%% -*- mode: erlang; tab-width: 4; indent-tabs-mode: 1; st-rulers: [70] -*-
%% vim: ts=4 sw=4 ft=erlang noet
%%%-------------------------------------------------------------------
%%% @author Andrew Bennett <potatosaladx@gmail.com>
%%% @copyright 2014-2019, Andrew Bennett
%%% @doc ChaCha20 and Poly1305 for IETF Protocols
%%% See https://tools.ietf.org/html/rfc7539
%%% @end
%%% Created :  31 May 2016 by Andrew Bennett <potatosaladx@gmail.com>
%%%-------------------------------------------------------------------
-module(jose_jwa_chacha20).

%% API
-export([quarter_round/1]).
-export([column_round/1]).
-export([diagonal_round/1]).
-export([block/3]).
-export([encrypt/4]).

%% Macros
-define(math, jose_jwa_math).
-define(p, 16#100000000).
-define(rotl(X, R), ?math:mod((X bsl R) bor (X bsr (32 - R)), ?p)).

%%====================================================================
%% API functions
%%====================================================================

quarter_round({A0, B0, C0, D0}) ->
	A1 = ?math:mod(A0 + B0, ?p),
	D1 = ?rotl(D0 bxor A1, 16),
	C1 = ?math:mod(C0 + D1, ?p),
	B1 = ?rotl(B0 bxor C1, 12),
	A = ?math:mod(A1 + B1, ?p),
	D = ?rotl(D1 bxor A, 8),
	C = ?math:mod(C1 + D, ?p),
	B = ?rotl(B1 bxor C, 7),
	{A, B, C, D}.

column_round({X00, X01, X02, X03, X04, X05, X06, X07, X08, X09, X10, X11, X12, X13, X14, X15}) ->
	{Y00, Y04, Y08, Y12} = quarter_round({X00, X04, X08, X12}),
	{Y01, Y05, Y09, Y13} = quarter_round({X01, X05, X09, X13}),
	{Y02, Y06, Y10, Y14} = quarter_round({X02, X06, X10, X14}),
	{Y03, Y07, Y11, Y15} = quarter_round({X03, X07, X11, X15}),
	{Y00, Y01, Y02, Y03, Y04, Y05, Y06, Y07, Y08, Y09, Y10, Y11, Y12, Y13, Y14, Y15}.

diagonal_round({Y00, Y01, Y02, Y03, Y04, Y05, Y06, Y07, Y08, Y09, Y10, Y11, Y12, Y13, Y14, Y15}) ->
	{Z00, Z05, Z10, Z15} = quarter_round({Y00, Y05, Y10, Y15}),
	{Z01, Z06, Z11, Z12} = quarter_round({Y01, Y06, Y11, Y12}),
	{Z02, Z07, Z08, Z13} = quarter_round({Y02, Y07, Y08, Y13}),
	{Z03, Z04, Z09, Z14} = quarter_round({Y03, Y04, Y09, Y14}),
	{Z00, Z01, Z02, Z03, Z04, Z05, Z06, Z07, Z08, Z09, Z10, Z11, Z12, Z13, Z14, Z15}.

block(Key, Counter, Nonce)
		when is_binary(Key)
		andalso bit_size(Key) =:= 256
		andalso is_integer(Counter)
		andalso Counter >= 0
		andalso is_binary(Nonce)
		andalso bit_size(Nonce) =:= 96 ->
	State = <<
		"expand 32-byte k",
		Key:256/bitstring,
		Counter:32/unsigned-little-integer-unit:1,
		Nonce:96/bitstring
	>>,
	WS0 = list_to_tuple([Word || << Word:32/unsigned-little-integer-unit:1 >> <= State]),
	WS1 = rounds(WS0, 10),
	WS2 = add(WS1, WS0),
	serialize(WS2).

encrypt(Key, Counter, Nonce, Plaintext) ->
	encrypt(Key, Counter, Nonce, Plaintext, 0, <<>>).

encrypt(_Key, _Counter, _Nonce, <<>>, _J, EncryptedMessage) ->
	EncryptedMessage;
encrypt(Key, Counter, Nonce, << Block:64/binary, Rest/binary >>, J, EncryptedMessage) ->
	KeyStream = block(Key, Counter + J, Nonce),
	encrypt(Key, Counter, Nonce, Rest, J + 1, << EncryptedMessage/binary, (crypto:exor(Block, KeyStream))/binary >>);
encrypt(Key, Counter, Nonce, Block, J, EncryptedMessage) ->
	BlockBytes = byte_size(Block),
	<< KeyStream:BlockBytes/binary, _/binary >> = block(Key, Counter + J, Nonce),
	<< EncryptedMessage/binary, (crypto:exor(Block, KeyStream))/binary >>.

%%%-------------------------------------------------------------------
%%% Internal functions
%%%-------------------------------------------------------------------

%% @private
inner_block(State0)
		when is_tuple(State0)
		andalso tuple_size(State0) =:= 16 ->
	State1 = column_round(State0),
	State2 = diagonal_round(State1),
	State2.

%% @private
rounds(S, 0) ->
	S;
rounds(S, N)
		when is_integer(N)
		andalso N > 0 ->
	rounds(inner_block(S), N - 1).

%% @private
add({X00, X01, X02, X03, X04, X05, X06, X07, X08, X09, X10, X11, X12, X13, X14, X15},
	{Y00, Y01, Y02, Y03, Y04, Y05, Y06, Y07, Y08, Y09, Y10, Y11, Y12, Y13, Y14, Y15}) ->
	{X00 + Y00, X01 + Y01, X02 + Y02, X03 + Y03, X04 + Y04, X05 + Y05, X06 + Y06, X07 + Y07, X08 + Y08, X09 + Y09, X10 + Y10, X11 + Y11, X12 + Y12, X13 + Y13, X14 + Y14, X15 + Y15}.

%% @private
serialize({Z00, Z01, Z02, Z03, Z04, Z05, Z06, Z07, Z08, Z09, Z10, Z11, Z12, Z13, Z14, Z15}) ->
	<<
		Z00:32/unsigned-little-integer-unit:1,
		Z01:32/unsigned-little-integer-unit:1,
		Z02:32/unsigned-little-integer-unit:1,
		Z03:32/unsigned-little-integer-unit:1,
		Z04:32/unsigned-little-integer-unit:1,
		Z05:32/unsigned-little-integer-unit:1,
		Z06:32/unsigned-little-integer-unit:1,
		Z07:32/unsigned-little-integer-unit:1,
		Z08:32/unsigned-little-integer-unit:1,
		Z09:32/unsigned-little-integer-unit:1,
		Z10:32/unsigned-little-integer-unit:1,
		Z11:32/unsigned-little-integer-unit:1,
		Z12:32/unsigned-little-integer-unit:1,
		Z13:32/unsigned-little-integer-unit:1,
		Z14:32/unsigned-little-integer-unit:1,
		Z15:32/unsigned-little-integer-unit:1
	>>.
