Spygram

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

public class Program
{
    public static void Main()
    {
        var privateKey = Console.ReadLine();
        var messages = new List<Message>();
        var rgx = new Regex(@"^TO: ([A-Z]+); MESSAGE: (.+);$");
        string input;
        while (!(input = Console.ReadLine()).Equals("END"))
        {
            var m = rgx.Match(input);
            if (m.Success)
                messages.Add(new Message(m.Groups[1].Value, m.Groups[2].Value));
        }

        var sorted = messages
        .OrderBy(m => m.Sender);
        foreach (var message in sorted)
        {
            var encrypted = Message.Encrypt(message, privateKey);
            Console.WriteLine(encrypted);
        }
    }
}

public class Message
{
    public Message(string sender, string content)
    {
        Sender = sender;
        Content = content;
    }

    public string Sender { get; }
    public string Content { get; }

    public static string Encrypt(Message message, string privateKey)
    {
        var toEncrypt = $"TO: {message.Sender}; MESSAGE: {message.Content};";
        var result = new StringBuilder(toEncrypt.Length);
        var keyIndex = 0;
        foreach (var letter in toEncrypt)
        result.Append((char) (letter + privateKey[keyIndex++ % privateKey.Length] - 48));

        return result.ToString();
    }
}