1 /* $NetBSD: ssh-ecdsa-sk.c,v 1.3 2020/12/04 18:42:50 christos Exp $ */ 2 /* $OpenBSD: ssh-ecdsa-sk.c,v 1.8 2020/06/22 23:44:27 djm Exp $ */ 3 4 /* 5 * Copyright (c) 2000 Markus Friedl. All rights reserved. 6 * Copyright (c) 2010 Damien Miller. All rights reserved. 7 * Copyright (c) 2019 Google Inc. All rights reserved. 8 * 9 * Redistribution and use in source and binary forms, with or without 10 * modification, are permitted provided that the following conditions 11 * are met: 12 * 1. Redistributions of source code must retain the above copyright 13 * notice, this list of conditions and the following disclaimer. 14 * 2. Redistributions in binary form must reproduce the above copyright 15 * notice, this list of conditions and the following disclaimer in the 16 * documentation and/or other materials provided with the distribution. 17 * 18 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 19 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 20 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 21 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 22 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 23 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 24 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 25 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 26 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 27 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 28 */ 29 #include "includes.h" 30 __RCSID("$NetBSD: ssh-ecdsa-sk.c,v 1.3 2020/12/04 18:42:50 christos Exp $"); 31 32 /* #define DEBUG_SK 1 */ 33 34 #include <sys/types.h> 35 36 #include <openssl/bn.h> 37 #include <openssl/ec.h> 38 #include <openssl/ecdsa.h> 39 #include <openssl/evp.h> 40 41 #include <string.h> 42 #include <stdio.h> /* needed for DEBUG_SK only */ 43 44 #include "sshbuf.h" 45 #include "ssherr.h" 46 #include "digest.h" 47 #define SSHKEY_INTERNAL 48 #include "sshkey.h" 49 50 /* 51 * Check FIDO/W3C webauthn signatures clientData field against the expected 52 * format and prepare a hash of it for use in signature verification. 53 * 54 * webauthn signatures do not sign the hash of the message directly, but 55 * instead sign a JSON-like "clientData" wrapper structure that contains the 56 * message hash along with a other information. 57 * 58 * Fortunately this structure has a fixed format so it is possible to verify 59 * that the hash of the signed message is present within the clientData 60 * structure without needing to implement any JSON parsing. 61 */ 62 static int 63 webauthn_check_prepare_hash(const u_char *data, size_t datalen, 64 const char *origin, const struct sshbuf *wrapper, 65 uint8_t flags, const struct sshbuf *extensions, 66 u_char *msghash, size_t msghashlen) 67 { 68 int r = SSH_ERR_INTERNAL_ERROR; 69 struct sshbuf *chall = NULL, *m = NULL; 70 71 if ((m = sshbuf_new()) == NULL || 72 (chall = sshbuf_from(data, datalen)) == NULL) { 73 r = SSH_ERR_ALLOC_FAIL; 74 goto out; 75 } 76 /* 77 * Ensure origin contains no quote character and that the flags are 78 * consistent with what we received 79 */ 80 if (strchr(origin, '\"') != NULL || 81 (flags & 0x40) != 0 /* AD */ || 82 ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) { 83 r = SSH_ERR_INVALID_FORMAT; 84 goto out; 85 } 86 87 /* 88 * Prepare the preamble to clientData that we expect, poking the 89 * challenge and origin into their canonical positions in the 90 * structure. The crossOrigin flag and any additional extension 91 * fields present are ignored. 92 */ 93 #define WEBAUTHN_0 "{\"type\":\"webauthn.get\",\"challenge\":\"" 94 #define WEBAUTHN_1 "\",\"origin\":\"" 95 #define WEBAUTHN_2 "\"" 96 if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 || 97 (r = sshbuf_dtourlb64(chall, m, 0)) != 0 || 98 (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 || 99 (r = sshbuf_put(m, origin, strlen(origin))) != 0 || 100 (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0) 101 goto out; 102 #ifdef DEBUG_SK 103 fprintf(stderr, "%s: received origin: %s\n", __func__, origin); 104 fprintf(stderr, "%s: received clientData:\n", __func__); 105 sshbuf_dump(wrapper, stderr); 106 fprintf(stderr, "%s: expected clientData premable:\n", __func__); 107 sshbuf_dump(m, stderr); 108 #endif 109 /* Check that the supplied clientData has the preamble we expect */ 110 if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0) 111 goto out; 112 113 /* Prepare hash of clientData */ 114 if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper, 115 msghash, msghashlen)) != 0) 116 goto out; 117 118 /* success */ 119 r = 0; 120 out: 121 sshbuf_free(chall); 122 sshbuf_free(m); 123 return r; 124 } 125 126 /* ARGSUSED */ 127 int 128 ssh_ecdsa_sk_verify(const struct sshkey *key, 129 const u_char *signature, size_t signaturelen, 130 const u_char *data, size_t datalen, u_int compat, 131 struct sshkey_sig_details **detailsp) 132 { 133 ECDSA_SIG *sig = NULL; 134 BIGNUM *sig_r = NULL, *sig_s = NULL; 135 u_char sig_flags; 136 u_char msghash[32], apphash[32], sighash[32]; 137 u_int sig_counter; 138 int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR; 139 struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL; 140 struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL; 141 char *ktype = NULL, *webauthn_origin = NULL; 142 struct sshkey_sig_details *details = NULL; 143 #ifdef DEBUG_SK 144 char *tmp = NULL; 145 #endif 146 147 if (detailsp != NULL) 148 *detailsp = NULL; 149 if (key == NULL || key->ecdsa == NULL || 150 sshkey_type_plain(key->type) != KEY_ECDSA_SK || 151 signature == NULL || signaturelen == 0) 152 return SSH_ERR_INVALID_ARGUMENT; 153 154 if (key->ecdsa_nid != NID_X9_62_prime256v1) 155 return SSH_ERR_INTERNAL_ERROR; 156 157 /* fetch signature */ 158 if ((b = sshbuf_from(signature, signaturelen)) == NULL) 159 return SSH_ERR_ALLOC_FAIL; 160 if ((details = calloc(1, sizeof(*details))) == NULL) { 161 ret = SSH_ERR_ALLOC_FAIL; 162 goto out; 163 } 164 if (sshbuf_get_cstring(b, &ktype, NULL) != 0) { 165 ret = SSH_ERR_INVALID_FORMAT; 166 goto out; 167 } 168 if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0) 169 is_webauthn = 1; 170 else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) { 171 ret = SSH_ERR_INVALID_FORMAT; 172 goto out; 173 } 174 if (sshbuf_froms(b, &sigbuf) != 0 || 175 sshbuf_get_u8(b, &sig_flags) != 0 || 176 sshbuf_get_u32(b, &sig_counter) != 0) { 177 ret = SSH_ERR_INVALID_FORMAT; 178 goto out; 179 } 180 if (is_webauthn) { 181 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 || 182 sshbuf_froms(b, &webauthn_wrapper) != 0 || 183 sshbuf_froms(b, &webauthn_exts) != 0) { 184 ret = SSH_ERR_INVALID_FORMAT; 185 goto out; 186 } 187 } 188 if (sshbuf_len(b) != 0) { 189 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 190 goto out; 191 } 192 193 /* parse signature */ 194 if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 || 195 sshbuf_get_bignum2(sigbuf, &sig_s) != 0) { 196 ret = SSH_ERR_INVALID_FORMAT; 197 goto out; 198 } 199 if (sshbuf_len(sigbuf) != 0) { 200 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA; 201 goto out; 202 } 203 204 #ifdef DEBUG_SK 205 fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen); 206 /* sshbuf_dump_data(data, datalen, stderr); */ 207 fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r))); 208 free(tmp); 209 fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s))); 210 free(tmp); 211 fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n", 212 __func__, sig_flags, sig_counter); 213 if (is_webauthn) { 214 fprintf(stderr, "%s: webauthn origin: %s\n", __func__, 215 webauthn_origin); 216 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__); 217 sshbuf_dump(webauthn_wrapper, stderr); 218 } 219 #endif 220 if ((sig = ECDSA_SIG_new()) == NULL) { 221 ret = SSH_ERR_ALLOC_FAIL; 222 goto out; 223 } 224 if (!ECDSA_SIG_set0(sig, sig_r, sig_s)) { 225 ret = SSH_ERR_LIBCRYPTO_ERROR; 226 goto out; 227 } 228 sig_r = sig_s = NULL; /* transferred */ 229 230 /* Reconstruct data that was supposedly signed */ 231 if ((original_signed = sshbuf_new()) == NULL) { 232 ret = SSH_ERR_ALLOC_FAIL; 233 goto out; 234 } 235 if (is_webauthn) { 236 if ((ret = webauthn_check_prepare_hash(data, datalen, 237 webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts, 238 msghash, sizeof(msghash))) != 0) 239 goto out; 240 } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen, 241 msghash, sizeof(msghash))) != 0) 242 goto out; 243 /* Application value is hashed before signature */ 244 if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application, 245 strlen(key->sk_application), apphash, sizeof(apphash))) != 0) 246 goto out; 247 #ifdef DEBUG_SK 248 fprintf(stderr, "%s: hashed application:\n", __func__); 249 sshbuf_dump_data(apphash, sizeof(apphash), stderr); 250 fprintf(stderr, "%s: hashed message:\n", __func__); 251 sshbuf_dump_data(msghash, sizeof(msghash), stderr); 252 #endif 253 if ((ret = sshbuf_put(original_signed, 254 apphash, sizeof(apphash))) != 0 || 255 (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 || 256 (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 || 257 (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 || 258 (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0) 259 goto out; 260 /* Signature is over H(original_signed) */ 261 if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed, 262 sighash, sizeof(sighash))) != 0) 263 goto out; 264 details->sk_counter = sig_counter; 265 details->sk_flags = sig_flags; 266 #ifdef DEBUG_SK 267 fprintf(stderr, "%s: signed buf:\n", __func__); 268 sshbuf_dump(original_signed, stderr); 269 fprintf(stderr, "%s: signed hash:\n", __func__); 270 sshbuf_dump_data(sighash, sizeof(sighash), stderr); 271 #endif 272 273 /* Verify it */ 274 switch (ECDSA_do_verify(sighash, sizeof(sighash), sig, key->ecdsa)) { 275 case 1: 276 ret = 0; 277 break; 278 case 0: 279 ret = SSH_ERR_SIGNATURE_INVALID; 280 goto out; 281 default: 282 ret = SSH_ERR_LIBCRYPTO_ERROR; 283 goto out; 284 } 285 /* success */ 286 if (detailsp != NULL) { 287 *detailsp = details; 288 details = NULL; 289 } 290 out: 291 explicit_bzero(&sig_flags, sizeof(sig_flags)); 292 explicit_bzero(&sig_counter, sizeof(sig_counter)); 293 explicit_bzero(msghash, sizeof(msghash)); 294 explicit_bzero(sighash, sizeof(msghash)); 295 explicit_bzero(apphash, sizeof(apphash)); 296 sshkey_sig_details_free(details); 297 sshbuf_free(webauthn_wrapper); 298 sshbuf_free(webauthn_exts); 299 free(webauthn_origin); 300 sshbuf_free(original_signed); 301 sshbuf_free(sigbuf); 302 sshbuf_free(b); 303 ECDSA_SIG_free(sig); 304 BN_clear_free(sig_r); 305 BN_clear_free(sig_s); 306 free(ktype); 307 return ret; 308 } 309