Curve25519

Elliptic-curve Diffie-Hellman key agreement on Curve25519.

Available Implementations

5164

#include "Hacl_Curve25519_51.h"

A portable implementation.

API Reference

Example

#define HACL_DH_CURVE25519_SECRETKEY_LEN 32
#define HACL_DH_CURVE25519_PUBLICKEY_LEN 32
#define HACL_DH_CURVE25519_SHARED_LEN 32

// Alice and Bob want to agree on a shared secret via X25519.

// Thus, Alice needs a secret and public key ...
uint8_t alice_sk[HACL_DH_CURVE25519_SECRETKEY_LEN];
uint8_t alice_pk[HACL_DH_CURVE25519_PUBLICKEY_LEN];
// Note: This function is not in HACL*.
//       You need to bring your own random.
generate_random(alice_sk, HACL_DH_CURVE25519_SECRETKEY_LEN);
Hacl_Curve25519_51_secret_to_public(alice_pk, alice_sk);

// ... and Bob does as well.
uint8_t bob_sk[HACL_DH_CURVE25519_SECRETKEY_LEN];
uint8_t bob_pk[HACL_DH_CURVE25519_PUBLICKEY_LEN];
// Note: This function is not in HACL*.
//       You need to bring your own random.
generate_random(bob_sk, HACL_DH_CURVE25519_SECRETKEY_LEN);
Hacl_Curve25519_51_secret_to_public(bob_pk, bob_sk);

// Now, Alice and Bob exchange their public keys so that
// Alice can compute her shared secret as ...
uint8_t shared_alice[HACL_DH_CURVE25519_SHARED_LEN];
bool res_alice = Hacl_Curve25519_51_ecdh(shared_alice, alice_sk, bob_pk);

// ... and Bob can compute his shared secret as ...
uint8_t shared_bob[HACL_DH_CURVE25519_SHARED_LEN];
bool res_bob = Hacl_Curve25519_51_ecdh(shared_bob, bob_sk, alice_pk);

// Now, both Alice and Bob should share the same secret value, i.e.,
//
//     `shared_alice` == `shared_bob`
//
// ... and can use this to derive, e.g., an encryption key.

5164

void Hacl_Curve25519_51_scalarmult(uint8_t *out, uint8_t *priv, uint8_t *pub)

Compute the scalar multiple of a point.

Parameters

• out – Pointer to 32 bytes of memory, allocated by the caller, where the resulting point is written to.

• priv – Pointer to 32 bytes of memory where the secret/private key is read from.

• pub – Pointer to 32 bytes of memory where the public point is read from.

5164

void Hacl_Curve25519_51_secret_to_public(uint8_t *pub, uint8_t *priv)

Calculate a public point from a secret/private key.

This computes a scalar multiplication of the secret/private key with the curve’s basepoint.

Parameters

• pub – Pointer to 32 bytes of memory, allocated by the caller, where the resulting point is written to.

• priv – Pointer to 32 bytes of memory where the secret/private key is read from.

5164

bool Hacl_Curve25519_51_ecdh(uint8_t *out, uint8_t *priv, uint8_t *pub)

Execute the diffie-hellmann key exchange.

Parameters

• out – Pointer to 32 bytes of memory, allocated by the caller, where the resulting point is written to.

• priv – Pointer to 32 bytes of memory where our secret/private key is read from.

• pub – Pointer to 32 bytes of memory where their public point is read from.