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-rw-r--r--src/lib/math/bigint/big_rand.cpp21
-rw-r--r--src/lib/math/bigint/bigint.cpp4
-rw-r--r--src/lib/math/bigint/bigint.h16
-rw-r--r--src/tests/catchy/test_bigint.cpp97
4 files changed, 128 insertions, 10 deletions
diff --git a/src/lib/math/bigint/big_rand.cpp b/src/lib/math/bigint/big_rand.cpp
index 074c368e0..cfc1facee 100644
--- a/src/lib/math/bigint/big_rand.cpp
+++ b/src/lib/math/bigint/big_rand.cpp
@@ -15,7 +15,7 @@ namespace Botan {
* Randomize this number
*/
void BigInt::randomize(RandomNumberGenerator& rng,
- size_t bitsize)
+ size_t bitsize, bool set_high_bit)
{
set_sign(Positive);
@@ -27,9 +27,13 @@ void BigInt::randomize(RandomNumberGenerator& rng,
{
secure_vector<byte> array = rng.random_vec(round_up(bitsize, 8) / 8);
+ // Always cut unwanted bits
if(bitsize % 8)
array[0] &= 0xFF >> (8 - (bitsize % 8));
- array[0] |= 0x80 >> ((bitsize % 8) ? (8 - bitsize % 8) : 0);
+
+ // Set the highest bit if wanted
+ if (set_high_bit)
+ array[0] |= 0x80 >> ((bitsize % 8) ? (8 - bitsize % 8) : 0);
binary_decode(array);
}
@@ -41,12 +45,19 @@ void BigInt::randomize(RandomNumberGenerator& rng,
BigInt BigInt::random_integer(RandomNumberGenerator& rng,
const BigInt& min, const BigInt& max)
{
- BigInt range = max - min;
+ BigInt delta_upper_bound = max - min - 1;
- if(range <= 0)
+ if(delta_upper_bound < 0)
throw Invalid_Argument("random_integer: invalid min/max values");
- return (min + (BigInt(rng, range.bits() + 2) % range));
+ // Choose x in [0, delta_upper_bound]
+ BigInt x;
+ do {
+ auto bitsize = delta_upper_bound.bits();
+ x.randomize(rng, bitsize, false);
+ } while(x > delta_upper_bound);
+
+ return min + x;
}
}
diff --git a/src/lib/math/bigint/bigint.cpp b/src/lib/math/bigint/bigint.cpp
index 242a7fe0b..0a068c53e 100644
--- a/src/lib/math/bigint/bigint.cpp
+++ b/src/lib/math/bigint/bigint.cpp
@@ -87,9 +87,9 @@ BigInt::BigInt(const byte input[], size_t length, Base base)
/*
* Construct a BigInt from an encoded BigInt
*/
-BigInt::BigInt(RandomNumberGenerator& rng, size_t bits)
+BigInt::BigInt(RandomNumberGenerator& rng, size_t bits, bool set_high_bit)
{
- randomize(rng, bits);
+ randomize(rng, bits, set_high_bit);
}
/*
diff --git a/src/lib/math/bigint/bigint.h b/src/lib/math/bigint/bigint.h
index d866698b3..776884ad9 100644
--- a/src/lib/math/bigint/bigint.h
+++ b/src/lib/math/bigint/bigint.h
@@ -74,11 +74,15 @@ class BOTAN_DLL BigInt
BigInt(const byte buf[], size_t length, Base base = Binary);
/**
- * Create a random BigInt of the specified size
+ * \brief Create a random BigInt of the specified size
+ *
* @param rng random number generator
* @param bits size in bits
+ * @param set_high_bit if true, the highest bit is always set
+ *
+ * @see randomize
*/
- BigInt(RandomNumberGenerator& rng, size_t bits);
+ BigInt(RandomNumberGenerator& rng, size_t bits, bool set_high_bit = true);
/**
* Create BigInt of specified size, all zeros
@@ -431,10 +435,16 @@ class BOTAN_DLL BigInt
/**
* Fill BigInt with a random number with size of bitsize
+ *
+ * If \p set_high_bit is true, the highest bit will be set, which causes
+ * the entropy to be \a bits-1. Otherwise the highest bit is randomly choosen
+ * by the rng, causing the entropy to be \a bits.
+ *
* @param rng the random number generator to use
* @param bitsize number of bits the created random value should have
+ * @param set_high_bit if true, the highest bit is always set
*/
- void randomize(RandomNumberGenerator& rng, size_t bitsize = 0);
+ void randomize(RandomNumberGenerator& rng, size_t bitsize, bool set_high_bit = true);
/**
* Store BigInt-value in a given byte array
diff --git a/src/tests/catchy/test_bigint.cpp b/src/tests/catchy/test_bigint.cpp
index 5f4c960cf..9bd99e919 100644
--- a/src/tests/catchy/test_bigint.cpp
+++ b/src/tests/catchy/test_bigint.cpp
@@ -71,4 +71,101 @@ TEST_CASE("Bigint basics", "[bigint]")
}
}
+TEST_CASE("Bigint random_integer", "[bigint]")
+ {
+ RandomNumberGenerator *rng = RandomNumberGenerator::make_rng();
+
+ SECTION("min is 0")
+ {
+ // 0–9
+ const size_t MIN = 0;
+ const size_t MAX = 10; // excluded
+ const int ITERATIONS = 10000;
+
+ std::vector<int> counts(MAX, 0);
+ std::vector<double> ratios(MAX, 1.0);
+
+ for (size_t i = 0; i < ITERATIONS; i++)
+ {
+ BigInt b = BigInt::random_integer(*rng, MIN, MAX);
+ size_t x = b.to_u32bit();
+ counts[x]++;
+ }
+
+ std::stringstream debug;
+ for (size_t d = MIN; d < MAX; ++d)
+ {
+ auto ratio = static_cast<double>(counts[d]) / ITERATIONS;
+ ratios[d] = ratio;
+
+ if (!debug.str().empty())
+ {
+ debug << ", ";
+ }
+ debug << d << ": " << std::setprecision(3) << ratio;
+ }
+
+ INFO( debug.str() )
+
+ // Have ~ 10 % on each digit from 0-9
+ CHECK(( 0.09 < ratios[0] )); CHECK(( ratios[0] < 0.11 ));
+ CHECK(( 0.09 < ratios[1] )); CHECK(( ratios[1] < 0.11 ));
+ CHECK(( 0.09 < ratios[2] )); CHECK(( ratios[2] < 0.11 ));
+ CHECK(( 0.09 < ratios[3] )); CHECK(( ratios[3] < 0.11 ));
+ CHECK(( 0.09 < ratios[4] )); CHECK(( ratios[4] < 0.11 ));
+ CHECK(( 0.09 < ratios[5] )); CHECK(( ratios[5] < 0.11 ));
+ CHECK(( 0.09 < ratios[6] )); CHECK(( ratios[6] < 0.11 ));
+ CHECK(( 0.09 < ratios[7] )); CHECK(( ratios[7] < 0.11 ));
+ CHECK(( 0.09 < ratios[8] )); CHECK(( ratios[8] < 0.11 ));
+ CHECK(( 0.09 < ratios[9] )); CHECK(( ratios[9] < 0.11 ));
+ //CHECK( false );
+ }
+
+ SECTION("min is 10")
+ {
+ // 10–19
+ const size_t MIN = 10;
+ const size_t MAX = 20; // excluded
+ const size_t ITERATIONS = 10000;
+
+ std::vector<int> counts(MAX, 0);
+ std::vector<double> ratios(MAX, 1.0);
+
+ for (size_t i = 0; i < ITERATIONS; i++)
+ {
+ BigInt b = BigInt::random_integer(*rng, MIN, MAX);
+ size_t x = b.to_u32bit();
+ counts[x]++;
+ }
+
+ std::stringstream debug;
+ for (size_t d = MIN; d < MAX; ++d)
+ {
+ auto ratio = static_cast<double>(counts[d]) / ITERATIONS;
+ ratios[d] = ratio;
+
+ if (!debug.str().empty())
+ {
+ debug << ", ";
+ }
+ debug << d << ": " << std::setprecision(3) << ratio;
+ }
+
+ INFO( debug.str() )
+
+ // Have ~ 10 % on each digit from 10-19
+ CHECK(( 0.09 < ratios[10] )); CHECK(( ratios[10] < 0.11 ));
+ CHECK(( 0.09 < ratios[11] )); CHECK(( ratios[11] < 0.11 ));
+ CHECK(( 0.09 < ratios[12] )); CHECK(( ratios[12] < 0.11 ));
+ CHECK(( 0.09 < ratios[13] )); CHECK(( ratios[13] < 0.11 ));
+ CHECK(( 0.09 < ratios[14] )); CHECK(( ratios[14] < 0.11 ));
+ CHECK(( 0.09 < ratios[15] )); CHECK(( ratios[15] < 0.11 ));
+ CHECK(( 0.09 < ratios[16] )); CHECK(( ratios[16] < 0.11 ));
+ CHECK(( 0.09 < ratios[17] )); CHECK(( ratios[17] < 0.11 ));
+ CHECK(( 0.09 < ratios[18] )); CHECK(( ratios[18] < 0.11 ));
+ CHECK(( 0.09 < ratios[19] )); CHECK(( ratios[19] < 0.11 ));
+ //CHECK( false );
+ }
+ }
+
#endif
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