]> GreenArrow Email

todd@someguyinva.com

Internet-Draft and associated documents describe the DomainKeys Identified Mail v2 (DKIM2) email authentication protocol. DKIM2 is designed to address shortcomings in email authentication protocols and mechanisms released prior to DKIM2, specifically SPF , DKIM , DMARC , and ARC . Those shortcomings most commonly manifested themselves in email messages that passed through one or more intermediary hosts (e.g., forwarders or mailing list servers) while transiting from origination to final destination. Although these messages were properly authenticated when sent, the alteration of their path and/or content by the intermediary hosts would cause authentication checks to fail at the final destination. In addition, DKIM was susceptible to "replay" of signatures, when attackers would construct abusive messages in such a way that they would pass validation checks for a DKIM signature that had been imported from another message entirely. To address these shortcomings, DKIM2 provides methods not only for intermediary systems to provide details of the changes that they make to a message in transit, but also for receiving hosts to validate those changes in addition to the original message. DKIM2 also allows recipients to detect when messages have been unexpectedly "replayed" and can also ensure that delivery status notifications (DSNs) are only sent to entities that were involved in the transmission of a message. This document describes the recommended usage of the DKIM2 protocol for sending hosts, intermediary hosts, and receiving hosts.

Introduction DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or organization to document that they have handled an email message by associating a domain name with the message . A public key signature is used to record that they have been able to read the contents of the message and write to it. Verification of claims is achieved by fetching a public key stored in the DNS under the relevant domain and then checking the signature. Message transit from author to recipient is through Forwarders that typically make no substantive change to the message content and thus preserve the DKIM2 signature. Where they do make a change the changes they have made are documented so that these can be "undone" and the original signature validated. When a message is forwarded from one system to another an additional DKIM2 signature is added on each occasion. This chain of custody assists validators in distinguishing between messages that were intended to be sent to a particular email address and those that are being "replayed" to that address. The chain of custody can also be used to ensure that delivery status notifications are only sent to entities that were involved in the transmission of a message. Organizations that process a message can add to their signature a request for feedback as to any opinion (for example, that the email was considered to be spam) that the eventual recipient of the message wishes to share. This document discusses best practices for signing, handling, and validating messages that have a DKIM2 signature. These best practices are based in large part on the many years of experience the email community has with the authentication protocols that DKIM2 is intended to replace.

Terminology and Definitions This section defines terms used in the rest of the document. DKIM2 is designed to operate within the Internet Mail service, as defined in . Basic email terminology is taken from that specification. DKIM2 inherits many ideas from DKIM () which, for clarity we refer to in this specification as DKIM1. In addition, some features were influenced by experience from (see ) the experimental ARC protocol (). 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 . These words take their normative meanings only when they are presented in ALL UPPERCASE.

Signers Elements in the mail system that sign messages on behalf of a domain are referred to as Signers. These may be MUAs (Mail User Agents), MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other agents such as mailing list "exploders". In general, any Signer will be involved in the injection of a message into the message system in some way. The key point is that a message must be signed before it leaves the administrative domain of the Signer.

Forwarder defines a Relay as transmitting or retransmitting a message but states that it will not modify the envelope information or the message content semantics. It also defines a Gateway as a hybrid of User and Relay that connects heterogeneous mail services. In this document we use the concept of a Forwarder which is an MTA that receives a message and then, as an alternative to delivering it into a destination mailbox, can forward it on to another system in an automated, pre-determined, manner.

Reviser As will be seen, a Forwarder may alter the message content or header fields, in such a way that existing signatures on the message will no longer validate. If so, then a record will be made of these changes. We call a Forwarder that makes such changes a Reviser.

Verifiers Elements in the mail system that verify signatures are referred to as Verifiers. These may be Forwarders, Revisers, MTAs, Mail Delivery Agents (MDAs), or MUAs. It is an expectation of DKIM2 that a recipient of a message will wish to verify some or all signatures before determining whether or not to accept the message or pass it on to another entity.

Signing Domain A domain name associated with a signature. This domain may be associated with the author of an email, their organization, a company hired to deliver the email, a mailing list operator, or some other entity that handles email. What they have in common is that at some point they had access to the entire contents of the email and were in a position to add their signature to the email.

Selectors To support multiple concurrent public keys per signing domain, the key namespace is subdivided using "selectors". The number of public keys and corresponding selectors for each domain is determined by the domain owner. Many domain owners will use just one selector, whereas administratively distributed organizations can choose to manage disparate selectors and key pairs in different regions or on different email servers. Selectors can also be used to delegate a signing authority, which can be withdraw at any time. Selectors also make it possible to seamlessly replace keys on a routine basis by signing with a new selector, while keeping the key associated with the old selector available.

Senders defines a path for messages that starts with hops from an Author to an Originator to a Relay, before transiting additional hops on the way to their final destination. In this document, we will collectively refer to those first three hops (Author-->Originator-->Relay) as Sender.

Signing and Verification Cryptographic Algorithms DKIM2 supports multiple hashing and digital signature algorithms. One hash function (SHA256) if specified here and two signing algorithms are defined by the DKIM2 protocol: RSA-SHA256 and Ed25519-SHA256. Signers and Verifiers MUST implement SHA256. Signers SHOULD implement both RSA-SHA256 and Ed25519-SHA256. Verifiers MUST implement both RSA-SHA256 and Ed25519-SHA256.

The SHA256 Hashing Algorithm The SHA256 hashing algorithm is used to compute body and header hashes as defined in and The resultant values are stored within Message-Instance header fields.

The RSA-SHA256 Signing Algorithm The RSA-SHA256 Signing Algorithm computes a hash over all the Message-Instance and DKIM2-Signature header fields as described in using SHA-256 (FIPS-180-4-2015) as the hash-alg. That hash is then signed by the Signer using the RSA algorithm (defined in PKCS#1 version 1.5 ) as the crypt-alg and the Signer's private key. The hash MUST NOT be truncated or converted into any form other than the native binary form before being signed. The signing algorithm MUST use a public exponent of 65537. Signers MUST use RSA keys of at least 1024 bits. Verifiers MUST be able to validate signatures with keys ranging from 1024 bits to 2048 bits, and they MAY be able to validate signatures with larger keys.

The Ed25519-SHA256 Signing Algorithm The Ed25519-SHA256 Signing Algorithm computes a hash over all the Message-Instance and DKIM2-Signature fields as described in using SHA-256 (FIPS-180-4-2015) as the hash-alg. It signs the hash with the PureEdDSA variant Ed25519, as defined in Section 5.1 of .

Other Algorithms Other algorithms MAY be defined in the future. Verifiers MUST ignore any hashes or signatures using algorithms that they do not implement.

Key Management Some level of assurance is required that a public key is associated with the claimed Signer. DKIM2 does this by fetching the key from the DNS for the domain specified in the d= field. DKIM2 keys are stored in a subdomain named "_domainkey". Given a DKIM2-Signature field with a "d=" tag of "example.com" and an "s1=" tag of "foo.bar", the DNS query will be for "foo.bar._domainkey.example.com". NOTE: these keys are no different, and are stored in the same locations as those for DKIM1 (). Further details can be found in .

Sender Actions In order to participate in DKIM2, Senders *MUST be Signers. The following sections describe best practices for such Senders.

Sign all outgoing messages with DKIM2 and DKIM1 signatures. Because it is expected that the transition from DKIM1 to DKIM2 across the email ecosystem will be gradual, Senders SHOULD sign messages with both DKIM1 and DKIM2 keys until such time as the deployment of DKIM2 is effectively ubiquitous.

Sign with multiple keys To ensure maximum interoperability, Senders SHOULD sign messages with multiple DKIM2 signatures, with each such signature using a different cryptographic algorithm.

Only Sign On Exit Many Senders originate messages from infrastructure that requires the message transit multiple hops before reaching its egress point to travel to its destination. Senders MUST only apply DKIM2 signatures to messsages only at this last hop before it leaves their infrastructure.

Regularly Rotate DKIM Signing Keys Senders SHOULD follow best practices for rotating DKIM keys, both for DKIM1 and DKIM2 - https://www.m3aawg.org/sites/default/files/m3aawg-dkim-key-rotation-bp-2019-03.pdf

Forwarder Actions A Forwarder participating in DKIM2 MUST be a Signer. Further, as mentioned above, a Forwarder might also function as a Reviser and/or a Verifier before passing a message along to the next hop. This section describes the best practices for a Forwarder participating in DKIM2.

Attempt Verification of Existing DKIM Signatures A Forwarder MUST attempt to Verify the DKIM signatures found in a message when it first arrives to the Forwarder.

Sign all outgoing messages with DKIM2 and DKIM1 signatures. Forwarders participating in DKIM2 MUST be Signers, In addition, for the reason mentioned above in the Sender Actions section of this document, Forwarders SHOULD sign messages with both DKIM1 and DKIM2 keys until such time as the deployment of DKIM2 is effectively ubiquitous.

Sign Even If Just Forwarding and Not Revising Forwarders participating in DKIM2 MUST DKIM2 sign any message they handle, regardless of whether or not they Revise the message. Such signatures maintain a proper DKIM2 "chain of custody" and allow for cleaner verification and unwinding of changes at future hops.

Sign with multiple keys To ensure maximum interoperability, Forwarders SHOULD sign messages with multiple DKIM2 signatures, with each such signature using a different cryptographic algorithm.

Only Sign On Exit As with Senders, if the Fowarder's infrastructure requires the message to transit multiple hops before reaching its egress point to travel to its destination, then the Forwarder MUST only apply DKIM2 signatures to messsages only at this last hop before it leaves their infrastructure.

Regularly Rotate DKIM Signing Keys Forwarders SHOULD follow best practices for rotating DKIM keys, both for DKIM1 and DKIM2 - https://www.m3aawg.org/sites/default/files/m3aawg-dkim-key-rotation-bp-2019-03.pdf

Continue doing From munging When a message reaches a Forwarder, if the Forwarder determines that the Domain Owner of the RFC5322.From header domain has published a DMARC record for that domain, the Fowarder SHOULD alter the RFC5322.From header in such a way as to ensure that the message will not fail DMARC validation when it reaches its destination. Some strategies for doing this are discussed in Section 7.4 of

Bron's idea - Privacy preseving forwarder - remove mention of forward target from bounces Need some text for this...

Receiving Host Actions Receiving Hosts in the DKIM2 ecosystem are those that host mailboxes for the domain to which the message is ultimately routed. Receiving Hosts are Verifiers, and depending on local policy, the disposition of the message may be influenced by whether or not the message passes DKIM2 verification checks. Verification of messages will rely in part on whether or not a full DKIM2 "chain of custody" exists for the message, meaning whether or not the Verifier can reliably determine if each hop that handled the message prior to its reaching the Receiving Host applied a proper DKIM2 signature to the message. We will define three conditions, as follows: A message that was DKIM2 signed by each host handling the message prior to its reaching the Receiving Host is one that "Never Left The DKIM2 Ecosystem" A message that was DKIM2 signed by some, but not all, hosts handling the message prior to its reaching the Receiving Host is one that was "In and Out of The DKIM2 Ecosystem" A message that contains no DKIM2 signatures when reaching the Receiving Host is one that "Never Entered The DKIM2 Ecosystem" Each condition will come with its own set of recommendations for the Receiving Host.

Messages That Never Left The DKIM2 Ecosystem If such a message passes DKIM2 Verification performed by the Receiving Host, the Receiving Host should apply local policy for determining message handling and disposition. If such a message fails DKIM2 Verification performed by the Receiving Host, the Receiving Host MAY safely reject the message under assumption that the DSN(s) will only be sent to entities responsible for transmission of the message. The Receiving Host's local policy, however, will dictate final handling and disposition.

Messages That Were In and Out of the DKIM2 Ecosystem Examples of messages that match this condition would be: The message was DKIM2 signed by the Sender, and passed through one or more Forwarders on its way to the Receiver, and at least one Forwarder did not DKIM2 sign the message. The message was not DKIM2 signed by the Sender, and passed through one or more Forwarders on its way to the Receiver, and at least one Forwarder did DKIM sign the message. Need text on how to detect that message left DKIM2 Ecosystem? For such messages, local policy will dictate handling. Receivers will have to decide whether or not a successful DKIM2 Verification is a condition of message acceptance or whether or not a failed Verification might not preclude acceptance but might influence message disposition. As DKIM2 deployment widens and protocol usage matures, Receivers might alter their local policies to be more reliant on DKIM2 Verification.

Messages Never Entered The DKIM2 Ecosystem Receivers participating in DKIM2 will have to establish local policy to dictate what to do with messages that arrive bearing no DKIM2 signatures. This policy may change over time, as Receivers observe what percentage of mail arrives each day bearing DKIM2 signatures vice the percentage that arrives without, and how their mailbox holders engage with each kind.

DKIM1 and DKIM2 Interoperability (From Wei) When DKIM2 is first deployed, there will be a mix of DKIM-only (RFC6376) and DKIM2-capable participants and there will be significant interaction between these two groups. To maximize compatibility with the DKIM receivers, many DKIM2-capable participants will sign and verify DKIM in addition to DKIM2. At origination, DKIM2-capable sender should sign with DKIM and DKIM2 as some receivers may only support DKIM, and others both DKIM and DKIM2. DKIM2-capable receivers should verify both DKIM and DKIM2 signatures when found, but that said, should not treat results equivalently. When forwarding, DKIM2-capable sender should perform DKIM2 signing only when the DKIM2 security properties to protect against replay and backscatter are met. This property is met when any prior DKIM2 signatures are be valid per draft-clayton-dkim2-spec. DKIM does not provide the same anti-replay and backscatter protections that DKIM2 can. Consequently DKIM2-capable participant should not DKIM2 sign a forwarded message with the same consideration if there is only DKIM signatures available. Under certain circumstances a DKIM2-capable forwarder may choose to DKIM2 sign a message with a prior DKIM signature if it can be certain that the message was not replayed or introduce backscatter using local-policy. Some properties to consider are looking for the same anti-replay and backscatter properties described in draft-clayton-dkim2-spec except signed with the DKIM signature. Typically this only is only apparent for DKIM signed messages at origination, meaning when the payload From header is DMARC aligned with the DKIM signed domain and the signature is valid. Forwarders may wish to consider other validations under local-policy as well.

Notes Pull stuff from RFC 6376 Keys are same, but some features in keys in DKIM1 will be deprecated; unknown fields will be ignored, as with DKIM1 DKIM2 and DMARC Discuss implications of requesting feedback

This RFC is the revised basic definition of The Domain Name System. It obsoletes RFC-882. This memo describes the domain style names and their used for host address look up and electronic mail forwarding. It discusses the clients and servers in the domain name system and the protocol used between them. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies the Internet Message Format (IMF), a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This specification is a revision of Request For Comments (RFC) 2822, which itself superseded Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. [STANDARDS-TRACK] DomainKeys Identified Mail (DKIM) permits a person, role, or organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message. This can be an author's organization, an operational relay, or one of their agents. DKIM separates the question of the identity of the Signer of the message from the purported author of the message. Assertion of responsibility is validated through a cryptographic signature and by querying the Signer's domain directly to retrieve the appropriate public key. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the DKIM signature. This memo obsoletes RFC 4871 and RFC 5672. [STANDARDS-TRACK] Email on the Internet can be forged in a number of ways. In particular, existing protocols place no restriction on what a sending host can use as the "MAIL FROM" of a message or the domain given on the SMTP HELO/EHLO commands. This document describes version 1 of the Sender Policy Framework (SPF) protocol, whereby ADministrative Management Domains (ADMDs) can explicitly authorize the hosts that are allowed to use their domain names, and a receiving host can check such authorization. This document obsoletes RFC 4408. Domain-based Message Authentication, Reporting, and Conformance (DMARC) is a scalable mechanism by which a mail-originating organization can express domain-level policies and preferences for message validation, disposition, and reporting, that a mail-receiving organization can use to improve mail handling. Originators of Internet Mail need to be able to associate reliable and authenticated domain identifiers with messages, communicate policies about messages that use those identifiers, and report about mail using those identifiers. These abilities have several benefits: Receivers can provide feedback to Domain Owners about the use of their domains; this feedback can provide valuable insight about the management of internal operations and the presence of external domain name abuse. DMARC does not produce or encourage elevated delivery privilege of authenticated email. DMARC is a mechanism for policy distribution that enables increasingly strict handling of messages that fail authentication checks, ranging from no action, through altered delivery, up to message rejection. Valimail Standcore LLC This document describes the Domain-based Message Authentication, Reporting, and Conformance (DMARC) protocol. DMARC permits the owner of an email's Author Domain to enable validation of the domain's use, to indicate the Domain Owner's or Public Suffix Operator's message handling preference regarding failed validation, and to request reports about the use of the domain name. Mail receiving organizations can use this information when evaluating handling choices for incoming mail. This document obsoletes RFCs 7489 and 9091. Yahoo Google Fastmail Pty Ltd DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or organization that owns a signing domain to document that it has handled an email message by associating their domain with the message. This is achieved by providing a hash value that has been calculated on the current contents of the message and then applying a cryptographic signature that covers the hash values and other details about the transmission of the message. Verification is performed by querying an entry within the signing domain's DNS space to retrieve an appropriate public key. As a message is transferred from author to recipient systems that alter the body or header fields will provide details of their changes and calculate new hash values. Further signatures will be added to provide a validatable "chain". This permits validators to identify the nature of changes made by intermediaries and apply a reputation to the systems that made changed. DKIM2 also allows recipients to detect when messages have been unexpectedly "replayed" and will ensure that Delivery Status Notifications are only sent to entities that were involved in the transmission of a message. Google The updated DomainKeys Identified Mail (DKIM2) permits an organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message through a digital signature. This is done by publishing to Domain Name Service (DNS) of the domain a public key that is then associated to the domain and where messages can be signed by the corresponding private key. Assertion of responsibility is validated through a cryptographic signature and by querying the Signer’s domain directly to retrieve the appropriate public key. This document describes DKIM2 DNS record format and how to find the record. Over its thirty-five-year history, Internet Mail has changed significantly in scale and complexity, as it has become a global infrastructure service. These changes have been evolutionary, rather than revolutionary, reflecting a strong desire to preserve both its installed base and its usefulness. To collaborate productively on this large and complex system, all participants need to work from a common view of it and use a common language to describe its components and the interactions among them. But the many differences in perspective currently make it difficult to know exactly what another participant means. To serve as the necessary common frame of reference, this document describes the enhanced Internet Mail architecture, reflecting the current service. This memo provides information for the Internet community. This document provides recommendations for the implementation of public-key cryptography based on the RSA algorithm, covering cryptographic primitives, encryption schemes, signature schemes with appendix, and ASN.1 syntax for representing keys and for identifying the schemes. This document represents a republication of PKCS #1 v2.2 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document also obsoletes RFC 3447. This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. The Authenticated Received Chain (ARC) protocol provides an authenticated "chain of custody" for a message, allowing each entity that handles the message to see what entities handled it before and what the message's authentication assessment was at each step in the handling. ARC allows Internet Mail Handlers to attach assertions of message authentication assessment to individual messages. As messages traverse ARC-enabled Internet Mail Handlers, additional ARC assertions can be attached to messages to form ordered sets of ARC assertions that represent the authentication assessment at each step of the message-handling paths. ARC-enabled Internet Mail Handlers can process sets of ARC assertions to inform message disposition decisions, identify Internet Mail Handlers that might break existing authentication mechanisms, and convey original authentication assessments across trust boundaries. Proofpoint Taughannock Networks This document calls for a conclusion to the experiment defined by “The Authenticated Received Chain (ARC) Protocol,” (RFC8617) and recommends that ARC no longer be deployed or relied upon between disparate senders and receivers. The document summarizes what ARC set out to do, reports on operational experience, and explains how the experience gained during the experiment is being incorporated into the proposed DKIM2 work as the successor to DomainKeys Identified Mail (DKIM). To avoid any future confusion, it is therefore requested that ARC (RFC8617) be marked “Obsolete” by the publication of this Internet-Draft.

Changes from Earlier Versions [[This section to be removed by RFC Editor]]