Monocypher

Boring crypto that simply works
CRYPTO_X25519(3MONOCYPHER) 3MONOCYPHER CRYPTO_X25519(3MONOCYPHER)

X25519 key exchange (Public Key Cryptography)
#include <monocypher.h>

void
crypto_x25519(uint8_t raw_shared_secret[32], const uint8_t your_secret_key[32], const uint8_t their_public_key[32]);

void
crypto_x25519_public_key(uint8_t your_public_key[32], const uint8_t your_secret_key[32]);

crypto_x25519() computes a shared secret with your_secret_key and their_public_key. It is a low-level primitive; X25519 is a building block for protocols. To perform a key exchange, use a higher level protocol, such as the X3DH key agreement protocol.

crypto_x25519_public_key() deterministically computes the public key from a random secret key.

The arguments are:

raw_shared_secret
The shared secret, known only to those who know a relevant secret key (yours or theirs). It is not cryptographically random. Do not use it directly as a key. Hash it concatenated with your_public_key and their_public_key using crypto_blake2b() for key derivation.
your_secret_key
A 32-byte secret random number. See intro() for advice about generating random bytes (use the operating system's random number generator).
their_public_key
The public key of the other party.

raw_shared_secret and your_secret_key may overlap if your secret is no longer required.

Some protocols, such as some password-authenticated key exchange (PAKE) protocols and oblivious pseudo-random functions (OPRF), may require “contributory” behaviour, which ensures that no untrusted party forces the shared secret to a known constant. If a protocol requires contributory behaviour, compare the output of crypto_x25519() to an all-zero buffer using crypto_verify32(), then abort the protocol if the output and the all-zero buffer are equal.

Do not use the same secret key for both key exchanges and signatures. The public keys are different and revealing both may leak information. If there really is no room to store or derive two different secret keys, consider generating a key pair for signatures and then converting it with crypto_from_eddsa_private() and crypto_from_eddsa_public().

crypto_x25519() and crypto_x25519_public_key() return nothing.

The following example assumes the existence of arc4random_buf(), which fills the given buffer with cryptographically secure random bytes. If arc4random_buf() does not exist on your system, see intro() for advice about how to generate cryptographically secure random bytes.

Generate a pair of shared keys with your secret key and their public key (this can help nonce management for full duplex communications).

const uint8_t their_pk     [32]; /* Their public key          */
uint8_t       your_sk      [32]; /* Your secret key           */
uint8_t       your_pk      [32]; /* Your public key           */
uint8_t       shared_secret[32]; /* Shared secret (NOT a key) */
arc4random_buf(your_sk, 32);
crypto_x25512(your_pk, your_sk);
crypto_x25519(shared_secret, your_sk, their_pk);
/* Wipe secrets if they are no longer needed */
crypto_wipe(your_sk, 32);

uint8_t shared_keys[64]; /* Two shared session keys */
crypto_blake2b_ctx ctx;
crypto_blake2b_init  (&ctx);
crypto_blake2b_update(&ctx, shared_secret, 32);
crypto_blake2b_update(&ctx, your_pk      , 32);
crypto_blake2b_update(&ctx, their_pk     , 32);
crypto_blake2b_final (&ctx, shared_keys);
const uint8_t *key_1 = shared_keys;      /* Shared key 1 */
const uint8_t *key_2 = shared_keys + 32; /* Shared key 2 */
/* Wipe secrets if they are no longer needed */
crypto_wipe(shared_secret, 32);

intro()

This function implements X25519, described in RFC 7748.

The crypto_x25519() and crypto_x25519_public_key() functions first appeared in Monocypher 0.1.

If either of the long-term secret keys leaks, it may compromise all past messages. This can be avoided by using protocols that provide forward secrecy, such as the X3DH key agreement protocol.

Many (private, public) key pairs produce the same shared secret. Therefore, not including the public keys in the key derivation can lead to subtle vulnerabilities. This can be avoided by hashing the shared secret concatenated with both public keys. For example,

BLAKE2b(shared_secret || your_pk || their_pk)
using crypto_blake2b().

The most significant bit of the public key is systematically ignored. It is not needed because every public key should be smaller than 2^255-19, which fits in 255 bits. If another implementation of X25519 gives you a key that is not fully reduced and has its high bit set, the computation will fail. On the other hand, it also means you may use this bit for other purposes (such as parity flipping for Ed25519 compatibility).
February 23, 2022 Debian