166 lines
5.6 KiB
Markdown
166 lines
5.6 KiB
Markdown
Ed25519
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=======
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This is a portable implementation of [Ed25519](http://ed25519.cr.yp.to/) based
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on the SUPERCOP "ref10" implementation. Additionally there is key exchanging
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and scalar addition included to further aid building a PKI using Ed25519. All
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code is in the public domain.
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All code is pure ANSI C without any dependencies, except for the random seed
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generation which uses standard OS cryptography APIs (`CryptGenRandom` on
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Windows, `/dev/urandom` on nix). If you wish to be entirely portable define
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`ED25519_NO_SEED`. This disables the `ed25519_create_seed` function, so if your
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application requires key generation you must supply your own seeding function
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(which is simply a 256 bit (32 byte) cryptographic random number generator).
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Performance
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-----------
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On a Windows machine with an Intel Pentium B970 @ 2.3GHz I got the following
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speeds (running on only one a single core):
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Seed generation: 64us (15625 per second)
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Key generation: 88us (11364 per second)
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Message signing (short message): 87us (11494 per second)
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Message verifying (short message): 228us (4386 per second)
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Scalar addition: 100us (10000 per second)
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Key exchange: 220us (4545 per second)
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The speeds on other machines may vary. Sign/verify times will be higher with
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longer messages. The implementation significantly benefits from 64 bit
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architectures, if possible compile as 64 bit.
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Usage
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-----
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Simply add all .c and .h files in the `src/` folder to your project and include
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`ed25519.h` in any file you want to use the API. If you prefer to use a shared
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library, only copy `ed25519.h` and define `ED25519_DLL` before importing. A
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windows DLL is pre-built.
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There are no defined types for seeds, private keys, public keys, shared secrets
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or signatures. Instead simple `unsigned char` buffers are used with the
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following sizes:
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```c
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unsigned char seed[32];
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unsigned char signature[64];
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unsigned char public_key[32];
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unsigned char private_key[64];
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unsigned char scalar[32];
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unsigned char shared_secret[32];
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```
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API
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---
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```c
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int ed25519_create_seed(unsigned char *seed);
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```
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Creates a 32 byte random seed in `seed` for key generation. `seed` must be a
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writable 32 byte buffer. Returns 0 on success, and nonzero on failure.
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```c
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void ed25519_create_keypair(unsigned char *public_key, unsigned char *private_key, const unsigned char *seed);
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```
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Creates a new key pair from the given seed. `public_key` must be a writable 32
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byte buffer, `private_key` must be a writable 64 byte buffer and `seed` must be
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a 32 byte buffer.
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```c
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void ed25519_sign(unsigned char *signature,
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const unsigned char *message, size_t message_len,
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const unsigned char *public_key, const unsigned char *private_key);
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```
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Creates a signature of the given message with the given key pair. `signature`
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must be a writable 64 byte buffer. `message` must have at least `message_len`
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bytes to be read.
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```c
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int ed25519_verify(const unsigned char *signature,
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const unsigned char *message, size_t message_len,
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const unsigned char *public_key);
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```
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Verifies the signature on the given message using `public_key`. `signature`
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must be a readable 64 byte buffer. `message` must have at least `message_len`
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bytes to be read. Returns 1 if the signature matches, 0 otherwise.
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```c
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void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key,
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const unsigned char *scalar);
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```
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Adds `scalar` to the given key pair where scalar is a 32 byte buffer (possibly
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generated with `ed25519_create_seed`), generating a new key pair. You can
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calculate the public key sum without knowing the private key and vice versa by
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passing in `NULL` for the key you don't know. This is useful for enforcing
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randomness on a key pair by a third party while only knowing the public key,
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among other things. Warning: the last bit of the scalar is ignored - if
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comparing scalars make sure to clear it with `scalar[31] &= 127`.
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```c
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void ed25519_key_exchange(unsigned char *shared_secret,
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const unsigned char *public_key, const unsigned char *private_key);
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```
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Performs a key exchange on the given public key and private key, producing a
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shared secret. It is recommended to hash the shared secret before using it.
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`shared_secret` must be a 32 byte writable buffer where the shared secret will
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be stored.
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Example
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-------
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```c
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unsigned char seed[32], public_key[32], private_key[64], signature[64];
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unsigned char other_public_key[32], other_private_key[64], shared_secret[32];
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const unsigned char message[] = "TEST MESSAGE";
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/* create a random seed, and a key pair out of that seed */
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if (ed25519_create_seed(seed)) {
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printf("error while generating seed\n");
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exit(1);
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}
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ed25519_create_keypair(public_key, private_key, seed);
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/* create signature on the message with the key pair */
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ed25519_sign(signature, message, strlen(message), public_key, private_key);
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/* verify the signature */
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if (ed25519_verify(signature, message, strlen(message), public_key)) {
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printf("valid signature\n");
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} else {
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printf("invalid signature\n");
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}
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/* create a dummy keypair to use for a key exchange, normally you'd only have
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the public key and receive it through some communication channel */
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if (ed25519_create_seed(seed)) {
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printf("error while generating seed\n");
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exit(1);
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}
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ed25519_create_keypair(other_public_key, other_private_key, seed);
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/* do a key exchange with other_public_key */
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ed25519_key_exchange(shared_secret, other_public_key, private_key);
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/*
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the magic here is that ed25519_key_exchange(shared_secret, public_key,
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other_private_key); would result in the same shared_secret
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*/
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```
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License
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-------
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All code is in the public domain.
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