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 <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 static int 216 ssh_ecdsa_sk_verify(const struct sshkey *key, 217 const u_char *sig, size_t siglen, 218 const u_char *data, size_t dlen, const char *alg, u_int compat, 219 struct sshkey_sig_details **detailsp) 220 { 221 ECDSA_SIG *esig = NULL; 222 EVP_MD_CTX *md_ctx = NULL; 223 BIGNUM *sig_r = NULL, *sig_s = NULL; 224 u_char sig_flags; 225 u_char msghash[32], apphash[32]; 226 u_int sig_counter; 227 u_char *sigb = NULL, *cp; 228 int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR, len = 0; 229 struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL; 230 struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL; 231 char *ktype = NULL, *webauthn_origin = NULL; 232 struct sshkey_sig_details *details = NULL; 233 #ifdef DEBUG_SK 234 char *tmp = NULL; 235 #endif 236 237 if (detailsp != NULL) 238 *detailsp = NULL; 239 if (key == NULL || key->pkey == NULL || 240 sshkey_type_plain(key->type) != KEY_ECDSA_SK || 241 sig == NULL || siglen == 0) 242 return SSH_ERR_INVALID_ARGUMENT; 243 244 if (key->ecdsa_nid != NID_X9_62_prime256v1) 245 return SSH_ERR_INTERNAL_ERROR; 246 247 /* fetch signature */ 248 if ((b = sshbuf_from(sig, siglen)) == NULL) 249 return SSH_ERR_ALLOC_FAIL; 250 if ((details = calloc(1, sizeof(*details))) == NULL) { 251 ret = SSH_ERR_ALLOC_FAIL; 252 goto out; 253 } 254 if (sshbuf_get_cstring(b, &ktype, NULL) != 0) { 255 ret = SSH_ERR_INVALID_FORMAT; 256 goto out; 257 } 258 if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0) 259 is_webauthn = 1; 260 else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) { 261 ret = SSH_ERR_INVALID_FORMAT; 262 goto out; 263 } 264 if (sshbuf_froms(b, &sigbuf) != 0 || 265 sshbuf_get_u8(b, &sig_flags) != 0 || 266 sshbuf_get_u32(b, &sig_counter) != 0) { 267 ret = SSH_ERR_INVALID_FORMAT; 268 goto out; 269 } 270 if (is_webauthn) { 271 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 || 272 sshbuf_froms(b, &webauthn_wrapper) != 0 || 273 sshbuf_froms(b, &webauthn_exts) != 0) { 274 ret = SSH_ERR_INVALID_FORMAT; 275 goto out; 276 } 277 } 278 if (sshbuf_len(b) != 0) { 279 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 280 goto out; 281 } 282 283 /* parse signature */ 284 if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 || 285 sshbuf_get_bignum2(sigbuf, &sig_s) != 0) { 286 ret = SSH_ERR_INVALID_FORMAT; 287 goto out; 288 } 289 if (sshbuf_len(sigbuf) != 0) { 290 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 291 goto out; 292 } 293 294 #ifdef DEBUG_SK 295 fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen); 296 /* sshbuf_dump_data(data, datalen, stderr); */ 297 fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r))); 298 free(tmp); 299 fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s))); 300 free(tmp); 301 fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n", 302 __func__, sig_flags, sig_counter); 303 if (is_webauthn) { 304 fprintf(stderr, "%s: webauthn origin: %s\n", __func__, 305 webauthn_origin); 306 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__); 307 sshbuf_dump(webauthn_wrapper, stderr); 308 } 309 #endif 310 if ((esig = ECDSA_SIG_new()) == NULL) { 311 ret = SSH_ERR_ALLOC_FAIL; 312 goto out; 313 } 314 if (!ECDSA_SIG_set0(esig, sig_r, sig_s)) { 315 ret = SSH_ERR_LIBCRYPTO_ERROR; 316 goto out; 317 } 318 sig_r = sig_s = NULL; /* transferred */ 319 320 /* Reconstruct data that was supposedly signed */ 321 if ((original_signed = sshbuf_new()) == NULL) { 322 ret = SSH_ERR_ALLOC_FAIL; 323 goto out; 324 } 325 if (is_webauthn) { 326 if ((ret = webauthn_check_prepare_hash(data, dlen, 327 webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts, 328 msghash, sizeof(msghash))) != 0) 329 goto out; 330 } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, dlen, 331 msghash, sizeof(msghash))) != 0) 332 goto out; 333 /* Application value is hashed before signature */ 334 if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application, 335 strlen(key->sk_application), apphash, sizeof(apphash))) != 0) 336 goto out; 337 #ifdef DEBUG_SK 338 fprintf(stderr, "%s: hashed application:\n", __func__); 339 sshbuf_dump_data(apphash, sizeof(apphash), stderr); 340 fprintf(stderr, "%s: hashed message:\n", __func__); 341 sshbuf_dump_data(msghash, sizeof(msghash), stderr); 342 #endif 343 if ((ret = sshbuf_put(original_signed, 344 apphash, sizeof(apphash))) != 0 || 345 (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 || 346 (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 || 347 (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 || 348 (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0) 349 goto out; 350 details->sk_counter = sig_counter; 351 details->sk_flags = sig_flags; 352 #ifdef DEBUG_SK 353 fprintf(stderr, "%s: signed buf:\n", __func__); 354 sshbuf_dump(original_signed, stderr); 355 #endif 356 357 if ((md_ctx = EVP_MD_CTX_new()) == NULL) { 358 ret = SSH_ERR_ALLOC_FAIL; 359 goto out; 360 } 361 if ((len = i2d_ECDSA_SIG(esig, NULL)) <= 0) { 362 len = 0; 363 ret = SSH_ERR_LIBCRYPTO_ERROR; 364 goto out; 365 } 366 if ((sigb = calloc(1, len)) == NULL) { 367 ret = SSH_ERR_ALLOC_FAIL; 368 goto out; 369 } 370 cp = sigb; /* ASN1_item_i2d increments the pointer past the object */ 371 if (i2d_ECDSA_SIG(esig, &cp) != len) { 372 ret = SSH_ERR_LIBCRYPTO_ERROR; 373 goto out; 374 } 375 #ifdef DEBUG_SK 376 fprintf(stderr, "%s: signed hash:\n", __func__); 377 sshbuf_dump_data(sigb, len, stderr); 378 #endif 379 /* Verify it */ 380 if (EVP_DigestVerifyInit(md_ctx, NULL, EVP_sha256(), NULL, 381 key->pkey) != 1) { 382 ret = SSH_ERR_LIBCRYPTO_ERROR; 383 goto out; 384 } 385 switch (EVP_DigestVerify(md_ctx, sigb, len, 386 sshbuf_ptr(original_signed), sshbuf_len(original_signed))) { 387 case 1: 388 ret = 0; 389 break; 390 case 0: 391 ret = SSH_ERR_SIGNATURE_INVALID; 392 goto out; 393 default: 394 ret = SSH_ERR_LIBCRYPTO_ERROR; 395 goto out; 396 } 397 /* success */ 398 if (detailsp != NULL) { 399 *detailsp = details; 400 details = NULL; 401 } 402 out: 403 explicit_bzero(&sig_flags, sizeof(sig_flags)); 404 explicit_bzero(&sig_counter, sizeof(sig_counter)); 405 explicit_bzero(msghash, sizeof(msghash)); 406 explicit_bzero(apphash, sizeof(apphash)); 407 sshkey_sig_details_free(details); 408 sshbuf_free(webauthn_wrapper); 409 sshbuf_free(webauthn_exts); 410 free(webauthn_origin); 411 sshbuf_free(original_signed); 412 sshbuf_free(sigbuf); 413 sshbuf_free(b); 414 ECDSA_SIG_free(esig); 415 BN_clear_free(sig_r); 416 BN_clear_free(sig_s); 417 free(ktype); 418 freezero(sigb, len); 419 EVP_MD_CTX_free(md_ctx); 420 return ret; 421 } 422 423 static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = { 424 /* .size = */ NULL, 425 /* .alloc = */ NULL, 426 /* .cleanup = */ ssh_ecdsa_sk_cleanup, 427 /* .equal = */ ssh_ecdsa_sk_equal, 428 /* .ssh_serialize_public = */ ssh_ecdsa_sk_serialize_public, 429 /* .ssh_deserialize_public = */ ssh_ecdsa_sk_deserialize_public, 430 /* .ssh_serialize_private = */ ssh_ecdsa_sk_serialize_private, 431 /* .ssh_deserialize_private = */ ssh_ecdsa_sk_deserialize_private, 432 /* .generate = */ NULL, 433 /* .copy_public = */ ssh_ecdsa_sk_copy_public, 434 /* .sign = */ NULL, 435 /* .verify = */ ssh_ecdsa_sk_verify, 436 }; 437 438 const struct sshkey_impl sshkey_ecdsa_sk_impl = { 439 /* .name = */ "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 = */ 0, 446 /* .keybits = */ 256, 447 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 448 }; 449 450 const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = { 451 /* .name = */ "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com", 452 /* .shortname = */ "ECDSA-SK-CERT", 453 /* .sigalg = */ NULL, 454 /* .type = */ KEY_ECDSA_SK_CERT, 455 /* .nid = */ NID_X9_62_prime256v1, 456 /* .cert = */ 1, 457 /* .sigonly = */ 0, 458 /* .keybits = */ 256, 459 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 460 }; 461 462 const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = { 463 /* .name = */ "webauthn-sk-ecdsa-sha2-nistp256@openssh.com", 464 /* .shortname = */ "ECDSA-SK", 465 /* .sigalg = */ NULL, 466 /* .type = */ KEY_ECDSA_SK, 467 /* .nid = */ NID_X9_62_prime256v1, 468 /* .cert = */ 0, 469 /* .sigonly = */ 1, 470 /* .keybits = */ 256, 471 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 472 }; 473