Introduce bn_rand_interval() that allows specifying an interval [a, b)

from which a a BIGNUM is chosen uniformly at random.

ok beck jsing

2 files changed, 30 insertions, 2 deletions

diff --git a/src/lib/libcrypto/bn/bn_lcl.h b/src/lib/libcrypto/bn/bn_lcl.h
index b8319dd700..d0f36822dc 100644
--- a/src/lib/libcrypto/bn/bn_lcl.h
+++ b/src/lib/libcrypto/bn/bn_lcl.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_lcl.h,v 1.29 2018/07/23 18:14:32 tb Exp $ */
+/* $OpenBSD: bn_lcl.h,v 1.30 2018/11/05 23:52:47 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -583,6 +583,7 @@ BN_ULONG bn_add_words(BN_ULONG *rp, const BN_ULONG *ap, const BN_ULONG *bp, int
 BN_ULONG bn_sub_words(BN_ULONG *rp, const BN_ULONG *ap, const BN_ULONG *bp, int num);
 
 int BN_bntest_rand(BIGNUM *rnd, int bits, int top, int bottom);
+int bn_rand_interval(BIGNUM *rnd, const BIGNUM *lower_inc, const BIGNUM *upper_exc);
 
 /* Explicitly const time / non-const time versions for internal use */
 int BN_mod_exp_ct(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
diff --git a/src/lib/libcrypto/bn/bn_rand.c b/src/lib/libcrypto/bn/bn_rand.c
index 8625757140..63b8af8b95 100644
--- a/src/lib/libcrypto/bn/bn_rand.c
+++ b/src/lib/libcrypto/bn/bn_rand.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_rand.c,v 1.20 2017/05/02 03:59:44 deraadt Exp $ */
+/* $OpenBSD: bn_rand.c,v 1.21 2018/11/05 23:52:47 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -280,6 +280,33 @@ BN_rand_range(BIGNUM *r, const BIGNUM *range)
 }
 
 int
+bn_rand_interval(BIGNUM *rnd, const BIGNUM *lower_inc, const BIGNUM *upper_exc)
+{
+        BIGNUM *len = NULL;
+        int ret = 0;
+
+        if (BN_cmp(lower_inc, upper_exc) <= 0)
+                goto err;
+
+        if ((len = BN_new()) == NULL)
+                goto err;
+
+        if (!BN_sub(len, upper_exc, lower_inc))
+                goto err;
+
+        if (!bn_rand_range(0, rnd, len))
+                goto err;
+
+        if (!BN_add(rnd, rnd, lower_inc))
+                goto err;
+
+        ret = 1;
+ err:
+        BN_free(len);
+        return ret;
+}
+
+int
 BN_pseudo_rand_range(BIGNUM *r, const BIGNUM *range)
 {
         return bn_rand_range(1, r, range);