/* $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 <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;
}