Network Working Group M. S. Kucherawy, Ed. Internet-Draft 1 October 2022 Intended status: Experimental Expires: 4 April 2023 Replay-Resistant DomainKeys Identified Mail (DKIM) Signatures draft-kucherawy-dkim-anti-replay-01 Abstract DomainKeys Identified Mail (DKIM) provides a digital signature mechanism for Internet messages, allowing a domain name owner to affix its domain name in a way that can be cryptographically validated. DKIM signatures protect the integrity of the message header and body only. By design, it decoupled itself from the transport and storage mechanisms used to handle messages. This gives rise to a possible replay attack, but the original DKIM specification fell short of providing a mitigation strategy. This document presents an optional method for binding a signature to a specific recipient or set of recipients so that broader replay attacks can be mitigated. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on 4 April 2023. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. Kucherawy Expires 4 April 2023 [Page 1] Internet-Draft DKIM Anti-Replay Canonicalization October 2022 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Recommended Reading . . . . . . . . . . . . . . . . . . . 3 2.2. Requirements Language . . . . . . . . . . . . . . . . . . 3 3. The 'e' Tag . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2. General Definition . . . . . . . . . . . . . . . . . . . 3 3.2.1. Modified Algorithm . . . . . . . . . . . . . . . . . 4 3.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Recipient Mutations . . . . . . . . . . . . . . . . . . . 7 4.2. Envelope Splitting . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . 8 7.2. Informative References . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction DomainKeys Identified Mail (DKIM) provides a digital signature mechanism for Internet messages, allowing a domain name owner to affix its domain name to a message in a way that can be cryptographically validated. [RFC4686] presents the original threat model DKIM was meant to address, and the environment in which it was expected to work. Notably, DKIM decoupled itself from the transport of the message. The theory suggests it should be possible to validate a signature whether a message is in situ (i.e., in an inbox on disk), in transit between mail servers, or being retrieved through a mailbox access protocol. Kucherawy Expires 4 April 2023 [Page 2] Internet-Draft DKIM Anti-Replay Canonicalization October 2022 In particular, this meant a DKIM signature can validate irrespective of what is in the SMTP [RFC5321] envelope containing it, or even when there is no envelope to consider. This means a message and its signature can be re-sent to anyone simply by changing the set of recipients in the envelope and passing the message back to a Mail Transport Agent (MTA) or Mail Submission Agent (MSA). As the message itself is unaltered, any DKIM signature(s) on it will continue to validate. This is a form of replay attack, and it relies for its success on the perceived value (i.e., reputation) of the domain(s) named in the signature(s). This document describes a mechanism by which a signature and a message can be coupled such that successful replays to other recipient sets are not possible, as the signature will no longer validate. 2. Definitions 2.1. Recommended Reading Several terms used in this document are based on their definitions in [RFC5598]. The term "envelope recipient" is, using the notation proposed in that document, an RFC5321.RcptTo address. 2.2. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. The 'e' Tag 3.1. Syntax Using ABNF [RFC5234], the syntax for the new tag is: sig-e-tag = %x65 [FWS] "=" %x79 3.2. General Definition This section introduces the "e" (for "envelope") tag, a new DKIM signature tag that can be used by a signer to indicate that signature will only validate for a specific envelope recipient set, namely the one associated with the message at the time it was signed. Kucherawy Expires 4 April 2023 [Page 3] Internet-Draft DKIM Anti-Replay Canonicalization October 2022 DKIM signers and verifiers to date have no reason to be interested in any aspect of the envelope used to transport a message. This sort of verification is not possible without that context being available, which may prove to be a challenge to some operating environments. Also, this will make it impossible to validate a DKIM signature using this algorithm in a context where no envelope exists, such as when retrieving a message from a mailbox. The expected value of the tag is simply the character "y", though other values may be introduced by future work. The value has no particular meaning; the presence of the tag is the important signal. [FOR DISCUSSION] Maybe this should be "r", indicating "recipients", to allow later extensions to include other parts of the envelope that might be helpful to include. The presence of this tag in a DKIM signature indicates that the signer executed a modified version of the algorithm described in Section 3.7 of [RFC6376], and the verifier MUST do the same. The modification inserts the envelope recipients available at signing or verification time into the data fed to the hash algorithm to either produce or verify the DKIM signature. 3.2.1. Modified Algorithm This section specifies the modified version of the algorithm defined in Section 3.7 of [RFC6376]. The pseudo-code of "data-hash" is replaced as follows: OLD: data-hash = hash-alg (h-headers, D-SIG, body-hash) NEW: data-hash = hash-alg (recipients, h-headers, D-SIG, body-hash) The definition of "data-hash" is replaced as follows: Kucherawy Expires 4 April 2023 [Page 4] Internet-Draft DKIM Anti-Replay Canonicalization October 2022 OLD: data-hash: is the output from using the hash-alg algorithm, to hash the header including the DKIM-Signature header, and the body hash. NEW: data-hash: is the output from using the hash-alg algorithm to hash the recipients, the header including the DKIM-Signature header, and the body hash. "recipients" is determined as follows: 1. Collect all envelope recipients into a list. 2. Sort them in typical lexical ASCII order. 3. Format the list by concatenating them all in this sorted order, separated by CRLF strings (ASCII 13 followed by ASCII 10), and with the last one terminated by a CRLF. The signing and verifying processes defined for DKIM are otherwise unmodified. 3.3. Example Consider the following SMTP transaction, wherein "C" denotes something sent by an SMTP client, "S" denotes something sent by an SMTP server, and terminating CRLFs in both directions are omitted: C: MAIL FROM: S: 250 Sender OK C: RCPT TO: S: 250 Recipient OK C: RCPT TO: S: 250 Recipient OK C: DATA S: 354 Go ahead [message header omitted] [message body omitted] . C: 250 Message delivered Kucherawy Expires 4 April 2023 [Page 5] Internet-Draft DKIM Anti-Replay Canonicalization October 2022 Compared to the standard signatures that would be generated or verified in the absence of this tag, the process described above would work the same way as the standard signing process would, except that the content fed to the hash algorithm would be preceded by: alice@example.com bob@example.com 4. Discussion Use of this tag guarantees that a signature will not verify unless sent to exactly the same set of envelope recipients as was present in the envelope when the message was prepared for signing. The fact that the recipient set is sorted allows verifiers to tolerate any reordering of the envelope that may be done in transit. However, if any original recipient is removed, or any new recipient added, the signature will not validate because the content passed to the hash step at the verifier will differ from what was done at the signer. Thus, in the replay scenario described in Section 1, the signature no longer validates. Anecdotal evidence suggests that the bulk of Internet message traffic is single-recipient traffic already, which implies the success of this proposal. However, since the messaging standards both permit and even encourage this "common factoring" of traffic, and this evidence has not been broadly verified, it is appropriate to consider all possibilities. In the absence of an SMTP envelope in the verification environment, the DKIM implementation SHOULD indicate that the signature cannot be verified, as distinct from considering such validation to have failed. If the need to be able to validate a signature from storage (without an envelope) needs to be preserved, the signer can still add a second signature not using this tag, which therefore does not need the envelope context to verify. This, however, requires the verifier to understand when it is appropriate to use which signature. Since [RFC6376] stipulates that unknown tags are to be ignored, there will be a possibly substantial time period during which the tag is unknown to receivers. Operators should expect these signatures to fail broadly during any early deployment period, even for non-replay messages, and it may be some time before meaningful signal begins to appear. Kucherawy Expires 4 April 2023 [Page 6] Internet-Draft DKIM Anti-Replay Canonicalization October 2022 Note that this mechanism is fragile in the modern Internet message ecosystem. Some scenarios that will yield false negatives with this method are described below. 4.1. Recipient Mutations If a receiving MTA notes that one of the envelope recipients refers to a mailbox in a domain for which it has administrative authority, but is known to be an alias, it may rewrite that envelope into its canonical form. For instance, if a receiving MTA is officially known as the mail server for "example.com", but also accepts mail for its users when addressed to "example.net", it may alter that latter address in the envelope to refer to its canonical name. This alters the recipient list, and thus alters the content passed to the hash algorithm when validating the signature, leading to a failure. Since hostnames are generally case-insensitive on the Internet, a relay MTA might (improperly) fold a hostname to lowercase. This too would invalidate a signature making use of this protocol. 4.2. Envelope Splitting If a message contains envelope recipients at domains served by separate MTAs, [RFC5321] compels the handling MTA to split the message, creating two envelopes containing identical content. The first of these will be addressed to one recipient and sent on its way; the second will be addressed to the other and sent via its own route. Upon arrival at either DKIM verifier, the recipient list has effectively been altered since signing. This alters the content passed to the hash algorithm when validating the signature, leading to a failure. This can be avoided by arranging that no envelope ever has more than a single recipient, but this renders useless an important "common factoring" feature of SMTP. In the case of a mailing list server that may need to distribute a single message to a very large number of recipients, this method can impose significant compute or storage costs. 5. IANA Considerations IANA is asked to make the following entry in the "DKIM-Signature Tag Specifications" sub-registry of the "DKIM Parameters" registry group: Type: e Kucherawy Expires 4 April 2023 [Page 7] Internet-Draft DKIM Anti-Replay Canonicalization October 2022 Reference: [this document] Status: active 6. Security Considerations All of the security considerations of [RFC6376] apply when applying the modification described here. A signer that is forced to generate independently signed messages for each recipient in a situation where large recipient lists are common could be exploited to cause a denial-of-service attack simply from the fact that there is an amplication of work being done. The loss of the ability to verify messages signed using this tag when extracted from their mailboxes will have unknown security impact. Although DKIM intentionally supports this capability, it is not known whether it is widely used. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008, . [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 7.2. Informative References Kucherawy Expires 4 April 2023 [Page 8] Internet-Draft DKIM Anti-Replay Canonicalization October 2022 [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686, September 2006, . [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July 2009, . Appendix A. Acknowledgments The author wishes to thank Dave Crocker for his contributions to this work. Author's Address Murray S. Kucherawy (editor) Email: superuser@gmail.com Kucherawy Expires 4 April 2023 [Page 9]