Yet another (simple) OCaml implementation of HMAC-SHA-512. Require cryptokit and sha opam packages. Cryptokit is needed for just two functions: xor_string and wipe_string.

hmac.ml

open Cryptokit

let ($) f x = f x
let (%) f g = fun x -> f (g x)

let to_hex = transform_string (Hexa.encode())
let of_hex = transform_string (Hexa.decode())

let hmac_sha512 key str = 
	let blocksize = 128 in
	let hmac_pad key byte =
		let key =
			if String.length key > blocksize
			then Sha512.to_bin % Sha512.string $ key
			else key
		in
		let r = String.make blocksize (Char.chr byte) in
		xor_string key 0 r 0 (String.length key); r
	in
	let ipad = hmac_pad key 0x36
	and opad = hmac_pad key 0x5c in
	let str = Sha512.to_bin % Sha512.string $ (ipad ^ str) in
	let str = Sha512.to_bin % Sha512.string $ (opad ^ str) in
	wipe_string ipad;
	wipe_string opad;
	str