1 /* $OpenBSD: kexc25519.c,v 1.17 2019/01/21 10:40:11 djm Exp $ */ 2 /* 3 * Copyright (c) 2019 Markus Friedl. All rights reserved. 4 * Copyright (c) 2010 Damien Miller. All rights reserved. 5 * Copyright (c) 2013 Aris Adamantiadis. All rights reserved. 6 * 7 * Redistribution and use in source and binary forms, with or without 8 * modification, are permitted provided that the following conditions 9 * are met: 10 * 1. Redistributions of source code must retain the above copyright 11 * notice, this list of conditions and the following disclaimer. 12 * 2. Redistributions in binary form must reproduce the above copyright 13 * notice, this list of conditions and the following disclaimer in the 14 * documentation and/or other materials provided with the distribution. 15 * 16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 17 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 18 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 19 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 21 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 22 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 23 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 24 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 25 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 26 */ 27 28 #include <sys/types.h> 29 30 #include <stdio.h> 31 #include <string.h> 32 #include <signal.h> 33 34 #include "sshkey.h" 35 #include "kex.h" 36 #include "sshbuf.h" 37 #include "digest.h" 38 #include "ssherr.h" 39 #include "ssh2.h" 40 41 extern int crypto_scalarmult_curve25519(u_char a[CURVE25519_SIZE], 42 const u_char b[CURVE25519_SIZE], const u_char c[CURVE25519_SIZE]) 43 __attribute__((__bounded__(__minbytes__, 1, CURVE25519_SIZE))) 44 __attribute__((__bounded__(__minbytes__, 2, CURVE25519_SIZE))) 45 __attribute__((__bounded__(__minbytes__, 3, CURVE25519_SIZE))); 46 47 void 48 kexc25519_keygen(u_char key[CURVE25519_SIZE], u_char pub[CURVE25519_SIZE]) 49 { 50 static const u_char basepoint[CURVE25519_SIZE] = {9}; 51 52 arc4random_buf(key, CURVE25519_SIZE); 53 crypto_scalarmult_curve25519(pub, key, basepoint); 54 } 55 56 int 57 kexc25519_shared_key_ext(const u_char key[CURVE25519_SIZE], 58 const u_char pub[CURVE25519_SIZE], struct sshbuf *out, int raw) 59 { 60 u_char shared_key[CURVE25519_SIZE]; 61 u_char zero[CURVE25519_SIZE]; 62 int r; 63 64 crypto_scalarmult_curve25519(shared_key, key, pub); 65 66 /* Check for all-zero shared secret */ 67 explicit_bzero(zero, CURVE25519_SIZE); 68 if (timingsafe_bcmp(zero, shared_key, CURVE25519_SIZE) == 0) 69 return SSH_ERR_KEY_INVALID_EC_VALUE; 70 71 #ifdef DEBUG_KEXECDH 72 dump_digest("shared secret", shared_key, CURVE25519_SIZE); 73 #endif 74 if (raw) 75 r = sshbuf_put(out, shared_key, CURVE25519_SIZE); 76 else 77 r = sshbuf_put_bignum2_bytes(out, shared_key, CURVE25519_SIZE); 78 explicit_bzero(shared_key, CURVE25519_SIZE); 79 return r; 80 } 81 82 int 83 kexc25519_shared_key(const u_char key[CURVE25519_SIZE], 84 const u_char pub[CURVE25519_SIZE], struct sshbuf *out) 85 { 86 return kexc25519_shared_key_ext(key, pub, out, 0); 87 } 88 89 int 90 kex_c25519_keypair(struct kex *kex) 91 { 92 struct sshbuf *buf = NULL; 93 u_char *cp = NULL; 94 int r; 95 96 if ((buf = sshbuf_new()) == NULL) 97 return SSH_ERR_ALLOC_FAIL; 98 if ((r = sshbuf_reserve(buf, CURVE25519_SIZE, &cp)) != 0) 99 goto out; 100 kexc25519_keygen(kex->c25519_client_key, cp); 101 #ifdef DEBUG_KEXECDH 102 dump_digest("client public key c25519:", cp, CURVE25519_SIZE); 103 #endif 104 kex->client_pub = buf; 105 buf = NULL; 106 out: 107 sshbuf_free(buf); 108 return r; 109 } 110 111 int 112 kex_c25519_enc(struct kex *kex, const struct sshbuf *client_blob, 113 struct sshbuf **server_blobp, struct sshbuf **shared_secretp) 114 { 115 struct sshbuf *server_blob = NULL; 116 struct sshbuf *buf = NULL; 117 const u_char *client_pub; 118 u_char *server_pub; 119 u_char server_key[CURVE25519_SIZE]; 120 int r; 121 122 *server_blobp = NULL; 123 *shared_secretp = NULL; 124 125 if (sshbuf_len(client_blob) != CURVE25519_SIZE) { 126 r = SSH_ERR_SIGNATURE_INVALID; 127 goto out; 128 } 129 client_pub = sshbuf_ptr(client_blob); 130 #ifdef DEBUG_KEXECDH 131 dump_digest("client public key 25519:", client_pub, CURVE25519_SIZE); 132 #endif 133 /* allocate space for encrypted KEM key and ECDH pub key */ 134 if ((server_blob = sshbuf_new()) == NULL) { 135 r = SSH_ERR_ALLOC_FAIL; 136 goto out; 137 } 138 if ((r = sshbuf_reserve(server_blob, CURVE25519_SIZE, &server_pub)) != 0) 139 goto out; 140 kexc25519_keygen(server_key, server_pub); 141 /* allocate shared secret */ 142 if ((buf = sshbuf_new()) == NULL) { 143 r = SSH_ERR_ALLOC_FAIL; 144 goto out; 145 } 146 if ((r = kexc25519_shared_key_ext(server_key, client_pub, buf, 0)) < 0) 147 goto out; 148 #ifdef DEBUG_KEXECDH 149 dump_digest("server public key 25519:", server_pub, CURVE25519_SIZE); 150 dump_digest("encoded shared secret:", sshbuf_ptr(buf), sshbuf_len(buf)); 151 #endif 152 *server_blobp = server_blob; 153 *shared_secretp = buf; 154 server_blob = NULL; 155 buf = NULL; 156 out: 157 explicit_bzero(server_key, sizeof(server_key)); 158 sshbuf_free(server_blob); 159 sshbuf_free(buf); 160 return r; 161 } 162 163 int 164 kex_c25519_dec(struct kex *kex, const struct sshbuf *server_blob, 165 struct sshbuf **shared_secretp) 166 { 167 struct sshbuf *buf = NULL; 168 const u_char *server_pub; 169 int r; 170 171 *shared_secretp = NULL; 172 173 if (sshbuf_len(server_blob) != CURVE25519_SIZE) { 174 r = SSH_ERR_SIGNATURE_INVALID; 175 goto out; 176 } 177 server_pub = sshbuf_ptr(server_blob); 178 #ifdef DEBUG_KEXECDH 179 dump_digest("server public key c25519:", server_pub, CURVE25519_SIZE); 180 #endif 181 /* shared secret */ 182 if ((buf = sshbuf_new()) == NULL) { 183 r = SSH_ERR_ALLOC_FAIL; 184 goto out; 185 } 186 if ((r = kexc25519_shared_key_ext(kex->c25519_client_key, server_pub, 187 buf, 0)) < 0) 188 goto out; 189 #ifdef DEBUG_KEXECDH 190 dump_digest("encoded shared secret:", sshbuf_ptr(buf), sshbuf_len(buf)); 191 #endif 192 *shared_secretp = buf; 193 buf = NULL; 194 out: 195 sshbuf_free(buf); 196 return r; 197 } 198