Cryptography | DES implementation in C
The Data Encryption Standard (DES) is a symmetric-key algorithm for the encryption of electronic data. Although now considered insecure, it was highly influential in the advancement of modern cryptography.
This article does not cover an overview of the DES Algorithm. We suggest going through the very nice tutorial given here for a detailed step-by-step explanation.
Input files to the program:
input.txt – Will contain our plain text (Max. Limit of plain text is 64kb).
key.txt – Will contain 64-bit key (Take the following key)
0001001100110100010101110111100110011011101111001101111111110001
key.txt – Will contain 64-bit key (Take the following key)
0001001100110100010101110111100110011011101111001101111111110001
DES Implementation:
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#include <stdio.h> #include <stdlib.h> #include <ctype.h> #include <math.h> #include <time.h> int 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 }; int 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 }; int 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 }; int 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 }; int S1[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 }; int S2[4][16] = { 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 }; int S3[4][16] = { 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 }; int S4[4][16] = { 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 }; int S5[4][16] = { 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 }; int S6[4][16] = { 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 }; int S7[4][16]= { 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 }; int S8[4][16]= { 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 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 }; int 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 }; int SHIFTS[] = { 1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1 }; FILE* out; int LEFT[17][32], RIGHT[17][32]; int IPtext[64]; int EXPtext[48]; int XORtext[48]; int X[8][6]; int X2[32]; int R[32]; int key56bit[56]; int key48bit[17][48]; int CIPHER[64]; int ENCRYPTED[64]; void expansion_function(int pos, int text) { for (int i = 0; i < 48; i++) { if (E[i] == pos + 1) { EXPtext[i] = text; } } } int initialPermutation(int pos, int text) { int i; for (i = 0; i < 64; i++) { if (IP[i] == pos + 1) { break; } } IPtext[i] = text; } int F1(int i) { int r, c, b[6]; for (int j = 0; j < 6; j++) { b[j] = X[i][j]; } r = b[0] * 2 + b[5]; c = 8 * b[1] + 4 * b[2] + 2 * b[3] + b[4]; if (i == 0) { return S1[r][c]; } else if (i == 1) { return S2[r][c]; } else if (i == 2) { return S3[r][c]; } else if (i == 3) { return S4[r][c]; } else if (i == 4) { return S5[r][c]; } else if (i == 5) { return S6[r][c]; } else if (i == 6) { return S7[r][c]; } else if (i == 7) { return S8[r][c]; } } int XOR(int a, int b) { return (a ^ b); } int ToBits(int value) { int k, j, m; static int i; if (i % 32 == 0) { i = 0; } for (j = 3; j >= 0; j--) { m = 1 << j; k = value & m; if (k == 0) { X2[3 - j + i] = '0' – 48; } else { X2[3 - j + i] = '1' – 48; } } i = i + 4; } int SBox(int XORtext[]) { int k = 0; for (int i = 0; i < 8; i++) { for (int j = 0; j < 6; j++) { X[i][j] = XORtext[k++]; } } int value; for (int i = 0; i < 8; i++) { value = F1(i); ToBits(value); } } int PBox(int pos, int text) { int i; for (i = 0; i < 32; i++) { if (P[i] == pos + 1) { break; } } R[i] = text; } void cipher(int Round, int mode) { for (int i = 0; i < 32; i++) { expansion_function(i, RIGHT[Round – 1][i]); } for (int i = 0; i < 48; i++) { if (mode == 0) { XORtext[i] = XOR(EXPtext[i], key48bit[Round][i]); } else { XORtext[i] = XOR(EXPtext[i], key48bit[17 – Round][i]); } } SBox(XORtext); for (int i = 0; i < 32; i++) { PBox(i, X2[i]); } for (int i = 0; i < 32; i++) { RIGHT[Round][i] = XOR(LEFT[Round – 1][i], R[i]); } } void finalPermutation(int pos, int text) { int i; for (i = 0; i < 64; i++) { if (FP[i] == pos + 1) { break; } } ENCRYPTED[i] = text; } void convertToBinary(int n) { int k, m; for (int i = 7; i >= 0; i--) { m = 1 << i; k = n & m; if (k == 0) { fprintf(out, "0"); } else { fprintf(out, "1"); } } } int convertCharToBit(long int n) { FILE* inp = fopen("input.txt", "rb"); out = fopen("bits.txt", "wb+"); char ch; int i = n * 8; while (i) { ch = fgetc(inp); if (ch == -1) { break; } i--; convertToBinary(ch); } fclose(out); fclose(inp); } void Encryption(long int plain[]) { out = fopen("cipher.txt", "ab+"); for (int i = 0; i < 64; i++) { initialPermutation(i, plain[i]); } for (int i = 0; i < 32; i++) { LEFT[0][i] = IPtext[i]; } for (int i = 32; i < 64; i++) { RIGHT[0][i – 32] = IPtext[i]; } for (int k = 1; k < 17; k++) { cipher(k, 0); for (int i = 0; i < 32; i++) LEFT[k][i] = RIGHT[k – 1][i]; } for (int i = 0; i < 64; i++) { if (i < 32) { CIPHER[i] = RIGHT[16][i]; } else { CIPHER[i] = LEFT[16][i – 32]; } finalPermutation(i, CIPHER[i]); } for (int i = 0; i < 64; i++) { fprintf(out, "%d", ENCRYPTED[i]); } fclose(out); } void Decryption(long int plain[]) { out = fopen("decrypted.txt", "ab+"); for (int i = 0; i < 64; i++) { initialPermutation(i, plain[i]); } for (int i = 0; i < 32; i++) { LEFT[0][i] = IPtext[i]; } for (int i = 32; i < 64; i++) { RIGHT[0][i – 32] = IPtext[i]; } for (int k = 1; k < 17; k++) { cipher(k, 1); for (int i = 0; i < 32; i++) { LEFT[k][i] = RIGHT[k – 1][i]; } } for (int i = 0; i < 64; i++) { if (i < 32) { CIPHER[i] = RIGHT[16][i]; } else { CIPHER[i] = LEFT[16][i – 32]; } finalPermutation(i, CIPHER[i]); } for (int i = 0; i < 64; i++) { fprintf(out, "%d", ENCRYPTED[i]); } fclose(out); } void convertToBits(int ch[]) { int value = 0; for (int i = 7; i >= 0; i--) { value += (int)pow(2, i) * ch[7 – i]; } fprintf(out, "%c", value); } int bittochar() { out = fopen("result.txt", "ab+"); for (int i = 0; i < 64; i = i + 8) { convertToBits(&ENCRYPTED[i]); } fclose(out); } void key56to48(int round, int pos, int text) { int i; for (i = 0; i < 56; i++) { if (PC2[i] == pos + 1) { break; } } key48bit[round][i] = text; } int key64to56(int pos, int text) { int i; for (i = 0; i < 56; i++) { if (PC1[i] == pos + 1) { break; } } key56bit[i] = text; } void key64to48(unsigned int key[]) { int k, backup[17][2]; int CD[17][56]; int C[17][28], D[17][28]; for (int i = 0; i < 64; i++) { key64to56(i, key[i]); } for (int i = 0; i < 56; i++) { if (i < 28) { C[0][i] = key56bit[i]; } else { D[0][i – 28] = key56bit[i]; } } for (int x = 1; x < 17; x++) { int shift = SHIFTS[x – 1]; for (int i = 0; i < shift; i++) { backup[x - 1][i] = C[x – 1][i]; } for (int i = 0; i < (28 – shift); i++) { C[x][i] = C[x – 1][i + shift]; } k = 0; for (int i = 28 – shift; i < 28; i++) { C[x][i] = backup[x – 1][k++]; } for (int i = 0; i < shift; i++) { backup[x - 1][i] = D[x – 1][i]; } for (int i = 0; i < (28 – shift); i++) { D[x][i] = D[x – 1][i + shift]; } k = 0; for (int i = 28 – shift; i < 28; i++) { D[x][i] = backup[x – 1][k++]; } } for (int j = 0; j < 17; j++) { for (int i = 0; i < 28; i++) { CD[j][i] = C[j][i]; } for (int i = 28; i < 56; i++) { CD[j][i] = D[j][i – 28]; } } for (int j = 1; j < 17; j++) { for (int i = 0; i < 56; i++) { key56to48(j, i, CD[j][i]); } } } void decrypt(long int n) { FILE* in = fopen("cipher.txt", "rb"); long int plain[n * 64]; int i = -1; char ch; while (!feof(in)) { ch = getc(in); plain[++i] = ch – 48; } for (int i = 0; i < n; i++) { Decryption(plain + i * 64); bittochar(); } fclose(in); } void encrypt(long int n) { FILE* in = fopen("bits.txt", "rb"); long int plain[n * 64]; int i = -1; char ch; while (!feof(in)) { ch = getc(in); plain[++i] = ch – 48; } for (int i = 0; i < n; i++) { Encryption(plain + 64 * i); } fclose(in); } void create16Keys() { FILE* pt = fopen("key.txt", "rb"); unsigned int key[64]; int i = 0, ch; while (!feof(pt)) { ch = getc(pt); key[i++] = ch – 48; } key64to48(key); fclose(pt); } long int findFileSize() { FILE* inp = fopen("input.txt", "rb"); long int size; if (fseek(inp, 0L, SEEK_END)) { perror("fseek() failed"); } // size will contain number of chars in the input file. else { size = ftell(inp); } fclose(inp); return size; } int main() { // destroy contents of these files (from previous runs, if any) out = fopen("result.txt", "wb+"); fclose(out); out = fopen("decrypted.txt", "wb+"); fclose(out); out = fopen("cipher.txt", "wb+"); fclose(out); create16Keys(); long int n = findFileSize() / 8; convertCharToBit(n); encrypt(n); decrypt(n); return 0; } |
OUTPUT FILE:
result.txt – IT WILL CONTAIN OUR DECRYPTED TEXT.
TEMP FILES:
bits.txt – IT WILL CONTAIN OUR PLAIN TEXT CONVERTED IN BITS.
cipher.txt – IT WILL CONTAIN OUR ENCRYPTED TEXT IN BITS.
decrypted.txt – IT WILL CONTAIN OUR DECRYPTED TEXT IN BITS (SAME AS bits.txt IN CONTENT)
The above program is tested in the Windows environment using Code::Blocks 16.01.
That’s all about DES implementation in C.
Exercise: Extend the solution to implement 3-DES algorithm.
Thanks for reading.
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