include/openssl/curve25519.h

/* Copyright (c) 2015, Google Inc.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */

#ifndef OPENSSL_HEADER_CURVE25519_H
#define OPENSSL_HEADER_CURVE25519_H

#include <openssl/base.h>

#if defined(__cplusplus)
extern "C" {
#endif


/* Curve25519.
 *
 * Curve25519 is an elliptic curve. See https://tools.ietf.org/html/rfc7748. */


/* X25519.
 *
 * X25519 is the Diffie-Hellman primitive built from curve25519. It is
 * sometimes referred to as “curve25519”, but “X25519” is a more precise name.
 * See http://cr.yp.to/ecdh.html and https://tools.ietf.org/html/rfc7748. */

/* X25519_keypair sets |out_public_value| and |out_private_key| to a freshly
 * generated, public–private key pair. */
OPENSSL_EXPORT void X25519_keypair(uint8_t out_public_value[32],
                                   uint8_t out_private_key[32]);

/* X25519 writes a shared key to |out_shared_key| that is calculated from the
 * given private key and the peer's public value. It returns one on success and
 * zero on error.
 *
 * Don't use the shared key directly, rather use a KDF and also include the two
 * public values as inputs. */
OPENSSL_EXPORT int X25519(uint8_t out_shared_key[32],
                          const uint8_t private_key[32],
                          const uint8_t peers_public_value[32]);

/* X25519_public_from_private calculates a Diffie-Hellman public value from the
 * given private key and writes it to |out_public_value|. */
OPENSSL_EXPORT void X25519_public_from_private(uint8_t out_public_value[32],
                                               const uint8_t private_key[32]);


/* Ed25519.
 *
 * Ed25519 is a signature scheme using a twisted-Edwards curve that is
 * birationally equivalent to curve25519. */

#define ED25519_PRIVATE_KEY_LEN 64
#define ED25519_PUBLIC_KEY_LEN 32
#define ED25519_SIGNATURE_LEN 64

/* ED25519_keypair sets |out_public_key| and |out_private_key| to a freshly
 * generated, public–private key pair. */
OPENSSL_EXPORT void ED25519_keypair(uint8_t out_public_key[32],
                                    uint8_t out_private_key[64]);

/* ED25519_sign sets |out_sig| to be a signature of |message_len| bytes from
 * |message| using |private_key|. It returns one on success or zero on
 * error. */
OPENSSL_EXPORT int ED25519_sign(uint8_t out_sig[64], const uint8_t *message,
                                size_t message_len,
                                const uint8_t private_key[64]);

/* ED25519_verify returns one iff |signature| is a valid signature, by
 * |public_key| of |message_len| bytes from |message|. It returns zero
 * otherwise. */
OPENSSL_EXPORT int ED25519_verify(const uint8_t *message, size_t message_len,
                                  const uint8_t signature[64],
                                  const uint8_t public_key[32]);


/* SPAKE2.
 *
 * SPAKE2 is a password-authenticated key-exchange. It allows two parties,
 * who share a low-entropy secret (i.e. password), to agree on a shared key.
 * An attacker can only make one guess of the password per execution of the
 * protocol.
 *
 * See https://tools.ietf.org/html/draft-irtf-cfrg-spake2-02. */

/* spake2_role_t enumerates the different “roles” in SPAKE2. The protocol
 * requires that the symmetry of the two parties be broken so one participant
 * must be “Alice” and the other be “Bob”. */
enum spake2_role_t {
  spake2_role_alice,
  spake2_role_bob,
};

/* SPAKE2_CTX_new creates a new |SPAKE2_CTX| (which can only be used for a
 * single execution of the protocol). SPAKE2 requires the symmetry of the two
 * parties to be broken which is indicated via |my_role| – each party must pass
 * a different value for this argument.
 *
 * The |my_name| and |their_name| arguments allow optional, opaque names to be
 * bound into the protocol. For example MAC addresses, hostnames, usernames
 * etc. These values are not exposed and can avoid context-confusion attacks
 * when a password is shared between several devices. */
OPENSSL_EXPORT SPAKE2_CTX *SPAKE2_CTX_new(
    enum spake2_role_t my_role,
    const uint8_t *my_name, size_t my_name_len,
    const uint8_t *their_name, size_t their_name_len);

/* SPAKE2_CTX_free frees |ctx| and all the resources that it has allocated. */
OPENSSL_EXPORT void SPAKE2_CTX_free(SPAKE2_CTX *ctx);

/* SPAKE2_MAX_MSG_SIZE is the maximum size of a SPAKE2 message. */
#define SPAKE2_MAX_MSG_SIZE 32

/* SPAKE2_generate_msg generates a SPAKE2 message given |password|, writes
 * it to |out| and sets |*out_len| to the number of bytes written.
 *
 * At most |max_out_len| bytes are written to |out| and, in order to ensure
 * success, |max_out_len| should be at least |SPAKE2_MAX_MSG_SIZE| bytes.
 *
 * This function can only be called once for a given |SPAKE2_CTX|.
 *
 * It returns one on success and zero on error. */
OPENSSL_EXPORT int SPAKE2_generate_msg(SPAKE2_CTX *ctx, uint8_t *out,
                                       size_t *out_len, size_t max_out_len,
                                       const uint8_t *password,
                                       size_t password_len);

/* SPAKE2_MAX_KEY_SIZE is the maximum amount of key material that SPAKE2 will
 * produce. */
#define SPAKE2_MAX_KEY_SIZE 64

/* SPAKE2_process_msg completes the SPAKE2 exchange given the peer's message in
 * |their_msg|, writes at most |max_out_key_len| bytes to |out_key| and sets
 * |*out_key_len| to the number of bytes written.
 *
 * The resulting keying material is suitable for:
 *   a) Using directly in a key-confirmation step: i.e. each side could
 *      transmit a hash of their role, a channel-binding value and the key
 *      material to prove to the other side that they know the shared key.
 *   b) Using as input keying material to HKDF to generate a variety of subkeys
 *      for encryption etc.
 *
 * If |max_out_key_key| is smaller than the amount of key material generated
 * then the key is silently truncated. If you want to ensure that no truncation
 * occurs then |max_out_key| should be at least |SPAKE2_MAX_KEY_SIZE|.
 *
 * You must call |SPAKE2_generate_msg| on a given |SPAKE2_CTX| before calling
 * this function. On successful return, |ctx| is complete and calling
 * |SPAKE2_CTX_free| is the only acceptable operation on it.
 *
 * Returns one on success or zero on error. */
OPENSSL_EXPORT int SPAKE2_process_msg(SPAKE2_CTX *ctx, uint8_t *out_key,
                                      size_t *out_key_len,
                                      size_t max_out_key_len,
                                      const uint8_t *their_msg,
                                      size_t their_msg_len);


#if defined(__cplusplus)
}  /* extern C */
#endif

#endif  /* OPENSSL_HEADER_CURVE25519_H */