關於RSA加密算法的長度限制問題

RSA是常用的非對稱加密算法。近來有學生在項目中使用System.Security類庫中的RSA加密算法時，出現了“不正確的長度”，這實際上是因為待加密的數據超長所致。.net Framework中提供的RSA算法規定，每次加密的字節數，不能超過密鑰的長度值減去11,而每次加密得到的密文長度，卻恰恰是密鑰的長度。所以，如果要加密較長的數據，可以采用數據截取的方法，分段加密，實現如下：

       RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();

       byte[] data = ........;         //要加密的數據

       string publicKey = .... ;  //獲取公鑰

       rsa.FromXmlString(publicKey);

       int keySize = rsa.KeySize / 8;

       int bufferSize = keySize - 11;

       byte[] buffer = new byte[bufferSize];

       MemoryStream msInput = new MemoryStream(data);

       MemoryStream msOuput = new MemoryStream();

       int readLen = msInput.Read(buffer, 0, bufferSize);

       while(readLen > 0)

       {

               byte[] dataToEnc = new byte[readLen];

               Array.Copy(buffer, 0 , dataToEnc, 0, readLen);

               byte[] encData = rsa.Encrypt(dataToEnc , false);

               msOutput.Write(encData, 0, encData.Length);

               readLen = msInput.Read(buffer, 0, bufferSize);

       }

       msInput.Close();

       byte[] result = msOutput.ToArray();    //得到加密結果

       msOutput.Close();

       rsa.Clear();

 

解密時肯定也要使用分段解密，算法如下：

       RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();

       byte[] key = .....;    //加載私鑰
       string privateKey = Encoding.ASCII.GetString(key);
       byte[] dataEnc = ...;   //加載密文

       rsa.FromXmlString(privateKey);

       int keySize = rsa.KeySize / 8;

       byte[] buffer = new byte[keySize];

       MemoryStream msInput = new MemoryStream(dataEnc );

       MemoryStream msOuput = new MemoryStream();

       int readLen = msInput.Read(buffer, 0, keySize);

       while(readLen > 0)

       {

               byte[] dataToDec = new byte[readLen];

               Array.Copy(buffer, 0 , dataToDec, 0, readLen);

               byte[] decData = rsa.Decrypt(dataToDec , false);

               msOutput.Write(decData, 0, decData.Length);

               readLen = msInput.Read(buffer, 0, keySize);

       }

       msInput.Close();

       byte[] result = msOutput.ToArray();    //得到解密結果

       msOutput.Close();

       rsa.Clear();