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  private string EncryptToCaesarHigh(string sText)
        {
            string temp = null;

            if (!chBDecrypt.Checked)
            {
                iChiffre = 0;

                for (int i = 0; i < sText.Length; i++)
                {
                    iChiffre++;
                    if (IsInValueRange(sText[i], cCryptoalphabet))
                    {
                        int index = GetIndexOfArray(sText[i], cCryptoalphabet);

                        if (index + iChiffre < cCryptoalphabet.Length)
                        {
                            temp += cCryptoalphabet[index + iChiffre];
                        }
                        else
                        {
                            temp += cCryptoalphabet[GetRestToCryptoalphabet(index)];
                        }
                    }
                    else
                    {
                        temp += sText[i];
                    }
                }

                return temp;
            }
            else
            {
                // Umkehralgorithmus
                return temp;
            }

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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Caesar_high
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void buttonDecrypt_Click(object sender, EventArgs e)
        {
            richTextBoxCrypto.Text = Crypto.CaesarHigh.Encrypt(richTextBoxPlain.Text, "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
        }

        private void buttonEncrypt_Click(object sender, EventArgs e)
        {
            richTextBoxCrypto.Text = Crypto.CaesarHigh.Decrypt(richTextBoxPlain.Text, "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
        }
    }
}

namespace Crypto
{
    public static class CaesarHigh
    {
        public static string Encrypt(string text, string alpabet)
        {
            string erg = "";
            for (int i = 1; i <= text.Length; i++)
                if (alpabet.Contains(text[i - 1]))
                    erg += alpabet[(alpabet.IndexOf(text[i - 1]) + i) % alpabet.Length];
                else erg += text[i - 1];
            return erg;
        }

        public static string Decrypt(string text, string alpabet)
        {
            string erg = "";
            for (int i = 1; i <= text.Length; i++)
                if (alpabet.Contains(text[i - 1]))
                    erg += alpabet[(((alpabet.IndexOf(text[i - 1]) - i) % alpabet.Length) + alpabet.Length) % alpabet.Length];
                else erg += text[i - 1];
            return erg;
        }
    }
}