1 /* $OpenBSD: ssh-ecdsa-sk.c,v 1.19 2024/08/15 00:51:51 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 "includes.h" 31 32 #include <sys/types.h> 33 34 #ifdef WITH_OPENSSL 35 #include <openssl/bn.h> 36 #include <openssl/ec.h> 37 #include <openssl/ecdsa.h> 38 #include <openssl/evp.h> 39 #endif 40 41 #include <string.h> 42 #include <stdio.h> /* needed for DEBUG_SK only */ 43 44 #include "openbsd-compat/openssl-compat.h" 45 46 #include "sshbuf.h" 47 #include "ssherr.h" 48 #include "digest.h" 49 #define SSHKEY_INTERNAL 50 #include "sshkey.h" 51 52 #ifndef OPENSSL_HAS_ECC 53 /* ARGSUSED */ 54 int 55 ssh_ecdsa_sk_verify(const struct sshkey *key, 56 const u_char *signature, size_t signaturelen, 57 const u_char *data, size_t datalen, u_int compat, 58 struct sshkey_sig_details **detailsp) 59 { 60 return SSH_ERR_FEATURE_UNSUPPORTED; 61 } 62 #else /* OPENSSL_HAS_ECC */ 63 64 /* Reuse some ECDSA internals */ 65 extern struct sshkey_impl_funcs sshkey_ecdsa_funcs; 66 67 static void 68 ssh_ecdsa_sk_cleanup(struct sshkey *k) 69 { 70 sshkey_sk_cleanup(k); 71 sshkey_ecdsa_funcs.cleanup(k); 72 } 73 74 static int 75 ssh_ecdsa_sk_equal(const struct sshkey *a, const struct sshkey *b) 76 { 77 if (!sshkey_sk_fields_equal(a, b)) 78 return 0; 79 if (!sshkey_ecdsa_funcs.equal(a, b)) 80 return 0; 81 return 1; 82 } 83 84 static int 85 ssh_ecdsa_sk_serialize_public(const struct sshkey *key, struct sshbuf *b, 86 enum sshkey_serialize_rep opts) 87 { 88 int r; 89 90 if ((r = sshkey_ecdsa_funcs.serialize_public(key, b, opts)) != 0) 91 return r; 92 if ((r = sshkey_serialize_sk(key, b)) != 0) 93 return r; 94 95 return 0; 96 } 97 98 static int 99 ssh_ecdsa_sk_serialize_private(const struct sshkey *key, struct sshbuf *b, 100 enum sshkey_serialize_rep opts) 101 { 102 int r; 103 104 if (!sshkey_is_cert(key)) { 105 if ((r = sshkey_ecdsa_funcs.serialize_public(key, 106 b, opts)) != 0) 107 return r; 108 } 109 if ((r = sshkey_serialize_private_sk(key, b)) != 0) 110 return r; 111 112 return 0; 113 } 114 115 static int 116 ssh_ecdsa_sk_copy_public(const struct sshkey *from, struct sshkey *to) 117 { 118 int r; 119 120 if ((r = sshkey_ecdsa_funcs.copy_public(from, to)) != 0) 121 return r; 122 if ((r = sshkey_copy_public_sk(from, to)) != 0) 123 return r; 124 return 0; 125 } 126 127 static int 128 ssh_ecdsa_sk_deserialize_public(const char *ktype, struct sshbuf *b, 129 struct sshkey *key) 130 { 131 int r; 132 133 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, b, key)) != 0) 134 return r; 135 if ((r = sshkey_deserialize_sk(b, key)) != 0) 136 return r; 137 return 0; 138 } 139 140 static int 141 ssh_ecdsa_sk_deserialize_private(const char *ktype, struct sshbuf *b, 142 struct sshkey *key) 143 { 144 int r; 145 146 if (!sshkey_is_cert(key)) { 147 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, 148 b, key)) != 0) 149 return r; 150 } 151 if ((r = sshkey_private_deserialize_sk(b, key)) != 0) 152 return r; 153 154 return 0; 155 } 156 157 /* 158 * Check FIDO/W3C webauthn signatures clientData field against the expected 159 * format and prepare a hash of it for use in signature verification. 160 * 161 * webauthn signatures do not sign the hash of the message directly, but 162 * instead sign a JSON-like "clientData" wrapper structure that contains the 163 * message hash along with a other information. 164 * 165 * Fortunately this structure has a fixed format so it is possible to verify 166 * that the hash of the signed message is present within the clientData 167 * structure without needing to implement any JSON parsing. 168 */ 169 static int 170 webauthn_check_prepare_hash(const u_char *data, size_t datalen, 171 const char *origin, const struct sshbuf *wrapper, 172 uint8_t flags, const struct sshbuf *extensions, 173 u_char *msghash, size_t msghashlen) 174 { 175 int r = SSH_ERR_INTERNAL_ERROR; 176 struct sshbuf *chall = NULL, *m = NULL; 177 178 if ((m = sshbuf_new()) == NULL || 179 (chall = sshbuf_from(data, datalen)) == NULL) { 180 r = SSH_ERR_ALLOC_FAIL; 181 goto out; 182 } 183 /* 184 * Ensure origin contains no quote character and that the flags are 185 * consistent with what we received 186 */ 187 if (strchr(origin, '\"') != NULL || 188 (flags & 0x40) != 0 /* AD */ || 189 ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) { 190 r = SSH_ERR_INVALID_FORMAT; 191 goto out; 192 } 193 194 /* 195 * Prepare the preamble to clientData that we expect, poking the 196 * challenge and origin into their canonical positions in the 197 * structure. The crossOrigin flag and any additional extension 198 * fields present are ignored. 199 */ 200 #define WEBAUTHN_0 "{\"type\":\"webauthn.get\",\"challenge\":\"" 201 #define WEBAUTHN_1 "\",\"origin\":\"" 202 #define WEBAUTHN_2 "\"" 203 if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 || 204 (r = sshbuf_dtourlb64(chall, m, 0)) != 0 || 205 (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 || 206 (r = sshbuf_put(m, origin, strlen(origin))) != 0 || 207 (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0) 208 goto out; 209 #ifdef DEBUG_SK 210 fprintf(stderr, "%s: received origin: %s\n", __func__, origin); 211 fprintf(stderr, "%s: received clientData:\n", __func__); 212 sshbuf_dump(wrapper, stderr); 213 fprintf(stderr, "%s: expected clientData premable:\n", __func__); 214 sshbuf_dump(m, stderr); 215 #endif 216 /* Check that the supplied clientData has the preamble we expect */ 217 if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0) 218 goto out; 219 220 /* Prepare hash of clientData */ 221 if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper, 222 msghash, msghashlen)) != 0) 223 goto out; 224 225 /* success */ 226 r = 0; 227 out: 228 sshbuf_free(chall); 229 sshbuf_free(m); 230 return r; 231 } 232 233 static int 234 ssh_ecdsa_sk_verify(const struct sshkey *key, 235 const u_char *sig, size_t siglen, 236 const u_char *data, size_t dlen, const char *alg, u_int compat, 237 struct sshkey_sig_details **detailsp) 238 { 239 ECDSA_SIG *esig = NULL; 240 EVP_MD_CTX *md_ctx = NULL; 241 BIGNUM *sig_r = NULL, *sig_s = NULL; 242 u_char sig_flags; 243 u_char msghash[32], apphash[32]; 244 u_int sig_counter; 245 u_char *sigb = NULL, *cp; 246 int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR, len = 0; 247 struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL; 248 struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL; 249 char *ktype = NULL, *webauthn_origin = NULL; 250 struct sshkey_sig_details *details = NULL; 251 #ifdef DEBUG_SK 252 char *tmp = NULL; 253 #endif 254 255 if (detailsp != NULL) 256 *detailsp = NULL; 257 if (key == NULL || key->pkey == NULL || 258 sshkey_type_plain(key->type) != KEY_ECDSA_SK || 259 sig == NULL || siglen == 0) 260 return SSH_ERR_INVALID_ARGUMENT; 261 262 if (key->ecdsa_nid != NID_X9_62_prime256v1) 263 return SSH_ERR_INTERNAL_ERROR; 264 265 /* fetch signature */ 266 if ((b = sshbuf_from(sig, siglen)) == NULL) 267 return SSH_ERR_ALLOC_FAIL; 268 if ((details = calloc(1, sizeof(*details))) == NULL) { 269 ret = SSH_ERR_ALLOC_FAIL; 270 goto out; 271 } 272 if (sshbuf_get_cstring(b, &ktype, NULL) != 0) { 273 ret = SSH_ERR_INVALID_FORMAT; 274 goto out; 275 } 276 if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0) 277 is_webauthn = 1; 278 else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) { 279 ret = SSH_ERR_INVALID_FORMAT; 280 goto out; 281 } 282 if (sshbuf_froms(b, &sigbuf) != 0 || 283 sshbuf_get_u8(b, &sig_flags) != 0 || 284 sshbuf_get_u32(b, &sig_counter) != 0) { 285 ret = SSH_ERR_INVALID_FORMAT; 286 goto out; 287 } 288 if (is_webauthn) { 289 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 || 290 sshbuf_froms(b, &webauthn_wrapper) != 0 || 291 sshbuf_froms(b, &webauthn_exts) != 0) { 292 ret = SSH_ERR_INVALID_FORMAT; 293 goto out; 294 } 295 } 296 if (sshbuf_len(b) != 0) { 297 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 298 goto out; 299 } 300 301 /* parse signature */ 302 if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 || 303 sshbuf_get_bignum2(sigbuf, &sig_s) != 0) { 304 ret = SSH_ERR_INVALID_FORMAT; 305 goto out; 306 } 307 if (sshbuf_len(sigbuf) != 0) { 308 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 309 goto out; 310 } 311 312 #ifdef DEBUG_SK 313 fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen); 314 /* sshbuf_dump_data(data, datalen, stderr); */ 315 fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r))); 316 free(tmp); 317 fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s))); 318 free(tmp); 319 fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n", 320 __func__, sig_flags, sig_counter); 321 if (is_webauthn) { 322 fprintf(stderr, "%s: webauthn origin: %s\n", __func__, 323 webauthn_origin); 324 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__); 325 sshbuf_dump(webauthn_wrapper, stderr); 326 } 327 #endif 328 if ((esig = ECDSA_SIG_new()) == NULL) { 329 ret = SSH_ERR_ALLOC_FAIL; 330 goto out; 331 } 332 if (!ECDSA_SIG_set0(esig, sig_r, sig_s)) { 333 ret = SSH_ERR_LIBCRYPTO_ERROR; 334 goto out; 335 } 336 sig_r = sig_s = NULL; /* transferred */ 337 338 /* Reconstruct data that was supposedly signed */ 339 if ((original_signed = sshbuf_new()) == NULL) { 340 ret = SSH_ERR_ALLOC_FAIL; 341 goto out; 342 } 343 if (is_webauthn) { 344 if ((ret = webauthn_check_prepare_hash(data, dlen, 345 webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts, 346 msghash, sizeof(msghash))) != 0) 347 goto out; 348 } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, dlen, 349 msghash, sizeof(msghash))) != 0) 350 goto out; 351 /* Application value is hashed before signature */ 352 if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application, 353 strlen(key->sk_application), apphash, sizeof(apphash))) != 0) 354 goto out; 355 #ifdef DEBUG_SK 356 fprintf(stderr, "%s: hashed application:\n", __func__); 357 sshbuf_dump_data(apphash, sizeof(apphash), stderr); 358 fprintf(stderr, "%s: hashed message:\n", __func__); 359 sshbuf_dump_data(msghash, sizeof(msghash), stderr); 360 #endif 361 if ((ret = sshbuf_put(original_signed, 362 apphash, sizeof(apphash))) != 0 || 363 (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 || 364 (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 || 365 (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 || 366 (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0) 367 goto out; 368 details->sk_counter = sig_counter; 369 details->sk_flags = sig_flags; 370 #ifdef DEBUG_SK 371 fprintf(stderr, "%s: signed buf:\n", __func__); 372 sshbuf_dump(original_signed, stderr); 373 #endif 374 375 if ((md_ctx = EVP_MD_CTX_new()) == NULL) { 376 ret = SSH_ERR_ALLOC_FAIL; 377 goto out; 378 } 379 if ((len = i2d_ECDSA_SIG(esig, NULL)) <= 0) { 380 len = 0; 381 ret = SSH_ERR_LIBCRYPTO_ERROR; 382 goto out; 383 } 384 if ((sigb = calloc(1, len)) == NULL) { 385 ret = SSH_ERR_ALLOC_FAIL; 386 goto out; 387 } 388 cp = sigb; /* ASN1_item_i2d increments the pointer past the object */ 389 if (i2d_ECDSA_SIG(esig, &cp) != len) { 390 ret = SSH_ERR_LIBCRYPTO_ERROR; 391 goto out; 392 } 393 #ifdef DEBUG_SK 394 fprintf(stderr, "%s: signed hash:\n", __func__); 395 sshbuf_dump_data(sigb, len, stderr); 396 #endif 397 /* Verify it */ 398 if (EVP_DigestVerifyInit(md_ctx, NULL, EVP_sha256(), NULL, 399 key->pkey) != 1) { 400 ret = SSH_ERR_LIBCRYPTO_ERROR; 401 goto out; 402 } 403 switch (EVP_DigestVerify(md_ctx, sigb, len, 404 sshbuf_ptr(original_signed), sshbuf_len(original_signed))) { 405 case 1: 406 ret = 0; 407 break; 408 case 0: 409 ret = SSH_ERR_SIGNATURE_INVALID; 410 goto out; 411 default: 412 ret = SSH_ERR_LIBCRYPTO_ERROR; 413 goto out; 414 } 415 /* success */ 416 if (detailsp != NULL) { 417 *detailsp = details; 418 details = NULL; 419 } 420 out: 421 explicit_bzero(&sig_flags, sizeof(sig_flags)); 422 explicit_bzero(&sig_counter, sizeof(sig_counter)); 423 explicit_bzero(msghash, sizeof(msghash)); 424 explicit_bzero(apphash, sizeof(apphash)); 425 sshkey_sig_details_free(details); 426 sshbuf_free(webauthn_wrapper); 427 sshbuf_free(webauthn_exts); 428 free(webauthn_origin); 429 sshbuf_free(original_signed); 430 sshbuf_free(sigbuf); 431 sshbuf_free(b); 432 ECDSA_SIG_free(esig); 433 BN_clear_free(sig_r); 434 BN_clear_free(sig_s); 435 free(ktype); 436 freezero(sigb, len); 437 EVP_MD_CTX_free(md_ctx); 438 return ret; 439 } 440 441 static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = { 442 /* .size = */ NULL, 443 /* .alloc = */ NULL, 444 /* .cleanup = */ ssh_ecdsa_sk_cleanup, 445 /* .equal = */ ssh_ecdsa_sk_equal, 446 /* .ssh_serialize_public = */ ssh_ecdsa_sk_serialize_public, 447 /* .ssh_deserialize_public = */ ssh_ecdsa_sk_deserialize_public, 448 /* .ssh_serialize_private = */ ssh_ecdsa_sk_serialize_private, 449 /* .ssh_deserialize_private = */ ssh_ecdsa_sk_deserialize_private, 450 /* .generate = */ NULL, 451 /* .copy_public = */ ssh_ecdsa_sk_copy_public, 452 /* .sign = */ NULL, 453 /* .verify = */ ssh_ecdsa_sk_verify, 454 }; 455 456 const struct sshkey_impl sshkey_ecdsa_sk_impl = { 457 /* .name = */ "sk-ecdsa-sha2-nistp256@openssh.com", 458 /* .shortname = */ "ECDSA-SK", 459 /* .sigalg = */ NULL, 460 /* .type = */ KEY_ECDSA_SK, 461 /* .nid = */ NID_X9_62_prime256v1, 462 /* .cert = */ 0, 463 /* .sigonly = */ 0, 464 /* .keybits = */ 256, 465 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 466 }; 467 468 const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = { 469 /* .name = */ "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com", 470 /* .shortname = */ "ECDSA-SK-CERT", 471 /* .sigalg = */ NULL, 472 /* .type = */ KEY_ECDSA_SK_CERT, 473 /* .nid = */ NID_X9_62_prime256v1, 474 /* .cert = */ 1, 475 /* .sigonly = */ 0, 476 /* .keybits = */ 256, 477 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 478 }; 479 480 const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = { 481 /* .name = */ "webauthn-sk-ecdsa-sha2-nistp256@openssh.com", 482 /* .shortname = */ "ECDSA-SK", 483 /* .sigalg = */ NULL, 484 /* .type = */ KEY_ECDSA_SK, 485 /* .nid = */ NID_X9_62_prime256v1, 486 /* .cert = */ 0, 487 /* .sigonly = */ 1, 488 /* .keybits = */ 256, 489 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 490 }; 491 492 #endif /* OPENSSL_HAS_ECC */ 493