DES-decryption.cpp

#include <bits/stdc++.h>
using namespace std;
#define int long long


typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<bool> vb;
typedef vector<vi> vvi;
typedef vector<pii> vpi;

#define forl(a, b, c) for (int(a) = (b); (a) < (c); (a)++)
#define fore(a, b, c) for (int(a) = (b); (a) <= (c); (a)++)
#define ford(a, b, c) for (int(a) = (b); (a) >= (c); --(a))
#define foreach(a, b) for (auto &(a) : (b))
#define reset(a, b) memset(a, b, sizeof(a))
#define all(v) v.begin(), v.end()

const char newl = '\n';
const int N = 2e5 + 10;
const int mod = 1e9+7;
const ll inf = 2e18;


string decToBin(int n)
{
	string bin = "";
	while (n > 0)
	{
		bin = (char)(n % 2 + '0') + bin;
		n /= 2;
	}
	while (bin.size() < 4) bin = '0' + bin;
	return bin;
} 

string binToHex(string s)
{
	string hex = "";
	for (int i = 0; i < s.size(); i += 4)
	{
		string k = s.substr(i, 4);
		if (k == "0000") hex += '0';
		else if (k == "0001") hex += '1';
		else if (k == "0010") hex += '2';
		else if (k == "0011") hex += '3';
		else if (k == "0100") hex += '4';
		else if (k == "0101") hex += '5';
		else if (k == "0110") hex += '6';
		else if (k == "0111") hex += '7';
		else if (k == "1000") hex += '8';
		else if (k == "1001") hex += '9';
		else if (k == "1010") hex += 'A';
		else if (k == "1011") hex += 'B';
		else if (k == "1100") hex += 'C';
		else if (k == "1101") hex += 'D';
		else if (k == "1110") hex += 'E';
		else if (k == "1111") hex += 'F';
	}
	return hex;
}

string hexToBin(string s)
{
	string bin = "";
	for (int i = 0; i < s.size(); i++)
	{
		switch (s[i])
		{
		case '0': bin += "0000"; break;
		case '1': bin += "0001"; break;
		case '2': bin += "0010"; break;
		case '3': bin += "0011"; break;
		case '4': bin += "0100"; break;
		case '5': bin += "0101"; break;
		case '6': bin += "0110"; break;
		case '7': bin += "0111"; break;
		case '8': bin += "1000"; break;
		case '9': bin += "1001"; break;
		case 'A': bin += "1010"; break;
		case 'B': bin += "1011"; break;
		case 'C': bin += "1100"; break;
		case 'D': bin += "1101"; break;
		case 'E': bin += "1110"; break;
		case 'F': bin += "1111"; break;

		}
	}
	return bin;
}


class DES_Decryption
{
	// constants regarding the keys

	const int permutationConstant_1[56] = {  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 };


	const int permutationConstant_2[48] = {  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 };

	// constants regarding the plain text

	const int initialPermutationTable[64] = { 	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 };

	const int expansionTable[48] = { 	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 };

	int Sbox[8][4][16] = {                       
		{
			{ 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 }
		}
	};

	int leftShift[16] = { 1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1 };

	const int P[32] = { 	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 };

	const int P_1[64] = { 	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 };



	string shift_bit(string s, int n)
	{
		string k = "";
		for (int i = n; i < s.size(); i++) k += s[i];
		for (int i = 0; i < n; i++) k += s[i];
		return k;
	}


	void expand_R(string r, string r32)
	{
		r = "";
		for (int j = 0; j < 48; j++) r += r32[expansionTable[j] - 1];
		
	}


	string xor_add(string s1, string s2)
	{
		string result = "";
		for (int j = 0; j < s1.size(); j++) {
			if (s1[j] != s2[j]) result += '1';
			else result += '0';
		}
		return result;
	}

	string get_element_from_box(string s, int k)
	{
		int dec1 = 0, dec2 = 0, pwr = 0;
		dec1 = (int)(s[0] - '0') * 2 + (int)(s[5] - '0');
		for (int i = s.size() - 2; i >= 1; i--) dec2 += (int)(s[i] - '0') * pow(2, pwr++);
		
		return decToBin(Sbox[k][dec1][dec2]);
	}

public:

	string decrypt( string& cipher_txt, string& key)
	{
		// making sub-keys

		string key_64 = hexToBin(key);
		string key_56 = "";

		for (int i = 0; i < 56; i++) key_56 += key_64[permutationConstant_1[i] - 1];

		string key_firstHalf = key_56.substr(0,28), key_secondHalf = key_56.substr(28, 28);
		string L_key[16], R_key[16];

		for (int i = 0; i < 16; i++)
		{
			L_key[i] = shift_bit(((i == 0) ? key_firstHalf : L_key[i - 1]), leftShift[i]);
			R_key[i] = shift_bit((( i == 0) ? key_secondHalf : R_key[i - 1]), leftShift[i]);
		}


		string key_48[16], keys_56[16];

		for (int i = 0; i < 16; i++)
		{
			keys_56[i] = L_key[i] + R_key[i]; // making 56 bits keys
		}
		for (int i = 0; i < 16; i++)
		{
			key_48[i] = "";
			for (int j = 0; j < 48; j++) key_48[i] += keys_56[i][permutationConstant_2[j] - 1]; // making 48 bits keys
		}

        // key swapper

        for ( int i = 0 ; i < 8 ; i++){
            swap(key_48[i], key_48[15-i]);
        }

		string cipher_txt_64 = hexToBin(cipher_txt); 

		string initialPermutation = "";

		for (int i = 0; i < 64; i++)
			initialPermutation += cipher_txt_64[initialPermutationTable[i] - 1];

		string L = initialPermutation.substr(0,32), R =initialPermutation.substr(32,32);

		string L_32[16], R_32[16];
		string R_xor_K[16];
		string R_48[16];
		string S_R[16], s[16][8];
		string s_1[16];
		string P_R[16]; 

		R_48[0] = "";
		for (int j = 0; j < 48; j++)
			R_48[0] += R[expansionTable[j] - 1];

		R_xor_K[0] = xor_add(R_48[0], key_48[0]);

		for (int j = 0; j < 48; j += 6) 
			for (int k = j; k < j + 6; k++)
				s[0][j / 6] += R_xor_K[0][k];

		s_1[0] = "";
		for (int j = 0; j < 8; j++)
			s_1[0] += get_element_from_box(s[0][j], j);

		for (int j = 0; j < 32; j++)
			P_R[0] += s_1[0][P[j] - 1];

		

		L_32[0] = R;
		R_32[0] = "";
		R_32[0] = xor_add(P_R[0], L);

		for (int i = 1; i < 16; i++){
			R_48[i] = "";
			for (int j = 0; j < 48; j++)
				R_48[i] += R_32[i - 1][expansionTable[j] - 1];

			R_xor_K[i] = xor_add(R_48[i], key_48[i]); 

			for (int j = 0; j <48; j += 6) 
				for (int k = j; k < j + 6; k++)
					s[i][j / 6] += R_xor_K[i][k];

			s_1[i] = "";
			for (int j = 0; j < 8; j++)
				s_1[i] += get_element_from_box(s[i][j], j);

			for (int j = 0; j < 32; j++)
				P_R[i] += s_1[i][P[j] - 1];

			L_32[i] = R_32[i - 1];
			R_32[i] = "";
			R_32[i] = xor_add(P_R[i], L_32[i - 1]);

		}

		string decryptedBin = "", RL;
		RL = R_32[15] + L_32[15];

		for (int i = 0; i < 64; i++) decryptedBin += RL[P_1[i] - 1];
		string plain_txt = binToHex(decryptedBin);

        return plain_txt;

	}
	
	string hex_to_string( string& in) {
		string output;
		int cnt = in.length() / 2;

		for (int i = 0; cnt > i; ++i) {
			int s = 0;
			stringstream ss;
			ss << hex << in.substr(i * 2, 2);
			ss >> s;
			output.push_back(static_cast<unsigned char>(s));
		}

		return output;
	}


};



int32_t main()
{

    DES_Decryption DES;
	string cipher_txt, key, plain_txt,M;

    cout << newl << "Enter the cipher text " << newl ;
    cin >> cipher_txt;

	cout << newl << "Enter key in hexadecimal of 16 characters " << newl;
    cin >> key;
	cout << newl;

	for (int i = 0; i < key.size(); i++) key[i] = toupper(key[i]);

	cout << "Decrypting..." << newl;
	cout << newl;
	sleep(1);

    for(int i = 0; i <cipher_txt.size(); i+=16){
        string subcipher = cipher_txt.substr(i,16);
        plain_txt += DES.decrypt(subcipher,key);
    }

	M = DES.hex_to_string(plain_txt);

    cout << "The message is " << newl;
	for (int i = 0; i < M.size(); i++) {
		if (M[i] == '.') continue;
		else cout << M[i];
	}
	cout << newl;

	return 0;
}