Error "InvalidCipherTextException: data start wrong 64" with Bouncy Castle

Question

I am encrypting and decrypting some Data using BouncyCastle, but when the lenght of the word is too long (i don´t know exactly the value), i got this error "InvalidCipherTextException: data start wrong 64"

This is my Class of Encription:

public static class Crypto
    {
        public static IAsymmetricBlockCipher CriarCipher(byte[] encodingParam)
        {
            // Creating the RSA algorithm object
            IAsymmetricBlockCipher cipher = new OaepEncoding(new RsaEngine(), new Sha256Digest(), encodingParam);

            return cipher;
        }

        public static AsymmetricCipherKeyPair CreatePair()
        {
            RsaKeyPairGenerator rsaKeyPairGnr = new RsaKeyPairGenerator();
            rsaKeyPairGnr.Init(new KeyGenerationParameters(new SecureRandom(), 1024));
            AsymmetricCipherKeyPair keyPair = rsaKeyPairGnr.GenerateKeyPair();

            return keyPair;
        }

        public static byte[] Encriptar(RsaKeyParameters publicKey, string texto, byte[] encodingParam)
        {
            // Creating the RSA algorithm object
            IAsymmetricBlockCipher cipher = new OaepEncoding(new RsaEngine(), new Sha256Digest(), encodingParam);
            var palavrabyte = Encoding.UTF8.GetBytes(texto);
             // Initializing the RSA object for Encryption with RSA public key. Remember, for encryption, public key is needed
            cipher.Init(true, publicKey);
            byte[] ciphered = cipher.ProcessBlock(palavrabyte, 0, palavrabyte.Length);

            return ciphered;
        }

        public static string Decriptar(RsaKeyParameters privateKey, string txtEncript, byte[] encodingParam)
        {
            // Creating the RSA algorithm object
            IAsymmetricBlockCipher cipher = new OaepEncoding(new RsaEngine(), new Sha256Digest(), encodingParam);
            // Initializing the RSA object for Encryption with RSA public key. Remember, for encryption, public key is needed
            cipher.Init(false, privateKey);
            byte[] txtEncriptBytes = Convert.FromBase64String(txtEncript);
            byte[] deciphered = cipher.ProcessBlock(txtEncriptBytes, 0, txtEncriptBytes.Length);
            string decipheredText = Encoding.UTF8.GetString(deciphered, 0, deciphered.Length);

            return decipheredText;
        }

    }

This is the code for OAEPE Encoding:

SHA256Managed Hash = new SHA256Managed();
byte[] ParamOEAP = Hash.ComputeHash("Example" + anotherdata);

And the class SHA256Managed:

public class SHA256Managed
    {
        public byte[] ComputeHash(string text)
        {
           Sha256Digest dig = new Sha256Digest();
           byte[] msgBytes = Encoding.UTF8.GetBytes(text);
           dig.BlockUpdate(msgBytes, 0, msgBytes.Length);
           byte[] result = new byte[dig.GetDigestSize()];
           dig.DoFinal(result, 0);

           return result;            
        }
    }

When i encrypt the word, per example, "Subtracão de Incapazes", the decryption its ok.

When i encrypt the word, per example, "Estelionato por Emissão de Cheque sem Suficiente Provisão de Fundos", the decryption brokes in the Decriptar codeline:

byte[] deciphered = cipher.ProcessBlock(txtEncriptBytes, 0, txtEncriptBytes.Length);

What i am doing wrong ?

Answer 1

Changing on CreatePair the line: rsaKeyPairGnr.Init(new KeyGenerationParameters(new SecureRandom(), 2048))

From 1024 to 2048 !! Now, big phrases are decrypted.