isc-hmac-fixup — fixes HMAC keys generated by older versions of BIND
Versions of BIND 9 up to and including BIND 9.6 had a bug causing HMAC-SHA* TSIG keys which were longer than the digest length of the hash algorithm (i.e., SHA1 keys longer than 160 bits, SHA256 keys longer than 256 bits, etc) to be used incorrectly, generating a message authentication code that was incompatible with other DNS implementations.
This bug was fixed in BIND 9.7. However, the fix may cause incompatibility between older and newer versions of BIND, when using long keys. isc-hmac-fixup modifies those keys to restore compatibility.
To modify a key, run isc-hmac-fixup and specify the key's algorithm and secret on the command line. If the secret is longer than the digest length of the algorithm (64 bytes for SHA1 through SHA256, or 128 bytes for SHA384 and SHA512), then a new secret will be generated consisting of a hash digest of the old secret. (If the secret did not require conversion, then it will be printed without modification.)
Secrets that have been converted by isc-hmac-fixup are shortened, but as this is how the HMAC protocol works in operation anyway, it does not affect security. RFC 2104 notes, "Keys longer than [the digest length] are acceptable but the extra length would not significantly increase the function strength."