ARC (Authenticated Received Chain)

ARC, short for Authenticated Received Chain, is an authentication protocol designed to bolster the security of email communication. It aims to address the issue of trustworthiness in message delivery, especially when they traverse through multiple mail servers before reaching their final destination.

ARC in Email Communication

Email authentication is a multi-layered approach to verify the authenticity and integrity of messages. ARC is a critical addition to the authentication ecosystem, enhancing the cooperation between SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance).

SPF and DKIM provide fundamental checks on the sender's domain and the data integrity, respectively. However, these two security solutions are not inerrant:

• SPF checks can fail when emails are forwarded or relayed through intermediary servers. For example, failing to set up SPF records for subdomains can lead to SPF failures for emails sent from those subdomains due to the break in SPF alignment.
• DKIM signatures are calculated based on the content of the message. Any modifications to the message, such as adding or removing content, can invalidate the DKIM signature.
• DMARC adds policy enforcement and reporting capabilities to further enhance control and visibility over email authentication. So, if a message fails both SPF and DKIM, it fails DMARC being considered suspicious, and will be rejected.

ARC ensures that these checks remain valid even when emails pass through intermediaries.

By working together, these protocols create a robust defense against spoofing, phishing attacks, and other email-based threats, ultimately improving security and trust in digital communication. Implementing all these protocols correctly is crucial for organizations aiming to protect their brand reputation and maintain secure email communication.

Unveiling ARC Mechanism and Operation

Authenticated Received Chain operates by creating a cryptographic chain of trust within email headers. Each mail server that processes a message adds a header containing a digital signature, allowing the recipient's server to verify the authenticity of the message's path. This chain of trust ensures that modifications made to the email during transit can be detected and authenticated.

Here are the steps involved in ARC signature implementation with examples:

Step 1: Initial Signature (First Server)

When an email is sent, the first mail server adds a DKIM (DomainKeys Identified Mail) signature to the message's header. This DKIM signature attests to the authenticity of the sender's domain. For example:

      DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com;
      s=selector1; h=from:subject:to; bh=abcdefg1234567=;
      b=XYzABCDeF1234==
    

Step 2: ARC Signature (Intermediate Servers)

As the email passes through intermediate servers (e.g., forwarding services or mailing lists), each server adds an ARC header with its own cryptographic signature containing the authentication result of the previous check. Thus, the header includes the original DKIM signature from the first step. For example:

      ARC-Authentication-Results: i=1; mx.google.com;
      dkim=pass header.i=@example.com header.s=selector1;
      arc=pass (i=0 spf=pass dkim=pass);
    

Signature Components Breakdown

Step 3: Final Signature (Last Server)

When the email reaches its final destination, the recipient's mail server validates all the ARC signatures in the chain. It also verifies the DKIM signature from the first step, ensuring that the message's path was legitimate and that no unauthorized modifications occurred during transit. If all signatures are valid, the email is delivered to the recipient's inbox.

Implementing ARC: Best Practices

By implementing an Authenticated Received Chain, organizations can ensure the integrity of their email communication, even when messages pass through multiple intermediate servers. It provides a clear chain of trust, enhancing security and reducing the risk of spoofing and phishing attacks.

If you're considering implementing ARC for your email domain, here are some best practices to follow:

• SPF and DKIM Setup: Ensure that Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) are correctly configured for your domain. ARC builds upon these existing authentication mechanisms.
• Choose a Reputable Email Service Provider: If you're not well-versed in email authentication protocols, consider using a service provider that supports ARC. They can guide you through the setup process.
• Monitoring and Testing: Regular monitoring of email traffic is strongly recommended for any issues related to ARC implementation. Testing is crucial to identify and resolve problems promptly.
• Keep Policies Updated: As your email infrastructure evolves, update your ARC policies to reflect these changes. Outdated policies can lead to authentication failures.