xref: /openssl/crypto/bn/bn_rand.c (revision 7ed6de99)
1 /*
2  * Copyright 1995-2024 The OpenSSL Project Authors. All Rights Reserved.
3  *
4  * Licensed under the Apache License 2.0 (the "License").  You may not use
5  * this file except in compliance with the License.  You can obtain a copy
6  * in the file LICENSE in the source distribution or at
7  * https://www.openssl.org/source/license.html
8  */
9 
10 #include <stdio.h>
11 #include <time.h>
12 #include "internal/cryptlib.h"
13 #include "crypto/rand.h"
14 #include "bn_local.h"
15 #include <openssl/rand.h>
16 #include <openssl/sha.h>
17 #include <openssl/evp.h>
18 
19 typedef enum bnrand_flag_e {
20     NORMAL, TESTING, PRIVATE
21 } BNRAND_FLAG;
22 
bnrand(BNRAND_FLAG flag,BIGNUM * rnd,int bits,int top,int bottom,unsigned int strength,BN_CTX * ctx)23 static int bnrand(BNRAND_FLAG flag, BIGNUM *rnd, int bits, int top, int bottom,
24                   unsigned int strength, BN_CTX *ctx)
25 {
26     unsigned char *buf = NULL;
27     int b, ret = 0, bit, bytes, mask;
28     OSSL_LIB_CTX *libctx = ossl_bn_get_libctx(ctx);
29 
30     if (bits == 0) {
31         if (top != BN_RAND_TOP_ANY || bottom != BN_RAND_BOTTOM_ANY)
32             goto toosmall;
33         BN_zero(rnd);
34         return 1;
35     }
36     if (bits < 0 || (bits == 1 && top > 0))
37         goto toosmall;
38 
39     bytes = (bits + 7) / 8;
40     bit = (bits - 1) % 8;
41     mask = 0xff << (bit + 1);
42 
43     buf = OPENSSL_malloc(bytes);
44     if (buf == NULL)
45         goto err;
46 
47     /* make a random number and set the top and bottom bits */
48     b = flag == NORMAL ? RAND_bytes_ex(libctx, buf, bytes, strength)
49                        : RAND_priv_bytes_ex(libctx, buf, bytes, strength);
50     if (b <= 0)
51         goto err;
52 
53     if (flag == TESTING) {
54         /*
55          * generate patterns that are more likely to trigger BN library bugs
56          */
57         int i;
58         unsigned char c;
59 
60         for (i = 0; i < bytes; i++) {
61             if (RAND_bytes_ex(libctx, &c, 1, strength) <= 0)
62                 goto err;
63             if (c >= 128 && i > 0)
64                 buf[i] = buf[i - 1];
65             else if (c < 42)
66                 buf[i] = 0;
67             else if (c < 84)
68                 buf[i] = 255;
69         }
70     }
71 
72     if (top >= 0) {
73         if (top) {
74             if (bit == 0) {
75                 buf[0] = 1;
76                 buf[1] |= 0x80;
77             } else {
78                 buf[0] |= (3 << (bit - 1));
79             }
80         } else {
81             buf[0] |= (1 << bit);
82         }
83     }
84     buf[0] &= ~mask;
85     if (bottom)                 /* set bottom bit if requested */
86         buf[bytes - 1] |= 1;
87     if (!BN_bin2bn(buf, bytes, rnd))
88         goto err;
89     ret = 1;
90  err:
91     OPENSSL_clear_free(buf, bytes);
92     bn_check_top(rnd);
93     return ret;
94 
95 toosmall:
96     ERR_raise(ERR_LIB_BN, BN_R_BITS_TOO_SMALL);
97     return 0;
98 }
99 
BN_rand_ex(BIGNUM * rnd,int bits,int top,int bottom,unsigned int strength,BN_CTX * ctx)100 int BN_rand_ex(BIGNUM *rnd, int bits, int top, int bottom,
101                unsigned int strength, BN_CTX *ctx)
102 {
103     return bnrand(NORMAL, rnd, bits, top, bottom, strength, ctx);
104 }
105 #ifndef FIPS_MODULE
BN_rand(BIGNUM * rnd,int bits,int top,int bottom)106 int BN_rand(BIGNUM *rnd, int bits, int top, int bottom)
107 {
108     return bnrand(NORMAL, rnd, bits, top, bottom, 0, NULL);
109 }
110 
BN_bntest_rand(BIGNUM * rnd,int bits,int top,int bottom)111 int BN_bntest_rand(BIGNUM *rnd, int bits, int top, int bottom)
112 {
113     return bnrand(TESTING, rnd, bits, top, bottom, 0, NULL);
114 }
115 #endif
116 
BN_priv_rand_ex(BIGNUM * rnd,int bits,int top,int bottom,unsigned int strength,BN_CTX * ctx)117 int BN_priv_rand_ex(BIGNUM *rnd, int bits, int top, int bottom,
118                     unsigned int strength, BN_CTX *ctx)
119 {
120     return bnrand(PRIVATE, rnd, bits, top, bottom, strength, ctx);
121 }
122 
123 #ifndef FIPS_MODULE
BN_priv_rand(BIGNUM * rnd,int bits,int top,int bottom)124 int BN_priv_rand(BIGNUM *rnd, int bits, int top, int bottom)
125 {
126     return bnrand(PRIVATE, rnd, bits, top, bottom, 0, NULL);
127 }
128 #endif
129 
130 /* random number r:  0 <= r < range */
bnrand_range(BNRAND_FLAG flag,BIGNUM * r,const BIGNUM * range,unsigned int strength,BN_CTX * ctx)131 static int bnrand_range(BNRAND_FLAG flag, BIGNUM *r, const BIGNUM *range,
132                         unsigned int strength, BN_CTX *ctx)
133 {
134     int n;
135     int count = 100;
136 
137     if (r == NULL) {
138         ERR_raise(ERR_LIB_BN, ERR_R_PASSED_NULL_PARAMETER);
139         return 0;
140     }
141 
142     if (range->neg || BN_is_zero(range)) {
143         ERR_raise(ERR_LIB_BN, BN_R_INVALID_RANGE);
144         return 0;
145     }
146 
147     n = BN_num_bits(range);     /* n > 0 */
148 
149     /* BN_is_bit_set(range, n - 1) always holds */
150 
151     if (n == 1)
152         BN_zero(r);
153     else if (!BN_is_bit_set(range, n - 2) && !BN_is_bit_set(range, n - 3)) {
154         /*
155          * range = 100..._2, so 3*range (= 11..._2) is exactly one bit longer
156          * than range
157          */
158         do {
159             if (!bnrand(flag, r, n + 1, BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY,
160                         strength, ctx))
161                 return 0;
162 
163             /*
164              * If r < 3*range, use r := r MOD range (which is either r, r -
165              * range, or r - 2*range). Otherwise, iterate once more. Since
166              * 3*range = 11..._2, each iteration succeeds with probability >=
167              * .75.
168              */
169             if (BN_cmp(r, range) >= 0) {
170                 if (!BN_sub(r, r, range))
171                     return 0;
172                 if (BN_cmp(r, range) >= 0)
173                     if (!BN_sub(r, r, range))
174                         return 0;
175             }
176 
177             if (!--count) {
178                 ERR_raise(ERR_LIB_BN, BN_R_TOO_MANY_ITERATIONS);
179                 return 0;
180             }
181 
182         }
183         while (BN_cmp(r, range) >= 0);
184     } else {
185         do {
186             /* range = 11..._2  or  range = 101..._2 */
187             if (!bnrand(flag, r, n, BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY,
188                         strength, ctx))
189                 return 0;
190 
191             if (!--count) {
192                 ERR_raise(ERR_LIB_BN, BN_R_TOO_MANY_ITERATIONS);
193                 return 0;
194             }
195         }
196         while (BN_cmp(r, range) >= 0);
197     }
198 
199     bn_check_top(r);
200     return 1;
201 }
202 
BN_rand_range_ex(BIGNUM * r,const BIGNUM * range,unsigned int strength,BN_CTX * ctx)203 int BN_rand_range_ex(BIGNUM *r, const BIGNUM *range, unsigned int strength,
204                      BN_CTX *ctx)
205 {
206     return bnrand_range(NORMAL, r, range, strength, ctx);
207 }
208 
209 #ifndef FIPS_MODULE
BN_rand_range(BIGNUM * r,const BIGNUM * range)210 int BN_rand_range(BIGNUM *r, const BIGNUM *range)
211 {
212     return bnrand_range(NORMAL, r, range, 0, NULL);
213 }
214 #endif
215 
BN_priv_rand_range_ex(BIGNUM * r,const BIGNUM * range,unsigned int strength,BN_CTX * ctx)216 int BN_priv_rand_range_ex(BIGNUM *r, const BIGNUM *range, unsigned int strength,
217                           BN_CTX *ctx)
218 {
219     return bnrand_range(PRIVATE, r, range, strength, ctx);
220 }
221 
222 #ifndef FIPS_MODULE
BN_priv_rand_range(BIGNUM * r,const BIGNUM * range)223 int BN_priv_rand_range(BIGNUM *r, const BIGNUM *range)
224 {
225     return bnrand_range(PRIVATE, r, range, 0, NULL);
226 }
227 
228 # ifndef OPENSSL_NO_DEPRECATED_3_0
BN_pseudo_rand(BIGNUM * rnd,int bits,int top,int bottom)229 int BN_pseudo_rand(BIGNUM *rnd, int bits, int top, int bottom)
230 {
231     return BN_rand(rnd, bits, top, bottom);
232 }
233 
BN_pseudo_rand_range(BIGNUM * r,const BIGNUM * range)234 int BN_pseudo_rand_range(BIGNUM *r, const BIGNUM *range)
235 {
236     return BN_rand_range(r, range);
237 }
238 # endif
239 #endif
240 
ossl_bn_priv_rand_range_fixed_top(BIGNUM * r,const BIGNUM * range,unsigned int strength,BN_CTX * ctx)241 int ossl_bn_priv_rand_range_fixed_top(BIGNUM *r, const BIGNUM *range,
242                                       unsigned int strength, BN_CTX *ctx)
243 {
244     int n;
245     int count = 100;
246 
247     if (r == NULL) {
248         ERR_raise(ERR_LIB_BN, ERR_R_PASSED_NULL_PARAMETER);
249         return 0;
250     }
251 
252     if (range->neg || BN_is_zero(range)) {
253         ERR_raise(ERR_LIB_BN, BN_R_INVALID_RANGE);
254         return 0;
255     }
256 
257     n = BN_num_bits(range);     /* n > 0 */
258 
259     /* BN_is_bit_set(range, n - 1) always holds */
260 
261     if (n == 1) {
262         BN_zero(r);
263     } else {
264         BN_set_flags(r, BN_FLG_CONSTTIME);
265         do {
266             if (!bnrand(PRIVATE, r, n + 1, BN_RAND_TOP_ONE, BN_RAND_BOTTOM_ANY,
267                         strength, ctx))
268                 return 0;
269 
270             if (!--count) {
271                 ERR_raise(ERR_LIB_BN, BN_R_TOO_MANY_ITERATIONS);
272                 return 0;
273             }
274             ossl_bn_mask_bits_fixed_top(r, n);
275         }
276         while (BN_ucmp(r, range) >= 0);
277 #ifdef BN_DEBUG
278         /* With BN_DEBUG on a fixed top number cannot be returned */
279         bn_correct_top(r);
280 #endif
281     }
282 
283     return 1;
284 }
285 
286 /*
287  * ossl_bn_gen_dsa_nonce_fixed_top generates a random number 0 <= out < range.
288  * Unlike BN_rand_range, it also includes the contents of |priv| and |message|
289  * in the generation so that an RNG failure isn't fatal as long as |priv|
290  * remains secret. This is intended for use in DSA and ECDSA where an RNG
291  * weakness leads directly to private key exposure unless this function is
292  * used.
293  */
ossl_bn_gen_dsa_nonce_fixed_top(BIGNUM * out,const BIGNUM * range,const BIGNUM * priv,const unsigned char * message,size_t message_len,BN_CTX * ctx)294 int ossl_bn_gen_dsa_nonce_fixed_top(BIGNUM *out, const BIGNUM *range,
295                                     const BIGNUM *priv,
296                                     const unsigned char *message,
297                                     size_t message_len, BN_CTX *ctx)
298 {
299     EVP_MD_CTX *mdctx = EVP_MD_CTX_new();
300     /*
301      * We use 512 bits of random data per iteration to ensure that we have at
302      * least |range| bits of randomness.
303      */
304     unsigned char random_bytes[64];
305     unsigned char digest[SHA512_DIGEST_LENGTH];
306     unsigned done, todo;
307     /* We generate |range|+1 bytes of random output. */
308     const unsigned num_k_bytes = BN_num_bytes(range) + 1;
309     unsigned char private_bytes[96];
310     unsigned char *k_bytes = NULL;
311     const int max_n = 64;           /* Pr(failure to generate) < 2^max_n */
312     int n;
313     int ret = 0;
314     EVP_MD *md = NULL;
315     OSSL_LIB_CTX *libctx = ossl_bn_get_libctx(ctx);
316 
317     if (mdctx == NULL)
318         goto end;
319 
320     k_bytes = OPENSSL_malloc(num_k_bytes);
321     if (k_bytes == NULL)
322         goto end;
323     /* Ensure top byte is set to avoid non-constant time in bin2bn */
324     k_bytes[0] = 0xff;
325 
326     /* We copy |priv| into a local buffer to avoid exposing its length. */
327     if (BN_bn2binpad(priv, private_bytes, sizeof(private_bytes)) < 0) {
328         /*
329          * No reasonable DSA or ECDSA key should have a private key this
330          * large and we don't handle this case in order to avoid leaking the
331          * length of the private key.
332          */
333         ERR_raise(ERR_LIB_BN, BN_R_PRIVATE_KEY_TOO_LARGE);
334         goto end;
335     }
336 
337     md = EVP_MD_fetch(libctx, "SHA512", NULL);
338     if (md == NULL) {
339         ERR_raise(ERR_LIB_BN, BN_R_NO_SUITABLE_DIGEST);
340         goto end;
341     }
342     for (n = 0; n < max_n; n++) {
343         unsigned char i = 0;
344 
345         for (done = 1; done < num_k_bytes;) {
346             if (RAND_priv_bytes_ex(libctx, random_bytes, sizeof(random_bytes),
347                                    0) <= 0)
348                 goto end;
349 
350             if (!EVP_DigestInit_ex(mdctx, md, NULL)
351                     || !EVP_DigestUpdate(mdctx, &i, sizeof(i))
352                     || !EVP_DigestUpdate(mdctx, private_bytes,
353                                          sizeof(private_bytes))
354                     || !EVP_DigestUpdate(mdctx, message, message_len)
355                     || !EVP_DigestUpdate(mdctx, random_bytes,
356                                          sizeof(random_bytes))
357                     || !EVP_DigestFinal_ex(mdctx, digest, NULL))
358                 goto end;
359 
360             todo = num_k_bytes - done;
361             if (todo > SHA512_DIGEST_LENGTH)
362                 todo = SHA512_DIGEST_LENGTH;
363             memcpy(k_bytes + done, digest, todo);
364             done += todo;
365             ++i;
366         }
367 
368         if (!BN_bin2bn(k_bytes, num_k_bytes, out))
369             goto end;
370 
371         /* Clear out the top bits and rejection filter into range */
372         BN_set_flags(out, BN_FLG_CONSTTIME);
373         ossl_bn_mask_bits_fixed_top(out, BN_num_bits(range));
374 
375         if (BN_ucmp(out, range) < 0) {
376             ret = 1;
377 #ifdef BN_DEBUG
378             /* With BN_DEBUG on a fixed top number cannot be returned */
379             bn_correct_top(out);
380 #endif
381             goto end;
382         }
383     }
384     /* Failed to generate anything */
385     ERR_raise(ERR_LIB_BN, ERR_R_INTERNAL_ERROR);
386 
387  end:
388     EVP_MD_CTX_free(mdctx);
389     EVP_MD_free(md);
390     OPENSSL_clear_free(k_bytes, num_k_bytes);
391     OPENSSL_cleanse(digest, sizeof(digest));
392     OPENSSL_cleanse(random_bytes, sizeof(random_bytes));
393     OPENSSL_cleanse(private_bytes, sizeof(private_bytes));
394     return ret;
395 }
396 
BN_generate_dsa_nonce(BIGNUM * out,const BIGNUM * range,const BIGNUM * priv,const unsigned char * message,size_t message_len,BN_CTX * ctx)397 int BN_generate_dsa_nonce(BIGNUM *out, const BIGNUM *range,
398                           const BIGNUM *priv, const unsigned char *message,
399                           size_t message_len, BN_CTX *ctx)
400 {
401     int ret;
402 
403     ret = ossl_bn_gen_dsa_nonce_fixed_top(out, range, priv, message,
404                                           message_len, ctx);
405     /*
406      * This call makes the BN_generate_dsa_nonce non-const-time, thus we
407      * do not use it internally. But fixed_top BNs currently cannot be returned
408      * from public API calls.
409      */
410     bn_correct_top(out);
411     return ret;
412 }
413