Erlang bindings for NaCl cryptography library as packaged by libsodium.
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### About.

  This package provides Erlang bindings for 'libsodium', a portable packaging
  of NaCl cryptography library. The bindings are pretty complete, covering all
  public APIs of all chosen primitives through NIF code in 'c_src/salt_nif.c'
  and supporting Erlang modules.

  NaCl provides high-speed cryptographic primitives whose implementations
  are resilient to side-channel attacks by design. The API exposes high-level
  operations with clear security contracts and minimal space for user to
  introduce undue risks accidentally. This is ideal for the working engineer
  wishing to improve upon security aspects of his/her designs but unable or
  unwilling to engage in fragile low-level cryptoplumbing.

  Most of the crypto is performed directly on scheduler threads without visible
  side effects (aside from allocation of result terms) and without performing
  any system or library calls (outside libsodium). Upper bound on execution
  latency is imposed indirectly by limiting input block sizes throughout.

  This is not considered to be a problematic decision as it is likely that
  networking applications will likewise prefer to limit maximum PDU size, and
  storage applications are likely to operate on blocks of constant size. Fine
  tuning of the limit may be desirable looking forward, current value of 16KB
  is chosen arbitrarily. Future changes should be supported by measurements on
  relevant CPUs with the target of one reduction per operation, or cca 1ms.
  The author defines "relevant CPUs" as "enteprise class amd64 chips" :-).

  Key generation and RNG routines are (at least potentially) blocking and
  are therefore perfomed on a worker thread via call too 'salt_server'. This
  means all these calls get serialized and incur somewhat higher latency. It
  is reasonable to expect key generation to only be performed at relatively
  low frequencies. The same hopefully applies to random bytes generation.

### References.


    * Home of NaCl project:
    * Home of libsodium:
    * Home of Salt:


    "The security impact of a new cryptographic library"
    Daniel J. Bernstein, Tanja Lange, Peter Schwabe

    "Cryptography in NaCl"
    Daniel J. Bernstein

### Credits and Licensing.

  The original NaCl code was released by Daniel J. Bernstein, Tanja Lange,
  Peter Schwabe and contributors into the public domain.

  Libsodium, by Frank Denis and contributors, is subject to ISC license.

  Salt, by Jachym Holecek and contributors, is subject to a 2-clause BSD

### Compiling.

  * Install 'libsodium':

    $ git clone
    $ ( cd libsodium && \
	./configure --prefix="/usr/local" --disable-ssp --disable-pie \
                    --disable-silent-rules && \
	make && make check && sudo make install && make clean )

  * Build 'salt', you'll need 'rebar' utility:

    $ git clone
    $ ( cd salt && rebar clean && rebar compile )

  * To run a simple self-test have a look at '_run' script.

  * The 'salt_test' module is also a good source of simple usage examples.

### TODO

  * Verify current message/block size limit of 16KB corresponds to reasonable
  * Also export BLAKE2b hash function, despite not having "chosen" status.
  * Perform 'libsodium' initialization from worker thread before app startup

### Data types.

  XXX document variables mentioned below, pretty obvious,
  XXX also see include/salt.hrl

### Public-key cryptography.

  crypto_box_keypair() ->
    {Public_key, Secret_key}.

  crypto_box(Plain_text, Nonce, Public_key, Secret_key) ->

  crypto_box_open(Cipher_text, Nonce, Public_key, Secret_key) ->
    {ok, Plain_text} | forged_or_garbled.

  crypto_box_beforenm(Public_key, Secret_key) ->

  crypto_box_afternm(Plain_text, Nonce, Context) ->

  crypto_box_open_afternm(Cipher_text, Nonce, Context) ->
    {ok, Plain_text} | forged_or_garbled.

## Scalar multiplication.

  crypto_scalarmult(Integer, Group_p) ->

  crypto_scalarmult_base(Integer) ->

## Signatures.

  crypto_sign_keypair() ->
    {Public_key, Secret_key}.

  crypto_sign(Message, Secret_key) ->

  crypto_sign_open(Signed_msg, Public_key) ->
    {ok, Verified_msg} | forged_or_garbled.

### Secret-key cryptography.

## Authenticated encryption.

  crypto_secretbox(Plain_text, Nonce, Secret_key) ->

  crypto_secretbox_open(Cipher_text, Nonce, Secret_key) ->
    {ok, Plain_text} | forged_or_garbled.

## Encryption.

  crypto_stream(Byte_cnt, Nonce, Secret_key) ->

  crypto_stream_xor(In_text, Nonce, Secret_key) ->

## Message authentication.

  crypto_auth(Message, Secret_key) ->

  crypto_auth_verify(Authenticator, Message, Secret_key) ->
    authenticated | forged_or_garbled.

## Single-message authentication.

  crypto_onetimeauth(Message, Secret_key) ->

  crypto_onetimeauth_verify(Authenticator, Message, Secret_key) ->
    authenticated | forged_or_garbled.

### Low-level functions.

## Hashing.

  crypto_hash(Message) ->

## String comparison.

  crypto_verify_16(Bin_x, Bin_y) ->
    equal | not_equal.

  crypto_verify_32(Bin_x, Bin_y) ->
    equal | not_equal.

## Random number generator.

  crypto_random_bytes(Cnt) ->