Question

I’m trying to encrypt an array of 256 bytes by using RSACryptoServiceProvider but I’m getting an exception of “Bad Length” each time.

byte[] seed = new byte[256];
byte[] cypherSeed = new byte[256];

RNGCryptoServiceProvider gen = new RNGCryptoServiceProvider();
gen.GetBytes(seed);

using (RSACryptoServiceProvider rsaCryptoServiceProvider = new RSACryptoServiceProvider(2560))
{
   RSAParameters param = new RSAParameters();
   param.Exponent = this.exponent;
   param.Modulus = this.modulus;

   // set public keys
   rsaCryptoServiceProvider.ImportParameters(param);
   cypherSeed = rsaCryptoServiceProvider.Encrypt(seed, false);
}

What am I doing wrong??!!

Answer 1

You forgot to account for the padding. The message size is smaller than the key size because RSA needs the rest of the block for padding.

If you use PKCS#1 v1.5 padding, you need 11 bytes of padding, if you use the stronger OAEP padding, you need 2*hashsize + 2 bytes as padding.

See RSACryptoServiceProvider.Encrypt Method

You should also consider using hybrid encryption, where you encrypt a random key with RSA, and the actual message with that key.