Encode.cs

using System;
using System.Collections.Generic;
using System.Linq;
class MainClass {
  public static Letter GetAlphabet(Letter code, Letter letter) {
    return (Letter)(((int)code + (int)letter) % 26);
  }
  
  public static void Main (string[] args) {
    var code = "INFO"
      .ToUpper()
      .Select(c => {
        switch (c) {
          case 'A': return Letter.A;
          case 'B': return Letter.B;
          case 'C': return Letter.C;
          case 'D': return Letter.D;
          case 'E': return Letter.E;
          case 'F': return Letter.F;
          case 'G': return Letter.G;
          case 'H': return Letter.H;
          case 'I': return Letter.I;
          case 'J': return Letter.J;
          case 'K': return Letter.K;
          case 'L': return Letter.L;
          case 'M': return Letter.M;
          case 'N': return Letter.N;
          case 'O': return Letter.O;
          case 'P': return Letter.P;
          case 'Q': return Letter.Q;
          case 'R': return Letter.R;
          case 'S': return Letter.S;
          case 'T': return Letter.T;
          case 'U': return Letter.U;
          case 'V': return Letter.V;
          case 'W': return Letter.W;
          case 'X': return Letter.X;
          case 'Y': return Letter.Y;
          case 'Z': return Letter.Z;
          default:
            return Letter.None;
        }
      })
      .Where(l => l != Letter.None)
      .ToArray();
    var text = "Lieberladysfirstalsjameslast";
    var letters = text
      .ToUpper()
      .Select(c => {
        switch (c) {
          case 'A': return Letter.A;
          case 'B': return Letter.B;
          case 'C': return Letter.C;
          case 'D': return Letter.D;
          case 'E': return Letter.E;
          case 'F': return Letter.F;
          case 'G': return Letter.G;
          case 'H': return Letter.H;
          case 'I': return Letter.I;
          case 'J': return Letter.J;
          case 'K': return Letter.K;
          case 'L': return Letter.L;
          case 'M': return Letter.M;
          case 'N': return Letter.N;
          case 'O': return Letter.O;
          case 'P': return Letter.P;
          case 'Q': return Letter.Q;
          case 'R': return Letter.R;
          case 'S': return Letter.S;
          case 'T': return Letter.T;
          case 'U': return Letter.U;
          case 'V': return Letter.V;
          case 'W': return Letter.W;
          case 'X': return Letter.X;
          case 'Y': return Letter.Y;
          case 'Z': return Letter.Z;
          default:
            return Letter.None;
        }
      })
      .Where(l => l != Letter.None);
    
    //Console.WriteLine("CODE="+code+"; TEXT=" + text);
    
    var iCode = 0;
    var encoded = letters
      .Select(l => {
        var newL = GetAlphabet(code[iCode], l);
        iCode = (iCode + 1) % code.Length;
        return newL;
      });
    
    foreach (var l in encoded)
      Console.Write(l.ToString());
    Console.WriteLine();
  }
  
  public enum Letter {
    A = 0,
    B = 1,
    C = 2,
    D = 3,
    E = 4,
    F = 5,
    G = 6,
    H = 7,
    I = 8,
    J = 9,
    K = 10,
    L = 11,
    M = 12,
    N = 13,
    O = 14,
    P = 15,
    Q = 16,
    R = 17,
    S = 18,
    T = 19,
    U = 20,
    V = 21,
    W = 22,
    X = 23,
    Y = 24,
    Z = 25,
    None = 99,
  }
}