Two useful hashing extension methods (MD5 and SHA1).

CryptoExtensions.cs

using System.Security.Cryptography;

namespace Extensions {
  public static class CryptoExtensions {
    private static readonly SHA1 Sha1 = SHA1.Create();
    private static readonly MD5 Md5 = MD5.Create();

    /// <summary>
    /// Performs an MD5 hash on the string and returns a 32 character hex format.
    /// </summary>
    /// <param name="str"></param>
    /// <returns>A 32 character hex-formatted string (e.g. ed076287532e86365e841e92bfc50d8c)</returns>
    /// <remarks>
    /// http://msdn.microsoft.com/en-us/library/system.security.cryptography.md5.aspx
    /// </remarks>
    public static string Md5HashHex(this string str) {
      return String.Join("",
                         Md5.ComputeHash(Encoding.Default.GetBytes(str))
                            .SelectMany(b => b.ToString("x2"))
                            .Select(c => c.ToString(CultureInfo.InvariantCulture))
                            .ToArray());
    }

    /// <summary>
    /// Performs a SHA1 hash on the string and returns a 40 character hex format.
    /// </summary>
    /// <param name="str"></param>
    /// <returns>A 40 character hex-formatted string (e.g. ca5c9c0732f9bb7d81f4eb8e49db81a809a26fdf)</returns>
    /// <remarks>
    /// http://msdn.microsoft.com/en-us/library/system.security.cryptography.sha1.aspx
    /// </remarks>
    public static string SHA1HashHex(this string str) {
      return String.Join("",
                         Sha1.ComputeHash(Encoding.Default.GetBytes(str))
                             .SelectMany(b => b.ToString("x2"))
                             .Select(c => c.ToString(CultureInfo.InvariantCulture))
                             .ToArray());
    }    
  }
}

Encryption.cs

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

namespace Infrastructure.Encryption {
    public static class AES {


        // Encrypt string using an IV (if null, AesManaged creates one automatically using RNGCryptoServiceProvider)

        public static byte[] EncryptAES(this string plainText, byte[] iv) {
            if (String.IsNullOrWhiteSpace(plainText))
                return null;

            byte[] encrypted;
            // Create an AesManaged object
            // with the specified key and IV.
            using (var aesAlg = new AesManaged()) {
                aesAlg.Key = // Use a key!;
                aesAlg.IV = iv;

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

                // Create the streams used for encryption.
                using (var msEncrypt = new MemoryStream()) {
                    using (var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write)) {
                        using (var 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;

        }

        // Decrypts using AES and an IV (use the same IV when encrypting/decrypting)

        public static string DecryptAES(this byte[] cipherText, byte[] iv) {
            if (cipherText == null || cipherText.Length <= 0)
                return null;

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

            // Create an AesManaged object
            // with the specified key and IV.
            using (var aesAlg = new AesManaged()) {
                aesAlg.Key = // use the same key!;
                aesAlg.IV = iv;

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

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

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

            }

            return plaintext;

        }
    }
}