Key exchange

Example (client-side)

unsigned char client_pk[crypto_kx_PUBLICKEYBYTES], client_sk[crypto_kx_SECRETKEYBYTES];
unsigned char client_rx[crypto_kx_SESSIONKEYBYTES], client_tx[crypto_kx_SESSIONKEYBYTES];

/* Generate the client's key pair */
crypto_kx_keypair(client_pk, client_sk);

/* Prerequisite after this point: the server's public key must be known by the client */

/* Compute two shared keys using the server's public key and the client's secret key.
   client_rx will be used by the client to receive data from the server,
   client_tx will be used by the client to send data to the server. */
if (crypto_kx_client_session_keys(client_rx, client_tx,
                                  client_pk, client_sk, server_pk) != 0) {
    /* Suspicious server public key, bail out */
}

Example (server-side)

unsigned char server_pk[crypto_kx_PUBLICKEYBYTES], server_sk[crypto_kx_SECRETKEYBYTES];
unsigned char server_rx[crypto_kx_SESSIONKEYBYTES], server_tx[crypto_kx_SESSIONKEYBYTES];

/* Generate the server's key pair */
crypto_kx_keypair(server_pk, server_sk);

/* Prerequisite after this point: the client's public key must be known by the server */

/* Compute two shared keys using the client's public key and the server's secret key.
   server_rx will be used by the server to receive data from the client,
   server_tx will be used by the server to send data to the client. */
if (crypto_kx_server_session_keys(server_rx, server_tx,
                                  server_pk, server_sk, client_pk) != 0) {
    /* Suspicious client public key, bail out */
}

Purpose

Using the key exchange API, two parties can securely compute a set of shared keys using their peer’s public key and their own secret key.

This API was introduced in libsodium 1.0.12.

Usage

int crypto_kx_keypair(unsigned char pk[crypto_kx_PUBLICKEYBYTES],
                      unsigned char sk[crypto_kx_SECRETKEYBYTES]);

The crypto_kx_keypair() function creates a new key pair. It puts the public key into pk and the secret key into sk.

int crypto_kx_seed_keypair(unsigned char pk[crypto_kx_PUBLICKEYBYTES],
                           unsigned char sk[crypto_kx_SECRETKEYBYTES],
                           const unsigned char seed[crypto_kx_SEEDBYTES]);

The crypto_kx_seed_keypair() function computes a deterministic key pair from the seed seed (crypto_kx_SEEDBYTES bytes).

int crypto_kx_client_session_keys(unsigned char rx[crypto_kx_SESSIONKEYBYTES],
                                  unsigned char tx[crypto_kx_SESSIONKEYBYTES],
                                  const unsigned char client_pk[crypto_kx_PUBLICKEYBYTES],
                                  const unsigned char client_sk[crypto_kx_SECRETKEYBYTES],
                                  const unsigned char server_pk[crypto_kx_PUBLICKEYBYTES]);

The crypto_kx_client_session_keys() function computes a pair of shared keys (rx and tx) using the client’s public key client_pk, the client’s secret key client_sk, and the server’s public key server_pk.

It returns 0 on success and -1 if the server’s public key is not acceptable.

These keys can be used by any functions requiring secret keys up to crypto_kx_SESSIONKEYBYTES bytes, including crypto_secretbox_*() and crypto_aead_*().

The shared secret key rx should be used by the client to receive data from the server, whereas tx should be used for data flowing in the opposite direction.

rx and tx are both crypto_kx_SESSIONKEYBYTES bytes long. If only one session key is required, either rx or tx can be set to NULL.

int crypto_kx_server_session_keys(unsigned char rx[crypto_kx_SESSIONKEYBYTES],
                                  unsigned char tx[crypto_kx_SESSIONKEYBYTES],
                                  const unsigned char server_pk[crypto_kx_PUBLICKEYBYTES],
                                  const unsigned char server_sk[crypto_kx_SECRETKEYBYTES],
                                  const unsigned char client_pk[crypto_kx_PUBLICKEYBYTES]);

The crypto_kx_server_session_keys() function computes a pair of shared keys (rx and tx) using the server’s public key server_pk, the server’s secret key server_sk, and the client’s public key client_pk.

It returns 0 on success and -1 if the client’s public key is not acceptable.

The shared secret key rx should be used by the server to receive data from the client, whereas tx should be used for data flowing in the opposite direction.

rx and tx are both crypto_kx_SESSIONKEYBYTES bytes long. If only one session key is required, either rx or tx can be set to NULL.

Constants

  • crypto_kx_PUBLICKEYBYTES

  • crypto_kx_SECRETKEYBYTES

  • crypto_kx_SEEDBYTES

  • crypto_kx_SESSIONKEYBYTES

  • crypto_kx_PRIMITIVE

Algorithm details

Let p.n be the crypto_scalarmult_curve25519_BYTES byte output of the X25519 key exchange operation. The 512-bit output of BLAKE2B-512 is split into two 256-bit keys rx and tx.

rx || tx = BLAKE2B-512(p.n || client_pk || server_pk)

Notes

For earlier versions of the library that didn’t implement this API, the X25519 function is accessible directly using the crypto_scalarmult_*() API.

Having different keys for each direction allows counters to be safely used as nonces without having to wait for an acknowledgment after every message.

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