C#将字母或数字加密成字母的方法

public class MD5
{
        static List<KeyValuePair<char,char>> MappingList;
        #region 加密      public static string Encrypt(string str)
        /// <summary>
        /// 加密
        /// </summary>
        /// <param name="str">要加密的字符串</param>
        /// <returns>返回结果的字符串</returns>
        public static string Encrypt(string str)
        {
            MappingList = new List<KeyValuePair<char,char>>();
            for (char c = '0'; c <= '9'; c++)
                MappingList.Add(new KeyValuePair<char,char>(c,(char)(c - '0' + 'a')));
            for (char c = 'a'; c <= 'f'; c++)
                MappingList.Add(new KeyValuePair<char,(char)(c - 'a' + 'u')));
            return Encoding.ASCII.GetBytes(str)
                .Select((b,i) => (b ^ ((byte)(0xa0 + i))).ToString("x2"))
                .Aggregate("",(s,c) => s + c)
                .ToCharArray().Select(c => MappingList.First(kv => kv.Key == c).Value)
                .Aggregate("",c) => s + c);
        }
        #endregion
        #region 解密       public static string Decrypt(string str)
        /// <summary>
        /// 解密
        /// </summary>
        /// <param name="str">解密的字符串</param>
        /// <returns>返回结果字符串</returns>
        public static string Decrypt(string str)
        {
            string base16 = str.ToCharArray()
                .Select(c => MappingList.First(kv => kv.Value == c).Key)
                .Aggregate("",c) => s + c);
            return Encoding.ASCII.GetString((new byte[base16.Length / 2])
                .Select((b,i) => (byte)(Convert.ToByte(base16.Substring(i * 2,2),16) ^ ((byte)(0xa0 + i)))).ToArray());
        }
        #endregion
}