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Uses
HKDF has two primary and potentially independent uses:
# To "extract" (condense/blend) entropy
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
from a larger random source to provide a more uniformly unbiased and higher entropy but smaller output (e.g., an encryption key
A key in cryptography is a piece of information, usually a string of numbers or letters that are stored in a file, which, when processed through a cryptographic algorithm, can encode or decode cryptographic data. Based on the used method, the key ...
). This is done by utilising the diffusion properties of cryptographic MACs.
# To "expand" the generated output of an already reasonably random input such as an existing shared key into a larger cryptographically independent output, thereby producing multiple keys deterministically from that initial shared key, so that the same process may produce those same secret keys safely on multiple devices, as long as the same inputs are utilised.
These two functions may also be combined and used to form a PRNG to improve a random number generator's potentially-biased output, as well as to protect it from analysis and help defend the random number generation from malicious inputs.
Example: Python implementation
#!/usr/bin/env python3
import hashlib
import hmac
from math import ceil
hash_len = 32
def hmac_sha256(key, data):
return hmac.new(key, data, hashlib.sha256).digest()
def hkdf(length: int, ikm, salt: bytes = b"", info: bytes = b"") -> bytes:
"""Key derivation function"""
if len(salt) 0:
salt = bytes( * hash_len)
prk = hmac_sha256(salt, ikm)
t = b""
okm = b""
for i in range(ceil(length / hash_len)):
t = hmac_sha256(prk, t + info + bytes( + 1)
okm += t
return okm length
okm = hkdf(length=42,
ikm=bytes.fromhex('0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b'),
salt=bytes.fromhex('000102030405060708090a0b0c'),
info=bytes.fromhex('f0f1f2f3f4f5f6f7f8f9'))
assert okm bytes.fromhex(
'3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865')
References
{{Reflist
Cryptography
Key derivation functions