Remove unneeded check in Integer::Randomize(bitCount) (GH #1206)

Update docs to specify case when bitCount==0
Add tests for Randomize function in debug builds

integer.cpp

@@ -3522,8 +3522,12 @@ void Integer::Randomize(RandomNumberGenerator &rng, size_t nbits)
 	const size_t nbytes = nbits/8 + 1;
 	SecByteBlock buf(nbytes);
 	rng.GenerateBlock(buf, nbytes);
-	if (nbytes)
-		buf[0] = (byte)Crop(buf[0], nbits % 8);
+
+	// https://github.com/weidai11/cryptopp/issues/1206
+	// if (nbytes)
+	//     buf[0] = (byte)Crop(buf[0], nbits % 8);
+
+	buf[0] = (byte)Crop(buf[0], nbits % 8);
 	Decode(buf, nbytes, UNSIGNED);
 }
 

integer.h

@@ -444,6 +444,7 @@ class CRYPTOPP_DLL Integer : private InitializeInteger, public ASN1Object
 		/// \param rng RandomNumberGenerator used to generate material
 		/// \param bitCount the number of bits in the resulting integer
 		/// \details The random integer created is uniformly distributed over <tt>[0, 2<sup>bitCount</sup>]</tt>.
+		/// \note If \p bitCount is 0, then this Integer is set to 0 (and not 0 or 1).
 		void Randomize(RandomNumberGenerator &rng, size_t bitCount);
 
 		/// \brief Set this Integer to random integer

validat2.cpp

@@ -1280,7 +1280,47 @@ bool TestIntegerOps()
        std::cout << "FAILED:";
     std::cout << "  Exponentiation operations\n";
 
-    return pass;
+    // ****************************** Integer Randomize ******************************
+
+	try
+	{
+		const word32 bitCounts[] = {
+            0,1,2,3,4,5,6,7,8,9,15,16,17,31,32,33,63,64,65,127,128,129
+		};
+
+		for (size_t i=0; i<COUNTOF(bitCounts); ++i)
+		{
+			result = true;
+            unsigned int maxBits = 0;
+			const size_t bitCount = bitCounts[i];
+			Integer n;
+
+			for (size_t j=0; j<128; ++j)
+			{
+				n.Randomize(prng, bitCount);
+				maxBits = (std::max)(maxBits, n.BitCount());
+			}
+
+			result &= (maxBits == bitCount);
+			if (!result)
+				std::cout << "FAILED:  Randomize " << bitCount << "-bits\n";
+
+			pass &= result;
+		}
+	}
+	catch (const Exception&)
+	{
+		pass = false;
+		result = false;
+	}
+
+	if (!pass)
+		std::cout << "FAILED:";
+	else
+		std::cout << "passed:";
+	std::cout << "  Randomize of various bit lengths\n";
+
+	return pass;
 }
 #endif