Ed25519

This is a portable implementation of Ed25519 based on the SUPERCOP "ref10" implementation. All code is in the public domain.

All code is pure ANSI C without any dependencies, except for the random seed generation which uses standard OS cryptography APIs. If you wish to be entirely portable define ED25519_NO_SEED. This disables the ed25519_create_seed function, so if your application requires key generation you must supply your own seeding function (simply a 32 byte random number generator).

Performance

On a machine with an Intel Pentium B970 @ 2.3GHz I got the following speeds (running on only one a single core):

Seed + key generation:             345us
Message signing (short message):   256us
Message verifying (short message): 777us

The speeds on other machines may vary. Sign/verify times will be higher with longer messages.

Usage

Simply add all .c and .h files in the src/ folder to your project and include ed25519.h in any file you want to use the API. If you prefer to use a shared library, only copy ed25519.h and define ED25519_DLL before importing. A windows DLL is pre-built.

There are no defined types for seeds, private keys, public keys or signatures. Instead simple unsigned char buffers are used with the following sizes:

unsigned char seed[32];
unsigned char signature[64];
unsigned char public_key[32];
unsigned char private_key[64];
unsigned char scalar[32];

API

int ed25519_create_seed(unsigned char *seed);

Creates a 32 byte random seed in seed for key generation. seed must be a writable 32 byte buffer. Returns 0 on success, and nonzero on failure.

void ed25519_create_keypair(unsigned char *public_key, unsigned char *private_key, const unsigned char *seed);

Creates a new key pair from the given seed. public_key must be a writable 32 byte buffer, private_key must be a writable 64 byte buffer and seed must be a 32 byte buffer.

void ed25519_sign(unsigned char *signature,
                 const unsigned char *message, size_t message_len,
                 const unsigned char *public_key, const unsigned char *private_key);

Creates a signature of the given message with the keypair (public_key, private_key). signature must be a writable 64 byte buffer. message must have at least message_len bytes to be read. The given keypair must be a keypair generated by ed25519_create_keypair.

int ed25519_verify(const unsigned char *signature,
                   const unsigned char *message, size_t message_len,
                   const unsigned char *public_key);

Verifies the signature on the given message using public_key. signature must be a readable 64 byte buffer. message must have at least message_len bytes to be read. public_key must be a 32 byte public key generated by ed25519_create_keypair. Returns 1 if the signature matches, 0 otherwise.

void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key,
                        const unsigned char *scalar);

Adds scalar to the given keypair where scalar is a 32 byte buffer (possibly generated with ed25519_create_seed), generating a new keypair. You can calculate the public key sum without knowing the private key and vice versa by passing in NULL. This is useful for enforcing randomness on a keypair while only knowing the public key, among other things.

Example

unsigned char seed[32], public_key[32], private_key[64], signature[64];
const unsigned char message[] = "TEST MESSAGE";

/* create a random seed, and a keypair out of that seed */
if (ed25519_create_seed(seed)) {
    printf("error while generating seed\n");
    exit(1);
}

ed25519_create_keypair(public_key, private_key, seed);

/* create signature on the message with the keypair */
ed25519_sign(signature, message, strlen(message), public_key, private_key);

/* verify the signature */
if (ed25519_verify(signature, message, strlen(message), public_key)) {
    printf("valid signature\n");
} else {
    printf("invalid signature\n");
}