When you're setting up email authentication, you'll encounter a lot of technical jargon and settings. One of the core technologies you'll work with is DKIM (DomainKeys Identified Mail). It's a powerful tool for verifying that an email was actually sent and authorized by the owner of that domain. Getting this right is crucial for deliverability and protecting your brand from phishing attacks.
DKIM works by adding a digital signature to the headers of your emails. This signature is created using a private key, and receiving mail servers can verify it by looking up a corresponding public key published in your domain's DNS records. The record itself is a simple text (TXT) record, but it's packed with information structured as tags.
These tags are essentially key-value pairs separated by semicolons, like v=DKIM1;. Each tag provides a specific piece of information for the verification process. While some tags are mandatory, others are optional, and one of the more obscure optional tags is the x tag.
The purpose of the 'x' tag
The x= tag in a DKIM signature specifies the signature's expiration date and time. It's a security feature that sets a deadline for when the signature is considered valid.
If a receiving mail server processes an email with a DKIM signature containing an x tag, it checks the timestamp in the tag against the current time. If the current time is past the expiration date, the DKIM signature is treated as invalid, and the check will fail, even if the cryptographic signature itself is correct.
How it works with other tags
The expiration tag doesn't work in isolation. It's designed to be used with the t= tag, which specifies the time the signature was created. Together, they create a validity window for the email signature.
- t= (Signature Timestamp): This tag marks the exact date and time the email was signed. It's a mandatory-to-implement but optional-to-verify tag.
- x= (Signature Expiration): This optional tag defines a future point in time after which the signature should be considered invalid.
This mechanism helps prevent a specific type of attack where a malicious actor might capture a validly signed email and resend it much later. Without an expiration, the signature would remain valid indefinitely. By setting an expiration date, you limit the window of opportunity for such abuse. This strengthens your overall email security posture and helps maintain your sender reputation, which is key to avoiding spam folders and staying off a dreaded email blacklist (or blocklist).
Should you use the 'x' tag?
The x tag is optional, and according to the official DKIM specification (RFC 6376), it is not required for a signature to be considered valid. Most email service providers do not include it by default in the DKIM signatures they generate.
For the vast majority of senders, not using the x tag is perfectly fine. Standard DKIM implementation provides a very high level of protection against spoofing and phishing. However, for organizations with extremely high-security requirements, such as financial institutions or government agencies, adding a short expiration time can provide an additional layer of protection against replay attacks.
In summary, the x tag is a tool for time-limiting the validity of a DKIM signature. While not commonly used or required, it serves a specific security purpose by adding an expiration date, making it a valuable option in high-security contexts.
