SHA256 与 HMAC 空消息

Question

why those functions return different values?

from hashlib import sha256
import hmac

seed = "seed".encode('utf-8')

def genSHA256():
    return = sha256(seed).hexdigest()
    #19b25856e1c150ca834cffc8b59b23adbd0ec0389e58eb22b3b64768098d002b

def genHMACsha256():
    return hmac.new(seed, b"", sha256).hexdigest()
    #2ad1ced5a9ef8e90bce26c0ac9fae5af5e4b4442b2315ed58bf772a54e24fd50

If I put empty string in message value in the HMAC one, why does not return same as simple SHA256? Are this two values related?

Answer 1

The seed is masked with ipad and opad when HMAC is used, and HMAC consists of two hash iterations, even when HMAC is used over an empty string.

Basically the only way to get the same results as HMAC from a hash function is to use it to build HMAC - as the HMAC definition only uses the hash as secure primitive function.