A lightweight, secure, easy-to-use crypto library suitable for constrained environments.
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The Hydrogen library is a small, easy-to-use, hard-to-misuse cryptographic library.


  • Consistent high-level API, inspired by libsodium. Instead of low-level primitives, it exposes simple functions to solve common problems that cryptography can solve.
  • 100% built using just two cryptographic building blocks: the Curve25519 elliptic curve, and the Gimli permutation.
  • Small and easy to audit. Implemented as one tiny file for every set of operation, and adding a single .c file to your project is all it takes to use libhydrogen in your project.
  • The whole code is released under a single, very liberal license (ISC).
  • Zero dynamic memory allocations and low stack requirements (median: 32 bytes, max: 128 bytes). This makes it usable in constrained environments such as microcontrollers.
  • Portable: written in standard C99. Supports Linux, *BSD, MacOS, Windows, and the Arduino IDE out of the box.
  • Can generate cryptographically-secure random numbers, even on Arduino boards.
  • Attempts to mitigate the implications of accidental misuse, even on systems with an unreliable PRG and/or no clock.


  • Having multiple primitives serving the same purpose, even to provide compatibility with other libraries.
  • Networking -- but a simple key exchange API based on the Noise protocol is available, and a STROBE-based transport API will be implemented.
  • Interoperability with other libraries.
  • Replacing libsodium. Libhydrogen tries to keep the number of APIs and the code size down to a minimum.

Libhydrogen documentation

The documentation is maintained in the libhydrogen wiki.

The legacy libhydrogen code (leveraging XChaCha20, SipHashX, BLAKE2SX, Curve25519) remains available in the v0 branch.