Implement a C program to decrypt a bmp image (main code given, need help writing functions)

/* Mini Data Encryption Standard (DES) Dan Ross Oct 2015 Performs a simplified version of DES encryption using an 8-bit key 1) Substitution - a discrete encryption function selected by the key 2) Transposition - a simple byte swap in this implementation 3) XOR - with the key 4) Repeat 3 more times */ #include  #include  #include  #include  #include  #include  #pragma warning( disable : 4267) #pragma warning( disable : 4996) using namespace std; /* The Encryption Functions f[0] = { (0, 1)(1, 18)...(255,117) } f[1] = { (0,112)(1,103)...(255, 96) } etc... */ unsigned char f[4][256] = { // 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 { 1, 18, 121, 32, 127, 137, 132, 136, 144, 152, 159, 167, 175, 176, 178, 6, 149, 179, 186, 112, 59, 64, 12, 9, 81, 41, 29, 16, 10, 5, 0, 31, //f[0] 76, 115, 165, 168, 74, 105, 48, 124, 172, 51, 182, 195, 63, 123, 38, 200, 205, 210, 28, 53, 212, 215, 217, 220, 109, 72, 99, 148, 158, 222, 229, 230, 57, 90, 104, 55, 8, 131, 169, 87, 180, 189, 213, 214, 194, 85, 133, 187, 73, 58, 66, 54, 141, 86, 15, 60, 82, 142, 47, 111, 157, 19, 36, 163, 166, 184, 71, 199, 201, 100, 39, 40, 95, 101, 139, 155, 62, 33, 177, 202, 25, 216, 223, 150, 227, 232, 235, 237, 238, 43, 240, 241, 2, 183, 122, 236, 24, 130, 161, 171, 23, 69, 181, 44, 190, 174, 219, 49, 75, 135, 208, 225, 234, 242, 243, 244, 67, 65, 146, 245, 239, 98, 7, 147, 120, 203, 207, 145, 80, 221, 160, 107, 61, 22, 92, 143, 193, 35, 185, 196, 204, 224, 231, 246, 248, 249, 250, 52, 251, 192, 197, 103, 119, 37, 77, 21, 129, 151, 170, 191, 198, 206, 209, 102, 11, 153, 88, 164, 226, 228, 116, 134, 233, 253, 126, 162, 42, 3, 138, 154, 78, 218, 247, 252, 211, 254, 96, 68, 110, 83, 125, 34, 128, 97, 106, 255, 156, 4, 26, 27, 30, 108, 56, 13, 93, 140, 17, 45, 173, 50, 118, 188, 91, 114, 46, 84, 89, 20, 14, 79, 94, 70, 113, 117}, { 112, 103, 224, 128, 140, 156, 60, 219, 25, 44, 213, 180, 187, 171, 169, 197, 228, 221, 236, 84, 164, 117, 225, 71, 1, 85, 27, 87, 99, 249, 118, 250, //f[1] 214, 106, 239, 57, 92, 20, 138, 148, 181, 21, 185, 46, 226, 159, 240, 251, 2, 166, 252, 82, 253, 254, 65, 4, 105, 39, 177, 231, 255, 12, 64, 170, 53, 36, 182, 26, 22, 42, 9, 54, 119, 137, 124, 37, 155, 31, 86, 35, 107, 186, 216, 101, 33, 69, 116, 136, 134, 30, 49, 142, 143, 160, 179, 193, 220, 235, 176, 131, 0, 61, 47, 81, 108, 233, 18, 11, 28, 45, 95, 98, 162, 174, 175, 125, 67, 120, 194, 102, 201, 211, 8, 38, 62, 158, 163, 237, 3, 79, 150, 205, 238, 58, 241, 68, 178, 40, 14, 114, 123, 129, 144, 195, 203, 243, 100, 199, 76, 157, 50, 167, 24, 208, 212, 196, 217, 121, 41, 59, 115, 135, 127, 149, 184, 190, 29, 97, 189, 215, 90, 133, 83, 72, 244, 248, 94, 152, 23, 89, 139, 191, 200, 32, 91, 202, 227, 48, 209, 75, 80, 7, 154, 210, 126, 141, 232, 70, 218, 165, 198, 55, 110, 147, 5, 6, 51, 153, 173, 204, 161, 206, 43, 207, 188, 19, 223, 234, 10, 63, 52, 66, 34, 93, 130, 242, 88, 183, 13, 146, 230, 229, 17, 132, 168, 245, 246, 145, 111, 74, 16, 56, 104, 247, 109, 113, 73, 192, 222, 77, 172, 15, 122, 151, 78, 96}, { 71, 68, 46, 109, 126, 147, 179, 62, 183, 195, 213, 67, 83, 191, 11, 37, 118, 193, 216, 14, 186, 73, 101, 114, 140, 142, 231, 0, 205, 232, 12, 40, //f[2] 123, 32, 137, 23, 25, 38, 132, 143, 27, 107, 121, 94, 203, 82, 131, 163, 196, 206, 149, 218, 50, 72, 81, 146, 111, 100, 219, 110, 75, 127, 44, 182, 155, 48, 76, 189, 79, 210, 56, 116, 164, 198, 20, 209, 214, 113, 138, 1, 220, 221, 34, 158, 225, 229, 86, 98, 234, 3, 42, 236, 237, 238, 241, 243, 16, 57, 9, 8, 28, 33, 13, 7, 80, 96, 103, 159, 17, 228, 150, 177, 184, 212, 239, 141, 244, 144, 90, 78, 171, 106, 152, 55, 176, 85, 122, 10, 54, 102, 26, 60, 139, 148, 153, 53, 4, 65, 84, 161, 169, 49, 187, 58, 222, 224, 226, 130, 136, 43, 245, 207, 35, 157, 246, 248, 105, 21, 249, 250, 251, 252, 253, 133, 41, 47, 6, 247, 254, 240, 128, 162, 63, 97, 173, 66, 15, 104, 69, 108, 119, 52, 129, 167, 170, 115, 59, 175, 180, 185, 145, 89, 135, 181, 22, 165, 112, 5, 124, 160, 87, 192, 200, 92, 201, 215, 30, 217, 51, 174, 223, 230, 70, 233, 227, 235, 178, 197, 99, 204, 242, 19, 255, 93, 64, 29, 18, 39, 95, 45, 151, 188, 190, 117, 194, 74, 199, 88, 168, 202, 125, 154, 208, 36, 134, 166, 77, 31, 156, 172, 2, 211, 120, 61, 24, 91}, { 224, 102, 161, 74, 213, 234, 212, 69, 246, 30, 64, 107, 66, 81, 82, 123, 76, 132, 112, 136, 105, 129, 194, 151, 215, 40, 120, 49, 83, 148, 100, 131, //f[3] 15, 65, 43, 134, 174, 189, 216, 138, 220, 214, 223, 225, 142, 71, 205, 48, 217, 227, 34, 12, 169, 17, 179, 237, 238, 218, 3, 16, 41, 61, 109, 45, 198, 239, 119, 92, 25, 230, 243, 244, 247, 248, 249, 250, 150, 253, 58, 236, 8, 94, 121, 167, 251, 52, 44, 97, 21, 36, 5, 42, 114, 32, 95, 166, 196, 201, 219, 26, 101, 57, 37, 140, 46, 7, 125, 77, 153, 171, 187, 203, 204, 56, 70, 207, 67, 115, 135, 173, 90, 195, 197, 209, 210, 211, 192, 221, 226, 157, 162, 20, 96, 147, 199, 98, 126, 80, 178, 182, 228, 103, 229, 128, 156, 13, 144, 23, 51, 139, 172, 183, 63, 110, 202, 208, 231, 35, 235, 155, 241, 252, 27, 254, 255, 68, 133, 9, 62, 1, 99, 87, 108, 130, 137, 54, 84, 149, 0, 104, 113, 93, 127, 145, 79, 154, 14, 164, 22, 143, 170, 177, 180, 181, 163, 186, 60, 124, 191, 193, 190, 33, 53, 175, 91, 240, 117, 242, 245, 176, 185, 29, 73, 152, 233, 18, 111, 116, 146, 75, 78, 89, 141, 158, 168, 200, 222, 50, 85, 88, 184, 86, 55, 232, 122, 6, 10, 38, 4, 39, 106, 118, 2, 19, 47, 160, 59, 24, 11, 31, 72, 165, 188, 206, 28, 159} }; /* The INVERSE Encryption Functions Please fill this programmatically using fill_fi() fi[0] = { (0, 30)(1, 0)...(255, 227) } fi[1] = { (0, 100)(1, 24)...(255, 60) } etc... */ unsigned char fi[4][256]; // fill and print the inverse function table void fill_fi() { // YOU WRITE THIS PLEASE } /* Swaps the high and low nibbles of a byte */ unsigned char swapbytes(unsigned char cIn) { unsigned char lownibble, highnibble, cOut = 0; lownibble = cIn & 0x0F; highnibble = cIn & 0xF0; cOut = highnibble >> 4; cOut = cOut | (lownibble << 4); return cOut; } /* Performs miniDES style encryption You will need to write a decrypt function */ unsigned char encrypt(unsigned char w, unsigned char key) { // intermediate values in the process unsigned char x0, y0, z0; unsigned char x1, y1, z1; unsigned char x2, y2, z2; unsigned char x3, y3, z3; // get base_4 digit values of key by parsing bits... every 2 bits is a base_4 digit unsigned char p, q, r, s; // key = s x 4^3 + r x 4^2 + q x 4^1 + p x 4^0 p = (key & 0x03); q = (key & 0x0C) >> 2; r = (key & 0x30) >> 4; s = (key & 0xC0) >> 6; // Stage 0 x0 = f[s][w]; // substitution y0 = swapbytes(x0); // transposition z0 = y0 ^ key; // XOR // Stage 1 x1 = f[r][z0]; y1 = swapbytes(x1); z1 = y1 ^ key; // Stage 2 x2 = f[q][z1]; y2 = swapbytes(x2); z2 = y2 ^ key; // Stage 3 x3 = f[p][z2]; y3 = swapbytes(x3); z3 = y3 ^ key; return z3; } /* Yo, sup? */ void main() { char c; // the byte to encrypt // open source file ifstream fin("pPic.bmp", ios_base::binary); if (!fin) { cerr << "Input file could not be opened "; exit(1); } // open destination file ofstream fout("ePic.bmp", ios_base::binary); if (!fout) { cerr << "Output file could not be opened "; exit(1); } // read, encrypt, write while (!fin.eof()) { fin.read(&c, 1); if (!fin.eof()) { //cout << (int) w << endl; c = encrypt(c, 42); // Not the real key, you must brute force guess the real key fout.write(&c, 1); } } fin.close(); fout.close(); /* // handy code to build a filename string for iterative (brute force) crack char filename[30]; char numstr[10]; strcpy(filename, "pPic"); itoa(key, numstr, 10); strcat(filename, numstr); strcat(filename, ".bmp"); */ }