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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.


  • New-ish Erlang installation. Tested back to version 22.3, but version 21 may work as well.
  • 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. In particular, this means your libsodium installation must be fairly recent as well.

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:



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


  • 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.


In general, consult the libsodium documentation at Libsodium documentation

The original NaCl documentation is nowadays largely superceded by the libsodium documentation, but it is still worth a visit NaCl website

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

rebar3 doc


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.




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.


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.


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.


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