1 /* $NetBSD: ssh-ecdsa-sk.c,v 1.5 2024/09/24 21:32:19 christos Exp $ */
2 /* $OpenBSD: ssh-ecdsa-sk.c,v 1.19 2024/08/15 00:51:51 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 1.5 2024/09/24 21:32:19 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 /* Reuse some ECDSA internals */
51 extern struct sshkey_impl_funcs sshkey_ecdsa_funcs;
52
53 static void
ssh_ecdsa_sk_cleanup(struct sshkey * k)54 ssh_ecdsa_sk_cleanup(struct sshkey *k)
55 {
56 sshkey_sk_cleanup(k);
57 sshkey_ecdsa_funcs.cleanup(k);
58 }
59
60 static int
ssh_ecdsa_sk_equal(const struct sshkey * a,const struct sshkey * b)61 ssh_ecdsa_sk_equal(const struct sshkey *a, const struct sshkey *b)
62 {
63 if (!sshkey_sk_fields_equal(a, b))
64 return 0;
65 if (!sshkey_ecdsa_funcs.equal(a, b))
66 return 0;
67 return 1;
68 }
69
70 static int
ssh_ecdsa_sk_serialize_public(const struct sshkey * key,struct sshbuf * b,enum sshkey_serialize_rep opts)71 ssh_ecdsa_sk_serialize_public(const struct sshkey *key, struct sshbuf *b,
72 enum sshkey_serialize_rep opts)
73 {
74 int r;
75
76 if ((r = sshkey_ecdsa_funcs.serialize_public(key, b, opts)) != 0)
77 return r;
78 if ((r = sshkey_serialize_sk(key, b)) != 0)
79 return r;
80
81 return 0;
82 }
83
84 static int
ssh_ecdsa_sk_serialize_private(const struct sshkey * key,struct sshbuf * b,enum sshkey_serialize_rep opts)85 ssh_ecdsa_sk_serialize_private(const struct sshkey *key, struct sshbuf *b,
86 enum sshkey_serialize_rep opts)
87 {
88 int r;
89
90 if (!sshkey_is_cert(key)) {
91 if ((r = sshkey_ecdsa_funcs.serialize_public(key,
92 b, opts)) != 0)
93 return r;
94 }
95 if ((r = sshkey_serialize_private_sk(key, b)) != 0)
96 return r;
97
98 return 0;
99 }
100
101 static int
ssh_ecdsa_sk_copy_public(const struct sshkey * from,struct sshkey * to)102 ssh_ecdsa_sk_copy_public(const struct sshkey *from, struct sshkey *to)
103 {
104 int r;
105
106 if ((r = sshkey_ecdsa_funcs.copy_public(from, to)) != 0)
107 return r;
108 if ((r = sshkey_copy_public_sk(from, to)) != 0)
109 return r;
110 return 0;
111 }
112
113 static int
ssh_ecdsa_sk_deserialize_public(const char * ktype,struct sshbuf * b,struct sshkey * key)114 ssh_ecdsa_sk_deserialize_public(const char *ktype, struct sshbuf *b,
115 struct sshkey *key)
116 {
117 int r;
118
119 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, b, key)) != 0)
120 return r;
121 if ((r = sshkey_deserialize_sk(b, key)) != 0)
122 return r;
123 return 0;
124 }
125
126 static int
ssh_ecdsa_sk_deserialize_private(const char * ktype,struct sshbuf * b,struct sshkey * key)127 ssh_ecdsa_sk_deserialize_private(const char *ktype, struct sshbuf *b,
128 struct sshkey *key)
129 {
130 int r;
131
132 if (!sshkey_is_cert(key)) {
133 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype,
134 b, key)) != 0)
135 return r;
136 }
137 if ((r = sshkey_private_deserialize_sk(b, key)) != 0)
138 return r;
139
140 return 0;
141 }
142
143 /*
144 * Check FIDO/W3C webauthn signatures clientData field against the expected
145 * format and prepare a hash of it for use in signature verification.
146 *
147 * webauthn signatures do not sign the hash of the message directly, but
148 * instead sign a JSON-like "clientData" wrapper structure that contains the
149 * message hash along with a other information.
150 *
151 * Fortunately this structure has a fixed format so it is possible to verify
152 * that the hash of the signed message is present within the clientData
153 * structure without needing to implement any JSON parsing.
154 */
155 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)156 webauthn_check_prepare_hash(const u_char *data, size_t datalen,
157 const char *origin, const struct sshbuf *wrapper,
158 uint8_t flags, const struct sshbuf *extensions,
159 u_char *msghash, size_t msghashlen)
160 {
161 int r = SSH_ERR_INTERNAL_ERROR;
162 struct sshbuf *chall = NULL, *m = NULL;
163
164 if ((m = sshbuf_new()) == NULL ||
165 (chall = sshbuf_from(data, datalen)) == NULL) {
166 r = SSH_ERR_ALLOC_FAIL;
167 goto out;
168 }
169 /*
170 * Ensure origin contains no quote character and that the flags are
171 * consistent with what we received
172 */
173 if (strchr(origin, '\"') != NULL ||
174 (flags & 0x40) != 0 /* AD */ ||
175 ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) {
176 r = SSH_ERR_INVALID_FORMAT;
177 goto out;
178 }
179
180 /*
181 * Prepare the preamble to clientData that we expect, poking the
182 * challenge and origin into their canonical positions in the
183 * structure. The crossOrigin flag and any additional extension
184 * fields present are ignored.
185 */
186 #define WEBAUTHN_0 "{\"type\":\"webauthn.get\",\"challenge\":\""
187 #define WEBAUTHN_1 "\",\"origin\":\""
188 #define WEBAUTHN_2 "\""
189 if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 ||
190 (r = sshbuf_dtourlb64(chall, m, 0)) != 0 ||
191 (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 ||
192 (r = sshbuf_put(m, origin, strlen(origin))) != 0 ||
193 (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0)
194 goto out;
195 #ifdef DEBUG_SK
196 fprintf(stderr, "%s: received origin: %s\n", __func__, origin);
197 fprintf(stderr, "%s: received clientData:\n", __func__);
198 sshbuf_dump(wrapper, stderr);
199 fprintf(stderr, "%s: expected clientData premable:\n", __func__);
200 sshbuf_dump(m, stderr);
201 #endif
202 /* Check that the supplied clientData has the preamble we expect */
203 if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0)
204 goto out;
205
206 /* Prepare hash of clientData */
207 if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper,
208 msghash, msghashlen)) != 0)
209 goto out;
210
211 /* success */
212 r = 0;
213 out:
214 sshbuf_free(chall);
215 sshbuf_free(m);
216 return r;
217 }
218
219 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)220 ssh_ecdsa_sk_verify(const struct sshkey *key,
221 const u_char *sig, size_t siglen,
222 const u_char *data, size_t dlen, const char *alg, u_int compat,
223 struct sshkey_sig_details **detailsp)
224 {
225 ECDSA_SIG *esig = NULL;
226 EVP_MD_CTX *md_ctx = NULL;
227 BIGNUM *sig_r = NULL, *sig_s = NULL;
228 u_char sig_flags;
229 u_char msghash[32], apphash[32];
230 u_int sig_counter;
231 u_char *sigb = NULL, *cp;
232 int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR, len = 0;
233 struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL;
234 struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL;
235 char *ktype = NULL, *webauthn_origin = NULL;
236 struct sshkey_sig_details *details = NULL;
237 #ifdef DEBUG_SK
238 char *tmp = NULL;
239 #endif
240
241 if (detailsp != NULL)
242 *detailsp = NULL;
243 if (key == NULL || key->pkey == NULL ||
244 sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
245 sig == NULL || siglen == 0)
246 return SSH_ERR_INVALID_ARGUMENT;
247
248 if (key->ecdsa_nid != NID_X9_62_prime256v1)
249 return SSH_ERR_INTERNAL_ERROR;
250
251 /* fetch signature */
252 if ((b = sshbuf_from(sig, siglen)) == NULL)
253 return SSH_ERR_ALLOC_FAIL;
254 if ((details = calloc(1, sizeof(*details))) == NULL) {
255 ret = SSH_ERR_ALLOC_FAIL;
256 goto out;
257 }
258 if (sshbuf_get_cstring(b, &ktype, NULL) != 0) {
259 ret = SSH_ERR_INVALID_FORMAT;
260 goto out;
261 }
262 if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0)
263 is_webauthn = 1;
264 else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) {
265 ret = SSH_ERR_INVALID_FORMAT;
266 goto out;
267 }
268 if (sshbuf_froms(b, &sigbuf) != 0 ||
269 sshbuf_get_u8(b, &sig_flags) != 0 ||
270 sshbuf_get_u32(b, &sig_counter) != 0) {
271 ret = SSH_ERR_INVALID_FORMAT;
272 goto out;
273 }
274 if (is_webauthn) {
275 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 ||
276 sshbuf_froms(b, &webauthn_wrapper) != 0 ||
277 sshbuf_froms(b, &webauthn_exts) != 0) {
278 ret = SSH_ERR_INVALID_FORMAT;
279 goto out;
280 }
281 }
282 if (sshbuf_len(b) != 0) {
283 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
284 goto out;
285 }
286
287 /* parse signature */
288 if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
289 sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
290 ret = SSH_ERR_INVALID_FORMAT;
291 goto out;
292 }
293 if (sshbuf_len(sigbuf) != 0) {
294 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
295 goto out;
296 }
297
298 #ifdef DEBUG_SK
299 fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen);
300 /* sshbuf_dump_data(data, datalen, stderr); */
301 fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r)));
302 free(tmp);
303 fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s)));
304 free(tmp);
305 fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
306 __func__, sig_flags, sig_counter);
307 if (is_webauthn) {
308 fprintf(stderr, "%s: webauthn origin: %s\n", __func__,
309 webauthn_origin);
310 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__);
311 sshbuf_dump(webauthn_wrapper, stderr);
312 }
313 #endif
314 if ((esig = ECDSA_SIG_new()) == NULL) {
315 ret = SSH_ERR_ALLOC_FAIL;
316 goto out;
317 }
318 if (!ECDSA_SIG_set0(esig, sig_r, sig_s)) {
319 ret = SSH_ERR_LIBCRYPTO_ERROR;
320 goto out;
321 }
322 sig_r = sig_s = NULL; /* transferred */
323
324 /* Reconstruct data that was supposedly signed */
325 if ((original_signed = sshbuf_new()) == NULL) {
326 ret = SSH_ERR_ALLOC_FAIL;
327 goto out;
328 }
329 if (is_webauthn) {
330 if ((ret = webauthn_check_prepare_hash(data, dlen,
331 webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts,
332 msghash, sizeof(msghash))) != 0)
333 goto out;
334 } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, dlen,
335 msghash, sizeof(msghash))) != 0)
336 goto out;
337 /* Application value is hashed before signature */
338 if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
339 strlen(key->sk_application), apphash, sizeof(apphash))) != 0)
340 goto out;
341 #ifdef DEBUG_SK
342 fprintf(stderr, "%s: hashed application:\n", __func__);
343 sshbuf_dump_data(apphash, sizeof(apphash), stderr);
344 fprintf(stderr, "%s: hashed message:\n", __func__);
345 sshbuf_dump_data(msghash, sizeof(msghash), stderr);
346 #endif
347 if ((ret = sshbuf_put(original_signed,
348 apphash, sizeof(apphash))) != 0 ||
349 (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 ||
350 (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 ||
351 (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 ||
352 (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0)
353 goto out;
354 details->sk_counter = sig_counter;
355 details->sk_flags = sig_flags;
356 #ifdef DEBUG_SK
357 fprintf(stderr, "%s: signed buf:\n", __func__);
358 sshbuf_dump(original_signed, stderr);
359 #endif
360
361 if ((md_ctx = EVP_MD_CTX_new()) == NULL) {
362 ret = SSH_ERR_ALLOC_FAIL;
363 goto out;
364 }
365 if ((len = i2d_ECDSA_SIG(esig, NULL)) <= 0) {
366 len = 0;
367 ret = SSH_ERR_LIBCRYPTO_ERROR;
368 goto out;
369 }
370 if ((sigb = calloc(1, len)) == NULL) {
371 ret = SSH_ERR_ALLOC_FAIL;
372 goto out;
373 }
374 cp = sigb; /* ASN1_item_i2d increments the pointer past the object */
375 if (i2d_ECDSA_SIG(esig, &cp) != len) {
376 ret = SSH_ERR_LIBCRYPTO_ERROR;
377 goto out;
378 }
379 #ifdef DEBUG_SK
380 fprintf(stderr, "%s: signed hash:\n", __func__);
381 sshbuf_dump_data(sigb, len, stderr);
382 #endif
383 /* Verify it */
384 if (EVP_DigestVerifyInit(md_ctx, NULL, EVP_sha256(), NULL,
385 key->pkey) != 1) {
386 ret = SSH_ERR_LIBCRYPTO_ERROR;
387 goto out;
388 }
389 switch (EVP_DigestVerify(md_ctx, sigb, len,
390 sshbuf_ptr(original_signed), sshbuf_len(original_signed))) {
391 case 1:
392 ret = 0;
393 break;
394 case 0:
395 ret = SSH_ERR_SIGNATURE_INVALID;
396 goto out;
397 default:
398 ret = SSH_ERR_LIBCRYPTO_ERROR;
399 goto out;
400 }
401 /* success */
402 if (detailsp != NULL) {
403 *detailsp = details;
404 details = NULL;
405 }
406 out:
407 explicit_bzero(&sig_flags, sizeof(sig_flags));
408 explicit_bzero(&sig_counter, sizeof(sig_counter));
409 explicit_bzero(msghash, sizeof(msghash));
410 explicit_bzero(apphash, sizeof(apphash));
411 sshkey_sig_details_free(details);
412 sshbuf_free(webauthn_wrapper);
413 sshbuf_free(webauthn_exts);
414 free(webauthn_origin);
415 sshbuf_free(original_signed);
416 sshbuf_free(sigbuf);
417 sshbuf_free(b);
418 ECDSA_SIG_free(esig);
419 BN_clear_free(sig_r);
420 BN_clear_free(sig_s);
421 free(ktype);
422 freezero(sigb, len);
423 EVP_MD_CTX_free(md_ctx);
424 return ret;
425 }
426
427 static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = {
428 /* .size = */ NULL,
429 /* .alloc = */ NULL,
430 /* .cleanup = */ ssh_ecdsa_sk_cleanup,
431 /* .equal = */ ssh_ecdsa_sk_equal,
432 /* .ssh_serialize_public = */ ssh_ecdsa_sk_serialize_public,
433 /* .ssh_deserialize_public = */ ssh_ecdsa_sk_deserialize_public,
434 /* .ssh_serialize_private = */ ssh_ecdsa_sk_serialize_private,
435 /* .ssh_deserialize_private = */ ssh_ecdsa_sk_deserialize_private,
436 /* .generate = */ NULL,
437 /* .copy_public = */ ssh_ecdsa_sk_copy_public,
438 /* .sign = */ NULL,
439 /* .verify = */ ssh_ecdsa_sk_verify,
440 };
441
442 const struct sshkey_impl sshkey_ecdsa_sk_impl = {
443 /* .name = */ "sk-ecdsa-sha2-nistp256@openssh.com",
444 /* .shortname = */ "ECDSA-SK",
445 /* .sigalg = */ NULL,
446 /* .type = */ KEY_ECDSA_SK,
447 /* .nid = */ NID_X9_62_prime256v1,
448 /* .cert = */ 0,
449 /* .sigonly = */ 0,
450 /* .keybits = */ 256,
451 /* .funcs = */ &sshkey_ecdsa_sk_funcs,
452 };
453
454 const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = {
455 /* .name = */ "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com",
456 /* .shortname = */ "ECDSA-SK-CERT",
457 /* .sigalg = */ NULL,
458 /* .type = */ KEY_ECDSA_SK_CERT,
459 /* .nid = */ NID_X9_62_prime256v1,
460 /* .cert = */ 1,
461 /* .sigonly = */ 0,
462 /* .keybits = */ 256,
463 /* .funcs = */ &sshkey_ecdsa_sk_funcs,
464 };
465
466 const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = {
467 /* .name = */ "webauthn-sk-ecdsa-sha2-nistp256@openssh.com",
468 /* .shortname = */ "ECDSA-SK",
469 /* .sigalg = */ NULL,
470 /* .type = */ KEY_ECDSA_SK,
471 /* .nid = */ NID_X9_62_prime256v1,
472 /* .cert = */ 0,
473 /* .sigonly = */ 1,
474 /* .keybits = */ 256,
475 /* .funcs = */ &sshkey_ecdsa_sk_funcs,
476 };
477