Pivoting Caesar cipher

PivotCaesar.cs

// Copyright (c) 2016 Christopher S. Charabaruk <chris.charabaruk%%outlook.com>

public class PivotCaesar
{
    public static void Main(string[] args)
    {
        if (args[0] == "-d" || args[0] == "--decrypt")
        {
            foreach (var message in args.Skip(1))
            {
                Console.Write("Cipher: \"{0}\"\n", message);
                Console.Write("Clear:  \"{0}\"\n\n", Decrypt(message));
            }
        }
        else
        {
            var original = string.Join(" ", args);
            var clear = original.ToUpperInvariant().Replace(".", "..");
            clear = (new Regex(@"[^A-Z0-9\.]")).Replace(clear, ".");
            
            Console.Write("Original: \"{0}\"\n", original);
            Console.Write("Clear:    \"{0}\"\n", clear);
            Console.Write("Cipher:   \"{0}\"\n", Encrypt(clear));
        }
    }
    
    public static string Encrypt(string cleartext)
    {
        var pivot = InitialPivot;
        var ciphertext = new StringBuilder(cleartext.Length);

        foreach (var c in cleartext.ToCharArray())
        {
            var pos = (Alphabet.IndexOf(c) + pivot) % AlphabetLength;
            if (pos == 0) pos = AlphabetLength;
            
            ciphertext.Append(Alphabet[pos]);

            if (Pivots.Keys.Contains(c))
            {
                pivot = (pivot + Pivots[c]) % AlphabetLength;
                if (pivot == 0) pivot = InitialPivot;
            }
        }
        
        return ciphertext.ToString();
    }
    
    public static string Decrypt(string ciphertext)
    {
        var pivot = InitialPivot;
        var cleartext = new StringBuilder(ciphertext.Length);

        foreach (var c in ciphertext.ToString().ToCharArray())
        {
            var pos = (Alphabet.IndexOf(c) - pivot);
            if (pos <= 0) pos += AlphabetLength;

            var cc = Alphabet[pos];
            cleartext.Append(cc);

            if (Pivots.Keys.Contains(cc))
            {
                pivot = (pivot + Pivots[cc]) % AlphabetLength;
                if (pivot == 0) pivot = InitialPivot;
            }
        }
        
        return cleartext.ToString();
    }

    public static readonly List<char> Alphabet = ("\0" + "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.")
        .ToCharArray()
        .ToList();
    public static readonly int AlphabetLength = Alphabet.Count - 1;

    public static readonly Dictionary<char, int> Pivots = new Dictionary<char, int>
    {
        { 'A',  2 },
        { 'E',  3 },
        { 'I',  5 },
        { 'O',  7 },
        { 'U', 11 },
        { '0', 13 },
        { '.', 17 }
    };
    
    public static readonly int InitialPivot = 19;
}