When you send an email, various mechanisms work behind the scenes to ensure its legitimacy and deliverability. One crucial component is the DomainKeys Identified Mail (DKIM) signature, which acts like a digital seal for your messages. Within this signature, you'll find several tags, each serving a specific purpose. Among them, the 't' tag holds particular importance, yet it's often overlooked or misunderstood.
At its core, the 't' tag provides a timestamp for when the DKIM signature was created. This simple piece of information plays a critical role in verifying the authenticity and integrity of your emails, helping to combat spam, phishing, and other malicious activities. Understanding its function is key to maintaining a strong email security posture and ensuring your messages consistently reach the inbox.
The 't' tag in detail
The 't' tag in a DKIM signature stands for 'timestamp'. It represents the time the DKIM signature was generated by the sending Mail Transfer Agent (MTA). This value is recorded as a Unix epoch timestamp, which is the number of seconds that have elapsed since January 1, 1970 (UTC). For example, a value like 1674489600 would correspond to a specific date and time.
While it's considered an optional tag according to the DKIM specification, including the 't' tag is widely recommended for robust email authentication. It provides valuable context for the receiving server, allowing it to assess the freshness of the signature and, by extension, the email itself.
Without this timestamp, it would be much harder for recipient servers to determine if a signature is current or if it's an old, potentially compromised signature being reused. This becomes especially important when combined with the expiration time tag ('x='), which we'll discuss next.
The 't' tag is a critical security measure against replay attacks. A replay attack involves an attacker capturing a legitimate email and then re-sending it at a later time to impersonate the sender or bypass security filters. By including a timestamp, the receiving server can check if the email was signed recently enough to be considered valid.
This tag works in conjunction with the 'x' tag, which specifies the expiration time of the signature. When a mail server receives an email, it compares the current time against both the 't' and 'x' values. If the current time is before the 'x' value but after the 't' value plus a reasonable buffer, the signature is deemed fresh and valid. If the signature has expired, or if the timestamp is suspiciously old, it can indicate a potential issue.
Detecting tampering and delays
A valid 't' tag ensures that the DKIM signature hasn't been created too far in the past. This helps prevent adversaries from stockpiling legitimate, but outdated, signatures to launch future attacks. It adds an extra layer of time-based security to your email authentication, crucial for overall email deliverability.
Parsing and validating the 't' tag
Receiving mail servers parse the DKIM-Signature header field to extract all the tags, including 't'. They then perform a series of checks. First, the 't' value is converted from a Unix timestamp to a human-readable date and time. This time is compared against the current time at the receiving server. If the signature is too old, or if it appears to be from the future (indicating a misconfigured sending server), it can lead to DKIM authentication failures.
For email senders
Ensure accurate server time: Keep your mail server's clock synchronized with a reliable Network Time Protocol (NTP) source to ensure correct timestamp generation.
Timestamp comparison: Receiving servers check if the 't' tag's value falls within an acceptable time window, typically defined by the 'x' tag and internal policies.
Expiration check: The 't' tag is used with the 'x' tag to ensure the DKIM signature has not yet expired, preventing replay attacks.
Properly parsing and validating the 't' tag is a standard procedure for mail servers, contributing significantly to the overall authentication process, including DMARC, SPF, and DKIM alignment.
Monitoring DKIM and the 't' tag
Manually monitoring DKIM signatures and their 't' tags across all your outgoing emails is virtually impossible for any organization with significant email volume. This is where DMARC reporting and monitoring tools become indispensable. These tools collect aggregate and forensic reports from receiving mail servers, providing insight into your email authentication results.
These reports can reveal if your DKIM signatures are failing authentication, which could be due to an incorrect 't' tag, expired signatures, or other issues. Identifying and addressing these problems quickly is crucial for protecting your domain's reputation and ensuring your emails are delivered, rather than being sent to spam or rejected.
Streamline DKIM monitoring with Suped
Our platform, Suped, excels at simplifying DMARC monitoring and reporting. With our generous free plan, you can gain immediate visibility into your DKIM authentication results. Suped provides AI-powered recommendations to help you fix any identified issues, including those related to the 't' tag, and strengthen your email authentication policies. We offer real-time alerts for immediate notification of critical failures, a unified platform for DMARC, SPF, and DKIM, and SPF flattening to ensure optimal deliverability.
Ensuring robust DKIM implementation
The 't' tag, while seemingly minor, is a foundational element in the robust architecture of DKIM. It ensures that your email signatures are not just valid, but also current, protecting your domain from various forms of email fraud, including replay attacks and impersonation.
As email security threats evolve, paying attention to every detail of your authentication protocols becomes increasingly important. Proper configuration of all DKIM tags, including the 't' tag, is a non-negotiable step for maintaining trust in your email communications.
By understanding and actively monitoring the 't' tag, along with other DKIM tags, you empower your domain with stronger email authentication, leading to improved deliverability and a more secure email ecosystem.
AI-powered recommendations to help you fix any identified issues, including those related to the 't' tag, and strengthen your email authentication policies. We offer real-time alerts for immediate notification of critical failures, a unified platform for DMARC, SPF, and DKIM, and SPF flattening to ensure optimal deliverability.
