7.5. Hash-Based Key Derivation Function (HKDF)

Hash-Based Key Derivation Functions (HKDFs) are keyed hash functions that can be used to deterministically derive multiple child keys from a single parent key. This can simplify your key management schemes.

Note that if the parent key is insecure because it has insufficient entropy, then the child keys will be insecure for the same reason.

In contrast to Password-Based Key Derivation Functions (PBKDFs) that use slow hash functions, HKDFs use cryptographic hash functions that are very quick to execute (i.e. not cpu and memory intensive).

Since user generated password/passphrases typically have low entropy, a quick hash function will make it easier for an adversary to brute force the input space and “crack” the password hashes. For this reason, HKDFs are not suitable for deriving cryptographic keys from passwords/passphrases.

Warning

DO NOT use HKDF to derive cryptographic keys from passwords.

Function:

HashKDF(key []byte, purpose []byte) ([]byte, error)

Returns a new child symmetric key deterministically derived from the given parent symmetric key and human readable explanation of the purpose of the new child key.

Parameters
  • key ([]byte) – 16-byte parent symmetric key

  • purpose ([]byte) – Arbitrary-length, human readable explanation of the purpose of the newly derived child key

Returns

16-byte derived symmetric key

Return type

[]byte, error

Example:

key := userlib.RandomBytes(16)

encryptionKey, err1 := userlib.HashKDF(key, []byte("encryption"))
encryptionKey = encryptionKey[:16]

macKey, err2 := userlib.HashKDF(key, []byte("mac"))
anotherKey = macKey[:16]

anotherKey, err3 := userlib.HashKDF(key, []byte("another purpose"))
anotherKey = anotherKey[:16]

Important

Remember: one key, one purpose. If we use a key for symmetric encryption or HMAC, we should not use the same key for HKDF. Similarly, we should not use the same key for both encryption and HMAC.