Encrypt/Decrypt String in C#

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

static byte[] keys = ASCIIEncoding.ASCII.GetBytes("friction");

public static string StringEncrypt(string originalString)
        {

            if (String.IsNullOrEmpty(originalString))
            {
                throw new ArgumentNullException
                       ("The string which needs to be encrypted can not be null.");
            }
            DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
            MemoryStream memoryStream = new MemoryStream();
            CryptoStream cryptoStream = new CryptoStream(memoryStream,
                cryptoProvider.CreateEncryptor(keys, keys), CryptoStreamMode.Write);
            StreamWriter writer = new StreamWriter(cryptoStream);
            writer.Write(originalString);
            writer.Flush();
            cryptoStream.FlushFinalBlock();
            writer.Flush();
            return Convert.ToBase64String(memoryStream.GetBuffer(), 0, (int)memoryStream.Length);
        }


public static string StringDecrypt(string cryptedString)
        {
            if (String.IsNullOrEmpty(cryptedString))
            {
                throw new ArgumentNullException
                   ("The string which needs to be decrypted can not be null.");
            }
            DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
            MemoryStream memoryStream = new MemoryStream
                    (Convert.FromBase64String(cryptedString));
            CryptoStream cryptoStream = new CryptoStream(memoryStream,
                cryptoProvider.CreateDecryptor(keys, keys), CryptoStreamMode.Read);
            StreamReader reader = new StreamReader(cryptoStream);
            return reader.ReadToEnd();
        }