Boring crypto that simply works


Monocypher is a cryptographic library. It provides functions for authenticated encryption, hashing, password key derivation, key exchange, and public key signatures.


Authenticated encryption

crypto_lock() and crypto_unlock() use the Chacha20 cipher and the Poly1305 one time authenticator. An incremental interface is also available.

Chacha20 is a stream cipher based on a cryptographic hash function. It runs efficiently on a wide variety of hardware, and unlike AES naturally runs in constant time on all hardware.

Poly1305 is a one time authenticator, derived from Carter & Wegman universal hashing. It is very fast and very simple.


crypto_blake2b() implements the Blake2b hash. Blake2b combines the security of SHA-3 and the speed of MD5. It is immune to length extension attacks and provides a keyed mode that makes it a safe, easy to use authenticator.

Password key derivation

crypto_argon2i() implements the Argon2i resource intensive hash algorithm. Argon2 won the password hashing competition in 2015. Unlike Scrypt, Argon2i is immune to timing attacks.

Key exchange

crypto_key_exchange() implements X25519, an elliptic curve Diffie Hellman key exchange algorithm based on Curve25519. X25519 derives a shared secret from two private/public key pairs. It is fast, simple, and relatively easy to implement securely.

Public key signatures

crypto_sign() and crypto_check() implement EdDSA, with Curve25519 and Blake2b. This is the same as the more famous Ed25519, with SHA-512 replaced by the faster and more secure Blake2b. Ed25519 (EdDSA with SHA-512), is supported as a compilation option.

Constant time comparison

crypto_verify16(), crypto_verify32(), and crypto_verify64() compare buffers in constant time. They should be used to compare secrets to prevent timing attacks.

Memory wipe

crypto_wipe() wipes a buffer. It is meant to erase secrets when they are no longer needed, to reduce the chances of leaks.

Security Considerations

Using cryptography securely is difficult. Flaws that never manifest under normal use might be exploited by a clever adversary. Cryptographic libraries are easy to misuse. Even Monocypher allows a number of fatal mistakes.

Users should follow a formal introduction to cryptography. We currently recommend the online course.

Random number generation

Use the facilities of your operating system. Avoid user space random number generators. They are easy to misuse, which has lead to countless vulnerabilities in the past. For instance, the random stream may be repeated if one is not careful with multi-threading, and forward secrecy is lost without proper key erasure.

Different system calls are available on different systems:

The /dev/urandom special file may be used on systems that do not provide an easy to use system call. Be careful though, being a file makes /dev/urandom hard to use correctly and securely. Reads may be interrupted, and more attacks are possible on a file than on a system call.

Timing attacks

Monocypher runs in "constant time". There is no flow from secrets to timings. No secret dependent indices, no secret dependent branches. Nevertheless, there are a couple important caveats.

Comparing secrets should be done with constant-time comparison functions, such as crypto_verify16(), crypto_verify32(), or crypto_verify64(). Do not use standard comparison functions. They tend to stop as soon as a difference is spotted. In many cases, this enables attackers to recover the secrets and destroy all security.

The Poly1305 authenticator, X25519, and EdDSA use multiplication. Some older processors do not multiply in constant time. If the target platform is something other than x86, x86_64, ARM or ARM64, double check how it handles multiplication.

Data compression

Encryption does not hide the length of the input plaintext. Most compression algorithms work by using fewer bytes to encode previously seen data or common characters. If an attacker can add data to the input before it is compressed and encrypted, they can observe changes to the ciphertext length to recover secrets from the input. Researchers have demonstrated an attack on HTTPS to steal session cookies when compression is enabled, dubbed "CRIME".

Forward secrecy

Long term secrets cannot be expected to stay safe indefinitely. Users may reveal them by mistake, or the host computer might have a vulnerability and be compromised. To mitigate this problem, some protocols guarantee that past messages are not compromised even if the long term keys are. This is done by generating temporary keys, then encrypting messages with them.

In general, secrets that went through a computer should not be compromised when this computer is stolen or infected at a later point.

A first layer of defence is to explicitly wipe secrets as soon as they are no longer used. Monocypher already wipes its own temporary buffers, and contexts are erased with the crypto_*_final() functions. The secret keys and messages however are the responsibility of the user. Use crypto_wipe() to erase them.

A second layer of defence is to ensure those secrets are not swapped to disk while they are used. There are several ways to do this. The most secure is to disable swapping entirely. Doing so is recommended on sensitive machines. Another way is to encrypt the swap partition (this is less safe). Finally, swap can be disabled locally—this is often the only way.

UNIX systems can disable swap for specific buffers with mlock(), and disable swap for the whole process with mlockall(). Windows can disable swap for specific buffers with VirtualLock().

Core dumps cause similar problems. Disable them. Also beware of suspend to disk (deep sleep mode), which writes all RAM to disk regardless of swap policy, as well as virtual machine snapshots. Erasing secrets with crypto_wipe() is often the only way to mitigate these dangers.

Undefined behaviour

Monocypher is a C library. C is notoriously unsafe. Using Monocypher incorrectly can trigger undefined behaviour. This can lead to data corruption, data theft, or even arbitrary code execution.

Consider binding to a safe language if possible.