CryptoWorkbench/affine.csharp

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

namespace AniNIX.Crypto {
    public class Affine : Cipher {

        public override String Description() { return "The Affine cipher\nKey format is two numbers, where the second number is coprime to the first."; } 
        public override String Command() {return "affine";} 
        public Affine(Workbench w) : base (w) {}

        public override String Encrypt(String workSpace,String inputText,String[] line) {
            if (line == null || line.Length != 4) { 
                Console.Error.WriteLine("Malformed!");
                return workSpace;
            }
            char[] changed = workSpace.ToCharArray();
            try { 
                int a = Int32.Parse(line[2]);
                try { 
                    MultiplicativeInverse(a);
                } catch (Exception e) {
                    Console.Error.WriteLine(String.Format("Value a <{0}> is not coprime to 26.\n{1}",a,e.Message));
                    return workSpace;
                }
                int b = Int32.Parse(line[3]);
                for (int i = 0; i <  changed.Length; i++) {
                    if (Char.IsLetter(changed[i])) {
                        int baseC = (Char.IsUpper(changed[i])) ? (int)'A' : (int)'a';
                        int modC = (int)changed[i] - baseC;
                        changed[i] = (char)(((a*modC+b)%26)+baseC);
                    }
                }
            } catch (Exception e) {
                Console.Error.WriteLine(String.Format("Failed!\n{0}",e.Message));
                return workSpace;
            }
            return new String(changed);
        }

        public  int MultiplicativeInverse(int a) {
            for (int x=1; x < 27; x++) {
                if ((a*x)%26 == 1) {
                    Console.WriteLine(String.Format("Found Multiplicative Inverse of {0}",x));
                    return x;
                }
            }
            throw new Exception("A is not coprime.");
        }

        public override String Decrypt(String workSpace,String inputText,String[] line) {
            if (line == null || line.Length != 4) { 
                Console.Error.WriteLine("Malformed!");
                return workSpace;
            }
            char[] changed = workSpace.ToCharArray();
            try { 
                int a = Int32.Parse(line[2]); 
                int b = Int32.Parse(line[3]);
                int multiinv = MultiplicativeInverse(a);
                for (int i = 0; i <  changed.Length; i++) {
                    if (Char.IsLetter(changed[i])) {
                        int baseC = (Char.IsUpper(changed[i])) ? (int)'A' : (int)'a';
                        int modC = (int)changed[i] - baseC;
                        int modResult = (multiinv * (modC-b))%26;
                        modResult = (modResult < 0) ? modResult+26 : modResult; // In case modResult is negative, add 26 back
                        changed[i] = (char)(modResult+baseC);
                    }
                }
            } catch (Exception e) {
                Console.Error.WriteLine(String.Format("Failed!\n{0}",e.Message));
                return workSpace;
            }
            return new String(changed);

        }

    }
}