Question

When I execute following code:

        const int blockSize = 64;
        const int keySize = 256;

        string inputText;
        byte[] inputBytes;
        using (StreamReader reader = new StreamReader(inputFile))
        {
            inputText = reader.ReadToEnd();
            inputBytes = StringToBytes(inputText);
        }
        byte[] outputBytes = new byte[inputBytes.Length];

        CipherKeyGenerator keygen = new CipherKeyGenerator();
        SecureRandom rand = new SecureRandom();

        KeyGenerationParameters keygenParams = new KeyGenerationParameters(rand, keySize);
        keygen.Init(keygenParams);
        byte[] key = keygen.GenerateKey();

        BufferedBlockCipher cipher = null;
        ICipherParameters cipherParams = null;

                byte[] iv = new byte[blockSize];
                rand.NextBytes(iv);
                cipherParams = new ParametersWithIV(
                    new ParametersWithSBox(
                        new KeyParameter(key),
                        Gost28147Engine.GetSBox("E-A")),
                    iv);
                cipher = new BufferedBlockCipher(
                    new CfbBlockCipher(new Gost28147Engine(), subblockLength));

        cipher.Init(true, cipherParams);
        int bytesLength = cipher.ProcessBytes(inputBytes, 0, inputBytes.Length, outputBytes, 0);
        cipher.DoFinal(outputBytes, bytesLength);

…I get following exception:

System.ArgumentOutOfRangeException was unhandled
  Source=mscorlib
  ParamName=dstIndex
  StackTrace:
       w System.Array.Copy(Array sourceArray, Int32 sourceIndex, Array destinationArray, Int32 destinationIndex, Int32 length, Boolean reliable)
       w System.Array.Copy(Array sourceArray, Int32 sourceIndex, Array destinationArray, Int32 destinationIndex, Int32 length)
       w Org.BouncyCastle.Crypto.Modes.CfbBlockCipher.Init(Boolean forEncryption, ICipherParameters parameters)
       w Org.BouncyCastle.Crypto.BufferedBlockCipher.Init(Boolean forEncryption, ICipherParameters parameters)
       ...

Answer 1

Already resolved problem. In new CfbBlockCipher(new Gost28147Engine(), subblockLength)) size of block should be passed as number of bits, not bytes. IV should be the same size as size of block passed to constructor of CfbBlockCipher so code have look like that:
byte[] iv = new byte[subblockLength]

A look into CfbBlockCipher.Init, exception is easy to track from here:

/**
* Initialise the cipher and, possibly, the initialisation vector (IV).
* If an IV isn't passed as part of the parameter, the IV will be all zeros.
* An IV which is too short is handled in FIPS compliant fashion.
*
* @param forEncryption if true the cipher is initialised for
*  encryption, if false for decryption.
* @param param the key and other data required by the cipher.
* @exception ArgumentException if the parameters argument is
* inappropriate.
*/
public void Init(
    bool forEncryption,
    ICipherParameters parameters)
{
    this.encrypting = forEncryption;
    if (parameters is ParametersWithIV)
    {
        ParametersWithIV ivParam = (ParametersWithIV) parameters;
        byte[] iv = ivParam.GetIV();
        int diff = IV.Length - iv.Length;
        Array.Copy(iv, 0, IV, diff, iv.Length);
        Array.Clear(IV, 0, diff);

        parameters = ivParam.Parameters;
    }
    Reset();
    cipher.Init(true, parameters);
}