The Authenticated Received Chain (ARC) Protocol
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J. Levine, T. Herkula · 2019-07
Abstract
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 prior handlers have asserted about it. The chain of authentication results is preserved even if the message is modified by intermediary handlers.
Why This RFC Matters
RFC 8617 introduced ARC to solve the problem of DMARC failures caused by legitimate mailing lists and forwarding services that modify email messages, breaking DKIM signatures and invalidating SPF checks. ARC creates a chain of signed headers recording each intermediary's authentication results, allowing the final receiver to evaluate whether an earlier trusted handler had verified the message. Large mailbox providers including Google and Microsoft have implemented ARC evaluation to reduce false positives for forwarded mail, making it an important part of the modern email authentication ecosystem despite its Experimental RFC status.