When you look at email headers, you're looking at the digital travel log of a message. One of the more complex, but increasingly important, sets of headers belongs to the Authenticated Received Chain, or ARC. ARC is designed to help preserve email authentication results (like SPF and DKIM) as an email passes through intermediaries, such as mailing lists or forwarding services, which are notorious for breaking authentication.
The process involves adding a series of ARC headers at each step, or 'hop', of the email's journey. The final header in this chain is the ARC-Seal header. This header effectively 'seals' the previous ARC headers, creating a verifiable chain of custody. Within this header, you'll find several tags, and the t= tag is one of them.
The ARC-Seal 't=' tag explained
Simply put, the t= tag in an ARC-Seal header represents the timestamp of the signature. It's a numerical value indicating when the ARC signature was created by the mail server that added the seal. This concept is not new to ARC; it's inherited directly from DKIM (DomainKeys Identified Mail).
The technical specification for ARC, as detailed in an IETF draft, confirms that the t= tag's definition is borrowed from the DKIM specification, RFC6376. This timestamp is crucial for several reasons:
- Auditing and debugging. The timestamp provides a chronological record of when each intermediary in the chain processed and sealed the message. This is invaluable for troubleshooting delivery issues, as it helps create a clear timeline of the email's path.
- Signature validation. While not used for the cryptographic validation itself, it gives context to the signature. A receiving server can see how old the signature is, which might be a factor in its overall trust assessment.
- Replay attack mitigation. In some security contexts, timestamps help prevent replay attacks, where a valid data transmission is maliciously repeated. While the primary defense here is the signature itself, an unusually old timestamp could be a signal for closer inspection.
How 't=' fits into the ARC header set
The ARC-Seal header is the final piece of a set of three ARC headers added by each intermediary. To understand the t= tag's role, it helps to know what the other parts do. As noted in a helpful post on ARC by VAND3RLINDEN, each ARC header includes an i= tag, which indicates the instance or position of that server in the chain.
The ARC-Seal header validates the ARC-Authentication-Results headers from the same instance (i=). The t= tag, along with others like the domain (d=), selector (s=), and the signature itself (b=), are all part of this sealing process. The timestamp provides a specific point in time when that validation occurred.
In summary, the t= tag is the signature timestamp within an ARC-Seal header. It marks the time when a specific mail server in the forwarding chain applied its ARC signature, providing a critical piece of chronological data for tracing an email's journey and verifying the integrity of the ARC chain.
