When delving into email authentication protocols, you will encounter the Authenticated Received Chain (ARC) standard. ARC plays a crucial role in preserving authentication results when an email is forwarded or redirected through various intermediaries, which can often break traditional authentication like SPF and DKIM. Within the ARC-Message-Signature header, one of the key components you'll find is the a= tag.
The primary purpose of the ARC a= tag is to specify the cryptographic algorithm used by the signing entity to create the ARC-Message-Signature. This algorithm is fundamental to ensuring the integrity and authenticity of the ARC seal. Without a clear indication of the algorithm, a receiving mail server would be unable to properly verify the signature, thus undermining the entire chain of authentication.
What is ARC and why it's needed?
Understanding ARC email authentication
ARC addresses a common problem in email forwarding, where the original email's authentication results can become invalid. When an email passes through a mailing list, a forwarder, or a gateway, it might undergo changes that break its SPF or DKIM signatures. This can lead to legitimate emails being marked as spam or rejected by the final recipient's mail server. ARC provides a way to establish a chain of custody for the email, preserving the authentication results across these intermediary steps.
Each intermediary server that supports ARC adds an ARC-Seal and an ARC-Message-Signature header to the email. The ARC-Seal header authenticates the entire chain of previous ARC-Seal and ARC-Message-Signature headers, forming a cryptographic link. This chain allows the final recipient's mail server to verify that the email passed through trusted intermediaries.
The core of ARC's security lies in these cryptographic signatures. By ensuring that each hop in the email's journey is cryptographically linked, ARC helps maintain sender reputation and improves email deliverability. Mailbox providers, such as Google and Microsoft, increasingly rely on ARC to make informed decisions about incoming email, especially when it would otherwise fail DMARC.
Deconstructing ARC-Message-Signature
Deconstructing the ARC-Message-Signature header
The ARC-Message-Signature header is where the actual cryptographic signature for a specific hop is recorded. It contains several tags that define how the signature was generated and what was included in the signing process. Key tags within this header include v= for version, a= for algorithm, s= for selector, and b= for the actual signature data.
The a= tag is particularly important because it tells the verifying server which cryptographic method was employed. This ensures that the recipient's system can use the correct decryption and hashing mechanisms to validate the signature. Common algorithms include RSA-SHA256, which specifies the use of RSA for public-key cryptography and SHA-256 for hashing. Other elements like the domain (d=) and instance (i=) also provide crucial context for validation.
Understanding these different ARC header fields is essential for diagnosing delivery issues and ensuring your emails are properly authenticated. The bh= tag, for instance, records a hash of the email body, preventing tampering. The s= tag also indicates the selector used to retrieve the public key for verification, similar to DKIM selectors.
The algorithm behind the 'a=' tag
Algorithms and the 'a=' tag
The algorithm specified by the a= tag dictates the mathematical process used to generate and verify the digital signature. For example, rsa-sha256 signifies that the RSA algorithm is used for encryption (signing) and SHA-256 for hashing the message content. This combination is widely used due to its robust security properties. The security of ARC, therefore, directly depends on the strength of these underlying cryptographic algorithms. If a weak or deprecated algorithm were used, it could potentially be exploited to forge ARC seals.
According to the ARC specification (RFC 8617), the choice of algorithm is critical. It must be strong enough to resist brute-force attacks and cryptographic weaknesses. As technology evolves, cryptographic algorithms are updated, and it's important for email infrastructure to keep pace. Using current, recommended algorithms ensures that the ARC chain remains trustworthy and effective against evolving threats.
Best practice for ARC signing algorithms
Use current standards: Always opt for rsa-sha256 as the signing algorithm for ARC-Message-Signatures.
Regularly review: Stay informed about cryptographic best practices and any updates to ARC specifications.
Monitor reports: Use a DMARC monitoring platform like Suped to identify any authentication failures related to ARC.
When an email is forwarded, the intermediary mail server creates a new ARC-Message-Signature, which includes a hash of the original message and a hash of the previous ARC-Seal header. The a= tag ensures that each link in this cryptographic chain is verifiable using a consistent and secure method. If the algorithm specified is not recognized or is considered weak, the entire ARC chain might be rejected, treating the email as unauthenticated.
Why the 'a=' tag matters for email authentication
Impact on email deliverability
Proper configuration of the a= tag within your ARC-Message-Signature header is directly tied to your email deliverability. When an email is forwarded and its original DKIM signature breaks, a properly signed ARC chain can provide the necessary trust signals to the recipient's mail server. If the ARC signature cannot be verified due to an incorrect or unsupported algorithm, the email could be flagged as suspicious, even if it originated from a legitimate sender.
For organizations using mailing lists, shared inboxes, or forwarding services, ARC is indispensable. It allows these legitimate email flows to bypass the strict DMARC policies that might otherwise cause delivery failures. Without ARC, these forwarded emails would frequently fail DMARC alignment checks, leading to them being sent to spam folders or rejected outright. The integrity of the ARC chain, upheld by tags like a=, is what makes this possible, distinguishing legitimate forwarded mail from malicious spoofing attempts.
Without ARC
DMARC failures: Forwarded emails often fail DMARC, even if legitimate.
Spam classification: Increased likelihood of emails landing in spam folders or being blocklisted.
Reduced trust:Recipient servers may view legitimate emails as suspicious.
With ARC
DMARC preservation: Maintains authentication results across intermediaries.
Improved deliverability: Higher chance of legitimate forwarded emails reaching the inbox.
Enhanced reputation: Supports sender reputation by validating the email's chain of custody.
Monitoring your ARC authentication is a critical part of maintaining strong email deliverability. With a platform like Suped, you can gain comprehensive insights into your ARC results, identify any failures, and ensure that your emails are consistently delivered to their intended recipients. Our AI-powered recommendations actively help you strengthen your policy and fix issues.
Conclusion
Ensuring email trust with ARC
The ARC a= tag, though a small part of a larger email header, is fundamental to the integrity and trustworthiness of the Authenticated Received Chain. It ensures that the cryptographic signatures within ARC-Message-Signature headers are correctly interpreted and validated by receiving mail servers. For anyone managing email delivery, understanding the nuances of ARC is essential to navigate the complexities of modern email authentication.
By ensuring that your email infrastructure correctly implements ARC with appropriate algorithms, you enhance your sender reputation and improve the chances of your emails reaching the inbox. Tools like Suped provide real-time alerts and a unified platform for monitoring DMARC, SPF, and DKIM, along with blocklist and deliverability insights, making it easier to manage and optimize your email authentication strategy.
Google and 
Microsoft, increasingly rely on ARC to make informed decisions about incoming email, especially when it would otherwise fail DMARC.
