/* $OpenBSD: kexmlkem768ecdh.c,v 1.4 2026/08/03 06:43:16 djm Exp $ */

/*

 * Copyright (c) 2025 Markus Friedl.  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 <stdint.h>

#include <stdbool.h>

#include <string.h>

#include <signal.h>

#include <endian.h>

#include "sshkey.h"

#include "kex.h"

#include "sshbuf.h"

#include "digest.h"

#include "ssherr.h"

#ifdef WITH_OPENSSL

int

kex_kem_mlkem768ecdh_keypair(struct kex *kex)

{

  struct sshbuf *buf = NULL;

  struct sshbuf *ec_blob = NULL;

  EC_KEY *client_key = NULL;

  const EC_GROUP *group;

  const EC_POINT *public_key;

  u_char *cp = NULL;

  size_t need;

  int r = SSH_ERR_INTERNAL_ERROR;

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

    return SSH_ERR_ALLOC_FAIL;

  need = MLKEM768_PUBLICKEYBYTES;

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

    goto out;

  if (crypto_kem_mlkem768_keypair(cp, kex->mlkem768_client_key) != 0) {

    r = SSH_ERR_INTERNAL_ERROR;

    goto out;

  }

#ifdef DEBUG_KEXECDH

  dump_digest("client public key mlkem768:", cp,

      MLKEM768_PUBLICKEYBYTES);

#endif

  if ((client_key = EC_KEY_new_by_curve_name(kex->ec_nid)) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if (EC_KEY_generate_key(client_key) != 1) {

    r = SSH_ERR_LIBCRYPTO_ERROR;

    goto out;

  }

  group = EC_KEY_get0_group(client_key);

  public_key = EC_KEY_get0_public_key(client_key);

  if ((ec_blob = sshbuf_new()) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if ((r = sshbuf_put_ec(ec_blob, public_key, group)) != 0 ||

      (r = sshbuf_get_u32(ec_blob, NULL)) != 0 ||

      (r = sshbuf_putb(buf, ec_blob)) != 0)

    goto out;

#ifdef DEBUG_KEXECDH

  fputs("client private key EC:\n", stderr);

  sshkey_dump_ec_key(client_key);

#endif

  /* success */

  r = 0;

  kex->ec_client_key = client_key;

  kex->ec_group = group;

  client_key = NULL; /* owned by the kex */

  kex->client_pub = buf;

  buf = NULL;

 out:

  sshbuf_free(buf);

  sshbuf_free(ec_blob);

  EC_KEY_free(client_key);

  return r;

}

int

kex_kem_mlkem768ecdh_enc(struct kex *kex,

   const struct sshbuf *client_blob, struct sshbuf **server_blobp,

   struct sshbuf **shared_secretp)

{

  const EC_GROUP *group;

  const EC_POINT *pub_key;

  EC_KEY *server_key = NULL;

  struct sshbuf *ec_pub = NULL;

  struct sshbuf *ec_blob = NULL;

  struct sshbuf *ec_shared = NULL;

  struct sshbuf *server_blob = NULL;

  struct sshbuf *buf = NULL;

  const u_char *client_pub;

  u_char hash[SSH_DIGEST_MAX_LENGTH];

  u_char ct[MLKEM768_CIPHERTEXTBYTES];

  u_char shared_secret[MLKEM768_BYTES];

  size_t need;

  int r = SSH_ERR_INTERNAL_ERROR;

  *server_blobp = NULL;

  *shared_secretp = NULL;

  /* client_blob contains both KEM and ECDH client pubkeys */

  need = MLKEM768_PUBLICKEYBYTES;

  if (sshbuf_len(client_blob) <= need) {

    r = SSH_ERR_SIGNATURE_INVALID;

    goto out;

  }

  client_pub = sshbuf_ptr(client_blob);

#ifdef DEBUG_KEXECDH

  dump_digest("client public key mlkem768:", client_pub,

      MLKEM768_PUBLICKEYBYTES);

#endif

  /* allocate buffer for concatenation of KEM key and ECDH shared key */

  /* the buffer will be hashed and the result is the shared secret */

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

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

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

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

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  /* generate and encrypt KEM key with client key */

  if (crypto_kem_mlkem768_enc(ct, shared_secret, client_pub) != 0) {

    r = SSH_ERR_INTERNAL_ERROR;

    goto out;

  }

  if ((r = sshbuf_put(buf, shared_secret, sizeof(shared_secret))) != 0 ||

      (r = sshbuf_put(server_blob, ct, sizeof(ct))) != 0)

    goto out;

  client_pub += MLKEM768_PUBLICKEYBYTES;

  if ((ec_pub = sshbuf_from(client_pub, sshbuf_len(client_blob) -

      MLKEM768_PUBLICKEYBYTES)) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  /* generate ECDH key pair */

  if ((server_key = EC_KEY_new_by_curve_name(kex->ec_nid)) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if (EC_KEY_generate_key(server_key) != 1) {

    r = SSH_ERR_LIBCRYPTO_ERROR;

    goto out;

  }

  group = EC_KEY_get0_group(server_key);

#ifdef DEBUG_KEXECDH

  fputs("server private key EC:\n", stderr);

  sshkey_dump_ec_key(server_key);

#endif

  /* store server pubkey after ciphertext */

  pub_key = EC_KEY_get0_public_key(server_key);

  if ((ec_blob = sshbuf_new()) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if ((r = sshbuf_put_ec(ec_blob, pub_key, group)) != 0 ||

      (r = sshbuf_get_u32(ec_blob, NULL)) != 0 ||

      (r = sshbuf_putb(server_blob, ec_blob)) != 0)

    goto out;

  /* append ECDH shared key */

  if ((r = kex_ecdh_dec_key_group(kex, ec_pub, server_key, group,

      1, &ec_shared)) != 0 ||

      (r = sshbuf_putb(buf, ec_shared)) != 0)

    goto out;

  if ((r = ssh_digest_buffer(kex->hash_alg, buf, hash, sizeof(hash))) != 0)

    goto out;

#ifdef DEBUG_KEXECDH

  dump_digest("server cipher text:", ct, sizeof(ct));

  dump_digest("server kem key:", shared_secret, sizeof(shared_secret));

  dump_digest("concatenation of KEM key and ECDH shared key:",

      sshbuf_ptr(buf), sshbuf_len(buf));

#endif

  /* string-encoded hash is resulting shared secret */

  sshbuf_reset(buf);

  if ((r = sshbuf_put_string(buf, hash,

      ssh_digest_bytes(kex->hash_alg))) != 0)

    goto out;

#ifdef DEBUG_KEXECDH

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

#endif

  /* success */

  r = 0;

  *server_blobp = server_blob;

  *shared_secretp = buf;

  server_blob = NULL;

  buf = NULL;

 out:

  explicit_bzero(hash, sizeof(hash));

  explicit_bzero(shared_secret, sizeof(shared_secret));

  EC_KEY_free(server_key);

  sshbuf_free(ec_pub);

  sshbuf_free(ec_blob);

  sshbuf_free(ec_shared);

  sshbuf_free(server_blob);

  sshbuf_free(buf);

  return r;

}

int

kex_kem_mlkem768ecdh_dec(struct kex *kex,

    const struct sshbuf *server_blob, struct sshbuf **shared_secretp)

{

  struct sshbuf *buf = NULL;

  struct sshbuf *ec_pub = NULL;

  struct sshbuf *ec_shared = NULL;

  u_char shared_secret[MLKEM768_BYTES];

  const u_char *ciphertext, *server_pub;

  u_char hash[SSH_DIGEST_MAX_LENGTH];

  size_t need;

  int r;

  *shared_secretp = NULL;

  need = MLKEM768_CIPHERTEXTBYTES;

  if (sshbuf_len(server_blob) <= need) {

    r = SSH_ERR_SIGNATURE_INVALID;

    goto out;

  }

  ciphertext = sshbuf_ptr(server_blob);

  server_pub = ciphertext + MLKEM768_CIPHERTEXTBYTES;

#ifdef DEBUG_KEXECDH

  dump_digest("server cipher text (dec):", ciphertext,

      MLKEM768_CIPHERTEXTBYTES);

#endif

  /* hash concatenation of KEM key and ECDH shared key */

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

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if (crypto_kem_mlkem768_dec(shared_secret, ciphertext,

      kex->mlkem768_client_key) != 0) {

    r = SSH_ERR_INTERNAL_ERROR;

    goto out;

  }

  if ((r = sshbuf_put(buf, shared_secret, sizeof(shared_secret))) != 0)

    goto out;

  if ((ec_pub = sshbuf_from(server_pub, sshbuf_len(server_blob) - need))

      == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if ((r = kex_ecdh_dec_key_group(kex, ec_pub, kex->ec_client_key,

      kex->ec_group, 1, &ec_shared)) != 0 ||

      (r = sshbuf_putb(buf, ec_shared)) != 0)

    goto out;

  if ((r = ssh_digest_buffer(kex->hash_alg, buf,

      hash, sizeof(hash))) != 0)

    goto out;

#ifdef DEBUG_KEXECDH

  dump_digest("client kem key:", shared_secret, sizeof(shared_secret));

  dump_digest("concatenation of KEM key and ECDH shared key:",

      sshbuf_ptr(buf), sshbuf_len(buf));

#endif

  sshbuf_reset(buf);

  if ((r = sshbuf_put_string(buf, hash,

      ssh_digest_bytes(kex->hash_alg))) != 0)

    goto out;

#ifdef DEBUG_KEXECDH

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

#endif

  /* success */

  r = 0;

  *shared_secretp = buf;

  buf = NULL;

 out:

  explicit_bzero(hash, sizeof(hash));

  explicit_bzero(shared_secret, sizeof(shared_secret));

  EC_KEY_free(kex->ec_client_key);

  kex->ec_client_key = NULL;

  sshbuf_free(ec_pub);

  sshbuf_free(ec_shared);

  sshbuf_free(buf);

  return r;

}

#endif  /* WITH_OPENSSL */