Triple DES Encrypt/decrypt extension method

I wanted to have a simple encryption method so that I could convert any string.

return credentials.Encrypt();

You should note that there is no constructor as it is better to initialise our key with a function call.

/// <summary>
/// The class will encrypt or decrpyt based on Triple DES algoritm
/// </summary>
public static class EncryptionService
{
    static byte[] _encryptionKey = InitialiseKey();

    /// <summary>
    /// Initialises the key.
    /// </summary>
    /// <returns></returns>
    private static byte[] InitialiseKey()
    {
        string encryptionKey = ConfigurationManager.AppSettings["EncryptionPassword"];
        if (string.IsNullOrEmpty(encryptionKey))
            throw new ConfigurationErrorsException(
		"'EncryptionPassword' is not defined in your config file.");

        byte[] encryptionKeyBytes = ASCIIEncoding.ASCII.GetBytes(encryptionKey);

        using (MD5CryptoServiceProvider hashmd5 = new MD5CryptoServiceProvider())
        {
            return hashmd5.ComputeHash(encryptionKeyBytes);
        }
    }

    /// <summary>
    /// Decrypts the specified input.
    /// </summary>
    /// <param name="input">The input.</param>
    /// <returns></returns>
    public static string Decrypt(this string input)
    {
        if (string.IsNullOrEmpty(input))
            return string.Empty;

        using (TripleDESCryptoServiceProvider des = new TripleDESCryptoServiceProvider())
        {
            des.Key = _encryptionKey;
            des.Mode = CipherMode.ECB;

            byte[] buff = null;
            try
            {
                buff = Convert.FromBase64String(input);
            }
            catch (FormatException)
            {
                return string.Empty;
            }

            return ASCIIEncoding.ASCII.GetString(
                des.CreateDecryptor().TransformFinalBlock(buff, 0, buff.Length));
        }
    }

    /// <summary>
    /// Encrypts the specified input.
    /// </summary>
    /// <param name="input">The input.</param>
    /// <returns></returns>
    public static string Encrypt(this string input)
    {
        using (TripleDESCryptoServiceProvider des = new TripleDESCryptoServiceProvider())
        {
            des.Key = _encryptionKey;
            des.Mode = CipherMode.ECB;

            byte[] buff = ASCIIEncoding.ASCII.GetBytes(input);

            return Convert.ToBase64String(
                des.CreateEncryptor().TransformFinalBlock(buff, 0, buff.Length));
        }
    }
}