the receiver should be suspicious if it does not.
4. B could use this information to bias its filters such that it looks much more suspicious.

4.  SSP Deployment Scenarios

Given the problems enumerated above for which we'd like SSP to provide information to recipients, there are a number of scenarios that are not related to the problems that are to be solved, per se, but the actual mechanics of implementing/deploying the information service that SSP would provide.

4.1.  Deployment Scenario 1: Outsourced Signing

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. One obvious use scenario is a domain holder from 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.

4.2.  Deployment Scenario 2: Determinism in Lookup Mechanism and Subdomain Coverage

SSP's client will generally be deployed on incoming mail streams to provide the information as proposed in the problem scenarios. The RFC2822.From address will be used as a basis for the lookup. More precisely the domain part of the first address of the RFC2822.From will form the trust basis to fetch the published information. A trivial attack to circumvent finding the published information could be mounted by simply using a subdomain of the parent which doesn't have published information. This attack is called the subdomain attack: that is, a domain needs to not only publish a policy for a given DNS label it controls, but it also may need to protect all subdomains of that label as well. If this characteristic is not met, an attacker would need only create a possibly fictitious subdomain which was not covered by SSP's information service. Thus, it would be advantageous for The Protocol to not only cover a given domain, but all subdomains of that domain as well.

4.3.  Deployment Scenario 3: Resent Original Mail

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 to Bob's mailing list. 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, perhaps corresponding to the Sender address.

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. It also has the side benefit that remailers that are not friendly to DKIM first party signatures (ie, break them) can be potentially assessed by the receiver based on the receiver's opinion of the signing domains that actually survived.

4.4.  Deployment Scenario 4: Incremental Deployment of Signing

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 given the complexities of the user population, outsourced vendors sending on its behalf, etc. This leaves open an exploit that high-value mail such as in Problem Scenario 2 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.

For example, bigbank.example.com might be ready to say that statements@bigbank.example.com 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 statements@bigbank.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 constrained practices while corporateusers.bigbank.example.com might publish more permissive practices.

4.5.  Deployment Scenario 5: Transport Scenarios

Email service provides an any-any mesh of potential connections: all that is required is the publication of an MX record and a SMTP listener to receive mail. Thus the use of SSP is likely to fall into two main scenarios, the first of which are large, well known domains who are in constant contact with one another. In this case caching of records is essential for performance, including the caching of the non-existence of records (ie, negative caching).

The second main scenario is when a domain exchanges mail with a much smaller volume domain. This scenario can be both perfectly normal as with the case of vanity domains, and sadly a vector for those sending mail for anti-social reasons. In this case we'd like the burden to SSP querier to be low, since many of the lookups will not provide a useful answer. Likewise, it would be advantageous to have upstream caching here as well so that, say, a mailing list exploder on a small domain does not result in an explosion of queries back at the root server for the small domain.

4.6.  Deployment Scenario 6: Human Legibility of Practices

While SSP records are likely to be primarily consumed by an automaton, for the foreseeable future they are also likely to be inspected by hand. It would be nice to have the practices stated in a fashion which is also intuitive to the human inspectors.

4.7.  Deployment Scenario 7: Cryptographic Downgrade Attacks

There is a downgrade attack possible when a DKIM signature is hashed with a previously acceptable but now insecure hash algorithm. This could allow attackers to send their chosen text which is apparently signed by the targeted domain. It would be advantageous for a domain to publish what the allowable signing/hashing algorithms are to prevent this downgrade attack.

4.8.  Deployment Scenario 8: Extensibility

While this document pertains only to requirements surrounding DKIM signing practices, it would be beneficial for the protocol to be able to extend to other protocols.

4.9.  Deployment Scenario 9: Security

SSP must be able to withstand life in a hostile open internet environment. These include DoS attacks, and especially DoS attacks that leverage themselves through amplification inherent in the protocol. In addition, while a useful protocol may be built without strong source authentication provided by the information service, a path to strong source authentication should be provided by the protocol, or underlying protocols.

5.  Requirements

This section defines the requirements for SSP. As with most requirements documents, these requirements define the MINIMUM requirements that a candidate protocol must provide. It should also be noted that SSP must fulfill all of the requirements.

5.1.  Discovery Requirements

Receivers need a means of obtaining information about a sender's DKIM practices. This requires a means of discovering where the information is and what it contains.

1. The author is the first-party sender of a message, as specified in the [rfc2822].From field. SSP's information is associated with the author's domain name and is published subordinate to that domain name.
2. In order to limit the cost of its use, any query service supplying SSP's information MUST provide a definitive responsive within a small, deterministic number of query exchanges.

   [Informative Note: this, for all intents and purposes is a prohibition on anything that might produce loops or result in extended delays and overhead; also though "deterministic" doesn't specify how many exchanges, the expectation is "few".]

   [Refs: Deployment Scenario 2]

3. SSP's publishing mechanism MUST be defined such that it does not lead to multiple records of different protocols residing at the same location.

   [Informative note: An example is multiple resource record of the same type within a common DNS leaf. Hence, uniquely defined leaf names or uniquely defined resource record types will ensure unambiguous reference.]

   [Refs: Deployment Scenario 2]

4. SSP must be capable of providing coverage for not only the domain but all of its subdomains as well. If all subdomains cannot be directly associated with the parent's information, the protocol MUST be able to communicate that the domain name is suspicious. The process of obtaining the parent domain's practices MUST complete in a deterministic number of steps, preferably few. In widening the scope to have the possibility of all subdomains inherit the parent practice, a number of algorithms could be employed -- all seemingly with their own set of engineering tradeoffs. A common theme in the production of this document was that the number of lookups, on average should be small, and that the discovery process should always be bound to some small finite number of queries.

   [Refs: Deployment Scenario 2

5.2.  SSP Transport Requirements

The publication and query mechanism will operate over the Internet message exchange. This lower layer service must exhibit basic characteristics:

1. Widespread deployment of the transport layer would be highly desirable, especially if riding on top of a true transport layer (eg, TCP, UDP).

   [Refs: Deployment Scenario 5, 8]

2. A low-cost query/response in terms of latency time and the number of packets involved is highly desirable.

   [Refs: Deployment Scenario 5]

3. If the infrastructure doesn't provide caching (ala DNS), the records retrieved will need time-to-live values to allow querying verifiers to maintain their own caches. Existing caching infrastructure is, however, highly desirable.

   [Refs: Deployment Scenario 5]

4. Multiple geographically and topologically diverse servers must be supported for high availability

   [Refs: Deployment Scenario 5, 8]

5.3.  Practice and Expectation Requirements

As stated in the definitions a Practice is a statement according to the [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).From domain holder of externally verifiable behavior in the email messages it sends. As a silly example, a Practice might be defined that all email messages will contain an X-Silly header. Since there is a possibility of alteration between what a sender sends and a receiver examines, an Expectation combines with a Practice to convey what the domain holder considers the likely outcome of the survivability of the Practice for at a receiver. For example, a Practice that X-Silly is present when it is sent from the domain, and an Expectation that it will remain present for receivers whether topologically adjacent or not.

1. SSP MUST be able to make Practices and Expectation assertions about the [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).From address in the context of DKIM. SSP will not make assertions about other addresses for DKIM at this time.

   [Refs: Problem Scenario 1,2]

2. SSP MUST provide a concise linkage between the [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).From and the identity in the DKIM i= tag, or its default if it is missing in the signature. That is, SSP MUST precisely define the semantics of what qualifies as a First Party Signature.

   [Refs: Problem Scenario 1,2]

3. SSP MUST be able to publish a Practice that the domain's signing behavior is "DKIM Signing Complete". That is, all messages were transmitted with a valid first party signature.

   [Refs: Problem Scenario 1]

4. SSP MUST be able to publish an Expectation that a verifiable first party DKIM Signature should be expected on receipt of a message.

   [Refs: Problem Scenario 2]

5. Practices and Expectations MUST be presented in SSP syntax using as intuitive a descriptor as possible. For example, p=? would be better represented as p=unknown.

   [Refs: Deployment Scenario 6]

6. Because DKIM uses DNS to store selectors, there is always the ability for a domain holder to delegate all or parts of the _domainkey subdomain to an affiliated 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 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. Last, a domain holder can even use CNAME's to delegate individual leaf nodes. Thus SSP need not invent a different means of allowing affiliated parties to sign on a domain's behalf at this time.
7. Practices and Expectations MUST be presented as an information service from the signing domain to be consumed as an added factor to the receiver's local policy. In particular, a Practice or Expectation MUST NOT mandate any disposition stance on the receiver.

   [Refs: Problem Scenario 1, 2, 3]

8. There is no requirement that SSP publish a Practices of any/all third parties that MUST NOT sign on the domain holder's behalf. This should be considered out of scope.

   [INFORMATIVE NOTE: this is essentially saying that the protocol doesn't have to concern itself with being a blacklist repository.]

   [Refs: Problem Scenario 1-2]

9. SSP MUST NOT be required to be invoked if a valid first party signature is found.

   [Refs: Deployment Scenario 2]

10. [PROVISIONAL] The signing policy statement MUST be capable of fully describing a signing practice in which multiple signatures are always provided such that the policy is of utility to any verifier is capable of verifying any of the signatures that are always provided. Such a mechanism MUST NOT:
    • Require the verifier to perform any additional DNS lookups
    • Require duplication of configuration data
    • In particular not require the policy record to provide for the description of any cryptographic or cannonicalization algorithm

    INFORMATIVE NOTE: The ability to specify multiple signatures is necessary in order to permit orderly transitions to new cryptographic and canonicalization algorithms. Unless the policy language is not sufficiently expressive to allow the signer to describe the actual signature practice in this case there is an opportunity for an attacker to exploit the fact that there are verifiers that do not yet support the new algorithm.

    [Refs: Deployment Scenario 7]

11. SSP MUST NOT provide a mechanism which impugns the existence of non-first party signatures in a message. A corollary of this requirement is that the protocol MUST NOT link practices of first party signers with the practices of third party signers.

    [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 meddle in what is ultimately the local policy of the receiver.]

    [Refs: Deployment Scenario 3]

5.4.  Extensibility and Forward Compatibility Requirements

1. SSP MUST NOT extend to any other protocol than DKIM for email at this time. SSP SHOULD be able to extend for protocols other than DKIM.

   [Refs: Deployment Scenario 8]

2. SSP MUST be able to add new Practices and Expectations within the existing discovery/transport/practices in a backward compatible fashion.

   [Refs: Deployment Scenario 8]

6.  Security Requirements

1. Minimize DoS potential: SSP for a high-value domain is potentially a high-value DoS target, especially since the unavailability of SSP's record could make unsigned messages less suspicious.
2. Amplification Attacks: SSP MUST NOT make highly leveraged amplification or make-work attacks possible. In particular any amplification must be O(1).

   [Author's question: is it really O(1)? or O(n)?]

   [Refs: Deployment Scenario 9]

3. Authenticity: SSP MUST have the ability for a domain holder to provide SSP's data such that a receiver can determine that it is authentically from the domain holder with a large degree of certainty. SSP may provide means which provide less certainty in trade off for ease of deployment.

   [Refs: Deployment Scenario 9]

7.  IANA Considerations

This document makes no request of IANA.

Note to RFC Editor: this section may be removed on publication as an RFC.

8.  Security Considerations

This document defines requirements for a new protocol and the security related requirements are defined above. There is an expectation that SSP will not always be required to have source authentication of the practices information which is noteworthy.

9.  Acknowledgments

Due to flipping in the market and raising interest rates, this home is no longer free. Dave Crocker and Jim Fenton helped me raise the walls on this document and are accorded a room at the inn. The inn is not yet full, however.

10.  References

10.1. Normative References

10.2. Informative References

Author's Address

Full Copyright Statement

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