xref: /openssh-portable/ssh-ecdsa-sk.c (revision bb52e70f)
1 /* $OpenBSD: ssh-ecdsa-sk.c,v 1.7 2020/06/22 05:58:35 djm Exp $ */
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 "includes.h"
31 
32 #include <sys/types.h>
33 
34 #ifdef WITH_OPENSSL
35 #include <openssl/bn.h>
36 #include <openssl/ec.h>
37 #include <openssl/ecdsa.h>
38 #include <openssl/evp.h>
39 #endif
40 
41 #include <string.h>
42 #include <stdio.h> /* needed for DEBUG_SK only */
43 
44 #include "openbsd-compat/openssl-compat.h"
45 
46 #include "sshbuf.h"
47 #include "ssherr.h"
48 #include "digest.h"
49 #define SSHKEY_INTERNAL
50 #include "sshkey.h"
51 
52 #ifndef OPENSSL_HAS_ECC
53 /* ARGSUSED */
54 int
55 ssh_ecdsa_sk_verify(const struct sshkey *key,
56     const u_char *signature, size_t signaturelen,
57     const u_char *data, size_t datalen, u_int compat,
58     struct sshkey_sig_details **detailsp)
59 {
60 	return SSH_ERR_FEATURE_UNSUPPORTED;
61 }
62 #else /* OPENSSL_HAS_ECC */
63 
64 /*
65  * Check FIDO/W3C webauthn signatures clientData field against the expected
66  * format and prepare a hash of it for use in signature verification.
67  *
68  * webauthn signatures do not sign the hash of the message directly, but
69  * instead sign a JSON-like "clientData" wrapper structure that contains the
70  * message hash along with a other information.
71  *
72  * Fortunately this structure has a fixed format so it is possible to verify
73  * that the hash of the signed message is present within the clientData
74  * structure without needing to implement any JSON parsing.
75  */
76 static int
77 webauthn_check_prepare_hash(const u_char *data, size_t datalen,
78     const char *origin, const struct sshbuf *wrapper,
79     uint8_t flags, const struct sshbuf *extensions,
80     u_char *msghash, size_t msghashlen)
81 {
82 	int r = SSH_ERR_INTERNAL_ERROR;
83 	struct sshbuf *chall = NULL, *m = NULL;
84 
85 	if ((m = sshbuf_new()) == NULL ||
86 	    (chall = sshbuf_from(data, datalen)) == NULL) {
87 		r = SSH_ERR_ALLOC_FAIL;
88 		goto out;
89 	}
90 	/*
91 	 * Ensure origin contains no quote character and that the flags are
92 	 * consistent with what we received
93 	 */
94 	if (strchr(origin, '\"') != NULL ||
95 	    (flags & 0x40) != 0 /* AD */ ||
96 	    ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) {
97 		r = SSH_ERR_INVALID_FORMAT;
98 		goto out;
99 	}
100 #define WEBAUTHN_0	"{\"type\":\"webauthn.get\",\"challenge\":\""
101 #define WEBAUTHN_1	"\",\"origin\":\""
102 #define WEBAUTHN_2	"\""
103 	if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 ||
104 	    (r = sshbuf_dtourlb64(chall, m, 0)) != 0 ||
105 	    (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 ||
106 	    (r = sshbuf_put(m, origin, strlen(origin))) != 0 ||
107 	    (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0)
108 		goto out;
109 #ifdef DEBUG_SK
110 	fprintf(stderr, "%s: received origin: %s\n", __func__, origin);
111 	fprintf(stderr, "%s: received clientData:\n", __func__);
112 	sshbuf_dump(wrapper, stderr);
113 	fprintf(stderr, "%s: expected clientData premable:\n", __func__);
114 	sshbuf_dump(m, stderr);
115 #endif
116 	/* Check that the supplied clientData matches what we expect */
117 	if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0)
118 		goto out;
119 
120 	/* Prepare hash of clientData */
121 	if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper,
122 	    msghash, msghashlen)) != 0)
123 		goto out;
124 
125 	/* success */
126 	r = 0;
127  out:
128 	sshbuf_free(chall);
129 	sshbuf_free(m);
130 	return r;
131 }
132 
133 /* ARGSUSED */
134 int
135 ssh_ecdsa_sk_verify(const struct sshkey *key,
136     const u_char *signature, size_t signaturelen,
137     const u_char *data, size_t datalen, u_int compat,
138     struct sshkey_sig_details **detailsp)
139 {
140 	ECDSA_SIG *sig = NULL;
141 	BIGNUM *sig_r = NULL, *sig_s = NULL;
142 	u_char sig_flags;
143 	u_char msghash[32], apphash[32], sighash[32];
144 	u_int sig_counter;
145 	int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR;
146 	struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL;
147 	struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL;
148 	char *ktype = NULL, *webauthn_origin = NULL;
149 	struct sshkey_sig_details *details = NULL;
150 #ifdef DEBUG_SK
151 	char *tmp = NULL;
152 #endif
153 
154 	if (detailsp != NULL)
155 		*detailsp = NULL;
156 	if (key == NULL || key->ecdsa == NULL ||
157 	    sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
158 	    signature == NULL || signaturelen == 0)
159 		return SSH_ERR_INVALID_ARGUMENT;
160 
161 	if (key->ecdsa_nid != NID_X9_62_prime256v1)
162 		return SSH_ERR_INTERNAL_ERROR;
163 
164 	/* fetch signature */
165 	if ((b = sshbuf_from(signature, signaturelen)) == NULL)
166 		return SSH_ERR_ALLOC_FAIL;
167 	if ((details = calloc(1, sizeof(*details))) == NULL) {
168 		ret = SSH_ERR_ALLOC_FAIL;
169 		goto out;
170 	}
171 	if (sshbuf_get_cstring(b, &ktype, NULL) != 0) {
172 		ret = SSH_ERR_INVALID_FORMAT;
173 		goto out;
174 	}
175 	if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0)
176 		is_webauthn = 1;
177 	else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) {
178 		ret = SSH_ERR_INVALID_FORMAT;
179 		goto out;
180 	}
181 	if (sshbuf_froms(b, &sigbuf) != 0 ||
182 	    sshbuf_get_u8(b, &sig_flags) != 0 ||
183 	    sshbuf_get_u32(b, &sig_counter) != 0) {
184 		ret = SSH_ERR_INVALID_FORMAT;
185 		goto out;
186 	}
187 	if (is_webauthn) {
188 		if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 ||
189 		    sshbuf_froms(b, &webauthn_wrapper) != 0 ||
190 		    sshbuf_froms(b, &webauthn_exts) != 0) {
191 			ret = SSH_ERR_INVALID_FORMAT;
192 			goto out;
193 		}
194 	}
195 	if (sshbuf_len(b) != 0) {
196 		ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
197 		goto out;
198 	}
199 
200 	/* parse signature */
201 	if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
202 	    sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
203 		ret = SSH_ERR_INVALID_FORMAT;
204 		goto out;
205 	}
206 	if (sshbuf_len(sigbuf) != 0) {
207 		ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
208 		goto out;
209 	}
210 
211 #ifdef DEBUG_SK
212 	fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen);
213 	/* sshbuf_dump_data(data, datalen, stderr); */
214 	fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r)));
215 	free(tmp);
216 	fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s)));
217 	free(tmp);
218 	fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
219 	    __func__, sig_flags, sig_counter);
220 	if (is_webauthn) {
221 		fprintf(stderr, "%s: webauthn origin: %s\n", __func__,
222 		    webauthn_origin);
223 		fprintf(stderr, "%s: webauthn_wrapper:\n", __func__);
224 		sshbuf_dump(webauthn_wrapper, stderr);
225 	}
226 #endif
227 	if ((sig = ECDSA_SIG_new()) == NULL) {
228 		ret = SSH_ERR_ALLOC_FAIL;
229 		goto out;
230 	}
231 	if (!ECDSA_SIG_set0(sig, sig_r, sig_s)) {
232 		ret = SSH_ERR_LIBCRYPTO_ERROR;
233 		goto out;
234 	}
235 	sig_r = sig_s = NULL; /* transferred */
236 
237 	/* Reconstruct data that was supposedly signed */
238 	if ((original_signed = sshbuf_new()) == NULL) {
239 		ret = SSH_ERR_ALLOC_FAIL;
240 		goto out;
241 	}
242 	if (is_webauthn) {
243 		if ((ret = webauthn_check_prepare_hash(data, datalen,
244 		    webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts,
245 		    msghash, sizeof(msghash))) != 0)
246 			goto out;
247 	} else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen,
248 	    msghash, sizeof(msghash))) != 0)
249 		goto out;
250 	/* Application value is hashed before signature */
251 	if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
252 	    strlen(key->sk_application), apphash, sizeof(apphash))) != 0)
253 		goto out;
254 #ifdef DEBUG_SK
255 	fprintf(stderr, "%s: hashed application:\n", __func__);
256 	sshbuf_dump_data(apphash, sizeof(apphash), stderr);
257 	fprintf(stderr, "%s: hashed message:\n", __func__);
258 	sshbuf_dump_data(msghash, sizeof(msghash), stderr);
259 #endif
260 	if ((ret = sshbuf_put(original_signed,
261 	    apphash, sizeof(apphash))) != 0 ||
262 	    (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 ||
263 	    (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 ||
264 	    (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 ||
265 	    (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0)
266 		goto out;
267 	/* Signature is over H(original_signed) */
268 	if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed,
269 	    sighash, sizeof(sighash))) != 0)
270 		goto out;
271 	details->sk_counter = sig_counter;
272 	details->sk_flags = sig_flags;
273 #ifdef DEBUG_SK
274 	fprintf(stderr, "%s: signed buf:\n", __func__);
275 	sshbuf_dump(original_signed, stderr);
276 	fprintf(stderr, "%s: signed hash:\n", __func__);
277 	sshbuf_dump_data(sighash, sizeof(sighash), stderr);
278 #endif
279 
280 	/* Verify it */
281 	switch (ECDSA_do_verify(sighash, sizeof(sighash), sig, key->ecdsa)) {
282 	case 1:
283 		ret = 0;
284 		break;
285 	case 0:
286 		ret = SSH_ERR_SIGNATURE_INVALID;
287 		goto out;
288 	default:
289 		ret = SSH_ERR_LIBCRYPTO_ERROR;
290 		goto out;
291 	}
292 	/* success */
293 	if (detailsp != NULL) {
294 		*detailsp = details;
295 		details = NULL;
296 	}
297  out:
298 	explicit_bzero(&sig_flags, sizeof(sig_flags));
299 	explicit_bzero(&sig_counter, sizeof(sig_counter));
300 	explicit_bzero(msghash, sizeof(msghash));
301 	explicit_bzero(sighash, sizeof(msghash));
302 	explicit_bzero(apphash, sizeof(apphash));
303 	sshkey_sig_details_free(details);
304 	sshbuf_free(webauthn_wrapper);
305 	sshbuf_free(webauthn_exts);
306 	free(webauthn_origin);
307 	sshbuf_free(original_signed);
308 	sshbuf_free(sigbuf);
309 	sshbuf_free(b);
310 	ECDSA_SIG_free(sig);
311 	BN_clear_free(sig_r);
312 	BN_clear_free(sig_s);
313 	free(ktype);
314 	return ret;
315 }
316 
317 #endif /* OPENSSL_HAS_ECC */
318