1 /* $OpenBSD: ssh-verify-attestation.c,v 1.2 2024/12/06 10:37:42 djm Exp $ */
2 /*
3 * Copyright (c) 2022-2024 Damien Miller
4 *
5 * Permission to use, copy, modify, and distribute this software for any
6 * purpose with or without fee is hereby granted, provided that the above
7 * copyright notice and this permission notice appear in all copies.
8 *
9 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
10 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
11 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
12 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
13 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
14 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
15 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
16 */
17
18 /*
19 * This is a small program to verify FIDO attestation objects that
20 * ssh-keygen(1) can record when enrolling a FIDO key. It requires that
21 * the attestation object and challenge used when creating the key be
22 * recorded.
23 *
24 * Example usage:
25 *
26 * $ # Generate a random challenge.
27 * $ dd if=/dev/urandom of=key_ecdsa_sk.challenge bs=32 count=1
28 * $ # Generate a key, record the attestation blob.
29 * $ ssh-keygen -f key_ecdsa_sk -t ecdsa-sk \
30 * -Ochallenge=key_ecdsa_sk.challenge \
31 * -Owrite-attestation=key_ecdsa_sk.attest -N ''
32 * $ # Validate the challenge (-A = print attestation CA cert)
33 * $ ./obj/ssh-verify-attestation -A key_ecdsa_sk key_ecdsa_sk.challenge \
34 * key_ecdsa_sk.attest
35 *
36 * Limitations/TODO:
37 *
38 * 1) It doesn't automatically detect the attestation statement format. It
39 * assumes the "packed" format used by FIDO2 keys. If that doesn't work,
40 * then try using the -U option to select the "fido-u2f" format.
41 * 2) It makes assumptions about RK, UV, etc status of the key/cred.
42 * 3) Probably bugs.
43 *
44 * Thanks to Markus Friedl and Pedro Martelletto for help getting this
45 * working.
46 */
47
48 #include "includes.h"
49
50 #include <stdint.h>
51 #include <inttypes.h>
52 #include <stdlib.h>
53 #include <stdio.h>
54 #include <unistd.h>
55 #include <stdarg.h>
56
57 #include "xmalloc.h"
58 #include "log.h"
59 #include "sshbuf.h"
60 #include "sshkey.h"
61 #include "authfile.h"
62 #include "ssherr.h"
63 #include "misc.h"
64 #include "digest.h"
65 #include "crypto_api.h"
66
67 #include <fido.h>
68 #include <openssl/x509.h>
69 #include <openssl/x509v3.h>
70 #include <openssl/bio.h>
71 #include <openssl/err.h>
72 #include <openssl/pem.h>
73
74 extern char *__progname;
75
76 #define ATTEST_MAGIC "ssh-sk-attest-v01"
77
78 static int
prepare_fido_cred(fido_cred_t * cred,int credtype,const char * attfmt,const char * rp_id,struct sshbuf * b,const struct sshbuf * challenge,struct sshbuf ** attestation_certp)79 prepare_fido_cred(fido_cred_t *cred, int credtype, const char *attfmt,
80 const char *rp_id, struct sshbuf *b, const struct sshbuf *challenge,
81 struct sshbuf **attestation_certp)
82 {
83 struct sshbuf *attestation_cert = NULL, *sig = NULL, *authdata = NULL;
84 char *magic = NULL;
85 int r = SSH_ERR_INTERNAL_ERROR;
86
87 *attestation_certp = NULL;
88
89 /* Make sure it's the format we're expecting */
90 if ((r = sshbuf_get_cstring(b, &magic, NULL)) != 0) {
91 error_fr(r, "parse header");
92 goto out;
93 }
94 if (strcmp(magic, ATTEST_MAGIC) != 0) {
95 error_f("unsupported format");
96 r = SSH_ERR_INVALID_FORMAT;
97 goto out;
98 }
99 /* Parse the remaining fields */
100 if ((r = sshbuf_froms(b, &attestation_cert)) != 0 ||
101 (r = sshbuf_froms(b, &sig)) != 0 ||
102 (r = sshbuf_froms(b, &authdata)) != 0 ||
103 (r = sshbuf_get_u32(b, NULL)) != 0 || /* reserved flags */
104 (r = sshbuf_get_string_direct(b, NULL, NULL)) != 0) { /* reserved */
105 error_fr(r, "parse body");
106 goto out;
107 }
108 debug3_f("attestation cert len=%zu, sig len=%zu, "
109 "authdata len=%zu challenge len=%zu", sshbuf_len(attestation_cert),
110 sshbuf_len(sig), sshbuf_len(authdata), sshbuf_len(challenge));
111
112 fido_cred_set_type(cred, credtype);
113 fido_cred_set_fmt(cred, attfmt);
114 fido_cred_set_clientdata(cred, sshbuf_ptr(challenge),
115 sshbuf_len(challenge));
116 fido_cred_set_rp(cred, rp_id, NULL);
117 fido_cred_set_authdata(cred, sshbuf_ptr(authdata),
118 sshbuf_len(authdata));
119 /* XXX set_extensions, set_rk, set_uv */
120 fido_cred_set_x509(cred, sshbuf_ptr(attestation_cert),
121 sshbuf_len(attestation_cert));
122 fido_cred_set_sig(cred, sshbuf_ptr(sig), sshbuf_len(sig));
123
124 /* success */
125 *attestation_certp = attestation_cert;
126 attestation_cert = NULL;
127 r = 0;
128 out:
129 free(magic);
130 sshbuf_free(attestation_cert);
131 sshbuf_free(sig);
132 sshbuf_free(authdata);
133 return r;
134 }
135
136 static uint8_t *
get_pubkey_from_cred_ecdsa(const fido_cred_t * cred,size_t * pubkey_len)137 get_pubkey_from_cred_ecdsa(const fido_cred_t *cred, size_t *pubkey_len)
138 {
139 const uint8_t *ptr;
140 uint8_t *pubkey = NULL, *ret = NULL;
141 BIGNUM *x = NULL, *y = NULL;
142 EC_POINT *q = NULL;
143 EC_GROUP *g = NULL;
144
145 if ((x = BN_new()) == NULL ||
146 (y = BN_new()) == NULL ||
147 (g = EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1)) == NULL ||
148 (q = EC_POINT_new(g)) == NULL) {
149 error_f("libcrypto setup failed");
150 goto out;
151 }
152 if ((ptr = fido_cred_pubkey_ptr(cred)) == NULL) {
153 error_f("fido_cred_pubkey_ptr failed");
154 goto out;
155 }
156 if (fido_cred_pubkey_len(cred) != 64) {
157 error_f("bad fido_cred_pubkey_len %zu",
158 fido_cred_pubkey_len(cred));
159 goto out;
160 }
161
162 if (BN_bin2bn(ptr, 32, x) == NULL ||
163 BN_bin2bn(ptr + 32, 32, y) == NULL) {
164 error_f("BN_bin2bn failed");
165 goto out;
166 }
167 if (EC_POINT_set_affine_coordinates_GFp(g, q, x, y, NULL) != 1) {
168 error_f("EC_POINT_set_affine_coordinates_GFp failed");
169 goto out;
170 }
171 *pubkey_len = EC_POINT_point2oct(g, q,
172 POINT_CONVERSION_UNCOMPRESSED, NULL, 0, NULL);
173 if (*pubkey_len == 0 || *pubkey_len > 2048) {
174 error_f("bad pubkey length %zu", *pubkey_len);
175 goto out;
176 }
177 if ((pubkey = malloc(*pubkey_len)) == NULL) {
178 error_f("malloc pubkey failed");
179 goto out;
180 }
181 if (EC_POINT_point2oct(g, q, POINT_CONVERSION_UNCOMPRESSED,
182 pubkey, *pubkey_len, NULL) == 0) {
183 error_f("EC_POINT_point2oct failed");
184 goto out;
185 }
186 /* success */
187 ret = pubkey;
188 pubkey = NULL;
189 out:
190 free(pubkey);
191 EC_POINT_free(q);
192 EC_GROUP_free(g);
193 BN_clear_free(x);
194 BN_clear_free(y);
195 return ret;
196 }
197
198 /* copied from sshsk_ecdsa_assemble() */
199 static int
cred_matches_key_ecdsa(const fido_cred_t * cred,const struct sshkey * k)200 cred_matches_key_ecdsa(const fido_cred_t *cred, const struct sshkey *k)
201 {
202 struct sshkey *key = NULL;
203 struct sshbuf *b = NULL;
204 EC_KEY *ec = NULL;
205 uint8_t *pubkey = NULL;
206 size_t pubkey_len;
207 int r;
208
209 if ((key = sshkey_new(KEY_ECDSA_SK)) == NULL) {
210 error_f("sshkey_new failed");
211 r = SSH_ERR_ALLOC_FAIL;
212 goto out;
213 }
214 key->ecdsa_nid = NID_X9_62_prime256v1;
215 if ((key->pkey = EVP_PKEY_new()) == NULL ||
216 (ec = EC_KEY_new_by_curve_name(key->ecdsa_nid)) == NULL ||
217 (b = sshbuf_new()) == NULL) {
218 error_f("allocation failed");
219 r = SSH_ERR_ALLOC_FAIL;
220 goto out;
221 }
222 if ((pubkey = get_pubkey_from_cred_ecdsa(cred, &pubkey_len)) == NULL) {
223 error_f("get_pubkey_from_cred_ecdsa failed");
224 r = SSH_ERR_INVALID_FORMAT;
225 goto out;
226 }
227 if ((r = sshbuf_put_string(b, pubkey, pubkey_len)) != 0) {
228 error_fr(r, "sshbuf_put_string");
229 goto out;
230 }
231 if ((r = sshbuf_get_eckey(b, ec)) != 0) {
232 error_fr(r, "parse");
233 r = SSH_ERR_INVALID_FORMAT;
234 goto out;
235 }
236 if (sshkey_ec_validate_public(EC_KEY_get0_group(ec),
237 EC_KEY_get0_public_key(ec)) != 0) {
238 error("Authenticator returned invalid ECDSA key");
239 r = SSH_ERR_KEY_INVALID_EC_VALUE;
240 goto out;
241 }
242 if (EVP_PKEY_set1_EC_KEY(key->pkey, ec) != 1) {
243 /* XXX assume it is a allocation error */
244 error_f("allocation failed");
245 r = SSH_ERR_ALLOC_FAIL;
246 goto out;
247 }
248 key->sk_application = xstrdup(k->sk_application); /* XXX */
249 if (!sshkey_equal_public(key, k)) {
250 error("sshkey_equal_public failed");
251 r = SSH_ERR_INVALID_ARGUMENT;
252 goto out;
253 }
254 r = 0; /* success */
255 out:
256 EC_KEY_free(ec);
257 free(pubkey);
258 sshkey_free(key);
259 sshbuf_free(b);
260 return r;
261 }
262
263
264 /* copied from sshsk_ed25519_assemble() */
265 static int
cred_matches_key_ed25519(const fido_cred_t * cred,const struct sshkey * k)266 cred_matches_key_ed25519(const fido_cred_t *cred, const struct sshkey *k)
267 {
268 struct sshkey *key = NULL;
269 const uint8_t *ptr;
270 int r = -1;
271
272 if ((ptr = fido_cred_pubkey_ptr(cred)) == NULL) {
273 error_f("fido_cred_pubkey_ptr failed");
274 goto out;
275 }
276 if (fido_cred_pubkey_len(cred) != ED25519_PK_SZ) {
277 error_f("bad fido_cred_pubkey_len %zu",
278 fido_cred_pubkey_len(cred));
279 goto out;
280 }
281
282 if ((key = sshkey_new(KEY_ED25519_SK)) == NULL) {
283 error_f("sshkey_new failed");
284 r = SSH_ERR_ALLOC_FAIL;
285 goto out;
286 }
287 if ((key->ed25519_pk = malloc(ED25519_PK_SZ)) == NULL) {
288 error_f("malloc failed");
289 r = SSH_ERR_ALLOC_FAIL;
290 goto out;
291 }
292 memcpy(key->ed25519_pk, ptr, ED25519_PK_SZ);
293 key->sk_application = xstrdup(k->sk_application); /* XXX */
294 if (!sshkey_equal_public(key, k)) {
295 error("sshkey_equal_public failed");
296 r = SSH_ERR_INVALID_ARGUMENT;
297 goto out;
298 }
299 r = 0; /* success */
300 out:
301 sshkey_free(key);
302 return r;
303 }
304
305 static int
cred_matches_key(const fido_cred_t * cred,const struct sshkey * k)306 cred_matches_key(const fido_cred_t *cred, const struct sshkey *k)
307 {
308 switch (sshkey_type_plain(k->type)) {
309 case KEY_ECDSA_SK:
310 switch (k->ecdsa_nid) {
311 case NID_X9_62_prime256v1:
312 return cred_matches_key_ecdsa(cred, k);
313 break;
314 default:
315 fatal("Unsupported ECDSA key size");
316 }
317 break;
318 case KEY_ED25519_SK:
319 return cred_matches_key_ed25519(cred, k);
320 default:
321 error_f("key type %s not supported", sshkey_type(k));
322 return -1;
323 }
324 }
325
326 int
main(int argc,char ** argv)327 main(int argc, char **argv)
328 {
329 LogLevel log_level = SYSLOG_LEVEL_INFO;
330 int r, ch, credtype = -1;
331 struct sshkey *k = NULL;
332 struct sshbuf *attestation = NULL, *challenge = NULL;
333 struct sshbuf *attestation_cert = NULL;
334 char *fp;
335 const char *attfmt = "packed", *style = NULL;
336 fido_cred_t *cred = NULL;
337 int write_attestation_cert = 0;
338 extern int optind;
339 /* extern char *optarg; */
340
341 ERR_load_crypto_strings();
342
343 sanitise_stdfd();
344 log_init(__progname, log_level, SYSLOG_FACILITY_AUTH, 1);
345
346 while ((ch = getopt(argc, argv, "UAv")) != -1) {
347 switch (ch) {
348 case 'U':
349 attfmt = "fido-u2f";
350 break;
351 case 'A':
352 write_attestation_cert = 1;
353 break;
354 case 'v':
355 if (log_level == SYSLOG_LEVEL_ERROR)
356 log_level = SYSLOG_LEVEL_DEBUG1;
357 else if (log_level < SYSLOG_LEVEL_DEBUG3)
358 log_level++;
359 break;
360 default:
361 goto usage;
362 }
363 }
364 log_init(__progname, log_level, SYSLOG_FACILITY_AUTH, 1);
365 argv += optind;
366 argc -= optind;
367
368 if (argc < 3) {
369 usage:
370 fprintf(stderr, "usage: %s [-vAU] "
371 "pubkey challenge attestation-blob\n", __progname);
372 exit(1);
373 }
374 if ((r = sshkey_load_public(argv[0], &k, NULL)) != 0)
375 fatal_r(r, "load key %s", argv[0]);
376 if ((fp = sshkey_fingerprint(k, SSH_FP_HASH_DEFAULT,
377 SSH_FP_DEFAULT)) == NULL)
378 fatal("sshkey_fingerprint failed");
379 debug2("key %s: %s %s", argv[2], sshkey_type(k), fp);
380 free(fp);
381 if ((r = sshbuf_load_file(argv[1], &challenge)) != 0)
382 fatal_r(r, "load challenge %s", argv[1]);
383 if ((r = sshbuf_load_file(argv[2], &attestation)) != 0)
384 fatal_r(r, "load attestation %s", argv[2]);
385 if ((cred = fido_cred_new()) == NULL)
386 fatal("fido_cred_new failed");
387
388 switch (sshkey_type_plain(k->type)) {
389 case KEY_ECDSA_SK:
390 switch (k->ecdsa_nid) {
391 case NID_X9_62_prime256v1:
392 credtype = COSE_ES256;
393 break;
394 default:
395 fatal("Unsupported ECDSA key size");
396 }
397 break;
398 case KEY_ED25519_SK:
399 credtype = COSE_EDDSA;
400 break;
401 default:
402 fatal("unsupported key type %s", sshkey_type(k));
403 }
404
405 if ((r = prepare_fido_cred(cred, credtype, attfmt, k->sk_application,
406 attestation, challenge, &attestation_cert)) != 0)
407 fatal_r(r, "prepare_fido_cred %s", argv[2]);
408 if (fido_cred_x5c_ptr(cred) != NULL) {
409 debug("basic attestation");
410 if ((r = fido_cred_verify(cred)) != FIDO_OK)
411 fatal("basic attestation failed");
412 style = "basic";
413 } else {
414 debug("self attestation");
415 if ((r = fido_cred_verify_self(cred)) != FIDO_OK)
416 fatal("self attestation failed");
417 style = "self";
418 }
419 if (cred_matches_key(cred, k) != 0)
420 fatal("cred authdata does not match key");
421
422 fido_cred_free(&cred);
423
424 if (write_attestation_cert) {
425 PEM_write(stdout, "CERTIFICATE", NULL,
426 sshbuf_ptr(attestation_cert), sshbuf_len(attestation_cert));
427 }
428 sshbuf_free(attestation_cert);
429
430 logit("%s: verified %s attestation", argv[2], style);
431
432 return (0);
433 }
434