1 /* $OpenBSD: ssh-ecdsa-sk.c,v 1.17 2022/10/28 00:44:44 djm Exp $ */ 2 /* 3 * Copyright (c) 2000 Markus Friedl. All rights reserved. 4 * Copyright (c) 2010 Damien Miller. All rights reserved. 5 * Copyright (c) 2019 Google Inc. 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 /* #define DEBUG_SK 1 */ 29 30 #include <sys/types.h> 31 32 #include <openssl/bn.h> 33 #include <openssl/ec.h> 34 #include <openssl/ecdsa.h> 35 #include <openssl/evp.h> 36 37 #include <string.h> 38 #include <stdio.h> /* needed for DEBUG_SK only */ 39 40 #include "sshbuf.h" 41 #include "ssherr.h" 42 #include "digest.h" 43 #define SSHKEY_INTERNAL 44 #include "sshkey.h" 45 46 /* Reuse some ECDSA internals */ 47 extern struct sshkey_impl_funcs sshkey_ecdsa_funcs; 48 49 static void 50 ssh_ecdsa_sk_cleanup(struct sshkey *k) 51 { 52 sshkey_sk_cleanup(k); 53 sshkey_ecdsa_funcs.cleanup(k); 54 } 55 56 static int 57 ssh_ecdsa_sk_equal(const struct sshkey *a, const struct sshkey *b) 58 { 59 if (!sshkey_sk_fields_equal(a, b)) 60 return 0; 61 if (!sshkey_ecdsa_funcs.equal(a, b)) 62 return 0; 63 return 1; 64 } 65 66 static int 67 ssh_ecdsa_sk_serialize_public(const struct sshkey *key, struct sshbuf *b, 68 enum sshkey_serialize_rep opts) 69 { 70 int r; 71 72 if ((r = sshkey_ecdsa_funcs.serialize_public(key, b, opts)) != 0) 73 return r; 74 if ((r = sshkey_serialize_sk(key, b)) != 0) 75 return r; 76 77 return 0; 78 } 79 80 static int 81 ssh_ecdsa_sk_serialize_private(const struct sshkey *key, struct sshbuf *b, 82 enum sshkey_serialize_rep opts) 83 { 84 int r; 85 86 if (!sshkey_is_cert(key)) { 87 if ((r = sshkey_ecdsa_funcs.serialize_public(key, 88 b, opts)) != 0) 89 return r; 90 } 91 if ((r = sshkey_serialize_private_sk(key, b)) != 0) 92 return r; 93 94 return 0; 95 } 96 97 static int 98 ssh_ecdsa_sk_copy_public(const struct sshkey *from, struct sshkey *to) 99 { 100 int r; 101 102 if ((r = sshkey_ecdsa_funcs.copy_public(from, to)) != 0) 103 return r; 104 if ((r = sshkey_copy_public_sk(from, to)) != 0) 105 return r; 106 return 0; 107 } 108 109 static int 110 ssh_ecdsa_sk_deserialize_public(const char *ktype, struct sshbuf *b, 111 struct sshkey *key) 112 { 113 int r; 114 115 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, b, key)) != 0) 116 return r; 117 if ((r = sshkey_deserialize_sk(b, key)) != 0) 118 return r; 119 return 0; 120 } 121 122 static int 123 ssh_ecdsa_sk_deserialize_private(const char *ktype, struct sshbuf *b, 124 struct sshkey *key) 125 { 126 int r; 127 128 if (!sshkey_is_cert(key)) { 129 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, 130 b, key)) != 0) 131 return r; 132 } 133 if ((r = sshkey_private_deserialize_sk(b, key)) != 0) 134 return r; 135 136 return 0; 137 } 138 139 /* 140 * Check FIDO/W3C webauthn signatures clientData field against the expected 141 * format and prepare a hash of it for use in signature verification. 142 * 143 * webauthn signatures do not sign the hash of the message directly, but 144 * instead sign a JSON-like "clientData" wrapper structure that contains the 145 * message hash along with a other information. 146 * 147 * Fortunately this structure has a fixed format so it is possible to verify 148 * that the hash of the signed message is present within the clientData 149 * structure without needing to implement any JSON parsing. 150 */ 151 static int 152 webauthn_check_prepare_hash(const u_char *data, size_t datalen, 153 const char *origin, const struct sshbuf *wrapper, 154 uint8_t flags, const struct sshbuf *extensions, 155 u_char *msghash, size_t msghashlen) 156 { 157 int r = SSH_ERR_INTERNAL_ERROR; 158 struct sshbuf *chall = NULL, *m = NULL; 159 160 if ((m = sshbuf_new()) == NULL || 161 (chall = sshbuf_from(data, datalen)) == NULL) { 162 r = SSH_ERR_ALLOC_FAIL; 163 goto out; 164 } 165 /* 166 * Ensure origin contains no quote character and that the flags are 167 * consistent with what we received 168 */ 169 if (strchr(origin, '\"') != NULL || 170 (flags & 0x40) != 0 /* AD */ || 171 ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) { 172 r = SSH_ERR_INVALID_FORMAT; 173 goto out; 174 } 175 176 /* 177 * Prepare the preamble to clientData that we expect, poking the 178 * challenge and origin into their canonical positions in the 179 * structure. The crossOrigin flag and any additional extension 180 * fields present are ignored. 181 */ 182 #define WEBAUTHN_0 "{\"type\":\"webauthn.get\",\"challenge\":\"" 183 #define WEBAUTHN_1 "\",\"origin\":\"" 184 #define WEBAUTHN_2 "\"" 185 if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 || 186 (r = sshbuf_dtourlb64(chall, m, 0)) != 0 || 187 (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 || 188 (r = sshbuf_put(m, origin, strlen(origin))) != 0 || 189 (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0) 190 goto out; 191 #ifdef DEBUG_SK 192 fprintf(stderr, "%s: received origin: %s\n", __func__, origin); 193 fprintf(stderr, "%s: received clientData:\n", __func__); 194 sshbuf_dump(wrapper, stderr); 195 fprintf(stderr, "%s: expected clientData premable:\n", __func__); 196 sshbuf_dump(m, stderr); 197 #endif 198 /* Check that the supplied clientData has the preamble we expect */ 199 if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0) 200 goto out; 201 202 /* Prepare hash of clientData */ 203 if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper, 204 msghash, msghashlen)) != 0) 205 goto out; 206 207 /* success */ 208 r = 0; 209 out: 210 sshbuf_free(chall); 211 sshbuf_free(m); 212 return r; 213 } 214 215 /* ARGSUSED */ 216 static int 217 ssh_ecdsa_sk_verify(const struct sshkey *key, 218 const u_char *sig, size_t siglen, 219 const u_char *data, size_t dlen, const char *alg, u_int compat, 220 struct sshkey_sig_details **detailsp) 221 { 222 ECDSA_SIG *esig = NULL; 223 BIGNUM *sig_r = NULL, *sig_s = NULL; 224 u_char sig_flags; 225 u_char msghash[32], apphash[32], sighash[32]; 226 u_int sig_counter; 227 int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR; 228 struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL; 229 struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL; 230 char *ktype = NULL, *webauthn_origin = NULL; 231 struct sshkey_sig_details *details = NULL; 232 #ifdef DEBUG_SK 233 char *tmp = NULL; 234 #endif 235 236 if (detailsp != NULL) 237 *detailsp = NULL; 238 if (key == NULL || key->ecdsa == NULL || 239 sshkey_type_plain(key->type) != KEY_ECDSA_SK || 240 sig == NULL || siglen == 0) 241 return SSH_ERR_INVALID_ARGUMENT; 242 243 if (key->ecdsa_nid != NID_X9_62_prime256v1) 244 return SSH_ERR_INTERNAL_ERROR; 245 246 /* fetch signature */ 247 if ((b = sshbuf_from(sig, siglen)) == NULL) 248 return SSH_ERR_ALLOC_FAIL; 249 if ((details = calloc(1, sizeof(*details))) == NULL) { 250 ret = SSH_ERR_ALLOC_FAIL; 251 goto out; 252 } 253 if (sshbuf_get_cstring(b, &ktype, NULL) != 0) { 254 ret = SSH_ERR_INVALID_FORMAT; 255 goto out; 256 } 257 if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0) 258 is_webauthn = 1; 259 else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) { 260 ret = SSH_ERR_INVALID_FORMAT; 261 goto out; 262 } 263 if (sshbuf_froms(b, &sigbuf) != 0 || 264 sshbuf_get_u8(b, &sig_flags) != 0 || 265 sshbuf_get_u32(b, &sig_counter) != 0) { 266 ret = SSH_ERR_INVALID_FORMAT; 267 goto out; 268 } 269 if (is_webauthn) { 270 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 || 271 sshbuf_froms(b, &webauthn_wrapper) != 0 || 272 sshbuf_froms(b, &webauthn_exts) != 0) { 273 ret = SSH_ERR_INVALID_FORMAT; 274 goto out; 275 } 276 } 277 if (sshbuf_len(b) != 0) { 278 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 279 goto out; 280 } 281 282 /* parse signature */ 283 if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 || 284 sshbuf_get_bignum2(sigbuf, &sig_s) != 0) { 285 ret = SSH_ERR_INVALID_FORMAT; 286 goto out; 287 } 288 if (sshbuf_len(sigbuf) != 0) { 289 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 290 goto out; 291 } 292 293 #ifdef DEBUG_SK 294 fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen); 295 /* sshbuf_dump_data(data, datalen, stderr); */ 296 fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r))); 297 free(tmp); 298 fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s))); 299 free(tmp); 300 fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n", 301 __func__, sig_flags, sig_counter); 302 if (is_webauthn) { 303 fprintf(stderr, "%s: webauthn origin: %s\n", __func__, 304 webauthn_origin); 305 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__); 306 sshbuf_dump(webauthn_wrapper, stderr); 307 } 308 #endif 309 if ((esig = ECDSA_SIG_new()) == NULL) { 310 ret = SSH_ERR_ALLOC_FAIL; 311 goto out; 312 } 313 if (!ECDSA_SIG_set0(esig, sig_r, sig_s)) { 314 ret = SSH_ERR_LIBCRYPTO_ERROR; 315 goto out; 316 } 317 sig_r = sig_s = NULL; /* transferred */ 318 319 /* Reconstruct data that was supposedly signed */ 320 if ((original_signed = sshbuf_new()) == NULL) { 321 ret = SSH_ERR_ALLOC_FAIL; 322 goto out; 323 } 324 if (is_webauthn) { 325 if ((ret = webauthn_check_prepare_hash(data, dlen, 326 webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts, 327 msghash, sizeof(msghash))) != 0) 328 goto out; 329 } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, dlen, 330 msghash, sizeof(msghash))) != 0) 331 goto out; 332 /* Application value is hashed before signature */ 333 if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application, 334 strlen(key->sk_application), apphash, sizeof(apphash))) != 0) 335 goto out; 336 #ifdef DEBUG_SK 337 fprintf(stderr, "%s: hashed application:\n", __func__); 338 sshbuf_dump_data(apphash, sizeof(apphash), stderr); 339 fprintf(stderr, "%s: hashed message:\n", __func__); 340 sshbuf_dump_data(msghash, sizeof(msghash), stderr); 341 #endif 342 if ((ret = sshbuf_put(original_signed, 343 apphash, sizeof(apphash))) != 0 || 344 (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 || 345 (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 || 346 (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 || 347 (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0) 348 goto out; 349 /* Signature is over H(original_signed) */ 350 if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed, 351 sighash, sizeof(sighash))) != 0) 352 goto out; 353 details->sk_counter = sig_counter; 354 details->sk_flags = sig_flags; 355 #ifdef DEBUG_SK 356 fprintf(stderr, "%s: signed buf:\n", __func__); 357 sshbuf_dump(original_signed, stderr); 358 fprintf(stderr, "%s: signed hash:\n", __func__); 359 sshbuf_dump_data(sighash, sizeof(sighash), stderr); 360 #endif 361 362 /* Verify it */ 363 switch (ECDSA_do_verify(sighash, sizeof(sighash), esig, key->ecdsa)) { 364 case 1: 365 ret = 0; 366 break; 367 case 0: 368 ret = SSH_ERR_SIGNATURE_INVALID; 369 goto out; 370 default: 371 ret = SSH_ERR_LIBCRYPTO_ERROR; 372 goto out; 373 } 374 /* success */ 375 if (detailsp != NULL) { 376 *detailsp = details; 377 details = NULL; 378 } 379 out: 380 explicit_bzero(&sig_flags, sizeof(sig_flags)); 381 explicit_bzero(&sig_counter, sizeof(sig_counter)); 382 explicit_bzero(msghash, sizeof(msghash)); 383 explicit_bzero(sighash, sizeof(msghash)); 384 explicit_bzero(apphash, sizeof(apphash)); 385 sshkey_sig_details_free(details); 386 sshbuf_free(webauthn_wrapper); 387 sshbuf_free(webauthn_exts); 388 free(webauthn_origin); 389 sshbuf_free(original_signed); 390 sshbuf_free(sigbuf); 391 sshbuf_free(b); 392 ECDSA_SIG_free(esig); 393 BN_clear_free(sig_r); 394 BN_clear_free(sig_s); 395 free(ktype); 396 return ret; 397 } 398 399 static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = { 400 /* .size = */ NULL, 401 /* .alloc = */ NULL, 402 /* .cleanup = */ ssh_ecdsa_sk_cleanup, 403 /* .equal = */ ssh_ecdsa_sk_equal, 404 /* .ssh_serialize_public = */ ssh_ecdsa_sk_serialize_public, 405 /* .ssh_deserialize_public = */ ssh_ecdsa_sk_deserialize_public, 406 /* .ssh_serialize_private = */ ssh_ecdsa_sk_serialize_private, 407 /* .ssh_deserialize_private = */ ssh_ecdsa_sk_deserialize_private, 408 /* .generate = */ NULL, 409 /* .copy_public = */ ssh_ecdsa_sk_copy_public, 410 /* .sign = */ NULL, 411 /* .verify = */ ssh_ecdsa_sk_verify, 412 }; 413 414 const struct sshkey_impl sshkey_ecdsa_sk_impl = { 415 /* .name = */ "sk-ecdsa-sha2-nistp256@openssh.com", 416 /* .shortname = */ "ECDSA-SK", 417 /* .sigalg = */ NULL, 418 /* .type = */ KEY_ECDSA_SK, 419 /* .nid = */ NID_X9_62_prime256v1, 420 /* .cert = */ 0, 421 /* .sigonly = */ 0, 422 /* .keybits = */ 256, 423 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 424 }; 425 426 const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = { 427 /* .name = */ "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com", 428 /* .shortname = */ "ECDSA-SK-CERT", 429 /* .sigalg = */ NULL, 430 /* .type = */ KEY_ECDSA_SK_CERT, 431 /* .nid = */ NID_X9_62_prime256v1, 432 /* .cert = */ 1, 433 /* .sigonly = */ 0, 434 /* .keybits = */ 256, 435 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 436 }; 437 438 const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = { 439 /* .name = */ "webauthn-sk-ecdsa-sha2-nistp256@openssh.com", 440 /* .shortname = */ "ECDSA-SK", 441 /* .sigalg = */ NULL, 442 /* .type = */ KEY_ECDSA_SK, 443 /* .nid = */ NID_X9_62_prime256v1, 444 /* .cert = */ 0, 445 /* .sigonly = */ 1, 446 /* .keybits = */ 256, 447 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 448 }; 449