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.\n\nExample: affine encrypt 5 3\n\n"; } 
        public override String Command() {return "affine";} 
        public Affine(Workbench w) : base (w) {}

        /// <summary>
        /// Encrypt a string with the Affine cipher
        /// </summary>
        /// <param name="workspace">A parameter for X</param>
        /// <param name="inputText">A parameter for Y</param>
        /// <param name="line">The line given by the user</param> 
        /// <returns>The return value</returns>
        public override String Encrypt(String workSpace,String inputText,String[] line) {
            // Affine requires two numbers
            if (line == null || line.Length != 4) { 
                Console.Error.WriteLine("Malformed!");
                return workSpace;
            }
            char[] changed = workSpace.ToCharArray();
            try { 
                // Convert the first number to an int
                int a = Int32.Parse(line[2]);
                // Validate the first number is coprime to 26.
                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;
                }
                // Convert the second number to an int
                int b = Int32.Parse(line[3]);
                // For each letter in the workSpace, apply the forumula e(x) = (ax + b) mod m 
                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);
                    }
                }
            // If there's a problem solving, tell the user.
            } catch (Exception e) {
                Console.Error.WriteLine(String.Format("Failed!\n{0}",e.Message));
                return workSpace;
            }
            return new String(changed);
        }

        /// <summary>
        /// Find the multiplicative inverse of a number. 
        /// </summary>
        /// <param name="a">A number</param>
        /// <returns>The inverse</returns>
        public int MultiplicativeInverse(int a) {
            for (int x=1; x < 27; x++) {
                //Try to find a number where the input times that number mod 26 is 1. If we roll through 26 numbers and don't find it, there isn't one.
                if ((a*x)%26 == 1) {
                    return x;
                }
            }
            throw new Exception("A is not coprime.");
        }

        /// <summary>
        /// Decrypt a string with the cipher
        /// </summary>
        /// <param name="workSpace">A parameter for X</param>
        /// <param name="inputText">A parameter for Y</param>
        /// <param name="line">The user input</param>
        /// <returns>The decrypted string</returns>
        public override String Decrypt(String workSpace,String inputText,String[] line) {
            // Decryption requires two numbers.
            if (line == null || line.Length != 4) { 
                Console.Error.WriteLine("Malformed!");
                return workSpace;
            }
            char[] changed = workSpace.ToCharArray();
            try {
                //Convert both numbers to ints
                int a = Int32.Parse(line[2]); 
                int b = Int32.Parse(line[3]);
                //Find the multiplicative inverse of the first.
                int multiinv = MultiplicativeInverse(a);
                // For each character, decrypt with d(x) = a-1(x - b) mod m
                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);

        }

    }
}