readme.md

Ed25519

This is a portable implementation of Ed25519 based on the SUPERCOP "ref10" implementation. Additionally there is key exchanging and scalar addition included to further aid building a PKI using Ed25519. All code is licensed under the permissive zlib license.

All code is pure ANSI C without any dependencies, except for the random seed generation which uses standard OS cryptography APIs (CryptGenRandom on Windows, /dev/urandom on nix). 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 (which is simply a 256 bit (32 byte) cryptographic random number generator).

Performance

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

Seed generation: 64us (15625 per second)
Key generation: 88us (11364 per second)
Message signing (short message): 87us (11494 per second)
Message verifying (short message): 228us (4386 per second)
Scalar addition: 100us (10000 per second)
Key exchange: 220us (4545 per second)

The speeds on other machines may vary. Sign/verify times will be higher with longer messages. The implementation significantly benefits from 64 bit architectures, if possible compile as 64 bit.

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, shared secrets 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];
unsigned char shared_secret[32];

Note: this library stores private keys in a different format than some other libraries, notably libsodium. They tend to store the concatenation of the seed and public_key as their private key representation. If you wish to be compatible with these libraries you must keep the seed around.

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 given key pair. signature must be a writable 64 byte buffer. message must have at least message_len bytes to be read.

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. 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 key pair where scalar is a 32 byte buffer (possibly generated with ed25519_create_seed), generating a new key pair. You can calculate the public key sum without knowing the private key and vice versa by passing in NULL for the key you don't know. This is useful for enforcing randomness on a key pair by a third party while only knowing the public key, among other things. Warning: the last bit of the scalar is ignored - if comparing scalars make sure to clear it with scalar[31] &= 127.

void ed25519_key_exchange(unsigned char *shared_secret,
                          const unsigned char *public_key, const unsigned char *private_key);

Performs a key exchange on the given public key and private key, producing a shared secret. It is recommended to hash the shared secret before using it. shared_secret must be a 32 byte writable buffer where the shared secret will be stored.

Example

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

/* create a random seed, and a key pair 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 key pair */
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");
}

/* create a dummy keypair to use for a key exchange, normally you'd only have
the public key and receive it through some communication channel */
if (ed25519_create_seed(seed)) {
    printf("error while generating seed\n");
    exit(1);
}

ed25519_create_keypair(other_public_key, other_private_key, seed);

/* do a key exchange with other_public_key */
ed25519_key_exchange(shared_secret, other_public_key, private_key);

/* 
    the magic here is that ed25519_key_exchange(shared_secret, public_key,
    other_private_key); would result in the same shared_secret
*/

License

All code is released under the zlib license. See license.txt for details.