Annotation of src/usr.bin/ssh/ssh-ecdsa-sk.c, Revision 1.8
1.8 ! djm 1: /* $OpenBSD: ssh-ecdsa-sk.c,v 1.7 2020/06/22 05:58:35 djm Exp $ */
1.1 djm 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:
1.7 djm 46: /*
47: * Check FIDO/W3C webauthn signatures clientData field against the expected
48: * format and prepare a hash of it for use in signature verification.
49: *
50: * webauthn signatures do not sign the hash of the message directly, but
51: * instead sign a JSON-like "clientData" wrapper structure that contains the
52: * message hash along with a other information.
53: *
54: * Fortunately this structure has a fixed format so it is possible to verify
55: * that the hash of the signed message is present within the clientData
56: * structure without needing to implement any JSON parsing.
57: */
58: static int
59: webauthn_check_prepare_hash(const u_char *data, size_t datalen,
60: const char *origin, const struct sshbuf *wrapper,
61: uint8_t flags, const struct sshbuf *extensions,
62: u_char *msghash, size_t msghashlen)
63: {
64: int r = SSH_ERR_INTERNAL_ERROR;
65: struct sshbuf *chall = NULL, *m = NULL;
66:
67: if ((m = sshbuf_new()) == NULL ||
68: (chall = sshbuf_from(data, datalen)) == NULL) {
69: r = SSH_ERR_ALLOC_FAIL;
70: goto out;
71: }
72: /*
73: * Ensure origin contains no quote character and that the flags are
74: * consistent with what we received
75: */
76: if (strchr(origin, '\"') != NULL ||
77: (flags & 0x40) != 0 /* AD */ ||
78: ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) {
79: r = SSH_ERR_INVALID_FORMAT;
80: goto out;
81: }
1.8 ! djm 82:
! 83: /*
! 84: * Prepare the preamble to clientData that we expect, poking the
! 85: * challenge and origin into their canonical positions in the
! 86: * structure. The crossOrigin flag and any additional extension
! 87: * fields present are ignored.
! 88: */
1.7 djm 89: #define WEBAUTHN_0 "{\"type\":\"webauthn.get\",\"challenge\":\""
90: #define WEBAUTHN_1 "\",\"origin\":\""
91: #define WEBAUTHN_2 "\""
92: if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 ||
93: (r = sshbuf_dtourlb64(chall, m, 0)) != 0 ||
94: (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 ||
95: (r = sshbuf_put(m, origin, strlen(origin))) != 0 ||
96: (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0)
97: goto out;
98: #ifdef DEBUG_SK
99: fprintf(stderr, "%s: received origin: %s\n", __func__, origin);
100: fprintf(stderr, "%s: received clientData:\n", __func__);
101: sshbuf_dump(wrapper, stderr);
102: fprintf(stderr, "%s: expected clientData premable:\n", __func__);
103: sshbuf_dump(m, stderr);
104: #endif
1.8 ! djm 105: /* Check that the supplied clientData has the preamble we expect */
1.7 djm 106: if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0)
107: goto out;
108:
109: /* Prepare hash of clientData */
110: if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper,
111: msghash, msghashlen)) != 0)
112: goto out;
113:
114: /* success */
115: r = 0;
116: out:
117: sshbuf_free(chall);
118: sshbuf_free(m);
119: return r;
120: }
121:
1.1 djm 122: /* ARGSUSED */
123: int
124: ssh_ecdsa_sk_verify(const struct sshkey *key,
125: const u_char *signature, size_t signaturelen,
1.4 djm 126: const u_char *data, size_t datalen, u_int compat,
127: struct sshkey_sig_details **detailsp)
1.1 djm 128: {
129: ECDSA_SIG *sig = NULL;
130: BIGNUM *sig_r = NULL, *sig_s = NULL;
131: u_char sig_flags;
132: u_char msghash[32], apphash[32], sighash[32];
133: u_int sig_counter;
1.7 djm 134: int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR;
1.1 djm 135: struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL;
1.7 djm 136: struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL;
137: char *ktype = NULL, *webauthn_origin = NULL;
1.4 djm 138: struct sshkey_sig_details *details = NULL;
1.1 djm 139: #ifdef DEBUG_SK
140: char *tmp = NULL;
141: #endif
142:
1.4 djm 143: if (detailsp != NULL)
144: *detailsp = NULL;
1.1 djm 145: if (key == NULL || key->ecdsa == NULL ||
146: sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
147: signature == NULL || signaturelen == 0)
148: return SSH_ERR_INVALID_ARGUMENT;
149:
150: if (key->ecdsa_nid != NID_X9_62_prime256v1)
151: return SSH_ERR_INTERNAL_ERROR;
152:
153: /* fetch signature */
154: if ((b = sshbuf_from(signature, signaturelen)) == NULL)
155: return SSH_ERR_ALLOC_FAIL;
1.6 djm 156: if ((details = calloc(1, sizeof(*details))) == NULL) {
157: ret = SSH_ERR_ALLOC_FAIL;
158: goto out;
159: }
160: if (sshbuf_get_cstring(b, &ktype, NULL) != 0) {
161: ret = SSH_ERR_INVALID_FORMAT;
162: goto out;
163: }
1.7 djm 164: if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0)
165: is_webauthn = 1;
166: else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) {
1.6 djm 167: ret = SSH_ERR_INVALID_FORMAT;
168: goto out;
169: }
170: if (sshbuf_froms(b, &sigbuf) != 0 ||
1.2 djm 171: sshbuf_get_u8(b, &sig_flags) != 0 ||
172: sshbuf_get_u32(b, &sig_counter) != 0) {
1.1 djm 173: ret = SSH_ERR_INVALID_FORMAT;
174: goto out;
175: }
1.7 djm 176: if (is_webauthn) {
177: if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 ||
178: sshbuf_froms(b, &webauthn_wrapper) != 0 ||
179: sshbuf_froms(b, &webauthn_exts) != 0) {
180: ret = SSH_ERR_INVALID_FORMAT;
181: goto out;
182: }
183: }
1.1 djm 184: if (sshbuf_len(b) != 0) {
185: ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
186: goto out;
187: }
188:
189: /* parse signature */
190: if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
1.2 djm 191: sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
1.1 djm 192: ret = SSH_ERR_INVALID_FORMAT;
193: goto out;
194: }
1.6 djm 195: if (sshbuf_len(sigbuf) != 0) {
196: ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
1.1 djm 197: goto out;
198: }
1.7 djm 199:
1.1 djm 200: #ifdef DEBUG_SK
1.5 djm 201: fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen);
202: /* sshbuf_dump_data(data, datalen, stderr); */
1.1 djm 203: fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r)));
204: free(tmp);
205: fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s)));
206: free(tmp);
207: fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
208: __func__, sig_flags, sig_counter);
1.7 djm 209: if (is_webauthn) {
210: fprintf(stderr, "%s: webauthn origin: %s\n", __func__,
211: webauthn_origin);
212: fprintf(stderr, "%s: webauthn_wrapper:\n", __func__);
213: sshbuf_dump(webauthn_wrapper, stderr);
214: }
1.1 djm 215: #endif
1.6 djm 216: if ((sig = ECDSA_SIG_new()) == NULL) {
217: ret = SSH_ERR_ALLOC_FAIL;
218: goto out;
219: }
220: if (!ECDSA_SIG_set0(sig, sig_r, sig_s)) {
221: ret = SSH_ERR_LIBCRYPTO_ERROR;
1.1 djm 222: goto out;
223: }
1.6 djm 224: sig_r = sig_s = NULL; /* transferred */
1.1 djm 225:
226: /* Reconstruct data that was supposedly signed */
1.3 djm 227: if ((original_signed = sshbuf_new()) == NULL) {
228: ret = SSH_ERR_ALLOC_FAIL;
229: goto out;
230: }
1.7 djm 231: if (is_webauthn) {
232: if ((ret = webauthn_check_prepare_hash(data, datalen,
233: webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts,
234: msghash, sizeof(msghash))) != 0)
235: goto out;
236: } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen,
1.1 djm 237: msghash, sizeof(msghash))) != 0)
238: goto out;
239: /* Application value is hashed before signature */
240: if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
241: strlen(key->sk_application), apphash, sizeof(apphash))) != 0)
242: goto out;
243: #ifdef DEBUG_SK
1.5 djm 244: fprintf(stderr, "%s: hashed application:\n", __func__);
245: sshbuf_dump_data(apphash, sizeof(apphash), stderr);
1.1 djm 246: fprintf(stderr, "%s: hashed message:\n", __func__);
247: sshbuf_dump_data(msghash, sizeof(msghash), stderr);
248: #endif
249: if ((ret = sshbuf_put(original_signed,
250: apphash, sizeof(apphash))) != 0 ||
251: (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 ||
252: (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 ||
1.7 djm 253: (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 ||
1.1 djm 254: (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0)
255: goto out;
256: /* Signature is over H(original_signed) */
257: if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed,
258: sighash, sizeof(sighash))) != 0)
259: goto out;
1.4 djm 260: details->sk_counter = sig_counter;
261: details->sk_flags = sig_flags;
1.1 djm 262: #ifdef DEBUG_SK
263: fprintf(stderr, "%s: signed buf:\n", __func__);
264: sshbuf_dump(original_signed, stderr);
265: fprintf(stderr, "%s: signed hash:\n", __func__);
266: sshbuf_dump_data(sighash, sizeof(sighash), stderr);
267: #endif
268:
269: /* Verify it */
270: switch (ECDSA_do_verify(sighash, sizeof(sighash), sig, key->ecdsa)) {
271: case 1:
272: ret = 0;
273: break;
274: case 0:
275: ret = SSH_ERR_SIGNATURE_INVALID;
276: goto out;
277: default:
278: ret = SSH_ERR_LIBCRYPTO_ERROR;
279: goto out;
280: }
1.4 djm 281: /* success */
282: if (detailsp != NULL) {
283: *detailsp = details;
284: details = NULL;
285: }
1.1 djm 286: out:
287: explicit_bzero(&sig_flags, sizeof(sig_flags));
288: explicit_bzero(&sig_counter, sizeof(sig_counter));
289: explicit_bzero(msghash, sizeof(msghash));
290: explicit_bzero(sighash, sizeof(msghash));
291: explicit_bzero(apphash, sizeof(apphash));
1.4 djm 292: sshkey_sig_details_free(details);
1.7 djm 293: sshbuf_free(webauthn_wrapper);
294: sshbuf_free(webauthn_exts);
295: free(webauthn_origin);
1.1 djm 296: sshbuf_free(original_signed);
297: sshbuf_free(sigbuf);
298: sshbuf_free(b);
299: ECDSA_SIG_free(sig);
300: BN_clear_free(sig_r);
301: BN_clear_free(sig_s);
302: free(ktype);
303: return ret;
304: }
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