using System;

using System.Linq;

using System.Text;

public class Program {

​

  private static char Rotate(char value, int shift, Range range) {

    int mod = (range.End.Value - range.Start.Value + 1);

​

    return (char) (range.Start.Value + 

      ((value - range.Start.Value + shift % mod) % mod + mod) % mod);

  }

  private static string doEncryption(string input, int key) { 

    if (input == null)

      return input;

​

    return string.Concat(input.Select(c =>

       c >= 'a' && c <= 'z' ? Rotate(c, key, 'a'..'z') 

     : c >= 'A' && c <= 'Z' ? Rotate(c, key, 'A'..'Z') 

     : c >= '0' && c <= '9' ? Rotate(c, key, '0'..'9') 

     : c));

  }

​

  private static string doDecryption(string input, int key) => 

    doEncryption(input, -key);  

  public static void Main() {

    string[] tests = new string[] {

      "Hello world",

      "key",

      "20",

      "output",

      "Byffi qilfx",

    };

​

    int key = 20;

​

    string result = String.Join(Environment.NewLine, tests

      .Select(test => (plain: test, code: doEncryption(test, key)))

      .Select(pair => $"{pair.plain,-12} => {pair.code,-12} => {doDecryption(pair.code, key)}"));

​

    Console.Write(result);

  }

}