Yes, Authenticated Received Chain (ARC) absolutely requires a private key for signing. The entire protocol is built upon the foundation of public-key cryptography, which is the same technology that powers DKIM (DomainKeys Identified Mail). Without a private key, a mail server cannot generate the cryptographic signature required to add its link to the chain of trust. This signature is essential for ensuring the authenticity of email messages as they are passed between different servers, especially through intermediaries like mailing lists or forwarding services.
How ARC signing works
When an email passes through a mail server that acts as an intermediary, standard authentication methods like SPF and DKIM can sometimes break. ARC was designed to solve this by creating a verifiable sequence of authentication results. Each server, or “hop,” in the email’s path that handles the message can add an ARC signature. This signature essentially says, “I received this message, these were its authentication results, and I am now passing it along.”
This process relies on public-key cryptography to prevent tampering. As Apple's developer documentation explains, you create a signature by hashing data and then encrypting that hash with your private key. This is exactly how ARC works.
- Private Key: Each mail server that participates in ARC has a secret, private key. This key is used to create the digital signature for the ARC-Signature header.
- Public Key: A corresponding public key is published in the domain's DNS records. Anyone can access this key.
- Verification: When a receiving server sees an ARC-signed message, it retrieves the public key from the DNS. It then uses this key to verify that the signature was created by the corresponding private key and that the signed content hasn't been altered.
What happens without a private key?
Without a private key, ARC signing is simply impossible. The server has no tool with which to create the cryptographic signature. Any attempt to apply an ARC seal will fail, and often a specific error will be generated. For example, some system logs might show an error message explicitly stating the issue, which Plesk highlights in their support documentation.
This error perfectly illustrates the point. The system attempted to sign the message with ARC but could not proceed because it was unable to read the necessary private key. The process was skipped entirely, leaving a gap in the chain of trust.
In summary, the private key is not just a component of ARC; it is the core of its security model. It provides the mechanism for creating the verifiable, tamper-evident signatures that allow receiving mail servers to trust the authentication results of messages that have passed through other systems. Without it, ARC cannot function.
