Annotation of src/usr.bin/ssh/ssh-ecdsa-sk.c, Revision 1.10
1.10 ! djm 1: /* $OpenBSD: ssh-ecdsa-sk.c,v 1.9 2022/10/28 00:35:40 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:
1.10 ! djm 46: /* Reuse some ECDSA internals */
! 47: extern struct sshkey_impl_funcs sshkey_ecdsa_funcs;
! 48:
1.9 djm 49: static void
50: ssh_ecdsa_sk_cleanup(struct sshkey *k)
51: {
1.10 ! djm 52: sshkey_sk_cleanup(k);
! 53: sshkey_ecdsa_funcs.cleanup(k);
! 54: }
! 55:
! 56: static int
! 57: ssh_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;
1.9 djm 64: }
65:
1.7 djm 66: /*
67: * Check FIDO/W3C webauthn signatures clientData field against the expected
68: * format and prepare a hash of it for use in signature verification.
69: *
70: * webauthn signatures do not sign the hash of the message directly, but
71: * instead sign a JSON-like "clientData" wrapper structure that contains the
72: * message hash along with a other information.
73: *
74: * Fortunately this structure has a fixed format so it is possible to verify
75: * that the hash of the signed message is present within the clientData
76: * structure without needing to implement any JSON parsing.
77: */
78: static int
79: webauthn_check_prepare_hash(const u_char *data, size_t datalen,
80: const char *origin, const struct sshbuf *wrapper,
81: uint8_t flags, const struct sshbuf *extensions,
82: u_char *msghash, size_t msghashlen)
83: {
84: int r = SSH_ERR_INTERNAL_ERROR;
85: struct sshbuf *chall = NULL, *m = NULL;
86:
87: if ((m = sshbuf_new()) == NULL ||
88: (chall = sshbuf_from(data, datalen)) == NULL) {
89: r = SSH_ERR_ALLOC_FAIL;
90: goto out;
91: }
92: /*
93: * Ensure origin contains no quote character and that the flags are
94: * consistent with what we received
95: */
96: if (strchr(origin, '\"') != NULL ||
97: (flags & 0x40) != 0 /* AD */ ||
98: ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) {
99: r = SSH_ERR_INVALID_FORMAT;
100: goto out;
101: }
1.8 djm 102:
103: /*
104: * Prepare the preamble to clientData that we expect, poking the
105: * challenge and origin into their canonical positions in the
106: * structure. The crossOrigin flag and any additional extension
107: * fields present are ignored.
108: */
1.7 djm 109: #define WEBAUTHN_0 "{\"type\":\"webauthn.get\",\"challenge\":\""
110: #define WEBAUTHN_1 "\",\"origin\":\""
111: #define WEBAUTHN_2 "\""
112: if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 ||
113: (r = sshbuf_dtourlb64(chall, m, 0)) != 0 ||
114: (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 ||
115: (r = sshbuf_put(m, origin, strlen(origin))) != 0 ||
116: (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0)
117: goto out;
118: #ifdef DEBUG_SK
119: fprintf(stderr, "%s: received origin: %s\n", __func__, origin);
120: fprintf(stderr, "%s: received clientData:\n", __func__);
121: sshbuf_dump(wrapper, stderr);
122: fprintf(stderr, "%s: expected clientData premable:\n", __func__);
123: sshbuf_dump(m, stderr);
124: #endif
1.8 djm 125: /* Check that the supplied clientData has the preamble we expect */
1.7 djm 126: if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0)
127: goto out;
128:
129: /* Prepare hash of clientData */
130: if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper,
131: msghash, msghashlen)) != 0)
132: goto out;
133:
134: /* success */
135: r = 0;
136: out:
137: sshbuf_free(chall);
138: sshbuf_free(m);
139: return r;
140: }
141:
1.1 djm 142: /* ARGSUSED */
143: int
144: ssh_ecdsa_sk_verify(const struct sshkey *key,
145: const u_char *signature, size_t signaturelen,
1.4 djm 146: const u_char *data, size_t datalen, u_int compat,
147: struct sshkey_sig_details **detailsp)
1.1 djm 148: {
149: ECDSA_SIG *sig = NULL;
150: BIGNUM *sig_r = NULL, *sig_s = NULL;
151: u_char sig_flags;
152: u_char msghash[32], apphash[32], sighash[32];
153: u_int sig_counter;
1.7 djm 154: int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR;
1.1 djm 155: struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL;
1.7 djm 156: struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL;
157: char *ktype = NULL, *webauthn_origin = NULL;
1.4 djm 158: struct sshkey_sig_details *details = NULL;
1.1 djm 159: #ifdef DEBUG_SK
160: char *tmp = NULL;
161: #endif
162:
1.4 djm 163: if (detailsp != NULL)
164: *detailsp = NULL;
1.1 djm 165: if (key == NULL || key->ecdsa == NULL ||
166: sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
167: signature == NULL || signaturelen == 0)
168: return SSH_ERR_INVALID_ARGUMENT;
169:
170: if (key->ecdsa_nid != NID_X9_62_prime256v1)
171: return SSH_ERR_INTERNAL_ERROR;
172:
173: /* fetch signature */
174: if ((b = sshbuf_from(signature, signaturelen)) == NULL)
175: return SSH_ERR_ALLOC_FAIL;
1.6 djm 176: if ((details = calloc(1, sizeof(*details))) == NULL) {
177: ret = SSH_ERR_ALLOC_FAIL;
178: goto out;
179: }
180: if (sshbuf_get_cstring(b, &ktype, NULL) != 0) {
181: ret = SSH_ERR_INVALID_FORMAT;
182: goto out;
183: }
1.7 djm 184: if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0)
185: is_webauthn = 1;
186: else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) {
1.6 djm 187: ret = SSH_ERR_INVALID_FORMAT;
188: goto out;
189: }
190: if (sshbuf_froms(b, &sigbuf) != 0 ||
1.2 djm 191: sshbuf_get_u8(b, &sig_flags) != 0 ||
192: sshbuf_get_u32(b, &sig_counter) != 0) {
1.1 djm 193: ret = SSH_ERR_INVALID_FORMAT;
194: goto out;
195: }
1.7 djm 196: if (is_webauthn) {
197: if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 ||
198: sshbuf_froms(b, &webauthn_wrapper) != 0 ||
199: sshbuf_froms(b, &webauthn_exts) != 0) {
200: ret = SSH_ERR_INVALID_FORMAT;
201: goto out;
202: }
203: }
1.1 djm 204: if (sshbuf_len(b) != 0) {
205: ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
206: goto out;
207: }
208:
209: /* parse signature */
210: if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
1.2 djm 211: sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
1.1 djm 212: ret = SSH_ERR_INVALID_FORMAT;
213: goto out;
214: }
1.6 djm 215: if (sshbuf_len(sigbuf) != 0) {
216: ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
1.1 djm 217: goto out;
218: }
1.7 djm 219:
1.1 djm 220: #ifdef DEBUG_SK
1.5 djm 221: fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen);
222: /* sshbuf_dump_data(data, datalen, stderr); */
1.1 djm 223: fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r)));
224: free(tmp);
225: fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s)));
226: free(tmp);
227: fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
228: __func__, sig_flags, sig_counter);
1.7 djm 229: if (is_webauthn) {
230: fprintf(stderr, "%s: webauthn origin: %s\n", __func__,
231: webauthn_origin);
232: fprintf(stderr, "%s: webauthn_wrapper:\n", __func__);
233: sshbuf_dump(webauthn_wrapper, stderr);
234: }
1.1 djm 235: #endif
1.6 djm 236: if ((sig = ECDSA_SIG_new()) == NULL) {
237: ret = SSH_ERR_ALLOC_FAIL;
238: goto out;
239: }
240: if (!ECDSA_SIG_set0(sig, sig_r, sig_s)) {
241: ret = SSH_ERR_LIBCRYPTO_ERROR;
1.1 djm 242: goto out;
243: }
1.6 djm 244: sig_r = sig_s = NULL; /* transferred */
1.1 djm 245:
246: /* Reconstruct data that was supposedly signed */
1.3 djm 247: if ((original_signed = sshbuf_new()) == NULL) {
248: ret = SSH_ERR_ALLOC_FAIL;
249: goto out;
250: }
1.7 djm 251: if (is_webauthn) {
252: if ((ret = webauthn_check_prepare_hash(data, datalen,
253: webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts,
254: msghash, sizeof(msghash))) != 0)
255: goto out;
256: } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen,
1.1 djm 257: msghash, sizeof(msghash))) != 0)
258: goto out;
259: /* Application value is hashed before signature */
260: if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
261: strlen(key->sk_application), apphash, sizeof(apphash))) != 0)
262: goto out;
263: #ifdef DEBUG_SK
1.5 djm 264: fprintf(stderr, "%s: hashed application:\n", __func__);
265: sshbuf_dump_data(apphash, sizeof(apphash), stderr);
1.1 djm 266: fprintf(stderr, "%s: hashed message:\n", __func__);
267: sshbuf_dump_data(msghash, sizeof(msghash), stderr);
268: #endif
269: if ((ret = sshbuf_put(original_signed,
270: apphash, sizeof(apphash))) != 0 ||
271: (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 ||
272: (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 ||
1.7 djm 273: (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 ||
1.1 djm 274: (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0)
275: goto out;
276: /* Signature is over H(original_signed) */
277: if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed,
278: sighash, sizeof(sighash))) != 0)
279: goto out;
1.4 djm 280: details->sk_counter = sig_counter;
281: details->sk_flags = sig_flags;
1.1 djm 282: #ifdef DEBUG_SK
283: fprintf(stderr, "%s: signed buf:\n", __func__);
284: sshbuf_dump(original_signed, stderr);
285: fprintf(stderr, "%s: signed hash:\n", __func__);
286: sshbuf_dump_data(sighash, sizeof(sighash), stderr);
287: #endif
288:
289: /* Verify it */
290: switch (ECDSA_do_verify(sighash, sizeof(sighash), sig, key->ecdsa)) {
291: case 1:
292: ret = 0;
293: break;
294: case 0:
295: ret = SSH_ERR_SIGNATURE_INVALID;
296: goto out;
297: default:
298: ret = SSH_ERR_LIBCRYPTO_ERROR;
299: goto out;
300: }
1.4 djm 301: /* success */
302: if (detailsp != NULL) {
303: *detailsp = details;
304: details = NULL;
305: }
1.1 djm 306: out:
307: explicit_bzero(&sig_flags, sizeof(sig_flags));
308: explicit_bzero(&sig_counter, sizeof(sig_counter));
309: explicit_bzero(msghash, sizeof(msghash));
310: explicit_bzero(sighash, sizeof(msghash));
311: explicit_bzero(apphash, sizeof(apphash));
1.4 djm 312: sshkey_sig_details_free(details);
1.7 djm 313: sshbuf_free(webauthn_wrapper);
314: sshbuf_free(webauthn_exts);
315: free(webauthn_origin);
1.1 djm 316: sshbuf_free(original_signed);
317: sshbuf_free(sigbuf);
318: sshbuf_free(b);
319: ECDSA_SIG_free(sig);
320: BN_clear_free(sig_r);
321: BN_clear_free(sig_s);
322: free(ktype);
323: return ret;
324: }
1.9 djm 325:
326: static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = {
327: /* .size = */ NULL,
328: /* .alloc = */ NULL,
329: /* .cleanup = */ ssh_ecdsa_sk_cleanup,
1.10 ! djm 330: /* .equal = */ ssh_ecdsa_sk_equal,
1.9 djm 331: };
332:
333: const struct sshkey_impl sshkey_ecdsa_sk_impl = {
334: /* .name = */ "sk-ecdsa-sha2-nistp256@openssh.com",
335: /* .shortname = */ "ECDSA-SK",
336: /* .sigalg = */ NULL,
337: /* .type = */ KEY_ECDSA_SK,
338: /* .nid = */ NID_X9_62_prime256v1,
339: /* .cert = */ 0,
340: /* .sigonly = */ 0,
341: /* .keybits = */ 256,
342: /* .funcs = */ &sshkey_ecdsa_sk_funcs,
343: };
344:
345: const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = {
346: /* .name = */ "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com",
347: /* .shortname = */ "ECDSA-SK-CERT",
348: /* .sigalg = */ NULL,
349: /* .type = */ KEY_ECDSA_SK_CERT,
350: /* .nid = */ NID_X9_62_prime256v1,
351: /* .cert = */ 1,
352: /* .sigonly = */ 0,
353: /* .keybits = */ 256,
354: /* .funcs = */ &sshkey_ecdsa_sk_funcs,
355: };
356:
357: const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = {
358: /* .name = */ "webauthn-sk-ecdsa-sha2-nistp256@openssh.com",
359: /* .shortname = */ "ECDSA-SK",
360: /* .sigalg = */ NULL,
361: /* .type = */ KEY_ECDSA_SK,
362: /* .nid = */ NID_X9_62_prime256v1,
363: /* .cert = */ 0,
364: /* .sigonly = */ 1,
365: /* .keybits = */ 256,
366: /* .funcs = */ &sshkey_ecdsa_sk_funcs,
367: };
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