Assume I am hashing user passwords with PBKDF2, bcrypt, scrypt, or some other secure key derivation function.
- tangible security benefits,
- precedents, and
- respected research
for protecting these hashes from offline attacks by either encrypting the hashes or otherwise applying a secret key to them via, say, an HMAC?
I believe this previous post attempted to ask a similar question, but the question was unclear and treated by most responders as if it asked whether or not passwords themselves should be HMACed instead of being passed through a salted KDF first.