]> Replay Resistant Authenticated Receiver Chain Google, Inc.
weihaw@google.com
Google, Inc.
arobins@google.com
Independent Stream DKIM ARC Replay DKIM (RFC6376) is an IETF standard for the cryptographic protocol to authenticate email at the domain level and protect the integrity of messages during transit. Section 8.6 defines a vulnerability called DKIM Replay as a spam message sent through a SMTP MTA DKIM signer, that then is sent to many more recipients, leveraging the reputation of the signer. We propose two Replay Resistant cryptographic domain based protocols that leverage ARC (RFC8617). The first technique discloses all SMTP recipients as signed RFC822 headers by the sender which allows a receiver to verify this. The second technique defines a SMTP extension that allows both the sender and receiver to participate in signing the message signature. It includes a path building technique that accurately defines the SMTP forwarding path of the message. Both techniques permit a receiver to detect DKIM and ARC replay.
Introduction This protocol provides two different techniques to authenticate email by domain that is replay resistant. It leverages the features of ARC to name ADMD in the email forwarding path and the intermediate results. The first technique discloses all SMTP recipients as signed RFC822 headers by the sender which allows a receiver to verify this. The second technique defines a SMTP extension that allows both the sender and receiver to participate in signing the message signature. These results may be used by spam filtering to apply some local policy, and/or applied to DMARC policy evaluation as one of its input email authenticators.
Terminology and Definitions Acronyms
  • ARC (RFC8617) - is a protocol that is meant to resolve some of the issues for DMARC (RFC7489) to fix the problems that DMARC policy rejects caused by mail forwarding, and is based on DKIM, but suffers from similar issues as DKIM replay. ARC defines digital signatures and Authentication-Results (RFC8601) by Administrative Management Domain (ADMD) and a versioning mechanism for them.
  • Authentication-Results (RFC8601)- A header containing a list of validation results and comments.
  • DKIM (RFC6376)- IETF standard for the cryptographic protocol to authenticate email at the domain level and protect the integrity of messages during transit.
  • DKIM replay- RFC6376 Section 8.6 defines a vulnerability called DKIM Replay as a spam message sent through a SMTP MTA DKIM signer, that then is sent to many more recipients, leveraging the reputation of the signer.
  • DMARC (RFC7489)- Defines a sender's domain identity and from that a sender's message handling policy when messages are being spoofed. It defines using SPF and DKIM as methods to determine the authenticity of messages.
  • signed RFC822 header recipients- These identities are defined by To, Cc and Forwarded-to headers, and MUST be a signed headers present in the ARC-Seal.
  • SMTP recipients- The RFC5321 MAIL FROM recipients are disclosed during the SMTP transmission. These identities define the inboxes that the message is intended for.
ARC Set Improvements This protocol uses ARC (RFC8617) to describe all ADMDs using the ARC set number as the ADMD version numbers. The initial ARC set "i=1" is defined for the message originator, and signs for the outbound message with an empty ARC-Authentication-Results. Traditionally ARC signing happens on inbound, however with these changes ARC implementations should be modified to sign on outbound as well that enables marking origination and forwarding. This protocol adds additional new ARC-Seal tag/values to describe protocol settings and new ARC-Authentication-Results status and comments. This protocol also heavily leverages ARC sealing and message signing which in turn leverages DKIM cryptographic primitives.
Declare All Recipients and Affirm (DARA)
Concepts We can harden the protocol against replay attacks by explicitly identifying all recipients in the headers, including when the recipient is "hidden" such as Bcc or Mailing-lists. That way when a signed message arrives, the receiver can check if the RCPT TO recipient correctly is a subset of the recipient in the signed message header. If not, then the message may be part of a replay attack. For blind carbon copy, while a Bcc header may be added, it can be stripped by subsequent forwarders. Instead we create a new Forwarded-to header that includes an ARC set versioning number to indicate which ADMD sent the message to a new recipient. The Forwarded-to header MUST be signed by the ARC-Message-Signature i.e. be present in the "h=", then prepended to the headers of the message. For privacy, if there are multiple recipients, the message must be split and signed exclusively for each Forwarded-to recipient to maintain privacy between recipients. Subsequent forwarders MUST not strip the Forwarded-to header from the message. To handle the email forwarder and mailing list scenario, we also use the Forwarded-to header to indicate that a message is sent to a new recipient. Messages sent to a new ADMD but with the same recipient identity disclosed by Forwarded-to MAY reuse the prior header. Senders and receivers may variously support the Declare All Recipients and Affirm (DARA) protocol or not, so the protocol needs to be tolerant of naive ADMDs. For example a naive mailing list sender sending to a protocol aware receiver shouldn't have traffic rejected simply because it didn't follow the protocol. Yet simultaneously, the DARA protocol needs to discourage abuse by spammers seeking to use the naive ADMD path for DKIM replay. In this protocol, that sender publishes their capability in the ARC-Seal as "dara=" tag/value, and whether the receiver should validate recipients. A value of "v" indicates that the receiver MUST validate the recipients, and if it fails verification, treat the message as DARA unauthenticated with the implication that the message is being replayed. As with other email authentication methods, the verifier is free to apply a locally defined policy against unauthenticated email. A value of "d" indicates that the receiver MAY choose to discretionally validate the recipients. If a receiver validates the recipients, it SHOULD treat recipient verification failure as neutral and SHOULD treat success as passes. The discretionary validation mode is to support the scenario of sending to a naive ADMD that does not support ARC or the DARA protocol. Because such naive forwarders may not add any indication of its presence e.g. adding an ARC set, the sender must protect subsequent DARA aware receivers from misinterpreting prior settings while allowing for recipient updates that may otherwise trigger false positive verification failures. All ADMD supporting the DARA protocol MUST publish a DNS txt policy record as described below. The sender fetches the receiver's policy record to determine whether to select the required verification "dara=v" which is done when the receiver supports the DARA protocol, otherwise the sender selects the optional "dara=d" validation profile. In addition when the receiver does not support the protocol, the sender always identifies the individual signed recipient. This may be needed when the recipient is in the To, or Cc headers, and in this case also adds a Forwarded-to header per recipient, then signs the message only for that recipient. Unique identification of the recipient and the receiving domain allows a receiver to adjust the reputation system in case there is a replay attack. Instead of penalizing the sender that is DARA aware, the receiver MAY elect to apply the reputation penalty to the receiving domain that is naive to DARA. The receiver's verification process is to collect all the recipients in the To, Cc and Forwarded-to headers. It verifies that they are signed appropriately in the sender's ARC-Message-Signature and if so, put them into a set of signed headers. The receiver then collects all the RCPT TO recipients as the envelope recipients. The receiver then verifies that the envelope recipients are a subset of the signed headers. It applies the policy depending on the sender's capabilities as described in the ARC-Seal "dara=" tag/value. The result of this check should be published in the ARC-Authentication-Results as dara=pass/fail/neutral. The DARA DNS policy record identifies whether an ADMD supports the protocol. It is a TXT DNS record located at the same domain name as the MX record. Quite likely it will share the policy record with SPF. Such a policy record starts with a SeRCi version number "dara_version=" which must be set to "ver1.0" indicating that ADMD supports DARA. While usually the sender looks up the DARA TXT DNS record, a receiver MAY elect to check the sender's policy if it suspects that a MiTM has stripped off the sender's DARA policy. If it detects a DARA declaration in the DNS policy, but not in the message, the receiver may elect to treat the message as spam.
Definitions DNS TXT Policy tags
  • "dara_version=": Value of "ver1.0" if the ADMD supports the DARA verification protocol.
ARC-Seal tags
  • "dara=": Value of "v" if the sender mandates that the receiver verify the recipients. Value "d" if the sender asks the receiver to optionally verify the recipients, and writes a "pass" if the recipient verification passes.
ARC-Authentication-Results tags
  • "dara=": Value of "pass" if recipient validation passes, otherwise "fail". In some circumstances this tag/value may not be set or be treated as "neutral".
Header Examples
MBP-Mailing-List-MBP First MBP outbound (after ARC seal) Mailing-List inbound (after ARC seal) Mailing-List outbound (after ARC seal) Final MBP inbound (after ARC seal)
MBP-MBP-Replay-MBP First MBP outbound (after ARC seal) Second MBP inbound (after ARC seal) Above message captured by spammer, modified (add additional headers) and then resent. A spammer might send the message to john.doe@example.com which would be unspecified in the headers. Victim (last) MBP inbound (after ARC seal)
MBP-Naive-Forwarder-MBP This describes a forwarder that doesn't not support DARA. First MBP outbound (after ARC seal) Forwarder headers will be the same as above as the forwarder is naive to the protocol. Final MBP inbound (after ARC seal). In this case the envelope recipient will change weihaw@example.com. The declared recipient user@other.example.com will mismatch the envelope recipient, and fail DARA. However the protocol is set to optional verification with DARA=d, and so does not report the failure.
Sender Receiver Co-Signing (SeRCi)
Concepts We can create a challenge response system using cryptographic signing orchestrated between the sender and receiver of an SMTP transaction. The receiver challenges the sender to sign a mutually agreed upon value with their secret key, and can demonstrate a proof of that SMTP client-server relationship to 3rd parties. One problem is that the receiver can't proactively issue the challenge, so as part of the EHLO, the server issues the challenge as an optional SMTP extension argument. The sender can respond with the signature incorporating the shared value as a SMTP extension verb. Another problem is preventing a malicious party from intercepting a message and trying to replicate the challenge. We propose using a timestamp that can't be used in the future i.e. both parties make sure the timestamp reasonably represents the current time. This cryptographic challenge needs to sign a hash that ties the signature back to the message, and for this proposal, we take the whole message hash from the ARC-message-signature. In addition the destination domain is specified to reduce the risk for this signature to be intercepted and reused for other communications with other destination domains. Such a protocol can help authenticate to a receiver that some sender sent a message without risk of replay via some third party. Sender Receiver Co-Signing (SeRCi pronounced Cersei as in the GoT character) could be used similarly to SPF (RFC7208) but without the risk of shared tenancy attack, IP reuse attack, and BPG vulnerabilities. Moreover a third party can independently verify the result that some sender and receiver sent the given message at the given time. This obviates the need to trust the ARC-Authentication-Results. Later we use SeRCi metadata to describe the forwarding path of the message. The SMTP extension for SeRCi for generating the hash and then publishing it is meant to prove that the sender and receiver collaborated to create the hash. The protocol is advertised as a SMTP extension in the SMTP EHLO named SeRCi with a timestamp argument. That timestamp will be in UTC seconds. If the timestamp is acceptable to the sender, then it should sign a tuple of base64 message hash used in the outbound ARC-Message-Signature, destination domain as defined in the next paragraph, and timestamp. That signature then should be base64 encoded and disclosed to the receiver as: ]]> where domain corresponds to the sender's DKIM domain and selector that is used to find the DKIM public key DNS record. If the timestamp is not acceptable, the sender can report this as SERCI-SIGNATURE "out-of-time" and potentially the receiver will return a new timestamp. The sender is allowed to do this once, and after that the receiver MUST report an error. To prevent eavesdropping and potential spoofing, this protocol must be secured by SMTP TLS. Upon obtaining the signature, the receiver MUST then validate the SeRCi signature. It looks at the sender's ARC-Message-Signature hash to see if that is acceptable, meaning matches a hash the receiver generates of the message. Next it checks if the timestamp is the same as provided to the sender, and if the destination domain is the same as the receiver's ARC-Seal "d=" domain. The SERCI-SIGNATURE command returns OK on success, otherwise some error code. An example SMTP transaction might look like: MAIL FROM: RCPT TO: DATA SERCI-SIGNATURE )))> ]]> The sender discovers the receiver's support for this protocol by a DNS txt policy lookup upon the recipient email address domain. Within this policy record may be a tag value indicating which SeRCi version number "SERCI_version=" which must be set to "ver1.0" when that ADMD indicates it supports SeRCi. The lookup also discovers the normalized destination domain name, and that destination domain MUST match the ADMD ARC-Seal "d=" domain (RFC8601) which enables tracing this domain from sender to receiver as described later. The domain name is specified serci=&lt;domain> in the DNS policy record. Once discovered this domain is put in the sender's ARC-Seal as serci=&lt;domain>, which indicates support by the receiver for the SeRCi as well as identify the intended receiver domain. If no such DNS txt policy record is found, then the receiver does not support the SeRCi protocol. This is indicated in the ARC-Seal by a SeRCi naive receiver tag/value of "snr=" and RFC822 FROM domain for path building described later. Further the "snr=" tag indicates to subsequent SeRCi aware receivers that there was an intermediate naive forwarder. If a domain advertises a SMTP SeRCi-SIGNATURE extension but does not publish a DNS txt policy, the sender MUST not call the SeRCi-SIGNATURE command as the receiver is declaring their intent to not participate in SeRCi. The SeRCi aware receiver will verify the signature after the SeRCi-SIGNATURE verb. Assuming the receiver agrees with the signature (i.e. verifies it), in addition to return OK , the receiver adds the signature as part of the SeRCi ARC-Authentication-Results as a comment: (,,, ,); ]]> The destination domain present in the signature is the same as that ADMD's ARC-Seal "d=" domain. The domain, selector, message-hash, timestamp and the signature should allow a 3rd party to verify the signature given a public key. Verification success or failure would also be indicated in the ARC-Authentication-Results for SeRCi as "serci=<pass|fail\>". The receiver indicates agreement i.e. verification success with the sender's signed signature by sealing the signature and indicating a passing result. A 3rd party can take the ARC-seal and SeRCi signatures and verify them to prove that a message was sent from the sender to receiver at the given time. If a sender does not provide a SERCI-SIGNATURE, most likely this is because the sender is unaware of the protocol. However a receiver MAY lookup the sender's SeRCi policy contained in the DNS txt record. If it detects such an active policy yet SERCI-SIGNATURE was not issued, then the receiver may note a SeRCi ARC-Authentication-Results verification failure. Lack of SERCI-SIGNATURE potentially may happen due to MiTM in the SMTP transaction even with TLS.
Definitions DNS TXT Policy tags
  • "serci_version=": Value of "ver1.0" if the ADMD supports the SeRCi verification protocol.
  • "serci=": Value of the receiver's ARC-Seal "d=" domain
ARC-Seal tags
  • "serci=": Value of the receiver's ARC-Seal "d=" domain when the receiver is SeRCi capable.
  • "snr=": Value of RFC822 recipient's domain name when the receiver is naive of SeRCi.
ARC-Authentication-Results tags
  • "serci=": Value of "pass" if sender/recipient signing validation succeeds, otherwise "fail".
Header Example
Chaining A receiver may check that a message was forwarded from the originator to it without discontinuities, and as intended by the originator. With SeRCi the sender defines a tag "serci=" with a value of the intended destination domain. If a discontinuity is detected, that indicates either there is a protocol unaware forwarder, or that there is a malicious sender attempting to take the message and reinject it along a new path outside the intent of the originator. Moreover the path that the message traverses can be used as the message flow identifier in a reputation system. It can help differentiate benign message flows from malicious ones to help identify a different form of replay. If the originator is malicious and intends for a message to be replayed even as part of an identified ADMD path, a path based reputation system can better segregate those malicious messages than a domain based one. We define nodes of the path as the ARC-Seal "d=" identities and form edges from SeRCi destination domain identities. Because building the edges of a path is recursive, we call this chain building. The edge is defined as a pair of nodes n1, n2 where n1 is some ARC-Seal "d=" domain and the "SeRCi=" domain is n2. This assumes that validation results from SeRCi and potentially DARA have already run and the results already populate the ARC-Authentication-Results. Chain building starts at the origination at ARC set "i=1", and walk through the ARC headers to the final receiver's ARC set "i=N". First the verifier looks up the SeRCi policy associated with the ARC set "i=1" ARC-Seal "d=" domain. If there is a valid SeRCi policy, then chain building can proceed. At ARC set "i=1" only the ARC seal and message-signature is checked. Starting at ARC set "i=2", path verifier MUST evaluate the local result, meaning the ARC seal, message-signature and SeRCi verification results, and the prior ARC set results are correct, and take the AND of the results. If any one of them is a failure or neutral, the path result is a failure or neutral. If the SeRCi result is missing, the path verifier checks if there was "snr=" tag at that node or prior node, then treats the SeRCi result as neutral, to tolerate the naive receiver. If not then treat the SeRCi result as missing and hence a fail. Otherwise if all the local results passed, then the current path results is a pass. Further local policy may modify the ARC message-signature result (perhaps due to future work around this draft or this one) The path verifier SHOULD incorporate meaning AND the DARA local results if available as well. In the local evaluation, the verifier SHOULD independently verify the SeRCi signature instead of taking the result from ARC-Authentication-Result and having to trust an externally generated result. The path verifier then evaluates the next ARC set results until it reaches the final recipient to compute a global result. That global result is published in the final receiver's at ARC set "i=N" ARC-Authentication-Results as a "chain=" result. The chain builder will internally collect the list of ARC-Seal "d=" nodes from ARC set 1 to N for when there is a "serci=" edge defined and passing result. The "snr=" domain may also be used to fill in a naive domain in the path. The path is for reputation building but this is not added to ARC-Authentication-Results.
Definitions ARC-Authentication-Results tags
  • "chain=": Value of "pass" if local results and prior nodes are all passes, otherwise if "snr=" was present in the flow then "neutral", else "fail".
Header Examples
MBP-Mailing-List-MBP First MBP outbound (after ARC seal) Mailing-List outbound (after ARC seal) Final MBP inbound (after ARC seal) The path verification result is "pass". The constructed path is (originator.example.com, mailinglist.example.com, terminator.example.com).
MBP-Naive-Forwarder-Aware-Forwarder-MBP This example includes naive forwarder naive.example.com that doesn't not support SeRCi. This is taken after the last MBP. The path verification result is "neutral". The constructed path is (source.example.com, naive.example.com, intermediary.example.com, destination.example.com).
MBP-MBP-Replay-MBP Final headers as seen by the victim MBP after replay injection to victim.example.com domain. Due to the path discontinuity, the path verification result is "fail". The constructed path is (source.example.com, destination.example.com).
DMARC These protocols can act as authenticators for DMARC (RFC7489). In particular SeRCi can act similarly to SPF in authenticating peers, while Chain's path is similar to DKIM in being tolerant of forwarding. This protocol modifies DMARC to permit the SeRCi and Chain to authenticate for DMARC. It requires alignment with the RFC822 domain for SeRCi sender's ARC-Seal "d=" domains. With Chain, that is further restricted to the originating sender's domain at ARC set "i=1".
Privacy Considerations
Security Considerations
IANA Considerations This document has no IANA actions yet.
Acknowledgments Thanks goes to Brandon Long, John R. Levine, Murray S. Kucherawy and Bruce Nan for their sage advice.