This library is a compact and portable (c99) implementation of two popular operations on the Curve25519:

• X25519: Diffie-Hellman Key Exchange
• Ed25519: EdDSA signature system

There are quite some advantages over older RSA based algorithms:

• compact key size (32 bytes for X25519 and 64bytes for Ed25519)
• Less opportunities to introduce side-channels
• Faster than other popular alternatives at the same 128bit security level

compact25519 targets smaller embedded devices, binary size is modest and all operations are on a byte level, no assumption of fast int32/int64 operations.

Since there is no single C package management system, compact25519 is released as a single pair of c & h file. Deployment options:

• Download release from Github and copy into your project directory
• TODO: Use PlatformIO and take a dependency on compact25519

The implementation of the X25519 and Ed25519 is extracted from Daniel Beer's c25519 version 2017-10-05. Daniel Beer released it under the Public Domain license. To remain in the same spirit, this library is also licensed as Public Domain using the CC0 license.

By default the library contains both X25519 and Ed25519. They share quite some code. compact25519 has several defines so that you can disable either one of the operations to reduce binary size.