DKIM bh= tag
The required tag in a DKIM signature holding the base64 hash of the canonicalized message body, checked before the signature itself.
What it means
The signer canonicalizes the body (simple or relaxed, per c=), hashes it with SHA-256, and records the result in bh=. The body hash is then covered by the signature in b=, since the DKIM-Signature header is part of what gets signed.
When verifying, a receiver recomputes the body hash first. A mismatch means the body changed after signing: a footer added by a mailing list or security gateway, a line rewrapped, an encoding converted, or a link rewritten. RFC 6376 names this failure "body hash did not verify"; it points to content modified in transit rather than to a key or DNS problem.
An empty body has a fixed hash, and RFC 6376 lists it for each canonicalization.
Example
bh=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=Related terms
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Sources
What this definition rests on. The wording is ours; the facts are theirs.
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