# Erlang bindings for NaCl/libsodium

This library provides bindings for the libsodium cryptographic library
for Erlang. Originally called NaCl by Bernstein, Lange and Schwabe[0],
Frank Denis took the source and made it far more portable in the
libsodium library. The enacl project is somewhat misnamed, as it uses
libsodium as the underlying driver.

## INSTALL/Requirements:

* Erlang/OTP above 17.3. This library *needs* the dirty scheduler
  implementation from 17.3 and onwards. The library relies on dirty
  scheduler support in order to handle long-running cryptography jobs,
  by moving them off the main Erlang scheduler and letting the dirty
  schedulers handle the work. This keeps the Erlang VM responsive while
  doing large amounts of cryptographic processing.
* Is tested with Erlang 22.3. Erlang version 21 *may* work, but that isn't
  the current testing target.
* *Requires* the libsodium library, and has been tested with version
  1.0.18. Lower versions might work, or they might fail to compile,
  due to missing functionality.
  *Note:* If installing on systems which cuts packages into
  subpackages, make sure you also get the "-dev" package containing
  the header files necessary in order to compile software linking to
  libsodium.

To build the software execute:

	make

or

	rebar compile

To build and run licensed eqc test execute:

	make eqc_run

To build and run eqc-mini version of test execute:

	make eqc_mini_run

## Features:

* Complete NaCl library, implementing all default functionality.
* Implements a large set of additional functionality from libsodium.
  Most notably access to a proper CSPRNG random source
* Tests created by aggressive use of Erlang QuickCheck.
* NaCl is a very fast cryptographic library. That is,
  crypto-operations runs quickly on modern CPUs, with ample security
  margins. This makes it highly useful on the server-side, where
  simultaneous concurrent load on the system means encryption can have
  a considerable overhead.
* Is tested on Linux, FreeBSD and Illumos (Omnios)

This package draws heavy inspiration from "erlang-nacl" by Tony
Garnock-Jones, and started its life with a gently nod in that
direction. However, it is a rewrite and it alters lots of code from
Tony's original work.

In addition, I would like to thank Steve Vinoski, Rickard Green, and
Sverker Eriksson for providing the Dirty Scheduler API in the first
place.

# USING:

In general, consult the libsodium documentation at

    https://download.libsodium.org/doc/

The original NaCl documentation is nowadays largely superceded by the
libsodium documentation, but it is still worth a visit

	https://nacl.cr.yp.to

but also note that our interface has full Edoc documentation,
generated by executing

	rebar3 doc

## Hints

In general, the primitives provided by NaCl are intermediate-level
primitives. Rather than you having to select a cipher suite, it is
selected for you, and primitives are provided at a higher level.
However, their correct use is still needed in order to be secure:

* Always make sure you obey the scheme of *nonce* values. If you ever
  reuse a nonce, and an attacker figures this out, the system will
  leak the XOR difference of messages sent with the same nonce. Given
  enough guessing, this can in turn leak the encryption stream of bits
  and every message hereafter, sent on the same keypair combination
  and reusing that nonce, will be trivially breakable.
* Use the beforenm/afternm primitives if using the `box` public-key
  encryption scheme. Precomputing the Curve25519 operations yields
  much faster operation in practice for a stream. Consult the `bench`
  directory for benchmarks in order to see how much faster it is for
  your system. The authors Core i7-4900MQ can process roughly 32
  Kilobyte data on the stream in the time it takes to do the
  Curve25519 computations. While NaCl is *fast*, this can make it even
  faster in practice.
* Encrypting very large blocks of data, several megabytes for
  instance, is problematic for two reasons. First, while the library
  attempts to avoid being a memory hog, you need at least a from-space
  and a to-space for the data, meaning you need at least double the
  memory for the operation. Furthermore, while such large blocks are
  executed on the dirty schedulers, they will never yield the DS for
  another piece of work. This means you end up blocking the dirty
  schedulers in turn. It is often better to build a framing scheme and
  encrypt data in smaller chunks, say 64 or 128 kilobytes at a time.
  In any case, it is important to measure. Especially for latency.
* The library should provide correct success type specifications. This
  means you can use the dialyzer on your code and get hints for
  incorrect usage of the library.
* Note that every "large" input to the library accepts `iodata()`
  rather than `binary()` data. The library itself will convert
  `iodata()` to binaries internally, so you don't have to do it at
  your end. It often yields simpler code since you can just build up
  an iolist of your data and shove it to the library. Key material,
  nonces and the like are generally *not* accepted as `iodata()`
  however but requires you to input binary data. This is a deliberate
  choice since most such material is not supposed to be broken up and
  constructed ever (except perhaps for the Nonce construction).
* The `enacl:randombytes/1` function provides portable access to the
  CSPRNG of your kernel. It is an *excellent* source of CSPRNG random
  data. If you need PRNG data with a seed for testing purposes, use
  the `rand` module of Erlang. The other alternative is the `crypto`
  module, which are bindings to OpenSSL with all its blessings and/or
  curses.
* Beware of timing attacks against your code! A typical area is string
  comparison, where the comparator function exits early. In that case,
  an attacker can time the response in order to guess at how many
  bytes where matched. This in turn enables some attacks where you use
  a foreign system as an oracle in order to learn the structure of a
  string, breaking the cryptograhic system in the process.

# Versions

See CHANGELOG.md

# Overview

The NaCl cryptographic library provides a number of different
cryptographic primitives. In the following, we split up the different
generic primitives and explain them briefly.

*A note on Nonces:* The crypto API makes use of "cryptographic
nonces", that is arbitrary numbers which are used only once. For these
primitives to be secure it is important to consult the NaCl
documentation on their choice. They are large values so generating
them randomly ensures security, provided the random number generator
uses a sufficiently large period. If you end up using, say, the nonce
`7` every time in communication while using the same keys, then the
security falls.

The reason you can pick the nonce values is because some uses are
better off using a nonce-construction based on monotonically
increasing numbers, while other uses do not. The advantage of a
sequence is that it can be used to reject older messages in the stream
and protect against replay attacks. So the correct use is up to the
application in many cases.

## Public Key cryptography

This implements standard Public/Secret key cryptography. The
implementation roughly consists of two major sections:

* *Authenticated encryption:* provides a `box` primitive which
  encrypts and then also authenticates a message. The reciever is only
  able to open the sealed box if they posses the secret key and the
  authentication from the sender is correct.
* *Signatures:* allows one party to sign a message (not encrypting it)
  so another party can verify the message has the right origin.

## Secret key cryptography

This implements cryptography where there is a shared secret key between parties.

* *Authenticated encryption:* provides a `secret box` primitive in
  which we can encrypt a message with a shared key `k`. The box also
  authenticates the message, so a message with an invalid key will be
  rejected as well. This protects against the application obtaining
  garbage data.
* *Encryption:* provides streams of bytes based on a Key and a Nonce.
  These streams can be used to `XOR` with a message to encrypt it. No
  authentication is provided. The API allows for the system to `XOR`
  the message for you while producing the stream.
* *Authentication:* Provides an implementation of a Message
  Authentication Code (MAC).
* *One Time Authentication:* Authenticate a message, but do so
  one-time. That is, a sender may *never* authenticate several
  messages under the same key. Otherwise an attacker can forge
  authenticators with enough time. The primitive is simpler and faster
  than the MAC authenticator however, so it is useful in some
  situations.

## Low-level functions

* *Hashing:* Cryptographically secure hashing
* *String comparison:* Implements guaranteed constant-time string
  comparisons to protect against timing attacks.

# Rationale

Doing crypto right in Erlang is not that easy. For one, the crypto
system has to be rather fast, which rules out Erlang as the main
vehicle. Second, cryptographic systems must be void of timing attacks.
This mandates we write the code in a language where we can avoid such
timing attacks, which leaves only C as a contender, more or less. The
obvious way to handle this is by the use of NIF implementations, but
most C code will run to its conclusion once set off for processing.
This is a major problem for a system which needs to keep its latency
in check. The solution taken by this library is to use the new Dirty
Scheduler API of Erlang in order to provide a safe way to handle the
long-running cryptographic processing. It keeps the cryptographic
primitives on the dirty schedulers and thus it avoids the major
problem.

Focus has first and foremost been on the correct use of dirty
schedulers, without any regard for speed. The plan is to extend the
underlying implementation, while keeping the API stable. We can
precompute keys for some operations for instance, which will yield a
speedup.

Also, while the standard `crypto` bindings in Erlang does a great job
at providing cryptographic primitives, these are based on OpenSSL,
which is known to be highly problematic in many ways. It is not as
easy to use the OpenSSL library correctly as it is with these
bindings. Rather than providing a low-level cipher suite, NaCl
provides intermediate level primitives constructed as to protect the
user against typical low-level cryptographic gotchas and problems.

## Scheduler handling

To avoid long running NIFs, the library switches to the use of dirty
schedulers for large encryption tasks. We investigated the Dirty
Scheduler switch overhead with DTrace on FreeBSD and found it to be
roughly 5μs in typical cases. Thus, we target calls taking at least
35μs is being easier to run directly on the dirty scheduler, as the
overhead for switching is thus going to be less than 15%. This means
very small operations are run directly on the BEAM scheduler, but as
soon as the operation takes a little longer, the switch overhead is
not large enough to warrant the current schedulers involvement.

In turn, some operations are *always* run on the dirty scheduler
because they take a long time in every case. This setup is far simpler
for most operations, unless the operation is performance sensitive and
allows small messages.

The tests were conducted on a Core 2 Duo machine, with newer machines
perhaps being able to switch faster. There are plans to rerun these
tests on OSX and Illumos as well, in order to investigate the numbers
on more platforms.

# Testing

Every primitive has been stress-tested through the use of Erlang
QuickCheck with both *positive* and *negative* testing. This has been
used to check against memory leaks as well as correct invocation.
Please report any error so we can extend the test cases to include a
randomized test which captures the problem so we generically catch
every problem in a given class of errors.

Positive and negative testing refers to Type I and Type II errors in
statistical testing. This means false positives—given a *valid* input
the function rejects it; as well as false negatives—given an *invalid*
input the functions fails to reject that input.

The problem however, is that while we are testing the API level, we
can't really test the strength of the cryptographic primitives. We can
verify their correctness by trying different standard correctness
tests for the primitives, verifying that the output matches the
expected one given a specific input. But there is no way we can show
that the cryptographic primitive has the strength we want. Thus, we
opted to mostly test the API and its invocation for stability.

Also, in addition to correctness, testing the system like this makes
sure we have no memory leaks as they will show themselves under the
extensive QuickCheck test cases we run. It has been verified there are
no leaks in the code.

# Notes

[0] Other people have worked on bits and pieces of NaCl. These are
just the 3 main authors. Please see the page

	http://nacl.cr.yp.to

for the full list of authors.