1/* $NetBSD: ssh-ecdsa-sk.c,v 1.4 2023/07/26 17:58:16 christos Exp $ */ 2/* $OpenBSD: ssh-ecdsa-sk.c,v 1.18 2023/03/08 04:43:12 guenther Exp $ */ 3/* 4 * Copyright (c) 2000 Markus Friedl. All rights reserved. 5 * Copyright (c) 2010 Damien Miller. All rights reserved. 6 * Copyright (c) 2019 Google Inc. All rights reserved. 7 * 8 * Redistribution and use in source and binary forms, with or without 9 * modification, are permitted provided that the following conditions 10 * are met: 11 * 1. Redistributions of source code must retain the above copyright 12 * notice, this list of conditions and the following disclaimer. 13 * 2. Redistributions in binary form must reproduce the above copyright 14 * notice, this list of conditions and the following disclaimer in the 15 * documentation and/or other materials provided with the distribution. 16 * 17 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 18 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 19 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 20 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 21 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 22 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 26 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27 */ 28#include "includes.h" 29__RCSID("$NetBSD: ssh-ecdsa-sk.c,v 1.4 2023/07/26 17:58:16 christos Exp $"); 30 31/* #define DEBUG_SK 1 */ 32 33#include <sys/types.h> 34 35#include <openssl/bn.h> 36#include <openssl/ec.h> 37#include <openssl/ecdsa.h> 38#include <openssl/evp.h> 39 40#include <string.h> 41#include <stdio.h> /* needed for DEBUG_SK only */ 42 43#include "sshbuf.h" 44#include "ssherr.h" 45#include "digest.h" 46#define SSHKEY_INTERNAL 47#include "sshkey.h" 48 49/* Reuse some ECDSA internals */ 50extern struct sshkey_impl_funcs sshkey_ecdsa_funcs; 51 52static void 53ssh_ecdsa_sk_cleanup(struct sshkey *k) 54{ 55 sshkey_sk_cleanup(k); 56 sshkey_ecdsa_funcs.cleanup(k); 57} 58 59static int 60ssh_ecdsa_sk_equal(const struct sshkey *a, const struct sshkey *b) 61{ 62 if (!sshkey_sk_fields_equal(a, b)) 63 return 0; 64 if (!sshkey_ecdsa_funcs.equal(a, b)) 65 return 0; 66 return 1; 67} 68 69static int 70ssh_ecdsa_sk_serialize_public(const struct sshkey *key, struct sshbuf *b, 71 enum sshkey_serialize_rep opts) 72{ 73 int r; 74 75 if ((r = sshkey_ecdsa_funcs.serialize_public(key, b, opts)) != 0) 76 return r; 77 if ((r = sshkey_serialize_sk(key, b)) != 0) 78 return r; 79 80 return 0; 81} 82 83static int 84ssh_ecdsa_sk_serialize_private(const struct sshkey *key, struct sshbuf *b, 85 enum sshkey_serialize_rep opts) 86{ 87 int r; 88 89 if (!sshkey_is_cert(key)) { 90 if ((r = sshkey_ecdsa_funcs.serialize_public(key, 91 b, opts)) != 0) 92 return r; 93 } 94 if ((r = sshkey_serialize_private_sk(key, b)) != 0) 95 return r; 96 97 return 0; 98} 99 100static int 101ssh_ecdsa_sk_copy_public(const struct sshkey *from, struct sshkey *to) 102{ 103 int r; 104 105 if ((r = sshkey_ecdsa_funcs.copy_public(from, to)) != 0) 106 return r; 107 if ((r = sshkey_copy_public_sk(from, to)) != 0) 108 return r; 109 return 0; 110} 111 112static int 113ssh_ecdsa_sk_deserialize_public(const char *ktype, struct sshbuf *b, 114 struct sshkey *key) 115{ 116 int r; 117 118 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, b, key)) != 0) 119 return r; 120 if ((r = sshkey_deserialize_sk(b, key)) != 0) 121 return r; 122 return 0; 123} 124 125static int 126ssh_ecdsa_sk_deserialize_private(const char *ktype, struct sshbuf *b, 127 struct sshkey *key) 128{ 129 int r; 130 131 if (!sshkey_is_cert(key)) { 132 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, 133 b, key)) != 0) 134 return r; 135 } 136 if ((r = sshkey_private_deserialize_sk(b, key)) != 0) 137 return r; 138 139 return 0; 140} 141 142/* 143 * Check FIDO/W3C webauthn signatures clientData field against the expected 144 * format and prepare a hash of it for use in signature verification. 145 * 146 * webauthn signatures do not sign the hash of the message directly, but 147 * instead sign a JSON-like "clientData" wrapper structure that contains the 148 * message hash along with a other information. 149 * 150 * Fortunately this structure has a fixed format so it is possible to verify 151 * that the hash of the signed message is present within the clientData 152 * structure without needing to implement any JSON parsing. 153 */ 154static int 155webauthn_check_prepare_hash(const u_char *data, size_t datalen, 156 const char *origin, const struct sshbuf *wrapper, 157 uint8_t flags, const struct sshbuf *extensions, 158 u_char *msghash, size_t msghashlen) 159{ 160 int r = SSH_ERR_INTERNAL_ERROR; 161 struct sshbuf *chall = NULL, *m = NULL; 162 163 if ((m = sshbuf_new()) == NULL || 164 (chall = sshbuf_from(data, datalen)) == NULL) { 165 r = SSH_ERR_ALLOC_FAIL; 166 goto out; 167 } 168 /* 169 * Ensure origin contains no quote character and that the flags are 170 * consistent with what we received 171 */ 172 if (strchr(origin, '\"') != NULL || 173 (flags & 0x40) != 0 /* AD */ || 174 ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) { 175 r = SSH_ERR_INVALID_FORMAT; 176 goto out; 177 } 178 179 /* 180 * Prepare the preamble to clientData that we expect, poking the 181 * challenge and origin into their canonical positions in the 182 * structure. The crossOrigin flag and any additional extension 183 * fields present are ignored. 184 */ 185#define WEBAUTHN_0 "{\"type\":\"webauthn.get\",\"challenge\":\"" 186#define WEBAUTHN_1 "\",\"origin\":\"" 187#define WEBAUTHN_2 "\"" 188 if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 || 189 (r = sshbuf_dtourlb64(chall, m, 0)) != 0 || 190 (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 || 191 (r = sshbuf_put(m, origin, strlen(origin))) != 0 || 192 (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0) 193 goto out; 194#ifdef DEBUG_SK 195 fprintf(stderr, "%s: received origin: %s\n", __func__, origin); 196 fprintf(stderr, "%s: received clientData:\n", __func__); 197 sshbuf_dump(wrapper, stderr); 198 fprintf(stderr, "%s: expected clientData premable:\n", __func__); 199 sshbuf_dump(m, stderr); 200#endif 201 /* Check that the supplied clientData has the preamble we expect */ 202 if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0) 203 goto out; 204 205 /* Prepare hash of clientData */ 206 if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper, 207 msghash, msghashlen)) != 0) 208 goto out; 209 210 /* success */ 211 r = 0; 212 out: 213 sshbuf_free(chall); 214 sshbuf_free(m); 215 return r; 216} 217 218static int 219ssh_ecdsa_sk_verify(const struct sshkey *key, 220 const u_char *sig, size_t siglen, 221 const u_char *data, size_t dlen, const char *alg, u_int compat, 222 struct sshkey_sig_details **detailsp) 223{ 224 ECDSA_SIG *esig = NULL; 225 BIGNUM *sig_r = NULL, *sig_s = NULL; 226 u_char sig_flags; 227 u_char msghash[32], apphash[32], sighash[32]; 228 u_int sig_counter; 229 int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR; 230 struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL; 231 struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL; 232 char *ktype = NULL, *webauthn_origin = NULL; 233 struct sshkey_sig_details *details = NULL; 234#ifdef DEBUG_SK 235 char *tmp = NULL; 236#endif 237 238 if (detailsp != NULL) 239 *detailsp = NULL; 240 if (key == NULL || key->ecdsa == NULL || 241 sshkey_type_plain(key->type) != KEY_ECDSA_SK || 242 sig == NULL || siglen == 0) 243 return SSH_ERR_INVALID_ARGUMENT; 244 245 if (key->ecdsa_nid != NID_X9_62_prime256v1) 246 return SSH_ERR_INTERNAL_ERROR; 247 248 /* fetch signature */ 249 if ((b = sshbuf_from(sig, siglen)) == NULL) 250 return SSH_ERR_ALLOC_FAIL; 251 if ((details = calloc(1, sizeof(*details))) == NULL) { 252 ret = SSH_ERR_ALLOC_FAIL; 253 goto out; 254 } 255 if (sshbuf_get_cstring(b, &ktype, NULL) != 0) { 256 ret = SSH_ERR_INVALID_FORMAT; 257 goto out; 258 } 259 if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0) 260 is_webauthn = 1; 261 else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) { 262 ret = SSH_ERR_INVALID_FORMAT; 263 goto out; 264 } 265 if (sshbuf_froms(b, &sigbuf) != 0 || 266 sshbuf_get_u8(b, &sig_flags) != 0 || 267 sshbuf_get_u32(b, &sig_counter) != 0) { 268 ret = SSH_ERR_INVALID_FORMAT; 269 goto out; 270 } 271 if (is_webauthn) { 272 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 || 273 sshbuf_froms(b, &webauthn_wrapper) != 0 || 274 sshbuf_froms(b, &webauthn_exts) != 0) { 275 ret = SSH_ERR_INVALID_FORMAT; 276 goto out; 277 } 278 } 279 if (sshbuf_len(b) != 0) { 280 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 281 goto out; 282 } 283 284 /* parse signature */ 285 if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 || 286 sshbuf_get_bignum2(sigbuf, &sig_s) != 0) { 287 ret = SSH_ERR_INVALID_FORMAT; 288 goto out; 289 } 290 if (sshbuf_len(sigbuf) != 0) { 291 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 292 goto out; 293 } 294 295#ifdef DEBUG_SK 296 fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen); 297 /* sshbuf_dump_data(data, datalen, stderr); */ 298 fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r))); 299 free(tmp); 300 fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s))); 301 free(tmp); 302 fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n", 303 __func__, sig_flags, sig_counter); 304 if (is_webauthn) { 305 fprintf(stderr, "%s: webauthn origin: %s\n", __func__, 306 webauthn_origin); 307 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__); 308 sshbuf_dump(webauthn_wrapper, stderr); 309 } 310#endif 311 if ((esig = ECDSA_SIG_new()) == NULL) { 312 ret = SSH_ERR_ALLOC_FAIL; 313 goto out; 314 } 315 if (!ECDSA_SIG_set0(esig, sig_r, sig_s)) { 316 ret = SSH_ERR_LIBCRYPTO_ERROR; 317 goto out; 318 } 319 sig_r = sig_s = NULL; /* transferred */ 320 321 /* Reconstruct data that was supposedly signed */ 322 if ((original_signed = sshbuf_new()) == NULL) { 323 ret = SSH_ERR_ALLOC_FAIL; 324 goto out; 325 } 326 if (is_webauthn) { 327 if ((ret = webauthn_check_prepare_hash(data, dlen, 328 webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts, 329 msghash, sizeof(msghash))) != 0) 330 goto out; 331 } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, dlen, 332 msghash, sizeof(msghash))) != 0) 333 goto out; 334 /* Application value is hashed before signature */ 335 if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application, 336 strlen(key->sk_application), apphash, sizeof(apphash))) != 0) 337 goto out; 338#ifdef DEBUG_SK 339 fprintf(stderr, "%s: hashed application:\n", __func__); 340 sshbuf_dump_data(apphash, sizeof(apphash), stderr); 341 fprintf(stderr, "%s: hashed message:\n", __func__); 342 sshbuf_dump_data(msghash, sizeof(msghash), stderr); 343#endif 344 if ((ret = sshbuf_put(original_signed, 345 apphash, sizeof(apphash))) != 0 || 346 (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 || 347 (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 || 348 (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 || 349 (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0) 350 goto out; 351 /* Signature is over H(original_signed) */ 352 if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed, 353 sighash, sizeof(sighash))) != 0) 354 goto out; 355 details->sk_counter = sig_counter; 356 details->sk_flags = sig_flags; 357#ifdef DEBUG_SK 358 fprintf(stderr, "%s: signed buf:\n", __func__); 359 sshbuf_dump(original_signed, stderr); 360 fprintf(stderr, "%s: signed hash:\n", __func__); 361 sshbuf_dump_data(sighash, sizeof(sighash), stderr); 362#endif 363 364 /* Verify it */ 365 switch (ECDSA_do_verify(sighash, sizeof(sighash), esig, key->ecdsa)) { 366 case 1: 367 ret = 0; 368 break; 369 case 0: 370 ret = SSH_ERR_SIGNATURE_INVALID; 371 goto out; 372 default: 373 ret = SSH_ERR_LIBCRYPTO_ERROR; 374 goto out; 375 } 376 /* success */ 377 if (detailsp != NULL) { 378 *detailsp = details; 379 details = NULL; 380 } 381 out: 382 explicit_bzero(&sig_flags, sizeof(sig_flags)); 383 explicit_bzero(&sig_counter, sizeof(sig_counter)); 384 explicit_bzero(msghash, sizeof(msghash)); 385 explicit_bzero(sighash, sizeof(msghash)); 386 explicit_bzero(apphash, sizeof(apphash)); 387 sshkey_sig_details_free(details); 388 sshbuf_free(webauthn_wrapper); 389 sshbuf_free(webauthn_exts); 390 free(webauthn_origin); 391 sshbuf_free(original_signed); 392 sshbuf_free(sigbuf); 393 sshbuf_free(b); 394 ECDSA_SIG_free(esig); 395 BN_clear_free(sig_r); 396 BN_clear_free(sig_s); 397 free(ktype); 398 return ret; 399} 400 401static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = { 402 /* .size = */ NULL, 403 /* .alloc = */ NULL, 404 /* .cleanup = */ ssh_ecdsa_sk_cleanup, 405 /* .equal = */ ssh_ecdsa_sk_equal, 406 /* .ssh_serialize_public = */ ssh_ecdsa_sk_serialize_public, 407 /* .ssh_deserialize_public = */ ssh_ecdsa_sk_deserialize_public, 408 /* .ssh_serialize_private = */ ssh_ecdsa_sk_serialize_private, 409 /* .ssh_deserialize_private = */ ssh_ecdsa_sk_deserialize_private, 410 /* .generate = */ NULL, 411 /* .copy_public = */ ssh_ecdsa_sk_copy_public, 412 /* .sign = */ NULL, 413 /* .verify = */ ssh_ecdsa_sk_verify, 414}; 415 416const struct sshkey_impl sshkey_ecdsa_sk_impl = { 417 /* .name = */ "sk-ecdsa-sha2-nistp256@openssh.com", 418 /* .shortname = */ "ECDSA-SK", 419 /* .sigalg = */ NULL, 420 /* .type = */ KEY_ECDSA_SK, 421 /* .nid = */ NID_X9_62_prime256v1, 422 /* .cert = */ 0, 423 /* .sigonly = */ 0, 424 /* .keybits = */ 256, 425 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 426}; 427 428const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = { 429 /* .name = */ "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com", 430 /* .shortname = */ "ECDSA-SK-CERT", 431 /* .sigalg = */ NULL, 432 /* .type = */ KEY_ECDSA_SK_CERT, 433 /* .nid = */ NID_X9_62_prime256v1, 434 /* .cert = */ 1, 435 /* .sigonly = */ 0, 436 /* .keybits = */ 256, 437 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 438}; 439 440const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = { 441 /* .name = */ "webauthn-sk-ecdsa-sha2-nistp256@openssh.com", 442 /* .shortname = */ "ECDSA-SK", 443 /* .sigalg = */ NULL, 444 /* .type = */ KEY_ECDSA_SK, 445 /* .nid = */ NID_X9_62_prime256v1, 446 /* .cert = */ 0, 447 /* .sigonly = */ 1, 448 /* .keybits = */ 256, 449 /* .funcs = */ &sshkey_ecdsa_sk_funcs, 450}; 451