Why does Bouncy Castle RSAEngine.processblock method always returns 255 bytes when decrypting? - rsa

Question

I am experiment with RSA using bouncy castle. I know this is the other way around to the general convention but to my understanding, it still should work theoretically.
I encrypt some data using an RSA private key. The length of the data being encrypted is 294 bytes. The encryption function outputs 512 bytes. I then call the decryption method by passing the above output cipher text and the corresponding public key. My problem is that the decryption always returns a buffer of 255 bytes whereas the actual input to the Encryption function was 294 bytes. What could be the reason for this ?
The following is the source code of the encryption and decryption functions.
public static byte[] RSAEncrypt(byte[] data, AsymmetricKeyParameter key)
{
try
{
RsaEngine e = new RsaEngine();
e.Init(true, key);
int blockSize = e.GetInputBlockSize();
List<byte> output = new List<byte>();
for (int chunkPosition = 0; chunkPosition < data.Length; chunkPosition += blockSize)
{
int chunkSize = Math.Min(blockSize, data.Length - (chunkPosition * blockSize));
output.AddRange(e.ProcessBlock(data, chunkPosition, chunkSize));
}
return output.ToArray();
}
catch (Exception ex)
{
return null;
}
}
public static byte[] RSADecrypt(byte[] data, AsymmetricKeyParameter key)
{
try
{
RsaEngine e = new RsaEngine();
e.Init(false, key);
int blockSize = e.GetInputBlockSize();
List<byte> output = new List<byte>();
for (int chunkPosition = 0; chunkPosition < data.Length; chunkPosition += blockSize)
{
int chunkSize = Math.Min(blockSize, data.Length - (chunkPosition * blockSize));
output.AddRange(e.ProcessBlock(data, chunkPosition, chunkSize));
}
return output.ToArray();
}
catch (Exception ex)
{
return null;
}
}

Answer 1

RSA is an asymmetric encryption method that encrypts a number less than the modulus of the RSA key (255 bytes would indicate that you're using a 256*8 = 2048 bit RSA key/modulus)
What you need to do to encrypt values greater than that is to generate a key, encrypt the data using a symmetric cipher (AES is not a bad choice) and encrypt the AES key using your private RSA key (preferably along with some other random data).
The AES key is a maximum of 256 bits, which will encrypt just fine with RSA, and AES does not have a size limit.