关于RSA加密算法的长度限制问题

RSA是常用的非对称加密算法。近来有学生在项目中使用System.Security类库中的RSA加密算法时，出现了“不正确的长度”，这实际上是因为待加密的数据超长所致。.net Framework中提供的RSA算法规定，每次加密的字节数，不能超过密钥的长度值减去11,而每次加密得到的密文长度，却恰恰是密钥的长度。所以，如果要加密较长的数据，可以采用数据截取的方法，分段加密，实现如下：

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();

byte[] data = ........;         //要加密的数据

string publicKey = .... ;  //获取公钥

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();    //得到加密结果

msOutput.Close();

rsa.Clear();

解密时肯定也要使用分段解密，算法如下：

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();

byte[] key = .....;    //加载私钥
       string privateKey = Encoding.ASCII.GetString(key);
       byte[] dataEnc = ...;   //加载密文

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();    //得到解密结果

msOutput.Close();

rsa.Clear();