What is the primary function of the DKIM 'bh=' tag?

The primary function of the bh= tag is to store a cryptographic hash of the email's body. This hash is used by the receiving mail server to verify that the email's content has not been altered or tampered with since the DKIM signature was applied by the sender.

What happens if the body hash verification fails?

If the body hash verification fails, it means the recalculated hash of the email body does not match the value in the bh= tag. This indicates that the email body has been modified in transit, either accidentally or maliciously. Such a failure typically results in the email being treated as suspicious, potentially leading to it being sent to spam or outright rejected.

Does DKIM sign the entire email body?

Yes, DKIM typically signs the entire email body, but it first undergoes a canonicalization process to standardize its format. This process accounts for variations in whitespace and line endings, ensuring that minor, non-substantive changes don't cause verification failures. The resulting canonicalized body is then hashed and included in the bh= tag.

How can I monitor for DKIM body hash failures?

You can monitor for DKIM body hash failures by analyzing your DMARC reports. These reports provide aggregated and forensic data on your email authentication results, including detailed information about DKIM passes and failures. Using a DMARC monitoring platform like Suped can simplify this process , offering clear visualizations and actionable recommendations to address any issues.

What DKIM tag specifies the body hash? - DKIM - Email authentication - Knowledge base

Magnifying glass examining an email, representing DKIM authentication and body hash verification

When an email arrives in your inbox, there's a lot happening behind the scenes to verify its legitimacy. One of the key mechanisms is DKIM, or DomainKeys Identified Mail, which uses digital signatures to assure recipients that the email hasn't been tampered with during transit and genuinely comes from the stated sender.

A crucial part of this verification process involves checking the integrity of the email's content. This is where the body hash comes into play. It's a cryptographic summary of the email's body, ensuring that even a single character change would invalidate the signature. But which specific tag within the DKIM signature is responsible for carrying this vital piece of information? Let's explore that.

The DKIM signature explained

The DKIM signature is added to an email as a header field, typically named DKIM-Signature. This header contains several tags, each serving a specific purpose in the authentication process. For instance, the d= tag specifies the domain that signed the email, while the s= tag indicates the selector used to locate the public key in DNS. These tags work together to guide the receiving server in verifying the signature.

The entire signature, including the b= tag, which holds the actual digital signature, is generated by the sending mail server. This signature is then validated by the receiving server against a public key published in the sender's DNS records. If the signature matches, it confirms that the email has not been altered during transmission and is authenticated.

The DKIM process

Sender generates a cryptographic hash of selected email headers and the email body.
Hash is encrypted using the sender's private key, creating the digital signature (the b= tag).
Signature added to the email headers as a DKIM-Signature header.
Recipient server retrieves the public key from the sender's DNS records using the d= and s= tags.
Server decrypts the signature and recalculates the email's hash. If they match, DKIM passes.

The 'bh=' tag explained

The specific DKIM tag that specifies the body hash is bh=. This tag contains a hash value that is computed from the canonicalized (standardized) body of the email message. The bh= tag provides a snapshot of the email body's content at the time the signature was created.

During DKIM verification, the receiving mail server recalculates the body hash of the incoming email using the same canonicalization algorithm (specified by the a= tag, which indicates the algorithm used for signing) and then compares it to the value found in the bh= tag of the DKIM-Signature header. If these two hash values don't match, it means the email body has been altered since the DKIM signature was applied.

Example DKIM-Signature header with 'bh=' tag

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com; s=s1;
  h=from:to:subject:date; bh=L6zE7p2T0yXbN3l0qZ7v9cD8g9e1f2h3i4j5k6l7m8n9o0p1q2r3s4t5u6v7w8x9y;
  b=Gf8dK4L9J6H2G1F0E5D4C3B2A1S8Q7P6O5N4M3L2K1J0I9H8G7F6E5D4C3B2A1S8...

Stack of documents with one locked, representing email body integrity

The calculation of the body hash follows a strict canonicalization process, which defines how whitespace, line endings, and other formatting elements are handled. This ensures that minor, insignificant changes (like those introduced by some mail transfer agents) don't inadvertently invalidate the DKIM signature. The c= tag in the signature specifies the canonicalization algorithms used for both the header and the body, allowing the receiver to replicate the hashing process precisely.

Why the body hash is crucial for email integrity

The body hash is an indispensable component of DKIM because it directly addresses email content integrity. Without it, a DKIM signature would only vouch for the authenticity of the sender and the immutability of the selected headers (specified by the h= tag). This means that malicious actors could potentially alter the email's body, inserting phishing links or harmful content, without invalidating the signature.

When an email's body hash verification fails, it immediately signals that the email's content has been tampered with. This could be due to a legitimate modification by an intermediary mail server (though less common with proper configuration) or, more concerningly, an attempt by an attacker to inject malicious content. Such failures can lead to emails being sent to spam, blocked, or flagged as suspicious by recipient mail servers.

Tampered email risk

If the body hash is compromised or missing, the integrity of the email's content is no longer guaranteed. Attackers can:

Inject phishing links or malware downloads.
Alter payment details in invoices or financial communications.
Modify messages to spread misinformation or damage sender reputation.

Body hash protection

The bh= tag ensures that the email body's content has not been altered. It provides:

Content integrity: Guarantees the message content seen by the recipient is what the sender intended.
Tamper detection: Any modification, however minor, will cause a DKIM body hash mismatch.
Anti-phishing: Prevents attackers from changing content in transit, a common phishing technique.

Monitoring DKIM and body hash integrity

Given the critical role of the bh= tag, it's essential to actively monitor your DKIM performance. DMARC reporting is the best way to get visibility into how your emails are being authenticated, including any instances of DKIM failing body hash verification.

We built Suped's DMARC monitoring platform to provide clear, actionable insights into your email authentication status. You can quickly identify issues related to DKIM, SPF, and DMARC, including specific details about body hash failures, so you can diagnose and fix problems efficiently. Our platform makes DMARC accessible to everyone, from SMBs to large enterprises and MSPs.

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Ensuring content integrity with DKIM's body hash

The bh= tag within a DKIM signature is more than just a technical detail, it's a cornerstone of email security and trust. By housing the cryptographic hash of the email body, it provides an undeniable mechanism to detect any alterations to the message content during its journey across the internet.

Understanding and correctly implementing DKIM, including the role of the body hash, is vital for maintaining a strong email reputation and ensuring your emails reach their intended recipients without being flagged as spam or fraudulent. Continual monitoring through tools like Suped helps maintain this crucial line of defense against email impersonation and content tampering.