/* $OpenBSD: kexc25519.c,v 1.18 2024/09/02 12:13:56 djm Exp $ */

/*

 * Copyright (c) 2019 Markus Friedl.  All rights reserved.

 * Copyright (c) 2010 Damien Miller.  All rights reserved.

 * Copyright (c) 2013 Aris Adamantiadis.  All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the above copyright

 *    notice, this list of conditions and the following disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 *

 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR

 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES

 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.

 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,

 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT

 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 */

#include "includes.h"

#include <sys/types.h>

#include <stdio.h>

#include <string.h>

#include <signal.h>

#include "sshkey.h"

#include "kex.h"

#include "sshbuf.h"

#include "digest.h"

#include "ssherr.h"

#include "ssh2.h"

extern int crypto_scalarmult_curve25519(u_char a[CURVE25519_SIZE],

    const u_char b[CURVE25519_SIZE], const u_char c[CURVE25519_SIZE])

  __attribute__((__bounded__(__minbytes__, 1, CURVE25519_SIZE)))

  __attribute__((__bounded__(__minbytes__, 2, CURVE25519_SIZE)))

  __attribute__((__bounded__(__minbytes__, 3, CURVE25519_SIZE)));

void

kexc25519_keygen(u_char key[CURVE25519_SIZE], u_char pub[CURVE25519_SIZE])

{

  static const u_char basepoint[CURVE25519_SIZE] = {9};

  arc4random_buf(key, CURVE25519_SIZE);

  crypto_scalarmult_curve25519(pub, key, basepoint);

}

int

kexc25519_shared_key_ext(const u_char key[CURVE25519_SIZE],

    const u_char pub[CURVE25519_SIZE], struct sshbuf *out, int raw)

{

  u_char shared_key[CURVE25519_SIZE];

  u_char zero[CURVE25519_SIZE];

  int r;

  crypto_scalarmult_curve25519(shared_key, key, pub);

  /* Check for all-zero shared secret */

  explicit_bzero(zero, CURVE25519_SIZE);

  if (timingsafe_bcmp(zero, shared_key, CURVE25519_SIZE) == 0)

    return SSH_ERR_KEY_INVALID_EC_VALUE;

#ifdef DEBUG_KEXECDH

  dump_digest("shared secret 25519", shared_key, CURVE25519_SIZE);

#endif

  if (raw)

    r = sshbuf_put(out, shared_key, CURVE25519_SIZE);

  else

    r = sshbuf_put_bignum2_bytes(out, shared_key, CURVE25519_SIZE);

  explicit_bzero(shared_key, CURVE25519_SIZE);

  return r;

}

int

kexc25519_shared_key(const u_char key[CURVE25519_SIZE],

    const u_char pub[CURVE25519_SIZE], struct sshbuf *out)

{

  return kexc25519_shared_key_ext(key, pub, out, 0);

}

int

kex_c25519_keypair(struct kex *kex)

{

  struct sshbuf *buf = NULL;

  u_char *cp = NULL;

  int r;

  if ((buf = sshbuf_new()) == NULL)

    return SSH_ERR_ALLOC_FAIL;

  if ((r = sshbuf_reserve(buf, CURVE25519_SIZE, &cp)) != 0)

    goto out;

  kexc25519_keygen(kex->c25519_client_key, cp);

#ifdef DEBUG_KEXECDH

  dump_digest("client public key c25519:", cp, CURVE25519_SIZE);

#endif

  kex->client_pub = buf;

  buf = NULL;

 out:

  sshbuf_free(buf);

  return r;

}

int

kex_c25519_enc(struct kex *kex, const struct sshbuf *client_blob,

   struct sshbuf **server_blobp, struct sshbuf **shared_secretp)

{

  struct sshbuf *server_blob = NULL;

  struct sshbuf *buf = NULL;

  const u_char *client_pub;

  u_char *server_pub;

  u_char server_key[CURVE25519_SIZE];

  int r;

  *server_blobp = NULL;

  *shared_secretp = NULL;

  if (sshbuf_len(client_blob) != CURVE25519_SIZE) {

    r = SSH_ERR_SIGNATURE_INVALID;

    goto out;

  }

  client_pub = sshbuf_ptr(client_blob);

#ifdef DEBUG_KEXECDH

  dump_digest("client public key 25519:", client_pub, CURVE25519_SIZE);

#endif

  /* allocate space for encrypted KEM key and ECDH pub key */

  if ((server_blob = sshbuf_new()) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if ((r = sshbuf_reserve(server_blob, CURVE25519_SIZE, &server_pub)) != 0)

    goto out;

  kexc25519_keygen(server_key, server_pub);

  /* allocate shared secret */

  if ((buf = sshbuf_new()) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if ((r = kexc25519_shared_key_ext(server_key, client_pub, buf, 0)) < 0)

    goto out;

#ifdef DEBUG_KEXECDH

  dump_digest("server public key 25519:", server_pub, CURVE25519_SIZE);

  dump_digest("encoded shared secret:", sshbuf_ptr(buf), sshbuf_len(buf));

#endif

  *server_blobp = server_blob;

  *shared_secretp = buf;

  server_blob = NULL;

  buf = NULL;

 out:

  explicit_bzero(server_key, sizeof(server_key));

  sshbuf_free(server_blob);

  sshbuf_free(buf);

  return r;

}

int

kex_c25519_dec(struct kex *kex, const struct sshbuf *server_blob,

    struct sshbuf **shared_secretp)

{

  struct sshbuf *buf = NULL;

  const u_char *server_pub;

  int r;

  *shared_secretp = NULL;

  if (sshbuf_len(server_blob) != CURVE25519_SIZE) {

    r = SSH_ERR_SIGNATURE_INVALID;

    goto out;

  }

  server_pub = sshbuf_ptr(server_blob);

#ifdef DEBUG_KEXECDH

  dump_digest("server public key c25519:", server_pub, CURVE25519_SIZE);

#endif

  /* shared secret */

  if ((buf = sshbuf_new()) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if ((r = kexc25519_shared_key_ext(kex->c25519_client_key, server_pub,

      buf, 0)) < 0)

    goto out;

#ifdef DEBUG_KEXECDH

  dump_digest("encoded shared secret:", sshbuf_ptr(buf), sshbuf_len(buf));

#endif

  *shared_secretp = buf;

  buf = NULL;

 out:

  sshbuf_free(buf);

  return r;

}