/* $OpenBSD: kexmlkem768ecdh.c,v */
/*
 * 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 <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"

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,
            &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(kex, ec_pub, &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;
}