1 /* $OpenBSD: ssh-ecdsa-sk.c,v 1.8 2020/06/22 23:44:27 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 /* 65 * Check FIDO/W3C webauthn signatures clientData field against the expected 66 * format and prepare a hash of it for use in signature verification. 67 * 68 * webauthn signatures do not sign the hash of the message directly, but 69 * instead sign a JSON-like "clientData" wrapper structure that contains the 70 * message hash along with a other information. 71 * 72 * Fortunately this structure has a fixed format so it is possible to verify 73 * that the hash of the signed message is present within the clientData 74 * structure without needing to implement any JSON parsing. 75 */ 76 static int 77 webauthn_check_prepare_hash(const u_char *data, size_t datalen, 78 const char *origin, const struct sshbuf *wrapper, 79 uint8_t flags, const struct sshbuf *extensions, 80 u_char *msghash, size_t msghashlen) 81 { 82 int r = SSH_ERR_INTERNAL_ERROR; 83 struct sshbuf *chall = NULL, *m = NULL; 84 85 if ((m = sshbuf_new()) == NULL || 86 (chall = sshbuf_from(data, datalen)) == NULL) { 87 r = SSH_ERR_ALLOC_FAIL; 88 goto out; 89 } 90 /* 91 * Ensure origin contains no quote character and that the flags are 92 * consistent with what we received 93 */ 94 if (strchr(origin, '\"') != NULL || 95 (flags & 0x40) != 0 /* AD */ || 96 ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) { 97 r = SSH_ERR_INVALID_FORMAT; 98 goto out; 99 } 100 101 /* 102 * Prepare the preamble to clientData that we expect, poking the 103 * challenge and origin into their canonical positions in the 104 * structure. The crossOrigin flag and any additional extension 105 * fields present are ignored. 106 */ 107 #define WEBAUTHN_0 "{\"type\":\"webauthn.get\",\"challenge\":\"" 108 #define WEBAUTHN_1 "\",\"origin\":\"" 109 #define WEBAUTHN_2 "\"" 110 if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 || 111 (r = sshbuf_dtourlb64(chall, m, 0)) != 0 || 112 (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 || 113 (r = sshbuf_put(m, origin, strlen(origin))) != 0 || 114 (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0) 115 goto out; 116 #ifdef DEBUG_SK 117 fprintf(stderr, "%s: received origin: %s\n", __func__, origin); 118 fprintf(stderr, "%s: received clientData:\n", __func__); 119 sshbuf_dump(wrapper, stderr); 120 fprintf(stderr, "%s: expected clientData premable:\n", __func__); 121 sshbuf_dump(m, stderr); 122 #endif 123 /* Check that the supplied clientData has the preamble we expect */ 124 if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0) 125 goto out; 126 127 /* Prepare hash of clientData */ 128 if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper, 129 msghash, msghashlen)) != 0) 130 goto out; 131 132 /* success */ 133 r = 0; 134 out: 135 sshbuf_free(chall); 136 sshbuf_free(m); 137 return r; 138 } 139 140 /* ARGSUSED */ 141 int 142 ssh_ecdsa_sk_verify(const struct sshkey *key, 143 const u_char *signature, size_t signaturelen, 144 const u_char *data, size_t datalen, u_int compat, 145 struct sshkey_sig_details **detailsp) 146 { 147 ECDSA_SIG *sig = NULL; 148 BIGNUM *sig_r = NULL, *sig_s = NULL; 149 u_char sig_flags; 150 u_char msghash[32], apphash[32], sighash[32]; 151 u_int sig_counter; 152 int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR; 153 struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL; 154 struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL; 155 char *ktype = NULL, *webauthn_origin = NULL; 156 struct sshkey_sig_details *details = NULL; 157 #ifdef DEBUG_SK 158 char *tmp = NULL; 159 #endif 160 161 if (detailsp != NULL) 162 *detailsp = NULL; 163 if (key == NULL || key->ecdsa == NULL || 164 sshkey_type_plain(key->type) != KEY_ECDSA_SK || 165 signature == NULL || signaturelen == 0) 166 return SSH_ERR_INVALID_ARGUMENT; 167 168 if (key->ecdsa_nid != NID_X9_62_prime256v1) 169 return SSH_ERR_INTERNAL_ERROR; 170 171 /* fetch signature */ 172 if ((b = sshbuf_from(signature, signaturelen)) == NULL) 173 return SSH_ERR_ALLOC_FAIL; 174 if ((details = calloc(1, sizeof(*details))) == NULL) { 175 ret = SSH_ERR_ALLOC_FAIL; 176 goto out; 177 } 178 if (sshbuf_get_cstring(b, &ktype, NULL) != 0) { 179 ret = SSH_ERR_INVALID_FORMAT; 180 goto out; 181 } 182 if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0) 183 is_webauthn = 1; 184 else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) { 185 ret = SSH_ERR_INVALID_FORMAT; 186 goto out; 187 } 188 if (sshbuf_froms(b, &sigbuf) != 0 || 189 sshbuf_get_u8(b, &sig_flags) != 0 || 190 sshbuf_get_u32(b, &sig_counter) != 0) { 191 ret = SSH_ERR_INVALID_FORMAT; 192 goto out; 193 } 194 if (is_webauthn) { 195 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 || 196 sshbuf_froms(b, &webauthn_wrapper) != 0 || 197 sshbuf_froms(b, &webauthn_exts) != 0) { 198 ret = SSH_ERR_INVALID_FORMAT; 199 goto out; 200 } 201 } 202 if (sshbuf_len(b) != 0) { 203 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 204 goto out; 205 } 206 207 /* parse signature */ 208 if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 || 209 sshbuf_get_bignum2(sigbuf, &sig_s) != 0) { 210 ret = SSH_ERR_INVALID_FORMAT; 211 goto out; 212 } 213 if (sshbuf_len(sigbuf) != 0) { 214 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 215 goto out; 216 } 217 218 #ifdef DEBUG_SK 219 fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen); 220 /* sshbuf_dump_data(data, datalen, stderr); */ 221 fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r))); 222 free(tmp); 223 fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s))); 224 free(tmp); 225 fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n", 226 __func__, sig_flags, sig_counter); 227 if (is_webauthn) { 228 fprintf(stderr, "%s: webauthn origin: %s\n", __func__, 229 webauthn_origin); 230 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__); 231 sshbuf_dump(webauthn_wrapper, stderr); 232 } 233 #endif 234 if ((sig = ECDSA_SIG_new()) == NULL) { 235 ret = SSH_ERR_ALLOC_FAIL; 236 goto out; 237 } 238 if (!ECDSA_SIG_set0(sig, sig_r, sig_s)) { 239 ret = SSH_ERR_LIBCRYPTO_ERROR; 240 goto out; 241 } 242 sig_r = sig_s = NULL; /* transferred */ 243 244 /* Reconstruct data that was supposedly signed */ 245 if ((original_signed = sshbuf_new()) == NULL) { 246 ret = SSH_ERR_ALLOC_FAIL; 247 goto out; 248 } 249 if (is_webauthn) { 250 if ((ret = webauthn_check_prepare_hash(data, datalen, 251 webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts, 252 msghash, sizeof(msghash))) != 0) 253 goto out; 254 } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen, 255 msghash, sizeof(msghash))) != 0) 256 goto out; 257 /* Application value is hashed before signature */ 258 if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application, 259 strlen(key->sk_application), apphash, sizeof(apphash))) != 0) 260 goto out; 261 #ifdef DEBUG_SK 262 fprintf(stderr, "%s: hashed application:\n", __func__); 263 sshbuf_dump_data(apphash, sizeof(apphash), stderr); 264 fprintf(stderr, "%s: hashed message:\n", __func__); 265 sshbuf_dump_data(msghash, sizeof(msghash), stderr); 266 #endif 267 if ((ret = sshbuf_put(original_signed, 268 apphash, sizeof(apphash))) != 0 || 269 (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 || 270 (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 || 271 (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 || 272 (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0) 273 goto out; 274 /* Signature is over H(original_signed) */ 275 if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed, 276 sighash, sizeof(sighash))) != 0) 277 goto out; 278 details->sk_counter = sig_counter; 279 details->sk_flags = sig_flags; 280 #ifdef DEBUG_SK 281 fprintf(stderr, "%s: signed buf:\n", __func__); 282 sshbuf_dump(original_signed, stderr); 283 fprintf(stderr, "%s: signed hash:\n", __func__); 284 sshbuf_dump_data(sighash, sizeof(sighash), stderr); 285 #endif 286 287 /* Verify it */ 288 switch (ECDSA_do_verify(sighash, sizeof(sighash), sig, key->ecdsa)) { 289 case 1: 290 ret = 0; 291 break; 292 case 0: 293 ret = SSH_ERR_SIGNATURE_INVALID; 294 goto out; 295 default: 296 ret = SSH_ERR_LIBCRYPTO_ERROR; 297 goto out; 298 } 299 /* success */ 300 if (detailsp != NULL) { 301 *detailsp = details; 302 details = NULL; 303 } 304 out: 305 explicit_bzero(&sig_flags, sizeof(sig_flags)); 306 explicit_bzero(&sig_counter, sizeof(sig_counter)); 307 explicit_bzero(msghash, sizeof(msghash)); 308 explicit_bzero(sighash, sizeof(msghash)); 309 explicit_bzero(apphash, sizeof(apphash)); 310 sshkey_sig_details_free(details); 311 sshbuf_free(webauthn_wrapper); 312 sshbuf_free(webauthn_exts); 313 free(webauthn_origin); 314 sshbuf_free(original_signed); 315 sshbuf_free(sigbuf); 316 sshbuf_free(b); 317 ECDSA_SIG_free(sig); 318 BN_clear_free(sig_r); 319 BN_clear_free(sig_s); 320 free(ktype); 321 return ret; 322 } 323 324 #endif /* OPENSSL_HAS_ECC */ 325