Elliptic Curve Diffie-Hellman key exchange (x25519).
#include <monocypher.h> int crypto_key_exchange(uint8_t shared_key , const uint8_t your_secret_key , const uint8_t their_public_key); void crypto_key_exchange_public_key( uint8_t your_public_key, const uint8_t your_secret_key);
crypto_key_exchange() computes a shared key with your secret key and
their public key.
crypto_key_exchange_public_key() deterministically computes the public
key from a random secret key.
The arguments are:
shared_key: The shared secret, known only to those who know a relevant secret key (yours or theirs). It is cryptographically random, and suitable for use with the
crypto_lock()family of functions.
your_secret_key: A 32-byte random number, known only to you. See the introduction about random number generation (use your operating system's random number generator). Do not use the same private key for both key exchanges and signatures. The public keys are different, and revealing both may leak information.
their_public_key: The public key of the other party.
your_public_key: Your public key, generated from
Some public keys force the shared key to a known constant.
crypto_key_exchange() returns -1 if it detects such a public key,
otherwise it returns 0. This never happens with legitimate public keys.
The return value has been deprecated.
void starting with the next major release of Monocypher. Some
poorly designed protocols require to test for "contributory" behaviour,
which ensures that no untrusted party forces the shared secret to a
known constant. Protocols should instead be designed in such a way that
no such check is necessary, namely by authenticating the other party or
exchanging keys over a trusted channel.
crypto_key_exchange_public_key returns nothing.
Generate a public key from a randomly generated secret key:
const uint8_t sk; /* Random secret key */ uint8_t pk; /* Public key */ crypto_key_exchange_public_key(pk, sk); /* Wipe secrets if they are no longer needed */ crypto_wipe(sk, 32);
Generate a shared, symmetric key with your secret key and their public key. (The other party will generate the same shared key with your public key and their secret key.)
const uint8_t their_pk ; /* Their public key */ const uint8_t your_sk ; /* Your secret key */ uint8_t shared_key; /* Shared session key */ crypto_key_exchange(shared_key, your_sk, their_pk) != 0; /* Wipe secrets if they are no longer needed */ crypto_wipe(your_sk, 32);
These functions implement X25519, described in RFC 7748.
crypto_key_exchange() uses HChacha20 as well.
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.
crypto_key_exchange_public_key() is an alias to