1/* $OpenBSD: ssh-ecdsa-sk.c,v 1.18 2023/03/08 04:43:12 guenther 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 */ 47extern struct sshkey_impl_funcs sshkey_ecdsa_funcs; 48 49static void 50ssh_ecdsa_sk_cleanup(struct sshkey *k) 51{ 52 sshkey_sk_cleanup(k); 53 sshkey_ecdsa_funcs.cleanup(k); 54} 55 56static int 57ssh_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 66static int 67ssh_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 80static int 81ssh_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 97static int 98ssh_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 109static int 110ssh_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 122static int 123ssh_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 */ 151static int 152webauthn_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 215static int 216ssh_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 BIGNUM *sig_r = NULL, *sig_s = NULL; 223 u_char sig_flags; 224 u_char msghash[32], apphash[32], sighash[32]; 225 u_int sig_counter; 226 int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR; 227 struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL; 228 struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL; 229 char *ktype = NULL, *webauthn_origin = NULL; 230 struct sshkey_sig_details *details = NULL; 231#ifdef DEBUG_SK 232 char *tmp = NULL; 233#endif 234 235 if (detailsp != NULL) 236 *detailsp = NULL; 237 if (key == NULL || key->ecdsa == NULL || 238 sshkey_type_plain(key->type) != KEY_ECDSA_SK || 239 sig == NULL || siglen == 0) 240 return SSH_ERR_INVALID_ARGUMENT; 241 242 if (key->ecdsa_nid != NID_X9_62_prime256v1) 243 return SSH_ERR_INTERNAL_ERROR; 244 245 /* fetch signature */ 246 if ((b = sshbuf_from(sig, siglen)) == NULL) 247 return SSH_ERR_ALLOC_FAIL; 248 if ((details = calloc(1, sizeof(*details))) == NULL) { 249 ret = SSH_ERR_ALLOC_FAIL; 250 goto out; 251 } 252 if (sshbuf_get_cstring(b, &ktype, NULL) != 0) { 253 ret = SSH_ERR_INVALID_FORMAT; 254 goto out; 255 } 256 if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0) 257 is_webauthn = 1; 258 else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) { 259 ret = SSH_ERR_INVALID_FORMAT; 260 goto out; 261 } 262 if (sshbuf_froms(b, &sigbuf) != 0 || 263 sshbuf_get_u8(b, &sig_flags) != 0 || 264 sshbuf_get_u32(b, &sig_counter) != 0) { 265 ret = SSH_ERR_INVALID_FORMAT; 266 goto out; 267 } 268 if (is_webauthn) { 269 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 || 270 sshbuf_froms(b, &webauthn_wrapper) != 0 || 271 sshbuf_froms(b, &webauthn_exts) != 0) { 272 ret = SSH_ERR_INVALID_FORMAT; 273 goto out; 274 } 275 } 276 if (sshbuf_len(b) != 0) { 277 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 278 goto out; 279 } 280 281 /* parse signature */ 282 if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 || 283 sshbuf_get_bignum2(sigbuf, &sig_s) != 0) { 284 ret = SSH_ERR_INVALID_FORMAT; 285 goto out; 286 } 287 if (sshbuf_len(sigbuf) != 0) { 288 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 289 goto out; 290 } 291 292#ifdef DEBUG_SK 293 fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen); 294 /* sshbuf_dump_data(data, datalen, stderr); */ 295 fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r))); 296 free(tmp); 297 fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s))); 298 free(tmp); 299 fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n", 300 __func__, sig_flags, sig_counter); 301 if (is_webauthn) { 302 fprintf(stderr, "%s: webauthn origin: %s\n", __func__, 303 webauthn_origin); 304 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__); 305 sshbuf_dump(webauthn_wrapper, stderr); 306 } 307#endif 308 if ((esig = ECDSA_SIG_new()) == NULL) { 309 ret = SSH_ERR_ALLOC_FAIL; 310 goto out; 311 } 312 if (!ECDSA_SIG_set0(esig, sig_r, sig_s)) { 313 ret = SSH_ERR_LIBCRYPTO_ERROR; 314 goto out; 315 } 316 sig_r = sig_s = NULL; /* transferred */ 317 318 /* Reconstruct data that was supposedly signed */ 319 if ((original_signed = sshbuf_new()) == NULL) { 320 ret = SSH_ERR_ALLOC_FAIL; 321 goto out; 322 } 323 if (is_webauthn) { 324 if ((ret = webauthn_check_prepare_hash(data, dlen, 325 webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts, 326 msghash, sizeof(msghash))) != 0) 327 goto out; 328 } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, dlen, 329 msghash, sizeof(msghash))) != 0) 330 goto out; 331 /* Application value is hashed before signature */ 332 if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application, 333 strlen(key->sk_application), apphash, sizeof(apphash))) != 0) 334 goto out; 335#ifdef DEBUG_SK 336 fprintf(stderr, "%s: hashed application:\n", __func__); 337 sshbuf_dump_data(apphash, sizeof(apphash), stderr); 338 fprintf(stderr, "%s: hashed message:\n", __func__); 339 sshbuf_dump_data(msghash, sizeof(msghash), stderr); 340#endif 341 if ((ret = sshbuf_put(original_signed, 342 apphash, sizeof(apphash))) != 0 || 343 (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 || 344 (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 || 345 (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 || 346 (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0) 347 goto out; 348 /* Signature is over H(original_signed) */ 349 if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed, 350 sighash, sizeof(sighash))) != 0) 351 goto out; 352 details->sk_counter = sig_counter; 353 details->sk_flags = sig_flags; 354#ifdef DEBUG_SK 355 fprintf(stderr, "%s: signed buf:\n", __func__); 356 sshbuf_dump(original_signed, stderr); 357 fprintf(stderr, "%s: signed hash:\n", __func__); 358 sshbuf_dump_data(sighash, sizeof(sighash), stderr); 359#endif 360 361 /* Verify it */ 362 switch (ECDSA_do_verify(sighash, sizeof(sighash), esig, key->ecdsa)) { 363 case 1: 364 ret = 0; 365 break; 366 case 0: 367 ret = SSH_ERR_SIGNATURE_INVALID; 368 goto out; 369 default: 370 ret = SSH_ERR_LIBCRYPTO_ERROR; 371 goto out; 372 } 373 /* success */ 374 if (detailsp != NULL) { 375 *detailsp = details; 376 details = NULL; 377 } 378 out: 379 explicit_bzero(&sig_flags, sizeof(sig_flags)); 380 explicit_bzero(&sig_counter, sizeof(sig_counter)); 381 explicit_bzero(msghash, sizeof(msghash)); 382 explicit_bzero(sighash, sizeof(msghash)); 383 explicit_bzero(apphash, sizeof(apphash)); 384 sshkey_sig_details_free(details); 385 sshbuf_free(webauthn_wrapper); 386 sshbuf_free(webauthn_exts); 387 free(webauthn_origin); 388 sshbuf_free(original_signed); 389 sshbuf_free(sigbuf); 390 sshbuf_free(b); 391 ECDSA_SIG_free(esig); 392 BN_clear_free(sig_r); 393 BN_clear_free(sig_s); 394 free(ktype); 395 return ret; 396} 397 398static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = { 399 /* .size = */ NULL, 400 /* .alloc = */ NULL, 401 /* .cleanup = */ ssh_ecdsa_sk_cleanup, 402 /* .equal = */ ssh_ecdsa_sk_equal, 403 /* .ssh_serialize_public = */ ssh_ecdsa_sk_serialize_public, 404 /* .ssh_deserialize_public = */ ssh_ecdsa_sk_deserialize_public, 405 /* .ssh_serialize_private = */ ssh_ecdsa_sk_serialize_private, 406 /* .ssh_deserialize_private = */ ssh_ecdsa_sk_deserialize_private, 407 /* .generate = */ NULL, 408 /* .copy_public = */ ssh_ecdsa_sk_copy_public, 409 /* .sign = */ NULL, 410 /* .verify = */ ssh_ecdsa_sk_verify, 411}; 412 413const struct sshkey_impl sshkey_ecdsa_sk_impl = { 414 /* .name = */ "sk-ecdsa-sha2-nistp256@openssh.com", 415 /* .shortname = */ "ECDSA-SK", 416 /* .sigalg = */ NULL, 417 /* .type = */ KEY_ECDSA_SK, 418 /* .nid = */ NID_X9_62_prime256v1, 419 /* .cert = */ 0, 420 /* .sigonly = */ 0, 421 /* .keybits = */ 256, 422 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 423}; 424 425const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = { 426 /* .name = */ "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com", 427 /* .shortname = */ "ECDSA-SK-CERT", 428 /* .sigalg = */ NULL, 429 /* .type = */ KEY_ECDSA_SK_CERT, 430 /* .nid = */ NID_X9_62_prime256v1, 431 /* .cert = */ 1, 432 /* .sigonly = */ 0, 433 /* .keybits = */ 256, 434 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 435}; 436 437const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = { 438 /* .name = */ "webauthn-sk-ecdsa-sha2-nistp256@openssh.com", 439 /* .shortname = */ "ECDSA-SK", 440 /* .sigalg = */ NULL, 441 /* .type = */ KEY_ECDSA_SK, 442 /* .nid = */ NID_X9_62_prime256v1, 443 /* .cert = */ 0, 444 /* .sigonly = */ 1, 445 /* .keybits = */ 256, 446 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 447}; 448