DKIM Working Group M. Thomas Internet-Draft Cisco Systems Intended status: Informational
September 2006 Expires:March 5,6, 2007 Expires: September 7, 2007 Requirements for a DKIM Signing Practices Protocol draft-ietf-dkim-ssp-requirements-02draft-ietf-dkim-ssp-requirements-03 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 5,September 7, 2007. Copyright Notice Copyright (C) The Internet Society (2006).IETF Trust (2007). Abstract DomainKeys Identified Mail [DKIM] [I-D.ietf-dkim-base](DKIM) provides a cryptographic mechanism for domains to assert responsibility for the messages they handle. A related mechanism wouldwill allow an administrator to publish various statements about their DKIM signing practices. This draftdocument defines the requirementrequirements 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 [RFC2119]. Table of Contents 1. Preface . .Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.SSP Problem Scenarios . . . . . . . . . . . . . . . . . . . . 8 4.1.7 3.1. Problem Scenario 1: All Mail Signed with DKIM . . . . . . 8 4.2.7 3.2. Problem Scenario 2: Illegitimate Domain Name Use . . . . . 9 4.3. Problem Scenario 3: Domain Sends No Mail . . . . . . . . . 10 5.8 4. SSP Deployment Scenarios . . . . . . . . . . . . . . . . . . . 11 5.1.10 4.1. Deployment Scenario 1: Outsourced Signing . . . . . . . . 11 5.2.10 4.2. Deployment Scenario 2: Determinism in Lookup Mechanism and Subdomain Coverage . . 11 5.3.. . . . . . . . . . . . . . . . 10 4.3. Deployment Scenario 3: Resent Original Mail . . . . . . . 11 5.4.10 4.4. Deployment Scenario 4: Incremental Deployment of Signing . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.5.11 4.5. Deployment Scenario 5: Transport Scenarios . . . . . . . . 12 5.6.11 4.6. Deployment Scenario 6: Human Legibility of Practices . . . 13 5.7.12 4.7. Deployment Scenario 7: Cryptographic Downgrade Attacks . . 13 5.8.12 4.8. Deployment Scenario 8: Extensibility . . . . . . . . . . . 13 5.9.12 4.9. Deployment Scenario 9: Security . . . . . . . . . . . . . 13 6.12 5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1.13 5.1. Discovery Requirements . . . . . . . . . . . . . . . . . . 15 6.2.13 5.2. SSP Transport requirements . .Requirements . . . . . . . . . . . . . . . . 16 6.3.14 5.3. Practice and Expectation Requirements . . . . . . . . . . 16 6.4.14 5.4. Extensibility and Forward Compatibility Requirements . . . 19 7.17 6. Security Requirements . . . . . . . . . . . . . . . . . . . . 20 8.18 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 9.19 8. Security Considerations . . . . . . . . . . . . . . . . . . . 22 10.20 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23 11.21 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 11.1.22 10.1. Normative References . . . . . . . . . . . . . . . . . . . 24 11.2.22 10.2. Informative References . . . . . . . . . . . . . . . . . . 2422 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 2523 Intellectual Property and Copyright Statements . . . . . . . . . . 2624 1. Preface The purpose of this draft is get out into the openIntroduction DomainKeys Identified Mail [I-D.ietf-dkim-base]defines a range of issues related to the perceived needmessage level signing and verification mechanism for email. While a signing practices information service primarily focused on DKIM. This documentDKIM signed message speaks for itself, there is intended to document well-agreed upon problems and requirements, in addition to less well-agreed upon requirements in an attemptambiguity if a message doesn't have a valid first party signature: is this to capture the issue as well as generalize the requirement as much as possible. These latter requirements willbe noted as "[PROVISIONAL]" to indicate that there is not yet solid consensus,expected or that thenot?. 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 well understood. A winnowing processassume that the lack of a valid signature is envisioned wherea priori invalid. Thus, an attacker can take advantage of the more difficult and/or speculative problems/requirement willambiguity and simply not sign messages. If a protocol could be eliminated unless concretedeveloped 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 with proven constituencies can be demonstrated, along with reasonable plausibilitythat they do not contradict moreSSP is intended to address as well agreed upon requirements.as the deployment issues surrounding the base problems. The second section is the Requirements that arise because of those scenarios. 2. Definitions o Domain Holder: the entity that ultimatelycontrols the contents of the DNS subtree starting at the domain, either directly or by delegation via NS records it controls. o First Party Address: For DKIM, a first party address is defined to be the [RFC2822].From address in the message header; a first party address is also known as an Author address o 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] o 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 does is not required to correspond to a third party address such as Sender or Listid, etc. o DKIM Signer Complete: the state where the domain holder believes that all legitimate mail purportedly from the domain was sent with a valid DKIM signature. o The Protocol: in this document, The Protocol is used as placeholder for a protocol that will meet the requirements set in this draft. 3. Introduction 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]. In addition there are two other documents [I-D.ietf-dkim-overview] and [I-D.ietf-dkim-threats] 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:valid signature that does not qualify as a Problem and Deployment Scenario section which describes the problemsFirst Party Signature. Note that The Protocola DKIM third party signature is intendednot required to correspond to a third party address such as well asSender or Listid, etc. o Practice: a statement according to the deployment issues surrounding[RFC2822].From domain holder of externally verifiable behavior in the base problems. The second section isemail messages it sends. A practice should always be true when received by a topologically adjacent SMTP. o Expectation: an Expectation combines with a Practice to convey what the Requirements that arise becausedomain holder considers the likely survivability of those scenarios. 4.the Practice for a non-topologically adjacent receiver. o 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. 220.127.116.11. 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 sendingfirst 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, DKIM signature breakage forunrestricted use patterns of mail (eg where users aremay 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 (quality)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].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. 18.104.22.168. Problem Scenario 2: Illegitimate Domain Name Use There seems to be aA class of mail -- mostlytypified by transactional mail from high value domains -- that areis the target of phishing attacks. In particular, many 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 give an enhanced guarantee that mail sent in their name should always arrive with the provable consent ofproof that the domain holder.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 also expects that legitimate mail fromplaces a very high value on the domain will be received withreceipt of a valid first party signature is quite interesting. This assertionfrom the signingthat domain is quite a bit stronger than the assertion in Problem Scenario 1; even thoughhelpful. Hence a signerreceiver can neverknow the true path mail will take before delivery, the implication isthat if the message is lacking a valid signaturethe message is either malicious or isdomain 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 responsibilityknowledge of the signing domainsender's expectations in hand might choose to process messages not conforming to avoid whatever brokethe signature.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].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. 4.3. Problem Scenario 3: Domain Sends No Mail A domain may not intend to send mail at all. In such a case, it could be advantageous for a receiver to know the domain's intent and would be likely to treat such mail very suspiciously. It has been noted that a solution to Problem Scenario 2 could potentially be used to emulate this practice. In reality, they are close but not precisely the same semantics. That is, a piece of email purporting to come from a domain which claims to send none is illegitimate on its face, whereas there may be some lingering doubt with Problem Scenario 2 given the possibility in deployments between whether one should publish scenario 1 and 2, etc. 5.4. SSP Deployment Scenarios Given the problems enumerated above for which we'd like The ProtocolSSP 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 The ProtocolSSP would provide. 22.214.171.124. 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. 126.96.36.199. Deployment Scenario 2: Determinism in Lookup Mechanism The Protocol'sand Subdomain Coverage SSP's client will generally be deployed on incoming mail streams to provide the information as proposed in the problem scenarios. As such, it is envisioned that theThe RFC2822.From address wouldwill be used as a basis for the lookup. In particular,More precisely the domain part of the first address would be consulted in some mannerof the RFC2822.From will form the trust basis to fetch the published information. There is a fairlyA trivial attack againstto circumvent finding the published information could be mounted by simply using a naive usesubdomain of this algorithmthe 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 The Protocol's information service 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 draft was that the number of lookups, on average should be small, and that the discovery process should alwayscovered by SSP's information service. Thus, it would be boundadvantageous for The Protocol to some small finite numbernot only cover a given domain, but all subdomains of queries. 5.3.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. 188.8.131.52. 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 email@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 firstname.lastname@example.org 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.comaccounts.bigbank.example.com would publish a strong practiceconstrained practices while *@bigbank.example.com couldcorporateusers.bigbank.example.com might publish amore permissive practice. 5.5.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 The ProtocolSSP 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 The ProtocolSSP 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. 184.108.40.206. Deployment Scenario 6: Human Legibility of Practices While The ProtocolSSP 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. [Author's $.02: it's been amply demonstrated that simple human readable labels are often misconstrued. Opaque symbols do have the advantage that you need to consult the definition to determine its meaning rather than just intuiting what it "ought" to mean. /mat] 220.127.116.11. Deployment Scenario 7: Cryptographic Downgrade Attacks There is a downgrade attack possible wherewhen 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. 18.104.22.168. 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. 22.214.171.124. Deployment Scenario 9: Security The protocolSSP 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. 6.5. Requirements This section defines the requirements for The Protocol.SSP. As with most requirements drafts,documents, these requirements define the MINIMUM requirements that a candidate protocol must provide. It should also be noted that The ProtocolSSP 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] 126.96.36.199. 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. The Protocol'sSSP'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 The Protocol'sSSP's information mustMUST 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. The Protocol'sSSP's publishing mechanism MUST be defined to produce unambiguous semantics, particularly with respect to other informationsuch that might shareit does not lead to multiple records of different protocols residing at the publication mechanism.same location. [Informative note: An example of ambiguityis sharingmultiple resource record typesof 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] 6.2.2] 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 requirementsRequirements 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] 188.8.131.52. Practice and Expectation Requirements In this section,As stated in the definitions a Practice is defined asa truestatement according to the [RFC2822].From domain holder of its intendedexternally visible behavior. Anverifiable 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 XX-Silly is truepresent when it leavesis sent from the domain, and an Expectation that it will|will-not|may|may-notwill remain truepresent for some/all receivers.receivers whether topologically adjacent or not. 1. The ProtocolSSP MUST be able to make Practices and Expectation assertions about the [RFC2822].From address in the context of DKIM. The ProtocolSSP will not make assertions about other addresses for DKIM at this time. [Refs: Problem Scenario 1,2] 2. [PROVISIONAL] The ProtocolSSP MUST be able to publish a Practice which is indicative that domain doesn't send mail. [Refs: Problem Scenario 3] 3. If the Discovery process would be shortened by publication of a "null" practice, the protocol SHOULDprovide a mechanism to publish such a practice. [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 shortconcise linkage between the policy lookup mechanism if it's published,[RFC2822].From and if that'sthe caseidentity in the DKIM i= tag, or its default if it seems worthwhile. Also, a null policy may have some forensic utility, such as gagingis missing in the numbersignature. That is, SSP MUST precisely define the semantics of domains considering/using DKIM for example.]what qualifies as a First Party Signature. [Refs: DeploymentProblem Scenario 2] 4. The Protocol1,2] 3. SSP MUST be able to publish a Practice that the domain's signing behavior is "DKIM Signing Complete"Complete". That is, all messages were transmitted with a valid first party signature. [Refs: Problem Scenario 1] 5. The Protocol4. SSP MUST be able to publish an Expectation that a verifiable First Partyfirst party DKIM Signature should be expected on receipt of a message. [Refs: Problem Scenario 2] 6.5. Practices and Expectations MUST be presented in the ProtocolSSP syntax using as intuitive a descriptor as possible. For example, p=? would be better represented as p=unknown. [Refs: Deployment Scenario 6] 7. The Protocol MUST NOT invent a different means of allowing affiliated parties to sign on a domain's behalf.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 howthird 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. 8. [PROVISIONAL] The protocol MUST have the ability to provide practices and expectations keyed off of the local part of the [RFC2822].From address. As with all provisional requirements, this requirement mustThus SSP need not be in conflict with other requirements, including DNS considerations, etc. [INFORMATIVE NOTE: this requirement seemsinvent a different means of allowing affiliated parties to have rather weak support. It's mainly been added so that it can be issue- tracked. /mat] [Refs: Deployment Scenario 4] 9.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] 10.8. There is no requirement that The ProtocolSSP 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] 11. The Protocol9. SSP MUST NOT be required to be invoked if a valid first party signature is found. [Refs: Deployment Scenario 2] 12.10. [PROVISIONAL] A domain holderThe signing policy statement MUST be able to publishcapable of fully describing a Practicesigning practice in which enumeratesmultiple signatures are always provided such that the acceptable cryptographic algorithmspolicy 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 purportedly fromis 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 domain.there are verifiers that do not yet support the new algorithm. [Refs: Deployment Scenario 7] 13. The protocol11. 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] 184.108.40.206. Extensibility and Forward Compatibility Requirements 1. The ProtocolSSP MUST NOT extend to any other protocol than DKIM for email at this time. The ProtocolSSP SHOULD be able to extend for protocols other than DKIM. [Refs: Deployment Scenario 8] 2. The ProtocolSSP MUST be able to add new Practices and Expectations within the existing discovery/transport/practices in a backward compatible fashion. [Refs: Deployment Scenario 8] 7.6. Security Requirements 1. Minimize DoS potential: The ProtocolSSP for a high-value domain is potentially a high-value DoS target, especially since the unavailability of The Protocol'sSSP's record could make unsigned messages less suspicious. 2. Amplification Attacks: The ProtocolSSP 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: The ProtocolSSP MUST have the ability for a domain holder to provide The Protocol'sSSP's data such that a receiver can determine that it is authentically from the domain holder with a large degree of certainty. The ProtocolSSP may provide means which provide less certainty in trade off for ease of deployment. [Refs: Deployment Scenario 9] 8.7. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. 9.8. Security Considerations This draftdocument defines requirements for a new protocol and the security related requirements are defined above. There is an expectation that The ProtocolSSP will not always be required to have source authentication of the practices information which is noteworthy. 10.9. 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 draftdocument and are accorded a room at the inn. The inn is not yet full, however. 11.10. References 220.127.116.11. Normative References [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. [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. 18.104.22.168. Informative References [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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