/* $OpenBSD: kexmlkem768x25519.c,v 1.3 2026/06/14 03:59:34 djm Exp $ */
/*
 * Copyright (c) 2023 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"
#include "log.h"

#include "crypto_api.h"

#ifdef USE_MLKEM768X25519

int
kex_kem_mlkem768x25519_keypair(struct kex *kex)
{
        struct sshbuf *buf = NULL;
        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 + CURVE25519_SIZE;
        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
        cp += MLKEM768_PUBLICKEYBYTES;
        kexc25519_keygen(kex->c25519_client_key, cp);
#ifdef DEBUG_KEXECDH
        dump_digest("client public key c25519:", cp, CURVE25519_SIZE);
#endif
        /* success */
        r = 0;
        kex->client_pub = buf;
        buf = NULL;
 out:
        sshbuf_free(buf);
        return r;
}

int
kex_kem_mlkem768x25519_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[CURVE25519_SIZE], server_key[CURVE25519_SIZE];
        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 + CURVE25519_SIZE;
        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);
        dump_digest("client public key 25519:",
            client_pub + MLKEM768_PUBLICKEYBYTES,
            CURVE25519_SIZE);
#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;
        }
        /* generate ECDH key pair, store server pubkey after ciphertext */
        kexc25519_keygen(server_key, server_pub);
        if ((r = sshbuf_put(buf, shared_secret, sizeof(shared_secret))) != 0 ||
            (r = sshbuf_put(server_blob, ct, sizeof(ct))) != 0 ||
            (r = sshbuf_put(server_blob, server_pub, sizeof(server_pub))) != 0)
                goto out;
        /* append ECDH shared key */
        client_pub += MLKEM768_PUBLICKEYBYTES;
        if ((r = kexc25519_shared_key_ext(server_key, client_pub, buf, 1)) < 0)
                goto out;
        if ((r = ssh_digest_buffer(kex->hash_alg, buf,
            hash, sizeof(hash))) != 0)
                goto out;
#ifdef DEBUG_KEXECDH
        dump_digest("server public key 25519:", server_pub, CURVE25519_SIZE);
        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(server_key, sizeof(server_key));
        explicit_bzero(shared_secret, sizeof(shared_secret));
        sshbuf_free(server_blob);
        sshbuf_free(buf);
        return r;
}

int
kex_kem_mlkem768x25519_dec(struct kex *kex,
    const struct sshbuf *server_blob, struct sshbuf **shared_secretp)
{
        struct sshbuf *buf = 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 + CURVE25519_SIZE;
        if (sshbuf_len(server_blob) != need) {
                r = SSH_ERR_SIGNATURE_INVALID;
                goto out;
        }
        ciphertext = sshbuf_ptr(server_blob);
        server_pub = ciphertext + MLKEM768_CIPHERTEXTBYTES;
        /* hash concatenation of KEM key and ECDH shared key */
        if ((buf = sshbuf_new()) == NULL) {
                r = SSH_ERR_ALLOC_FAIL;
                goto out;
        }
#ifdef DEBUG_KEXECDH
        dump_digest("server cipher text:", ciphertext,
            MLKEM768_CIPHERTEXTBYTES);
        dump_digest("server public key c25519:", server_pub, CURVE25519_SIZE);
#endif
        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 ((r = kexc25519_shared_key_ext(kex->c25519_client_key, server_pub,
            buf, 1)) < 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));
        sshbuf_free(buf);
        return r;
}
#else /* USE_MLKEM768X25519 */
int
kex_kem_mlkem768x25519_keypair(struct kex *kex)
{
        return SSH_ERR_SIGN_ALG_UNSUPPORTED;
}

int
kex_kem_mlkem768x25519_enc(struct kex *kex,
   const struct sshbuf *client_blob, struct sshbuf **server_blobp,
   struct sshbuf **shared_secretp)
{
        return SSH_ERR_SIGN_ALG_UNSUPPORTED;
}

int
kex_kem_mlkem768x25519_dec(struct kex *kex,
    const struct sshbuf *server_blob, struct sshbuf **shared_secretp)
{
        return SSH_ERR_SIGN_ALG_UNSUPPORTED;
}
#endif /* USE_MLKEM768X25519 */