xref: /openssh-portable/ssh-ecdsa-sk.c (revision 37f2da06)
1 /* $OpenBSD: ssh-ecdsa-sk.c,v 1.8 2020/06/22 23:44:27 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
ssh_ecdsa_sk_verify(const struct sshkey * key,const u_char * signature,size_t signaturelen,const u_char * data,size_t datalen,u_int compat,struct sshkey_sig_details ** detailsp)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
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)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 
101 	/*
102 	 * Prepare the preamble to clientData that we expect, poking the
103 	 * challenge and origin into their canonical positions in the
104 	 * structure. The crossOrigin flag and any additional extension
105 	 * fields present are ignored.
106 	 */
107 #define WEBAUTHN_0	"{\"type\":\"webauthn.get\",\"challenge\":\""
108 #define WEBAUTHN_1	"\",\"origin\":\""
109 #define WEBAUTHN_2	"\""
110 	if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 ||
111 	    (r = sshbuf_dtourlb64(chall, m, 0)) != 0 ||
112 	    (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 ||
113 	    (r = sshbuf_put(m, origin, strlen(origin))) != 0 ||
114 	    (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0)
115 		goto out;
116 #ifdef DEBUG_SK
117 	fprintf(stderr, "%s: received origin: %s\n", __func__, origin);
118 	fprintf(stderr, "%s: received clientData:\n", __func__);
119 	sshbuf_dump(wrapper, stderr);
120 	fprintf(stderr, "%s: expected clientData premable:\n", __func__);
121 	sshbuf_dump(m, stderr);
122 #endif
123 	/* Check that the supplied clientData has the preamble we expect */
124 	if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0)
125 		goto out;
126 
127 	/* Prepare hash of clientData */
128 	if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper,
129 	    msghash, msghashlen)) != 0)
130 		goto out;
131 
132 	/* success */
133 	r = 0;
134  out:
135 	sshbuf_free(chall);
136 	sshbuf_free(m);
137 	return r;
138 }
139 
140 /* ARGSUSED */
141 int
ssh_ecdsa_sk_verify(const struct sshkey * key,const u_char * signature,size_t signaturelen,const u_char * data,size_t datalen,u_int compat,struct sshkey_sig_details ** detailsp)142 ssh_ecdsa_sk_verify(const struct sshkey *key,
143     const u_char *signature, size_t signaturelen,
144     const u_char *data, size_t datalen, u_int compat,
145     struct sshkey_sig_details **detailsp)
146 {
147 	ECDSA_SIG *sig = NULL;
148 	BIGNUM *sig_r = NULL, *sig_s = NULL;
149 	u_char sig_flags;
150 	u_char msghash[32], apphash[32], sighash[32];
151 	u_int sig_counter;
152 	int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR;
153 	struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL;
154 	struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL;
155 	char *ktype = NULL, *webauthn_origin = NULL;
156 	struct sshkey_sig_details *details = NULL;
157 #ifdef DEBUG_SK
158 	char *tmp = NULL;
159 #endif
160 
161 	if (detailsp != NULL)
162 		*detailsp = NULL;
163 	if (key == NULL || key->ecdsa == NULL ||
164 	    sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
165 	    signature == NULL || signaturelen == 0)
166 		return SSH_ERR_INVALID_ARGUMENT;
167 
168 	if (key->ecdsa_nid != NID_X9_62_prime256v1)
169 		return SSH_ERR_INTERNAL_ERROR;
170 
171 	/* fetch signature */
172 	if ((b = sshbuf_from(signature, signaturelen)) == NULL)
173 		return SSH_ERR_ALLOC_FAIL;
174 	if ((details = calloc(1, sizeof(*details))) == NULL) {
175 		ret = SSH_ERR_ALLOC_FAIL;
176 		goto out;
177 	}
178 	if (sshbuf_get_cstring(b, &ktype, NULL) != 0) {
179 		ret = SSH_ERR_INVALID_FORMAT;
180 		goto out;
181 	}
182 	if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0)
183 		is_webauthn = 1;
184 	else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) {
185 		ret = SSH_ERR_INVALID_FORMAT;
186 		goto out;
187 	}
188 	if (sshbuf_froms(b, &sigbuf) != 0 ||
189 	    sshbuf_get_u8(b, &sig_flags) != 0 ||
190 	    sshbuf_get_u32(b, &sig_counter) != 0) {
191 		ret = SSH_ERR_INVALID_FORMAT;
192 		goto out;
193 	}
194 	if (is_webauthn) {
195 		if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 ||
196 		    sshbuf_froms(b, &webauthn_wrapper) != 0 ||
197 		    sshbuf_froms(b, &webauthn_exts) != 0) {
198 			ret = SSH_ERR_INVALID_FORMAT;
199 			goto out;
200 		}
201 	}
202 	if (sshbuf_len(b) != 0) {
203 		ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
204 		goto out;
205 	}
206 
207 	/* parse signature */
208 	if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
209 	    sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
210 		ret = SSH_ERR_INVALID_FORMAT;
211 		goto out;
212 	}
213 	if (sshbuf_len(sigbuf) != 0) {
214 		ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
215 		goto out;
216 	}
217 
218 #ifdef DEBUG_SK
219 	fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen);
220 	/* sshbuf_dump_data(data, datalen, stderr); */
221 	fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r)));
222 	free(tmp);
223 	fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s)));
224 	free(tmp);
225 	fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
226 	    __func__, sig_flags, sig_counter);
227 	if (is_webauthn) {
228 		fprintf(stderr, "%s: webauthn origin: %s\n", __func__,
229 		    webauthn_origin);
230 		fprintf(stderr, "%s: webauthn_wrapper:\n", __func__);
231 		sshbuf_dump(webauthn_wrapper, stderr);
232 	}
233 #endif
234 	if ((sig = ECDSA_SIG_new()) == NULL) {
235 		ret = SSH_ERR_ALLOC_FAIL;
236 		goto out;
237 	}
238 	if (!ECDSA_SIG_set0(sig, sig_r, sig_s)) {
239 		ret = SSH_ERR_LIBCRYPTO_ERROR;
240 		goto out;
241 	}
242 	sig_r = sig_s = NULL; /* transferred */
243 
244 	/* Reconstruct data that was supposedly signed */
245 	if ((original_signed = sshbuf_new()) == NULL) {
246 		ret = SSH_ERR_ALLOC_FAIL;
247 		goto out;
248 	}
249 	if (is_webauthn) {
250 		if ((ret = webauthn_check_prepare_hash(data, datalen,
251 		    webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts,
252 		    msghash, sizeof(msghash))) != 0)
253 			goto out;
254 	} else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen,
255 	    msghash, sizeof(msghash))) != 0)
256 		goto out;
257 	/* Application value is hashed before signature */
258 	if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
259 	    strlen(key->sk_application), apphash, sizeof(apphash))) != 0)
260 		goto out;
261 #ifdef DEBUG_SK
262 	fprintf(stderr, "%s: hashed application:\n", __func__);
263 	sshbuf_dump_data(apphash, sizeof(apphash), stderr);
264 	fprintf(stderr, "%s: hashed message:\n", __func__);
265 	sshbuf_dump_data(msghash, sizeof(msghash), stderr);
266 #endif
267 	if ((ret = sshbuf_put(original_signed,
268 	    apphash, sizeof(apphash))) != 0 ||
269 	    (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 ||
270 	    (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 ||
271 	    (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 ||
272 	    (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0)
273 		goto out;
274 	/* Signature is over H(original_signed) */
275 	if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed,
276 	    sighash, sizeof(sighash))) != 0)
277 		goto out;
278 	details->sk_counter = sig_counter;
279 	details->sk_flags = sig_flags;
280 #ifdef DEBUG_SK
281 	fprintf(stderr, "%s: signed buf:\n", __func__);
282 	sshbuf_dump(original_signed, stderr);
283 	fprintf(stderr, "%s: signed hash:\n", __func__);
284 	sshbuf_dump_data(sighash, sizeof(sighash), stderr);
285 #endif
286 
287 	/* Verify it */
288 	switch (ECDSA_do_verify(sighash, sizeof(sighash), sig, key->ecdsa)) {
289 	case 1:
290 		ret = 0;
291 		break;
292 	case 0:
293 		ret = SSH_ERR_SIGNATURE_INVALID;
294 		goto out;
295 	default:
296 		ret = SSH_ERR_LIBCRYPTO_ERROR;
297 		goto out;
298 	}
299 	/* success */
300 	if (detailsp != NULL) {
301 		*detailsp = details;
302 		details = NULL;
303 	}
304  out:
305 	explicit_bzero(&sig_flags, sizeof(sig_flags));
306 	explicit_bzero(&sig_counter, sizeof(sig_counter));
307 	explicit_bzero(msghash, sizeof(msghash));
308 	explicit_bzero(sighash, sizeof(msghash));
309 	explicit_bzero(apphash, sizeof(apphash));
310 	sshkey_sig_details_free(details);
311 	sshbuf_free(webauthn_wrapper);
312 	sshbuf_free(webauthn_exts);
313 	free(webauthn_origin);
314 	sshbuf_free(original_signed);
315 	sshbuf_free(sigbuf);
316 	sshbuf_free(b);
317 	ECDSA_SIG_free(sig);
318 	BN_clear_free(sig_r);
319 	BN_clear_free(sig_s);
320 	free(ktype);
321 	return ret;
322 }
323 
324 #endif /* OPENSSL_HAS_ECC */
325