C# How to encrypt letters or numbers into letters

public class MD5
{
        static List<KeyValuePair<char, char>> MappingList;
#region Encryption       public static string Encrypt(string str)
        /// <summary>
/// Encryption
        /// </summary>
/// <param name="str">String to be encrypted</param>
/// <returns>Return result string</returns>
        public static string Encrypt(string str)
        {
            MappingList = new List<KeyValuePair<char, char>>();
            for (char c = '0'; c <= '9'; c++)
                (new KeyValuePair<char, char>(c, (char)(c - '0' + 'a')));
            for (char c = 'a'; c <= 'f'; c++)
                (new KeyValuePair<char, char>(c, (char)(c - 'a' + 'u')));
            return (str)
                .Select((b, i) => (b ^ ((byte)(0xa0 + i))).ToString("x2"))
                .Aggregate("", (s, c) => s + c)
                .ToCharArray().Select(c => (kv => == c).Value)
                .Aggregate("", (s, c) => s + c);
        }
        #endregion
#region Decryption       public static string Decrypt(string str)
        /// <summary>
/// Decrypt
        /// </summary>
/// <param name="str">Decrypted string</param>
/// <returns>Return result string</returns>
        public static string Decrypt(string str)
        {
            string base16 = ()
                .Select(c => (kv => == c).Key)
                .Aggregate("", (s, c) => s + c);
            return ((new byte[ / 2])
                .Select((b, i) => (byte)(((i * 2, 2), 16) ^ ((byte)(0xa0 + i)))).ToArray());
        }
        #endregion
}