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/* $OpenBSD: kexc25519.c,v 1.17 2019/01/21 10:40:11 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", 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;
}
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