| Internet-Draft | DKIM2 Motivation | March 2025 |
| Gondwana, et al. | Expires 4 September 2025 | [Page] |
This memo provides a rationale for replacing the existing email security mechanisms with a new mechanism based around a more strongly authenticated email delivery pathway, including an asynchronous return channel.¶
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In 2007, [RFC4871] (Domain Key Identified Mail / DKIM) was published, outlining a mechanism for a domain to sign email in a way that recipients could ensure that the email had come from an entity possessing the secret key matching a public key published in the DNS by the source domain. For clarity in this document we call this established scheme "DKIM1".¶
[RFC4871] has been updated and extended many times since then, and a large amount of operational experience has been gained using it.¶
There are a number of things beyond authenticating email that would be useful for mail system operators, particularly when it travels through multiple hops. There have been other attempts to solve some of these problems, e.g. [RFC8617] (Authenticated Received Chain / ARC), however they have not achieved the same level of widespread use as DKIM1.¶
This document proposes solving these related problems in a holistic way, by having every hop in a forwarding chain responsible for:¶
verifying the path that messages have taken to get to it, including by being able to reverse modifications or by asserting that it trusts the previous hop unconditionally.¶
declaring (under protection of its own signature) where the message is being sent to next.¶
promising that it will pass control messages (including bounces, abuse reports and delivery notifications) back to the previous hop for a reasonable time.¶
There is no way to add these properties to existing DKIM1 in a way which is both backwards and forwards compatible, so any new specification will need new headers so that it not mis-interpreted by DKIM1 verifiers which are unaware of the new specfication.¶
By only allowing precisely one destination address per DKIM2-signed email, and encoding that address in the signature, messages will be unable to be replayed to any other destination. This allows recipients to confirm that they are the intended next hop for a message, and reject any message that is incorrectly formed.¶
Even if a message is BCC'd, the copy of that message sent to the BCC'd recipient will have their own address in the DKIM2 signature, so replay to arbitrary addresses is no longer possible.¶
During initial evaluation, there will be a period where receivers maintain a list of known dkim2-unaware forwarders and a longer term solution will be sought out; however we feel that the benefits of DKIM2 in identifying mail flow participants will help improve getting wanted email to their recipients, thereby driving adoption.¶
If the recipient wishes to forward the message on to another address, it must apply its own DKIM2 header, signed by a key which is aligned to the domain of the recipient address in the previous DKIM2 header, and with a bounce address which is in the same domain.¶
Both of the above requirements mean that every SMTP transaction for DKIM2 messages will, by necessity, have only a single recipient. Any email to multiple people, alias that expands to multiple addresses, or mailing list, will create a different next-hop signature for each distinct destination address.¶
The end result is, like ARC, a chain of domains which have handled the message; however unlike ARC, this chain MUST be fully linked in both directions, with every sending address aligning to the recipient address of the previous DKIM2 header.¶
In DKIM2, DSNs are passed back along the same path. This solves the issue with Email Service Providers (ESPs, entities that specialise in sending email on behalf of others) wanting error reports. The ESP will see the DSN and, depending on contractual arrangements, may be able to avoid passing this message any further back along the chain.¶
Since the DSN messages always go back up the DKIM2 chain, any hop can strip off the higher number (i=) records; including the sender and recipient addresses for them, and create a bounce as if the forwarder itself was doing the rejection. As asynchronous bounces will be common in DKIM2, this is indistinguishable to the sender, allowing privacy-preserving forwarders to seamlessly operate.¶
Passing bounces back along the outgoing path also allows mailing lists to take responsibility for the event and not bother the person who sent a message to the list.¶
Provided that an email is correctly signed when received, it can be rejected at a later point in time. The DSN will be sent to the immediately preceding intermediary. Since the bounce travels back along the (fully authenticated) incoming path it cannot be sent to an uninvolved third party.¶
DKIM2 will define an algebra for describing how to reverse any changes to create the prior binary data, by inspecting the diff between the two versions, recipients will be able to see who injected bad content.¶
Mailing lists (or alumni forwarders etc.) that alter the Subject header field (or other [RFC5322] headers) will record the previous header field contents. This is easy to undo for checking purposes.¶
Mailing lists that add text (either to a simple email body or one or more MIME parts within the body) will record details of the text they have added. This text can then be removed when checking earlier signatures.¶
We expect an "algebra" describing changes to be in a stand-alone document.¶
While this part is not strictly necessary to achieve the goals above, some headers must be signed to get the guarantees, so rather than say that certain headers must be listed, DKIM2 will specify a fixed set of always-signed headers in accordance with now well-established best practice (and insist they are unique) so there will be no need to list what is signed in the common case.¶
However, some exotic headers may need to be signed for unusual or future use-cases. DKIM2 will still allow for extended headers.¶
As the modification algebra allows for reversing changes to headers, it will not be necessary to oversign as any "extra" headers matching a signature must be either removed by the modification algebra, or break the signature.¶
Any future hop must continue to sign any headers that were signed by previous hops, even if they removed the header via modification algebra they needs to sign the lack of it, so that modifications can be correctly checked.¶
There are some types of alteration, for example by security gateways, that may be impractical to describe in a cost-effective manner.¶
We would expect that outgoing gateways that may be adding disclaimers or rewriting internal identifiers would be provided with appropriate signing keys so that they could be the "first hop" as far as the rest of the email handling chain is concerned.¶
Incoming security gateways may be making substantial changes. Typically they will remove problematic types of attachment and rewrite URLs to use "interstitials". Since this type of functionality is generally provided on a contracted basis further intermediaries will be fully aware of the presence of the security gateway and can be configured to implicitly trust that it has checked earlier signatures and found them to be correct. Hence there is no need to be able to "undo" these changes.¶
Because an email can currently be sent as "Bcc" such that there's no evidence in the message data of who the recipient is expected to be, it's possible to take a message that is correctly signed and replay it millions of times to different destination addresses as if they had been BCC'd. This message can be resent at any time.¶
DKIM2 headers will always have timestamps so that "old" signatures have no value.¶
A possibility to be investigated during testing is a "singleton" flag to allow senders to specify that this is a message for a single recipient (e.g. for authentication codes for billing transactions) and should not be expanded by mailing lists.¶
DKIM2 headers specify both "from" and "to" so that most opportunities to alter a message, re-sign it and replay it at scale will no longer be possible. Since the "to" address is always encoded in the email, any email to multiple recipients must be exploded by the sender, and each copy signed separately with different headers.¶
If the email is replayed (perhaps through a large system with many different customers) then if the email does not say that it has been duplicated then signatures can be assumed to be unique and hence simple caching (or Bloom filters) will identify replays. If the email has been duplicated then recipients can assign a reputation to the entity that did the duplication (along with the expected number of duplicates that will arrive from that entity) and assess duplicate signatures on that basis.¶
If the email is altered before duplication then it is again the case that this will be apparent to the recipient who can develop a reputation system for the entity that did the modification and replay.¶
The problem of backscatter, delivery status notifications sent to innocent third parties who had their address forged as the source of a message, has caused email recipients to implement a variety of countermeasures:¶
in-band scanning: performing detailed analysis of the email content before replying to the DATA phase of the SMTP transaction, allowing immediate rejection but consuming resources on both ends of the connection, and limiting the time that can be used for the analysis to avoid timeouts.¶
greylisting: replying with a temporary failure code to untrusted senders, allowing time to decide if the sender is trustworthy enough, but also delaying mail for an indeterminate period.¶
delivery to "Spam" or "Junk" mailboxes - in some jurisdictions it's not allowed to discard email that has been accepted, so providers must put the copy somewhere once they have accepted it, filling Junk mailboxes even if they're very sure it's bad.¶
By requiring bounce addresses to aligned with the most recent signature domain, we can avoid backscatter, allowing recipients to always take the message, and later return a bounce. This fulfils any legal obligation to inform the sender if the message isn't delivered, while also avoiding the timeout and greylisting re-connection issue that currently exists, so messages are spooled for less time on intermediates, and recipients can take their time to analyse messages; even delivering the message to a mailbox and then upon receiving further intelligence, undoing the delivery and generating a bounce.¶
Privacy-preserving forwarding services will also see every bounce from any dkim2-supporting destination mailbox, allowing them to strip off the details of the further hop(s) and generate a bounce as if they had been the terminal node of the delivery and were just making a delayed decision.¶
The final specification will require both RSA and elliptic curve be implemented for algorithmic agility. However this document acknowledges the long standing lack of adoption of elliptic curve ,and elliptic curve support may not be needed for development. The specifications will provide support for multiple algorithms. If there is IETF consensus around a "post-quantum" scheme then that will also be included.¶
Dexterity will become essential if advances in cryptanalysis cause a particular type of algorithm to become deprecated. To allow a phased switch away from such an algorithm we will make provision for more than one signature to be present in a single DKIM2 header. Systems capable of checking both signatures will require both to be correct. If only one signature is correct then email will be rejected with a clear message -- allowing interworking issues to be easily debugged.¶
Experience at large mailbox providers is that incoming messages can have large numbers of DKIM signatures all of which need to be checked. For DKIM2, in the common case where email has not been altered by earlier hops, it will only be necessary to check the first DKIM2 signature, the one applied by the previous hop and, if "feedback" is to be provided, the signatures of any entities that have requested feedback.¶
If DKIM-replay is felt to be an issue (and some providers will detect this by identifying non-unique signatures) then more DKIM2 headers may need to be processed to establish the veracity of an alleged forwarding path. Additionally any attempt to do forensics or to assign reputation to intermediaries will require more signatures to be checked.¶
[[This section to be removed by RFC Editor]]¶
initial version¶