.NET Framework Encryption / Cryptography Encrypt and decrypt data using AES (in C#)

Example

using System;
using System.IO;
using System.Security.Cryptography;

namespace Aes_Example
{
    class AesExample
    {
        public static void Main()
        {
            try
            {
                string original = "Here is some data to encrypt!";

                // Create a new instance of the Aes class.
                // This generates a new key and initialization vector (IV).
                using (Aes myAes = Aes.Create())
                {
                    // Encrypt the string to an array of bytes.
                    byte[] encrypted = EncryptStringToBytes_Aes(original, 
                                                                myAes.Key,
                                                                myAes.IV);

                    // Decrypt the bytes to a string.
                    string roundtrip = DecryptStringFromBytes_Aes(encrypted, 
                                                                  myAes.Key, 
                                                                  myAes.IV);

                    //Display the original data and the decrypted data.
                    Console.WriteLine("Original:   {0}", original);
                    Console.WriteLine("Round Trip: {0}", roundtrip);
                }
            }
            catch (Exception e)
            {
                Console.WriteLine("Error: {0}", e.Message);
            }
        }

        static byte[] EncryptStringToBytes_Aes(string plainText, byte[] Key, byte[] IV)
        {
            // Check arguments.
            if (plainText == null || plainText.Length <= 0)
                throw new ArgumentNullException("plainText");
            if (Key == null || Key.Length <= 0)
                throw new ArgumentNullException("Key");
            if (IV == null || IV.Length <= 0)
                throw new ArgumentNullException("IV");

            byte[] encrypted;

            // Create an Aes object with the specified key and IV.
            using (Aes aesAlg = Aes.Create())
            {
                aesAlg.Key = Key;
                aesAlg.IV = IV;

                // Create a decrytor to perform the stream transform.
                ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key,
                                                                    aesAlg.IV);

                // Create the streams used for encryption.
                using (MemoryStream msEncrypt = new MemoryStream())
                {
                    using (CryptoStream csEncrypt = new CryptoStream(msEncrypt,
                                                                     encryptor,
                                                                     CryptoStreamMode.Write))
                    {
                        using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                        {
                            //Write all data to the stream.
                            swEncrypt.Write(plainText);
                        }

                        encrypted = msEncrypt.ToArray();
                    }
                }
            }

            // Return the encrypted bytes from the memory stream.
            return encrypted;
        }

        static string DecryptStringFromBytes_Aes(byte[] cipherText, byte[] Key, byte[] IV)
        {
            // Check arguments.
            if (cipherText == null || cipherText.Length <= 0)
                throw new ArgumentNullException("cipherText");
            if (Key == null || Key.Length <= 0)
                throw new ArgumentNullException("Key");
            if (IV == null || IV.Length <= 0)
                throw new ArgumentNullException("IV");

            // Declare the string used to hold the decrypted text.
            string plaintext = null;

            // Create an Aes object with the specified key and IV.
            using (Aes aesAlg = Aes.Create())
            {
                aesAlg.Key = Key;
                aesAlg.IV = IV;

                // Create a decrytor to perform the stream transform.
                ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key,
                                                                    aesAlg.IV);

                // Create the streams used for decryption.
                using (MemoryStream msDecrypt = new MemoryStream(cipherText))
                {
                    using (CryptoStream csDecrypt = new CryptoStream(msDecrypt,
                                                                     decryptor,
                                                                     CryptoStreamMode.Read))
                    {
                        using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                        {

                            // Read the decrypted bytes from the decrypting stream
                            // and place them in a string.
                            plaintext = srDecrypt.ReadToEnd();
                        }
                    }
                }
            }

            return plaintext;
        }
    }
}

This example is from MSDN.

It is a console demo application, showing how to encrypt a string by using the standard AES encryption, and how to decrypt it afterwards.

(AES = Advanced Encryption Standard, a specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology (NIST) in 2001 which is still the de-facto standard for symmetric encryption)

Notes:

• In a real encryption scenario, you need to choose a proper cipher mode (can be assigned to the Mode property by selecting a value from the CipherMode enumeration). Never use the CipherMode.ECB (electronic codebook mode), since this procuces a weak cypher stream

• To create a good (and not a weak) Key, either use a cryptographic random generator or use the example above (Create a Key from a Password). The recommended KeySize is 256 bit. Supported key sizes are available via the LegalKeySizes property.

• To initialize the initialization vector IV, you can use a SALT as shown in the example above (Random SALT)

• Supported block sizes are available via the SupportedBlockSizes property, the block size can be assigned via the BlockSize property

Usage: see Main() method.