hmac 256 implementation

hmac256.erl

%%%-------------------------------------------------------------------
%%% File    : hmac256.erl
%%% Author  :  Sriram Krishnan<mail@sriramkrishnan.com>
%%% Description : HMAC-SHA256 implementation. Implementation based on Wikipedia's
%% pseudocode description of HMAC.  Relies on Steve Vinoski's SHA256 implementation
%%% from http://steve.vinoski.net/code/sha2.erl
%%%
%%% Created : 30 Dec 2008
%%%-------------------------------------------------------------------
%% @private
-module(hmac256).
-export([hexdigest/2,digest/2,digest_bin/2]).
-export([unhex/1]).
-version(1.0).

-spec hexdigest(string()|binary(),string()|binary()) -> string().
hexdigest(Key, Data)->
    digest(Key, Data, true).

sha256_str(Payload) ->
    [ hd(integer_to_list(I, 16)) || << I:4 >> <= erlsha2:sha256(Payload) ].

-spec digest(string()|binary(),string()|binary()) -> string().
digest(Key, Data) ->
    digest(Key, Data, false).

digest_bin(Key, Data) ->
  list_to_binary(digest(Key,Data)).


digest(Key, Data, Hex) when is_binary(Key) ->
  digest(binary_to_list(Key), Data, Hex);

digest(Key, Data, Hex) when is_binary(Data) ->
  digest(Key, binary_to_list(Data), Hex);


digest(Key, Data, Hex) when is_list(Key) and is_list(Data) ->
    BlockSize = 64,

    %% Initialize OPad and IPad arrays filled with magic 0x5c and 0x36 values
    %% respectively. The arrays need to be of the same size as the block length
    OPad = array:new( [{size,BlockSize},{fixed,true},{default,92}]),
    IPad = array:new ( [{size,BlockSize},{fixed,true},{default,54}]),



    %% If key is longer than block size, hash it to bring it below block size
    if
        length(Key)>BlockSize -> ShortHashKey = array:from_list(unhex(sha256_str(Key),[]));
        true-> ShortHashKey = array:from_list(Key)
    end,


    HashKey = array:resize(BlockSize, ShortHashKey), %% Zero-pad array


    PadUpdateFunc =  fun (Index, Term) ->
                             KeyTerm =  array:get(Index, HashKey),
                             if
                                 KeyTerm=:= undefined -> Term bxor 0;
                                 true -> Term bxor KeyTerm
                             end
                     end,

    OPadUpdated = array:map(PadUpdateFunc, OPad),
    IPadUpdated = array:map(PadUpdateFunc, IPad),

    FinalTransform = OPadUpdated:to_list() ++ unhex(sha256_str( IPadUpdated:to_list() ++ Data),[]),

    if
        Hex =:= true -> sha256_str(FinalTransform);
        Hex=:= false -> unhex(sha256_str( FinalTransform),[])
    end.

%%
%% Unhex functions adapted from ssl_debug and covered by the Erlang public license
%%
is_hex_digit(C) when C >= $0, C =< $9 -> true;
is_hex_digit(C) when C >= $A, C =< $F -> true;
is_hex_digit(C) when C >= $a, C =< $f -> true;
is_hex_digit(_) -> false.

-spec unhex(string()) -> string().
unhex(S) -> unhex(S, []).

unhex([], Acc) ->
    lists:reverse(Acc);
unhex([_], Acc) ->
    unhex([], Acc);
unhex([$  | Tl], Acc) ->
    unhex(Tl, Acc);
unhex([D1, D2 | Tl], Acc) ->
    case {is_hex_digit(D1), is_hex_digit(D2)} of
        {true, true} ->
            unhex(Tl, [erlang:list_to_integer([D1, D2], 16) | Acc]);
        _ ->
            unhex([], Acc)
    end.

Thanks!