1 /* $OpenBSD: schnorr.c,v 1.3 2009/03/05 07:18:19 djm Exp $ */
3 * Copyright (c) 2008 Damien Miller. All rights reserved.
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.
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.
19 * Implementation of Schnorr signatures / zero-knowledge proofs, based on
22 * F. Hao, P. Ryan, "Password Authenticated Key Exchange by Juggling",
23 * 16th Workshop on Security Protocols, Cambridge, April 2008
25 * http://grouper.ieee.org/groups/1363/Research/contributions/hao-ryan-2008.pdf
30 #include <sys/types.h>
36 #include <openssl/evp.h>
37 #include <openssl/bn.h>
45 /* #define SCHNORR_DEBUG */ /* Privacy-violating debugging */
46 /* #define SCHNORR_MAIN */ /* Include main() selftest */
49 # define SCHNORR_DEBUG_BN(a)
50 # define SCHNORR_DEBUG_BUF(a)
52 # define SCHNORR_DEBUG_BN(a) debug3_bn a
53 # define SCHNORR_DEBUG_BUF(a) debug3_buf a
54 #endif /* SCHNORR_DEBUG */
57 * Calculate hash component of Schnorr signature H(g || g^v || g^x || id)
58 * using the hash function defined by "evp_md". Returns signature as
59 * bignum or NULL on error.
62 schnorr_hash(const BIGNUM *p, const BIGNUM *q, const BIGNUM *g,
63 const EVP_MD *evp_md, const BIGNUM *g_v, const BIGNUM *g_x,
64 const u_char *id, u_int idlen)
72 if ((h = BN_new()) == NULL) {
73 error("%s: BN_new", __func__);
79 /* h = H(g || p || q || g^v || g^x || id) */
80 buffer_put_bignum2(&b, g);
81 buffer_put_bignum2(&b, p);
82 buffer_put_bignum2(&b, q);
83 buffer_put_bignum2(&b, g_v);
84 buffer_put_bignum2(&b, g_x);
85 buffer_put_string(&b, id, idlen);
87 SCHNORR_DEBUG_BUF((buffer_ptr(&b), buffer_len(&b),
88 "%s: hashblob", __func__));
89 if (hash_buffer(buffer_ptr(&b), buffer_len(&b), evp_md,
90 &digest, &digest_len) != 0) {
91 error("%s: hash_buffer", __func__);
94 if (BN_bin2bn(digest, (int)digest_len, h) == NULL) {
95 error("%s: BN_bin2bn", __func__);
99 SCHNORR_DEBUG_BN((h, "%s: h = ", __func__));
102 bzero(digest, digest_len);
112 * Generate Schnorr signature to prove knowledge of private value 'x' used
113 * in public exponent g^x, under group defined by 'grp_p', 'grp_q' and 'grp_g'
114 * using the hash function "evp_md".
115 * 'idlen' bytes from 'id' will be included in the signature hash as an anti-
118 * On success, 0 is returned. The signature values are returned as *e_p
119 * (g^v mod p) and *r_p (v - xh mod q). The caller must free these values.
120 * On failure, -1 is returned.
123 schnorr_sign(const BIGNUM *grp_p, const BIGNUM *grp_q, const BIGNUM *grp_g,
124 const EVP_MD *evp_md, const BIGNUM *x, const BIGNUM *g_x,
125 const u_char *id, u_int idlen, BIGNUM **r_p, BIGNUM **e_p)
128 BIGNUM *h, *tmp, *v, *g_v, *r;
131 SCHNORR_DEBUG_BN((x, "%s: x = ", __func__));
132 SCHNORR_DEBUG_BN((g_x, "%s: g_x = ", __func__));
134 /* Avoid degenerate cases: g^0 yields a spoofable signature */
135 if (BN_cmp(g_x, BN_value_one()) <= 0) {
136 error("%s: g_x < 1", __func__);
140 h = g_v = r = tmp = v = NULL;
141 if ((bn_ctx = BN_CTX_new()) == NULL) {
142 error("%s: BN_CTX_new", __func__);
145 if ((g_v = BN_new()) == NULL ||
146 (r = BN_new()) == NULL ||
147 (tmp = BN_new()) == NULL) {
148 error("%s: BN_new", __func__);
153 * v must be a random element of Zq, so 1 <= v < q
154 * we also exclude v = 1, since g^1 looks dangerous
156 if ((v = bn_rand_range_gt_one(grp_p)) == NULL) {
157 error("%s: bn_rand_range2", __func__);
160 SCHNORR_DEBUG_BN((v, "%s: v = ", __func__));
162 /* g_v = g^v mod p */
163 if (BN_mod_exp(g_v, grp_g, v, grp_p, bn_ctx) == -1) {
164 error("%s: BN_mod_exp (g^v mod p)", __func__);
167 SCHNORR_DEBUG_BN((g_v, "%s: g_v = ", __func__));
169 /* h = H(g || g^v || g^x || id) */
170 if ((h = schnorr_hash(grp_p, grp_q, grp_g, evp_md, g_v, g_x,
171 id, idlen)) == NULL) {
172 error("%s: schnorr_hash failed", __func__);
176 /* r = v - xh mod q */
177 if (BN_mod_mul(tmp, x, h, grp_q, bn_ctx) == -1) {
178 error("%s: BN_mod_mul (tmp = xv mod q)", __func__);
181 if (BN_mod_sub(r, v, tmp, grp_q, bn_ctx) == -1) {
182 error("%s: BN_mod_mul (r = v - tmp)", __func__);
185 SCHNORR_DEBUG_BN((g_v, "%s: e = ", __func__));
186 SCHNORR_DEBUG_BN((r, "%s: r = ", __func__));
204 * Generate Schnorr signature to prove knowledge of private value 'x' used
205 * in public exponent g^x, under group defined by 'grp_p', 'grp_q' and 'grp_g'
206 * using a SHA256 hash.
207 * 'idlen' bytes from 'id' will be included in the signature hash as an anti-
209 * On success, 0 is returned and *siglen bytes of signature are returned in
210 * *sig (caller to free). Returns -1 on failure.
213 schnorr_sign_buf(const BIGNUM *grp_p, const BIGNUM *grp_q, const BIGNUM *grp_g,
214 const BIGNUM *x, const BIGNUM *g_x, const u_char *id, u_int idlen,
215 u_char **sig, u_int *siglen)
220 if (schnorr_sign(grp_p, grp_q, grp_g, EVP_sha256(),
221 x, g_x, id, idlen, &r, &e) != 0)
224 /* Signature is (e, r) */
226 /* XXX sigtype-hash as string? */
227 buffer_put_bignum2(&b, e);
228 buffer_put_bignum2(&b, r);
229 *siglen = buffer_len(&b);
230 *sig = xmalloc(*siglen);
231 memcpy(*sig, buffer_ptr(&b), *siglen);
232 SCHNORR_DEBUG_BUF((buffer_ptr(&b), buffer_len(&b),
233 "%s: sigblob", __func__));
243 * Verify Schnorr signature { r (v - xh mod q), e (g^v mod p) } against
244 * public exponent g_x (g^x) under group defined by 'grp_p', 'grp_q' and
245 * 'grp_g' using hash "evp_md".
246 * Signature hash will be salted with 'idlen' bytes from 'id'.
247 * Returns -1 on failure, 0 on incorrect signature or 1 on matching signature.
250 schnorr_verify(const BIGNUM *grp_p, const BIGNUM *grp_q, const BIGNUM *grp_g,
251 const EVP_MD *evp_md, const BIGNUM *g_x, const u_char *id, u_int idlen,
252 const BIGNUM *r, const BIGNUM *e)
255 BIGNUM *h, *g_xh, *g_r, *expected;
258 SCHNORR_DEBUG_BN((g_x, "%s: g_x = ", __func__));
260 /* Avoid degenerate cases: g^0 yields a spoofable signature */
261 if (BN_cmp(g_x, BN_value_one()) <= 0) {
262 error("%s: g_x < 1", __func__);
266 h = g_xh = g_r = expected = NULL;
267 if ((bn_ctx = BN_CTX_new()) == NULL) {
268 error("%s: BN_CTX_new", __func__);
271 if ((g_xh = BN_new()) == NULL ||
272 (g_r = BN_new()) == NULL ||
273 (expected = BN_new()) == NULL) {
274 error("%s: BN_new", __func__);
278 SCHNORR_DEBUG_BN((e, "%s: e = ", __func__));
279 SCHNORR_DEBUG_BN((r, "%s: r = ", __func__));
281 /* h = H(g || g^v || g^x || id) */
282 if ((h = schnorr_hash(grp_p, grp_q, grp_g, evp_md, e, g_x,
283 id, idlen)) == NULL) {
284 error("%s: schnorr_hash failed", __func__);
289 if (BN_mod_exp(g_xh, g_x, h, grp_p, bn_ctx) == -1) {
290 error("%s: BN_mod_exp (g_x^h mod p)", __func__);
293 SCHNORR_DEBUG_BN((g_xh, "%s: g_xh = ", __func__));
296 if (BN_mod_exp(g_r, grp_g, r, grp_p, bn_ctx) == -1) {
297 error("%s: BN_mod_exp (g_x^h mod p)", __func__);
300 SCHNORR_DEBUG_BN((g_r, "%s: g_r = ", __func__));
302 /* expected = g^r * g_xh */
303 if (BN_mod_mul(expected, g_r, g_xh, grp_p, bn_ctx) == -1) {
304 error("%s: BN_mod_mul (expected = g_r mod p)", __func__);
307 SCHNORR_DEBUG_BN((expected, "%s: expected = ", __func__));
309 /* Check e == expected */
310 success = BN_cmp(expected, e) == 0;
317 BN_clear_free(expected);
322 * Verify Schnorr signature 'sig' of length 'siglen' against public exponent
323 * g_x (g^x) under group defined by 'grp_p', 'grp_q' and 'grp_g' using a
325 * Signature hash will be salted with 'idlen' bytes from 'id'.
326 * Returns -1 on failure, 0 on incorrect signature or 1 on matching signature.
329 schnorr_verify_buf(const BIGNUM *grp_p, const BIGNUM *grp_q,
331 const BIGNUM *g_x, const u_char *id, u_int idlen,
332 const u_char *sig, u_int siglen)
340 if ((e = BN_new()) == NULL ||
341 (r = BN_new()) == NULL) {
342 error("%s: BN_new", __func__);
346 /* Extract g^v and r from signature blob */
348 buffer_append(&b, sig, siglen);
349 SCHNORR_DEBUG_BUF((buffer_ptr(&b), buffer_len(&b),
350 "%s: sigblob", __func__));
351 buffer_get_bignum2(&b, e);
352 buffer_get_bignum2(&b, r);
353 rlen = buffer_len(&b);
356 error("%s: remaining bytes in signature %d", __func__, rlen);
360 ret = schnorr_verify(grp_p, grp_q, grp_g, EVP_sha256(),
361 g_x, id, idlen, r, e);
369 /* Helper functions */
372 * Generate uniformly distributed random number in range (1, high).
373 * Return number on success, NULL on failure.
376 bn_rand_range_gt_one(const BIGNUM *high)
381 if ((tmp = BN_new()) == NULL) {
382 error("%s: BN_new", __func__);
385 if ((r = BN_new()) == NULL) {
386 error("%s: BN_new failed", __func__);
389 if (BN_set_word(tmp, 2) != 1) {
390 error("%s: BN_set_word(tmp, 2)", __func__);
393 if (BN_sub(tmp, high, tmp) == -1) {
394 error("%s: BN_sub failed (tmp = high - 2)", __func__);
397 if (BN_rand_range(r, tmp) == -1) {
398 error("%s: BN_rand_range failed", __func__);
401 if (BN_set_word(tmp, 2) != 1) {
402 error("%s: BN_set_word(tmp, 2)", __func__);
405 if (BN_add(r, r, tmp) == -1) {
406 error("%s: BN_add failed (r = r + 2)", __func__);
419 * Hash contents of buffer 'b' with hash 'md'. Returns 0 on success,
420 * with digest via 'digestp' (caller to free) and length via 'lenp'.
421 * Returns -1 on failure.
424 hash_buffer(const u_char *buf, u_int len, const EVP_MD *md,
425 u_char **digestp, u_int *lenp)
427 u_char digest[EVP_MAX_MD_SIZE];
429 EVP_MD_CTX evp_md_ctx;
432 EVP_MD_CTX_init(&evp_md_ctx);
434 if (EVP_DigestInit_ex(&evp_md_ctx, md, NULL) != 1) {
435 error("%s: EVP_DigestInit_ex", __func__);
438 if (EVP_DigestUpdate(&evp_md_ctx, buf, len) != 1) {
439 error("%s: EVP_DigestUpdate", __func__);
442 if (EVP_DigestFinal_ex(&evp_md_ctx, digest, &digest_len) != 1) {
443 error("%s: EVP_DigestFinal_ex", __func__);
446 *digestp = xmalloc(digest_len);
448 memcpy(*digestp, digest, *lenp);
451 EVP_MD_CTX_cleanup(&evp_md_ctx);
452 bzero(digest, sizeof(digest));
457 /* print formatted string followed by bignum */
459 debug3_bn(const BIGNUM *n, const char *fmt, ...)
466 vasprintf(&out, fmt, args);
469 fatal("%s: vasprintf failed", __func__);
472 debug3("%s(null)", out);
475 debug3("%s0x%s", out, h);
481 /* print formatted string followed by buffer contents in hex */
483 debug3_buf(const u_char *buf, u_int len, const char *fmt, ...)
491 vasprintf(&out, fmt, args);
494 fatal("%s: vasprintf failed", __func__);
496 debug3("%s length %u%s", out, len, buf == NULL ? " (null)" : "");
502 for (i = j = 0; i < len; i++) {
503 snprintf(h + j, sizeof(h) - j, "%02x", buf[i]);
505 if (j >= sizeof(h) - 1 || i == len - 1) {
514 * Construct a MODP group from hex strings p (which must be a safe
515 * prime) and g, automatically calculating subgroup q as (p / 2)
518 modp_group_from_g_and_safe_p(const char *grp_g, const char *grp_p)
520 struct modp_group *ret;
522 ret = xmalloc(sizeof(*ret));
523 ret->p = ret->q = ret->g = NULL;
524 if (BN_hex2bn(&ret->p, grp_p) == 0 ||
525 BN_hex2bn(&ret->g, grp_g) == 0)
526 fatal("%s: BN_hex2bn", __func__);
527 /* Subgroup order is p/2 (p is a safe prime) */
528 if ((ret->q = BN_new()) == NULL)
529 fatal("%s: BN_new", __func__);
530 if (BN_rshift1(ret->q, ret->p) != 1)
531 fatal("%s: BN_rshift1", __func__);
537 modp_group_free(struct modp_group *grp)
540 BN_clear_free(grp->g);
542 BN_clear_free(grp->p);
544 BN_clear_free(grp->q);
545 bzero(grp, sizeof(*grp));
549 /* main() function for self-test */
553 schnorr_selftest_one(const BIGNUM *grp_p, const BIGNUM *grp_q,
554 const BIGNUM *grp_g, const BIGNUM *x)
561 if ((bn_ctx = BN_CTX_new()) == NULL)
562 fatal("%s: BN_CTX_new", __func__);
563 if ((g_x = BN_new()) == NULL)
564 fatal("%s: BN_new", __func__);
566 if (BN_mod_exp(g_x, grp_g, x, grp_p, bn_ctx) == -1)
567 fatal("%s: g_x", __func__);
568 if (schnorr_sign_buf(grp_p, grp_q, grp_g, x, g_x, "junk", 4,
570 fatal("%s: schnorr_sign", __func__);
571 if (schnorr_verify_buf(grp_p, grp_q, grp_g, g_x, "junk", 4,
573 fatal("%s: verify fail", __func__);
574 if (schnorr_verify_buf(grp_p, grp_q, grp_g, g_x, "JUNK", 4,
576 fatal("%s: verify should have failed (bad ID)", __func__);
578 if (schnorr_verify_buf(grp_p, grp_q, grp_g, g_x, "junk", 4,
580 fatal("%s: verify should have failed (bit error)", __func__);
587 schnorr_selftest(void)
590 struct modp_group *grp;
594 grp = jpake_default_group();
595 if ((x = BN_new()) == NULL)
596 fatal("%s: BN_new", __func__);
597 SCHNORR_DEBUG_BN((grp->p, "%s: grp->p = ", __func__));
598 SCHNORR_DEBUG_BN((grp->q, "%s: grp->q = ", __func__));
599 SCHNORR_DEBUG_BN((grp->g, "%s: grp->g = ", __func__));
602 for (i = 1; i < 20; i++) {
603 printf("x = %u\n", i);
605 if (BN_set_word(x, i) != 1)
606 fatal("%s: set x word", __func__);
607 schnorr_selftest_one(grp->p, grp->q, grp->g, x);
610 /* 100 x random [0, p) */
611 for (i = 0; i < 100; i++) {
612 if (BN_rand_range(x, grp->p) != 1)
613 fatal("%s: BN_rand_range", __func__);
615 printf("x = (random) 0x%s\n", hh);
618 schnorr_selftest_one(grp->p, grp->q, grp->g, x);
622 if (BN_set_word(x, 20) != 1)
623 fatal("%s: BN_set_word (x = 20)", __func__);
624 if (BN_sub(x, grp->q, x) != 1)
625 fatal("%s: BN_sub (q - x)", __func__);
626 for (i = 0; i < 19; i++) {
628 printf("x = (q - %d) 0x%s\n", 20 - i, hh);
631 schnorr_selftest_one(grp->p, grp->q, grp->g, x);
632 if (BN_add(x, x, BN_value_one()) != 1)
633 fatal("%s: BN_add (x + 1)", __func__);
639 main(int argc, char **argv)
641 log_init(argv[0], SYSLOG_LEVEL_DEBUG3, SYSLOG_FACILITY_USER, 1);