The short answer is yes, absolutely. While Authenticated Received Chain (ARC) wasn't designed primarily as a forensic tool, its core function creates a verifiable 'chain of custody' for an email. This makes it an invaluable asset for anyone conducting a forensic investigation into an email's origins and journey.
To understand how, we first need to define what we mean by email forensics and what ARC does.
What is email forensics?
Email forensics is the process of analyzing email evidence for legal or investigative purposes. It’s not just about reading the email content; it’s about dissecting the metadata, headers, and transmission data to piece together a story. As the security firm Salvation DATA puts it, email forensics is about collecting digital evidence to crack crimes or resolve disputes.
This involves verifying the authenticity of messages, tracing their path from sender to recipient, and identifying any tampering that may have occurred along the way. Traditionally, this relies heavily on analyzing email headers, which contain information about the servers an email passed through.
What is ARC?
Authenticated Received Chain (ARC) is an email authentication standard designed to solve a specific problem with DMARC, SPF, and DKIM. When an email is forwarded or passes through an intermediary like a mailing list, the forwarding server can break the original SPF alignment and DKIM signature. From the final recipient's perspective, the email might look like it fails authentication, even if it was legitimate to begin with.
ARC preserves the original authentication results. It allows each server in the handling chain to see the authentication status from the previous 'hop' and add its own cryptographic seal of approval. This creates a trusted chain, showing that even if the final checks fail, the message was authenticated at an earlier stage.
How ARC assists in email forensics
The very mechanism that allows ARC to preserve authentication results is what makes it so useful for forensic analysis. The sequence of ARC headers added to an email serves as a log of its journey through different mail servers.
In fact, according to email security specialists at Metaspike, this process is now a fundamental part of the job.
Each ARC seal acts as a digital signature from an intermediary, attesting to the validity of the authentication results it received. An investigator can cryptographically verify each seal in the chain, from the final recipient back to the earliest intermediary that supports ARC. This provides a strong, verifiable trail that can help confirm an email's path and demonstrate that it hasn't been tampered with in transit. If a seal is broken, it points to the exact spot in the chain where something went wrong.
Understanding the ARC headers
The ARC 'chain' is comprised of three key headers that are added at each hop:
- ARC-Authentication-Results (AAR): This header contains the results of the SPF, DKIM, and DMARC checks as seen by the intermediary server. It essentially says, 'This is what I saw when the email arrived here.'
- ARC-Message-Signature (AMS): This is a DKIM-like signature that covers the entire message, including its headers and body. It's a snapshot of the message content at that specific hop.
- ARC-Seal (AS): This is the crucial 'seal' that validates the integrity of the previous ARC headers (AAR and AMS). It signs the previous ARC-Seal header in the chain, creating a linked sequence that is difficult to forge.
By analyzing these three headers at each step, an investigator can reconstruct the email's journey and verify the authentication status at every point where ARC was applied.
Limitations to consider
While powerful, ARC is not a forensic silver bullet. Its effectiveness depends on its adoption. If an email passes through servers that don't support ARC, there will be gaps in the chain. The forensic trail goes cold at the point where a server fails to add its ARC headers.
Furthermore, the trust in the chain is only as strong as the trust in the intermediaries themselves. While cryptographically secure, the system relies on participating servers being configured correctly and not acting maliciously. However, for most investigations involving major email providers, who have widely adopted ARC, it provides a very reliable record.
Conclusion
ARC provides a robust and essential mechanism for email forensic analysis. By design, it creates a verifiable chain of custody that shows how an email traveled from one server to the next and what its authentication status was at each point.
For anyone needing to prove the path of an email or investigate potential tampering, the ARC headers are one of the first and most valuable pieces of evidence to examine.
