EQC Test improvements

* Generalize binary generation because it is used again and again.
* Use generalized binary generation in the hash functions since they are much faster as generators.

src/enacl_ext.erl

@@ -0,0 +1,25 @@
+%%% @doc module enacl_ext implements various enacl extensions.
+%%% <p>None of the extensions listed here are part of the official NaCl library.
+%%% Things may be removed without further notice if it suddenly ends up being
+%%% better to do something differently than the solution given here.
+%%% </p>
+-module(enacl_ext).
+
+-export([
+	scramble_block_16/2
+]).
+
+%% @doc scramble_block_16/2 scrambles (encrypt) a block under a given key
+%% The rules are that the block is 16 bytes and the key is 32 bytes. The block
+%% is scrambled by means of the (secret) key. This makes it impossible for an
+%% attacker to understand the original input for the scrambling. The intention
+%% of this method is to protect counters from leaking to the outside world, by
+%% scrambling them before they leave the system.
+%%
+%% Scrambling is done by means of the TEA algorithm (Tiny Encryption Algorithm)
+%% It has known weaknesses and should probably not be used long-term going
+%% forward, but CurveCP currently uses it for nonce scrambling.
+%% @end
+-spec scramble_block_16(binary(), binary()) -> binary().
+scramble_block_16(Block, Key) ->
+    enacl_nif:scramble_block_16(Block, Key).

src/enacl_nif.erl

@@ -78,6 +78,11 @@
 	randombytes_b/1
 ]).
 
+%% Undocumented features :>
+-export([
+	scramble_block_16/2
+]).
+
 -on_load(init/0).
 
 init() ->
@@ -153,3 +158,4 @@ crypto_verify_32(_X, _Y) -> not_loaded().
 randombytes(_RequestedSize) -> not_loaded().
 randombytes_b(_RequestedSize) -> not_loaded().
 
+scramble_block_16(_Block, _Key) -> not_loaded().