ARC-Seal, or Authenticated Received Chain Seal, is a part of the Authenticated Received Chain (ARC) protocol, which is a standard created to improve the handling of email messages that have been forwarded or that go through mailing lists.

When an email message is forwarded or goes through a mailing list, it can often break DKIM signatures, making it harder for receiving servers to verify the message’s authenticity. This can lead to legitimate messages being marked as spam or being rejected.


let’s dive a bit deeper into this.

DKIM, or DomainKeys Identified Mail, is an email authentication method that allows the receiving server to check if the email was indeed sent by the domain it claims to be sent from and if the content was tampered with during transit. It does this by verifying a digital signature in the email against the public DKIM key published in the sender’s DNS records.

However, when an email is forwarded or goes through a mailing list, the email’s content often gets modified. For example, an email forwarder might add “Fwd:” to the subject line, or a mailing list server might add a footer with unsubscribe information to the body of the email. These modifications, while often necessary or useful, change the email’s content and therefore break the DKIM signature, because the signature is created based on the content of the email.

When the DKIM signature is broken, the receiving server can no longer verify the DKIM signature, making it harder to confirm the email’s authenticity. As a result, the email might be marked as spam or even rejected outright, even if it’s a legitimate email. This is a significant problem, especially for mailing lists and auto-forwarding services, which often need to modify emails in transit.

This is where the Authenticated Received Chain (ARC) protocol, and specifically the ARC-Seal, comes in. It allows each server that handles the email to add its own seal and signature, preserving the authentication results from the previous servers. Each server adds an ARC-Seal, ARC-Message-Signature, and ARC-Authentication-Results to the message’s headers.

This way, even if the original DKIM signature is broken, the receiving server can still verify the email’s authenticity based on the ARC data.

The ARC-Seal specifically is a cryptographic seal that is added by each ARC participant. It covers the ARC message signature and the ARC authentication results of the previous hop in the email’s journey. This allows each server in the chain to verify that the ARC data added by the previous server has not been tampered with.

In summary, the ARC-Seal is a crucial part of the ARC protocol that helps to preserve email authentication results when messages are forwarded or go through mailing lists, improving email deliverability and reducing false positives in spam filtering.

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