ARC (Authenticated Received Chain)
acronymsecurityISP & telecom
A mechanism letting an intermediary - a forwarder or mailing list - record the authentication results it observed and sign that record, so a later receiver can tell that a message failed only because it was legitimately relayed.
It exists because forwarding breaks SPF and list processing breaks DKIM, which means strict DMARC enforcement would reject a great deal of legitimate mail. ARC does not restore authentication; it preserves a signed assertion about what was true earlier, so the final receiver has the option of trusting the intermediary. That is the catch worth stating: the security of the chain depends on judging which intermediaries deserve trust, which is a policy decision rather than a cryptographic one.
Also known as: authenticated received chain, forwarding, mailing list breakage