A Simple Decrypt Function

First Add This Namespaces

using System;
using System.Collections.Generic;
using System.Text;
using System.Security;
using System.Security.Cryptography;
using System.Runtime.InteropServices;
using System.Text.RegularExpressions;
using System.IO;
using System.Windows.Forms;

Now The Function:

public void Decrypt(string inputfile,string outputfile,string passPhrase,string saltValue,string hashAlgorithm,int passwordIterations,string initVector,int keySize)
// Convert strings defining encryption key characteristics into byte
// arrays. Let us assume that strings only contain ASCII codes.
// If strings include Unicode characters, use Unicode, UTF7, or UTF8
// encoding.
byte[] initVectorBytes = Encoding.ASCII.GetBytes(initVector);
byte[] saltValueBytes = Encoding.ASCII.GetBytes(saltValue);

// Convert our ciphertext into a byte array.
byte[] cipherTextBytes = ReadByteArrayFromFile(inputfile);

// First, we must create a password, from which the key will be
// derived. This password will be generated from the specified
// passphrase and salt value. The password will be created using
// the specified hash algorithm. Password creation can be done in
// several iterations.
PasswordDeriveBytes password = new PasswordDeriveBytes(

// Use the password to generate pseudo-random bytes for the encryption
// key. Specify the size of the key in bytes (instead of bits).
byte[] keyBytes = password.GetBytes(keySize / 8);

// Create uninitialized Rijndael encryption object.
RijndaelManaged symmetricKey = new RijndaelManaged();

// It is reasonable to set encryption mode to Cipher Block Chaining
// (CBC). Use default options for other symmetric key parameters.
symmetricKey.Mode = CipherMode.CBC;

// Generate decryptor from the existing key bytes and initialization
// vector. Key size will be defined based on the number of the key
// bytes.
ICryptoTransform decryptor = symmetricKey.CreateDecryptor(

// Define memory stream which will be used to hold encrypted data.
MemoryStream memoryStream = new MemoryStream(cipherTextBytes);

// Define cryptographic stream (always use Read mode for encryption).
CryptoStream cryptoStream = new CryptoStream(memoryStream,

// Since at this point we don't know what the size of decrypted data
// will be, allocate the buffer long enough to hold ciphertext;
// plaintext is never longer than ciphertext.
byte[] plainTextBytes = new byte[cipherTextBytes.Length];

// Start decrypting.
int decryptedByteCount = cryptoStream.Read(plainTextBytes,

// Close both streams.


// Convert decrypted data into a string.
// Let us assume that the original plaintext string was UTF8-encoded.
//string plainText = Encoding.UTF8.GetString(plainTextBytes,

// Return decrypted string.
//return plainText;

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