DES.java

import java.util.*;

class DES {
	// Initial Permutation table
	private static final byte[] IP = { 
		58, 50, 42, 34, 26, 18, 10, 2,
		60, 52, 44, 36, 28, 20, 12, 4,
		62, 54, 46, 38, 30, 22, 14, 6,
		64, 56, 48, 40, 32, 24, 16, 8,
		57, 49, 41, 33, 25, 17, 9,  1,
		59, 51, 43, 35, 27, 19, 11, 3,
		61, 53, 45, 37, 29, 21, 13, 5,
		63, 55, 47, 39, 31, 23, 15, 7
	};
	
	// Permuted Choice 1 table
	private static final byte[] PC1 = {
		57, 49, 41, 33, 25, 17, 9,
		1,  58, 50, 42, 34, 26, 18,
		10, 2,  59, 51, 43, 35, 27,
		19, 11, 3,  60, 52, 44, 36,
		63, 55, 47, 39, 31, 23, 15,
		7,  62, 54, 46, 38, 30, 22,
		14, 6,  61, 53, 45, 37, 29,
		21, 13, 5,  28, 20, 12, 4
	};
	
	// Permuted Choice 2 table
	private static final byte[] PC2 = {
		14, 17, 11, 24, 1,  5,
		3,  28, 15, 6,  21, 10,
		23, 19, 12, 4,  26, 8,
		16, 7,  27, 20, 13, 2,
		41, 52, 31, 37, 47, 55,
		30, 40, 51, 45, 33, 48,
		44, 49, 39, 56, 34, 53,
		46, 42, 50, 36, 29, 32
	};
	
	// Array to store the number of rotations that are to be done on each round
	private static final byte[] rotations = {
		1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1
	};
	
	// Expansion (aka P-box) table
	private static final byte[] E = {
		32, 1,  2,  3,  4,  5,
		4,  5,  6,  7,  8,  9,
		8,  9,  10, 11, 12, 13,
		12, 13, 14, 15, 16, 17,
		16, 17, 18, 19, 20, 21,
		20, 21, 22, 23, 24, 25,
		24, 25, 26, 27, 28, 29,
		28, 29, 30, 31, 32, 1
	};
	
	// S-boxes (i.e. Substitution boxes)
	private static final byte[][] S = { {
		14, 4,  13, 1,  2,  15, 11, 8,  3,  10, 6,  12, 5,  9,  0,  7,
		0,  15, 7,  4,  14, 2,  13, 1,  10, 6,  12, 11, 9,  5,  3,  8,
		4,  1,  14, 8,  13, 6,  2,  11, 15, 12, 9,  7,  3,  10, 5,  0,
		15, 12, 8,  2,  4,  9,  1,  7,  5,  11, 3,  14, 10, 0,  6,  13
	}, {
		15, 1,  8,  14, 6,  11, 3,  4,  9,  7,  2,  13, 12, 0,  5,  10,
		3,  13, 4,  7,  15, 2,  8,  14, 12, 0,  1,  10, 6,  9,  11, 5,
		0,  14, 7,  11, 10, 4,  13, 1,  5,  8,  12, 6,  9,  3,  2,  15,
		13, 8,  10, 1,  3,  15, 4,  2,  11, 6,  7,  12, 0,  5,  14, 9
	}, {
		10, 0,  9,  14, 6,  3,  15, 5,  1,  13, 12, 7,  11, 4,  2,  8,
		13, 7,  0,  9,  3,  4,  6,  10, 2,  8,  5,  14, 12, 11, 15, 1,
		13, 6,  4,  9,  8,  15, 3,  0,  11, 1,  2,  12, 5,  10, 14, 7,
		1,  10, 13, 0,  6,  9,  8,  7,  4,  15, 14, 3,  11, 5,  2,  12
	}, {
		7,  13, 14, 3,  0,  6,  9,  10, 1,  2,  8,  5,  11, 12, 4,  15,
		13, 8,  11, 5,  6,  15, 0,  3,  4,  7,  2,  12, 1,  10, 14, 9,
		10, 6,  9,  0,  12, 11, 7,  13, 15, 1,  3,  14, 5,  2,  8,  4,
		3,  15, 0,  6,  10, 1,  13, 8,  9,  4,  5,  11, 12, 7,  2,  14
	}, {
		2,  12, 4,  1,  7,  10, 11, 6,  8,  5,  3,  15, 13, 0,  14, 9,
		14, 11, 2,  12, 4,  7,  13, 1,  5,  0,  15, 10, 3,  9,  8,  6,
		4,  2,  1,  11, 10, 13, 7,  8,  15, 9,  12, 5,  6,  3,  0,  14,
		11, 8,  12, 7,  1,  14, 2,  13, 6,  15, 0,  9,  10, 4,  5,  3
	}, {
		12, 1,  10, 15, 9,  2,  6,  8,  0,  13, 3,  4,  14, 7,  5,  11,
		10, 15, 4,  2,  7,  12, 9,  5,  6,  1,  13, 14, 0,  11, 3,  8,
		9,  14, 15, 5,  2,  8,  12, 3,  7,  0,  4,  10, 1,  13, 11, 6,
		4,  3,  2,  12, 9,  5,  15, 10, 11, 14, 1,  7,  6,  0,  8,  13
	}, {
		4,  11, 2,  14, 15, 0,  8,  13, 3,  12, 9,  7,  5,  10, 6,  1,
		13, 0,  11, 7,  4,  9,  1,  10, 14, 3,  5,  12, 2,  15, 8,  6,
		1,  4,  11, 13, 12, 3,  7,  14, 10, 15, 6,  8,  0,  5,  9,  2,
		6,  11, 13, 8,  1,  4,  10, 7,  9,  5,  0,  15, 14, 2,  3,  12
	}, {
		13, 2,  8,  4,  6,  15, 11, 1,  10, 9,  3,  14, 5,  0,  12, 7,
		1,  15, 13, 8,  10, 3,  7,  4,  12, 5,  6,  11, 0,  14, 9,  2,
		7,  11, 4,  1,  9,  12, 14, 2,  0,  6,  10, 13, 15, 3,  5,  8,
		2,  1,  14, 7,  4,  10, 8,  13, 15, 12, 9,  0,  3,  5,  6,  11
	} };
	
	// Permutation table
	private static final byte[] P = {
		16, 7,  20, 21,
		29, 12, 28, 17,
		1,  15, 23, 26,
		5,  18, 31, 10,
		2,  8,  24, 14,
		32, 27, 3,  9,
		19, 13, 30, 6,
		22, 11, 4,  25
	};
	
	// Final permutation (aka Inverse permutation) table
	private static final byte[] FP = {
		40, 8, 48, 16, 56, 24, 64, 32,
		39, 7, 47, 15, 55, 23, 63, 31,
		38, 6, 46, 14, 54, 22, 62, 30,
		37, 5, 45, 13, 53, 21, 61, 29,
		36, 4, 44, 12, 52, 20, 60, 28,
		35, 3, 43, 11, 51, 19, 59, 27,
		34, 2, 42, 10, 50, 18, 58, 26,
		33, 1, 41, 9, 49, 17, 57, 25
	};
	
	// 28 bits each, used as storage in the KS (Key Structure) rounds to 
	// generate round keys (aka subkeys)
	private static int[] C = new int[28];
	private static int[] D = new int[28];
	
	// Decryption requires the 16 subkeys to be used in the exact same process
	// as encryption, with the only difference being that the keys are used
	// in reverse order, i.e. last key is used first and so on. Hence, during
	// encryption when the keys are first generated, they are stored in this
	// array. In case we wish to separate the encryption and decryption
	// programs, then we need to generate the subkeys first in order, store
	// them and then use them in reverse order.
	private static int[][] subkey = new int[16][48];
	
	public static void main(String args[]) {
		System.out.println("Enter the input as a 16 character hexadecimal value:");
		String input = new Scanner(System.in).nextLine();
		int inputBits[] = new int[64];
		// inputBits will store the 64 bits of the input as a an int array of
		// size 64. This program uses int arrays to store bits, for the sake
		// of simplicity. For efficient programming, use long data type. But
		// it increases program complexity which is unnecessary for this
		// context.
		for(int i=0 ; i < 16 ; i++) {
			// For every character in the 16 bit input, we get its binary value
			// by first parsing it into an int and then converting to a binary
			// string
			String s = Integer.toBinaryString(Integer.parseInt(input.charAt(i) + "", 16));
			
			// Java does not add padding zeros, i.e. 5 is returned as 111 but
			// we require 0111. Hence, this while loop adds padding 0's to the
			// binary value.
			while(s.length() < 4) {
				s = "0" + s;
			}
			// Add the 4 bits we have extracted into the array of bits.
			for(int j=0 ; j < 4 ; j++) {
				inputBits[(4*i)+j] = Integer.parseInt(s.charAt(j) + "");
			}
		}
		
		// Similar process is followed for the 16 bit key
		System.out.println("Enter the key as a 16 character hexadecimal value:");
		String key = new Scanner(System.in).nextLine();
		int keyBits[] = new int[64];
		for(int i=0 ; i < 16 ; i++) {
			String s = Integer.toBinaryString(Integer.parseInt(key.charAt(i) + "", 16));
			while(s.length() < 4) {
				s = "0" + s;
			}
			for(int j=0 ; j < 4 ; j++) {
				keyBits[(4*i)+j] = Integer.parseInt(s.charAt(j) + "");
			}
		}
		
		// permute(int[] inputBits, int[] keyBits, boolean isDecrypt)
		// method is used here. This allows encryption and decryption to be
		// done in the same method, reducing code.
		System.out.println("\n+++ ENCRYPTION +++");
		int outputBits[] = permute(inputBits, keyBits, false);
		System.out.println("\n+++ DECRYPTION +++");
		permute(outputBits, keyBits, true);
	}
	
	private static int[] permute(int[] inputBits, int[] keyBits, boolean isDecrypt) {
		// Initial permutation step takes input bits and permutes into the
		// newBits array
		int newBits[] = new int[inputBits.length];
		for(int i=0 ; i < inputBits.length ; i++) {
			newBits[i] = inputBits[IP[i]-1];
		}
		
		// 16 rounds will start here
		// L and R arrays are created to store the Left and Right halves of the
		// subkey respectively
		int L[] = new int[32];
		int R[] = new int[32];
		int i;
		
		// Permuted Choice 1 is done here
		for(i=0 ; i < 28 ; i++) {
			C[i] = keyBits[PC1[i]-1];
		}
		for( ; i < 56 ; i++) {
			D[i-28] = keyBits[PC1[i]-1];
		}
		
		// After PC1 the first L and R are ready to be used and hence looping
		// can start once L and R are initialized
		System.arraycopy(newBits, 0, L, 0, 32);
		System.arraycopy(newBits, 32, R, 0, 32);
		System.out.print("\nL0 = ");
		displayBits(L);
		System.out.print("R0 = ");
		displayBits(R);
		for(int n=0 ; n < 16 ; n++) {
			System.out.println("\n-------------");
			System.out.println("Round " + (n+1) + ":");
			// newR is the new R half generated by the Fiestel function. If it
			// is encrpytion then the KS method is called to generate the
			// subkey otherwise the stored subkeys are used in reverse order
			// for decryption.
			int newR[] = new int[0];
			if(isDecrypt) {
				newR = fiestel(R, subkey[15-n]);
				System.out.print("Round key = ");
				displayBits(subkey[15-n]);
			} else {
				newR = fiestel(R, KS(n, keyBits));
				System.out.print("Round key = ");
				displayBits(subkey[n]);
			}
			// xor-ing the L and new R gives the new L value. new L is stored
			// in R and new R is stored in L, thus exchanging R and L for the
			// next round.
			int newL[] = xor(L, newR);
			L = R;
			R = newL;
			System.out.print("L = ");
			displayBits(L);
			System.out.print("R = ");
			displayBits(R);
		}
		
		// R and L has the two halves of the output before applying the final
		// permutation. This is called the "Preoutput".
		int output[] = new int[64];
		System.arraycopy(R, 0, output, 0, 32);
		System.arraycopy(L, 0, output, 32, 32);
		int finalOutput[] = new int[64];
		// Applying FP table to the preoutput, we get the final output:
		// Encryption => final output is ciphertext
		// Decryption => final output is plaintext
		for(i=0 ; i < 64 ; i++) {
			finalOutput[i] = output[FP[i]-1];
		}
		
		// Since the final output is stored as an int array of bits, we convert
		// it into a hex string:
		String hex = new String();
		for(i=0 ; i < 16 ; i++) {
			String bin = new String();
			for(int j=0 ; j < 4 ; j++) {
				bin += finalOutput[(4*i)+j];
			}
			int decimal = Integer.parseInt(bin, 2);
			hex += Integer.toHexString(decimal);
		}
		if(isDecrypt) {
			System.out.print("Decrypted text: ");
		
		} else {
			System.out.print("Encrypted text: ");
		}
		System.out.println(hex.toUpperCase());
		return finalOutput;
	}
	
	private static int[] KS(int round, int[] key) {
		// The KS (Key Structure) function generates the round keys.
		// C1 and D1 are the new values of C and D which will be generated in
		// this round.
		int C1[] = new int[28];
		int D1[] = new int[28];
		
		// The rotation array is used to set how many rotations are to be done
		int rotationTimes = (int) rotations[round];
		// leftShift() method is used for rotation (the rotation is basically)
		// a left shift operation, hence the name.
		C1 = leftShift(C, rotationTimes);
		D1 = leftShift(D, rotationTimes);
		// CnDn stores the combined C1 and D1 halves
		int CnDn[] = new int[56];
		System.arraycopy(C1, 0, CnDn, 0, 28);
		System.arraycopy(D1, 0, CnDn, 28, 28);
		// Kn stores the subkey, which is generated by applying the PC2 table
		// to CnDn
		int Kn[] = new int[48];
		for(int i=0 ; i < Kn.length ; i++) {
			Kn[i] = CnDn[PC2[i]-1];
		}
		
		// Now we store C1 and D1 in C and D respectively, thus becoming the
		// old C and D for the next round. Subkey is stored and returned.
		subkey[round] = Kn;
		C = C1;
		D = D1;
		return Kn;
	}
	
	private static int[] fiestel(int[] R, int[] roundKey) {
		// Method to implement Fiestel function.
		// First the 32 bits of the R array are expanded using E table.
		int expandedR[] = new int[48];
		for(int i=0 ; i < 48 ; i++) {
			expandedR[i] = R[E[i]-1];
		}
		// We xor the expanded R and the generated round key
		int temp[] = xor(expandedR, roundKey);
		// The S boxes are then applied to this xor result and this is the
		// output of the Fiestel function.
		int output[] = sBlock(temp);
		return output;
	}
	
	private static int[] xor(int[] a, int[] b) {
		// Simple xor function on two int arrays
		int answer[] = new int[a.length];
		for(int i=0 ; i < a.length ; i++) {
			answer[i] = a[i]^b[i];
		}
		return answer;
	}
	
	private static int[] sBlock(int[] bits) {
		// S-boxes are applied in this method.
		int output[] = new int[32];
		// We know that input will be of 32 bits, hence we will loop 32/4 = 8
		// times (divided by 4 as we will take 4 bits of input at each
		// iteration).
		for(int i=0 ; i < 8 ; i++) {
			// S-box requires a row and a column, which is found from the
			// input bits. The first and 6th bit of the current iteration
			// (i.e. bits 0 and 5) gives the row bits.
			int row[] = new int [2];
			row[0] = bits[6*i];
			row[1] = bits[(6*i)+5];
			String sRow = row[0] + "" + row[1];
			// Similarly column bits are found, which are the 4 bits between
			// the two row bits (i.e. bits 1,2,3,4)
			int column[] = new int[4];
			column[0] = bits[(6*i)+1];
			column[1] = bits[(6*i)+2];
			column[2] = bits[(6*i)+3];
			column[3] = bits[(6*i)+4];
			String sColumn = column[0] +""+ column[1] +""+ column[2] +""+ column[3];
			// Converting binary into decimal value, to be given into the
			// array as input
			int iRow = Integer.parseInt(sRow, 2);
			int iColumn = Integer.parseInt(sColumn, 2);
			int x = S[i][(iRow*16) + iColumn];
			// We get decimal value of the S-box here, but we need to convert
			// it into binary:
			String s = Integer.toBinaryString(x);
			// Padding is required since Java returns a decimal '5' as '111' in
			// binary, when we require '0111'.
			while(s.length() < 4) {
				s = "0" + s;
			}
			// The binary bits are appended to the output
			for(int j=0 ; j < 4 ; j++) {
				output[(i*4) + j] = Integer.parseInt(s.charAt(j) + "");
			}
		}
		// P table is applied to the output and this is the final output of one
		// S-box round:
		int finalOutput[] = new int[32];
		for(int i=0 ; i < 32 ; i++) {
			finalOutput[i] = output[P[i]-1];
		}
		return finalOutput;
	}
	
	private static int[] leftShift(int[] bits, int n) {
		// Left shifting takes place here, i.e. each bit is rotated to the left
		// and the leftmost bit is stored at the rightmost bit. This is a left
		// shift operation.
		int answer[] = new int[bits.length];
		System.arraycopy(bits, 0, answer, 0, bits.length);
		for(int i=0 ; i < n ; i++) {
			int temp = answer[0];
			for(int j=0 ; j < bits.length-1 ; j++) {
				answer[j] = answer[j+1];
			}
			answer[bits.length-1] = temp;
		}
		return answer;
	}
	
	private static void displayBits(int[] bits) {
		// Method to display int array bits as a hexadecimal string.
		for(int i=0 ; i < bits.length ; i+=4) {
			String output = new String();
			for(int j=0 ; j < 4 ; j++) {
				output += bits[i+j];
			}
			System.out.print(Integer.toHexString(Integer.parseInt(output, 2)));
		}
		System.out.println();
	}
}