• Allows you to derive other possibly longer keys from another key.
• Easy for GPUs and ASICs to implement though.
• Its a standard but I prefer Blake2X over this.

Python code

#!/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:
    if len(salt) == 0:
        salt = bytes([0] * 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 + i]))
        okm += t
    return okm[:length]