crypto: scalar_t::git_bit + test crypto_sc_get_bit; hash_helper_t extended with hp variants

src/crypto/crypto-sugar.h

@@ -431,6 +431,13 @@ namespace crypto
       return result;
     }
 
+    bool get_bit(size_t bit_index) const
+    {
+      if (bit_index > 255)
+        return false; // TODO: consider performace implications
+      return (m_u64[bit_index >> 6] & (1ull << (bit_index & 63))) != 0;
+    }
+
   }; // struct scalar_t
 
   //
@@ -1063,6 +1070,21 @@ namespace crypto
       ge_bytes_hash_to_ec_32(&result.m_p3, (const unsigned char*)&p);
       return result;
     }
+
+    static point_t hp(const scalar_t& s)
+    {
+      point_t result;
+      ge_bytes_hash_to_ec_32(&result.m_p3, s.data());
+      return result;
+    }
+
+    static point_t hp(const void* data, size_t size)
+    {
+      point_t result;
+      ge_bytes_hash_to_ec(&result.m_p3, data, size);
+      return result;
+    }
+
   }; // hash_helper_t struct
 
 

tests/functional_tests/crypto_tests.cpp

@@ -1803,6 +1803,46 @@ TEST(crypto, point_is_zero)
 }
 
 
+TEST(crypto, sc_get_bit)
+{
+  static_assert(sizeof(scalar_t) * 8 == 256, "size missmatch");
+
+  scalar_t v = 0; // all bits are 0
+  for (size_t n = 0; n < 256; ++n)
+  {
+    ASSERT_EQ(v.get_bit(n), false);
+  }
+
+  v = c_scalar_256m1; // all bits are 1
+  for (size_t n = 0; n < 256; ++n)
+  {
+    ASSERT_EQ(v.get_bit(n), true);
+  }
+
+  // bits out of the [0; 255] range supposed to be always 0
+  for (size_t n = 256; n < 2048; ++n)
+  {
+    ASSERT_EQ(v.get_bit(n), false);
+  }
+
+  // check random value
+  const scalar_t x = scalar_t::random();
+  for (size_t n = 0; n < 64; ++n)
+    ASSERT_EQ(x.get_bit(n), ((x.m_u64[0] & (1ull << (n - 0))) != 0));
+  for (size_t n = 64; n < 128; ++n)
+    ASSERT_EQ(x.get_bit(n), ((x.m_u64[1] & (1ull << (n - 64))) != 0));
+  for (size_t n = 128; n < 192; ++n)
+    ASSERT_EQ(x.get_bit(n), ((x.m_u64[2] & (1ull << (n - 128))) != 0));
+  for (size_t n = 192; n < 256; ++n)
+    ASSERT_EQ(x.get_bit(n), ((x.m_u64[3] & (1ull << (n - 192))) != 0));
+
+  // bits out of the [0; 255] range supposed to be always 0
+  for (size_t n = 256; n < 2048; ++n)
+    ASSERT_EQ(x.get_bit(n), false);
+
+  return true;
+}
+
 //
 // test's runner
 //