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Correct. I understand the HMAC construction (which is really more of a workaround to address the length-extension attack weaknesses common to Merkle–Damgård construction hash functions), but the Un = PRF( PW, Un-1 ) construction is specific to PBKDF2. If just happens that if the PRF is an HMAC then the PW (which is typically short & low entropy) is used far more than the salt (which may have much better properties).
