gistfile1.txt

#include <stdio.h>
#include <stdint.h>
#include <string.h>

//addditive cipher
#define SHIFT 10
#define ALPHABET_COUNT 27
#define DECRYPT_STRING "BYLOBD"
#define ENCRYPT_STRING "ROBERT"

static char m_alphabet[ALPHABET_COUNT][2] = { {'A'},{ 'B'},{ 'C'},{'D'},{'E'},{'F'},{'G'},{'H'},{'I'},{'J'},{'K'},{'L'},{'M'},{'N'},{'O'},{'P'},{'Q'},{'R'},{'S'},{'T'},{'U'},{'V'},{'W'},{'X'},{'Y'},{'Z'}};
static char m_cipher_alphabet[ALPHABET_COUNT][2] = { {'\0'},{ '\0'},{ '\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'},{'\0'}};

static void setup_cipher_alphabet()
{
	int i = 0;
	for(i = 0; i < ALPHABET_COUNT; i++)
	{
		if(i+SHIFT < ALPHABET_COUNT)
			strcpy(m_cipher_alphabet[i], m_alphabet[i+SHIFT]);
		else
		{
			int modulo = (i+SHIFT) % ALPHABET_COUNT;
			strcpy(m_cipher_alphabet[i], m_alphabet[modulo]);
		}
	}
}

static void print_alphabets()
{
	printf("PLAIN ALPHABET: \n");

	int i = 0;
	for(i = 0; i < ALPHABET_COUNT; i++)
		printf("%s", m_alphabet[i]);
	
	printf("\n");

	printf("\nCIPHER ALPHABET (Shift by: %d)\n", SHIFT);

	for(i = 0; i < ALPHABET_COUNT; i++)
		printf("%s", m_cipher_alphabet[i]);
	
	printf("\n");
}

static unsigned lookup_alphabet_number(unsigned char * letter)
{
	switch(letter)
	{
		case 'A':
		case 'a':
			return 0;
		case 'B':
		case 'b':
			return 1;
		case 'C':
		case 'c':
			return 2;
		case 'D':
		case 'd':
			return 3;
		case 'E':
		case 'e':
			return 4;
		case 'F':
		case 'f':
			return 5;
		case 'G':
		case 'g':
			return 6;
		case 'H':
		case 'h':
			return 7;
		case 'I':
		case 'i':
			return 8;
		case 'J':
		case 'j':
			return 9;
		case 'K':
		case 'k':
			return 10;
		case 'L':
		case 'l':
			return 11;
		case 'M':
		case 'm':
			return 12;
		case 'N':
		case 'n':
			return 13;
		case 'O':
		case 'o':
			return 14;
		case 'P':
		case 'p':
			return 15;
		case 'Q':
		case 'q':
			return 16;
		case 'R':
		case 'r':
			return 17;
		case 'S':
		case 's':
			return 18;
		case 'T':
		case 't':
			return 19;
		case 'U':
		case 'u':
			return 20;
		case 'V':
		case 'v':
			return 21;
		case 'W':
		case 'w':
			return 22;
		case 'X':
		case 'x':
			return 23;
		case 'Y':
		case 'y':
			return 24;
		case 'Z':
		case 'z':
			return 25;
	}
}

static void encrypt_string(const char * m_text)
{
	printf("String (%s) will be encrypted...\n", m_text);
	printf("length of string: %d\n", strlen(m_text));
	int i;
	for(i = 0; i < strlen(m_text); i++)
	{
		uint32_t num_alpha = lookup_alphabet_number(m_text[i]);
		int32_t k = num_alpha + SHIFT;
		uint32_t modulo = k % 26;
		printf("num_alpha is:	%d	modulo is: %d	(%s)\n", num_alpha, modulo, m_alphabet[modulo]); 
	}
}

static void decrypt_string(const char * m_text)
{
	printf("Encrypted string (%s) will be decrypted...\n", m_text);
	int i;
	for(i = 0; i < strlen(m_text); i++)
	{
		uint32_t num_alpha = lookup_alphabet_number(m_text[i]);
		int32_t k = num_alpha - SHIFT;
		uint32_t modulo = k % 26;
		printf("num_alpha is:	%d	modulo is: %d	(%s)\n", num_alpha, modulo, m_alphabet[modulo]); 
	}
}

static void print_usage()
{
	printf("USAGE:\n");
	printf("print_alphabet [shiftvalue]: Print alphabet/cipher alphabet by given shift value\n");
	printf("encrypt_string [shiftvalue]: Encrypt string by given shift value\n");
}

void main(int argc, char **argv)
{
	printf("Additive cipher program\n\n");
	if(argc > 1)
	{
		if(strcmp(argv[1],"print_alphabets") == 0)
		{
			setup_cipher_alphabet();
			print_alphabets();
		}
		else if(strcmp(argv[1],"decrypt_string") == 0)
		{
			decrypt_string(DECRYPT_STRING);
			setup_cipher_alphabet();
			print_alphabets();
		}
		else if(strcmp(argv[1],"encrypt_string") == 0)
		{
			encrypt_string(ENCRYPT_STRING);
			setup_cipher_alphabet();
			print_alphabets();
		}
		else if(strcmp(argv[1],"char_to_number") == 0)
		{
			printf("A becomes : %d\n", 'A' - 0x41);
			printf("D becomes : %d\n", 'D' - 0x41);
		}
		else
		{
			printf("ERROR: Wrong parameter supplied!\n\n");
			print_usage();
		}
	}
	else
	{
		printf("ERROR: You forgot to launch this program with a parameter/argument.\n");
		print_usage();
	}
}