Introduce enacl:randombytes/1.

src/enacl.app.src

@@ -1,7 +1,7 @@
 {application, enacl,
   [
     {description, "Erlang NaCl bindings"},
-    {vsn, "0.9.0"},
+    {vsn, "0.10.0"},
     {registered, []},
     {applications, [kernel, stdlib]},
     {env, []}

src/enacl.erl

@@ -62,6 +62,11 @@
 	verify_32/2
 ]).
 
+%% Libsodium specific functions (which are also part of the "undocumented" interface to NaCl
+-export([
+	randombytes/1
+]).
+
 %% Other helper functions
 -export([
 	reds/1
@@ -85,6 +90,8 @@
 -define(AUTH_REDUCTIONS, 102 * 2).
 -define(ONETIME_AUTH_SIZE, 128 * 1024).
 -define(ONETIME_AUTH_REDUCTIONS, 105 * 2).
+-define(RANDOMBYTES_SIZE, 1024).
+-define(RANDOMBYTES_REDUCTIONS, 200).
 
 %% Count reductions and number of scheduler yields for Fun. Fun is assumed
 %% to be one of the above exor variants.
@@ -481,6 +488,24 @@ onetime_auth_size() -> enacl_nif:crypto_onetimeauth_BYTES().
 -spec onetime_auth_key_size() -> pos_integer().
 onetime_auth_key_size() -> enacl_nif:crypto_onetimeauth_KEYBYTES().
 
+%% Obtaining random bytes
+
+%% @doc randombytes/1 produces a stream of random bytes of the given size
+%% The security properties of the random stream are that of the libsodium library. Specifically,
+%% we use:
+%%
+%% * RtlGenRandom() on Windows systems
+%% * arc4random() on OpenBSD and Bitrig
+%% * /dev/urandom on other Unix environments
+%%
+%% It is up to you to pick a system with a appropriately strong (P)RNG for your purpose. We refer
+%% you to the underlying system implementations for random data.
+%% @end
+randombytes(N) when N =< ?RANDOMBYTES_SIZE ->
+    bump(enacl_nif:randombytes_b(N), ?RANDOMBYTES_REDUCTIONS, ?RANDOMBYTES_SIZE, N);
+randombytes(N) ->
+    enacl_nif:randombytes(N).
+
 %% Helpers
 p_zerobytes() ->
 	binary:copy(<<0>>, enacl_nif:crypto_box_ZEROBYTES()).

src/enacl_nif.erl

@@ -72,6 +72,12 @@
 	crypto_verify_32/2
 ]).
 
+%% Access to the RNG
+-export([
+	randombytes/1,
+	randombytes_b/1
+]).
+
 -on_load(init/0).
 
 init() ->
@@ -143,3 +149,7 @@ crypto_hash(Input) when is_binary(Input) -> not_loaded().
 crypto_hash_b(Input) when is_binary(Input) -> not_loaded().
 crypto_verify_16(_X, _Y) -> not_loaded().
 crypto_verify_32(_X, _Y) -> not_loaded().
+
+randombytes(_RequestedSize) -> not_loaded().
+randombytes_b(_RequestedSize) -> not_loaded().
+