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 */
50 extern struct sshkey_impl_funcs sshkey_ecdsa_funcs;
51
52 static void
ssh_ecdsa_sk_cleanup(struct sshkey * k)53 ssh_ecdsa_sk_cleanup(struct sshkey *k)
54 {
55 sshkey_sk_cleanup(k);
56 sshkey_ecdsa_funcs.cleanup(k);
57 }
58
59 static int
ssh_ecdsa_sk_equal(const struct sshkey * a,const struct sshkey * b)60 ssh_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
69 static int
ssh_ecdsa_sk_serialize_public(const struct sshkey * key,struct sshbuf * b,enum sshkey_serialize_rep opts)70 ssh_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
83 static int
ssh_ecdsa_sk_serialize_private(const struct sshkey * key,struct sshbuf * b,enum sshkey_serialize_rep opts)84 ssh_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
100 static int
ssh_ecdsa_sk_copy_public(const struct sshkey * from,struct sshkey * to)101 ssh_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
112 static int
ssh_ecdsa_sk_deserialize_public(const char * ktype,struct sshbuf * b,struct sshkey * key)113 ssh_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
125 static int
ssh_ecdsa_sk_deserialize_private(const char * ktype,struct sshbuf * b,struct sshkey * key)126 ssh_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 */
154 static int
webauthn_check_prepare_hash(const u_char * data,size_t datalen,const char * origin,const struct sshbuf * wrapper,uint8_t flags,const struct sshbuf * extensions,u_char * msghash,size_t msghashlen)155 webauthn_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
218 static int
ssh_ecdsa_sk_verify(const struct sshkey * key,const u_char * sig,size_t siglen,const u_char * data,size_t dlen,const char * alg,u_int compat,struct sshkey_sig_details ** detailsp)219 ssh_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
401 static 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
416 const 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
428 const 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
440 const 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