Ed25519
-------

This is a portable implementation of [Ed25519](http://ed25519.cr.yp.to/) 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:

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

API
---

```c
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.

```c
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.

```c
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. 

```c
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.

```c
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 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`.

Example
-------
```c
unsigned char seed[32], public_key[32], private_key[64], signature[64];
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");
}
```