Write a program to encrypt/decrypt a file using:

(1) An offset cipher: In an offset cipher each character from the source file is offset with a fixed value and then written to the target file. For example, if character read from the source file is ‘A’, then convert this into a new character by offsetting ‘A’ by a fixed value, say 128, and then writing the new character to the target file. 

(2) A substitution cipher: In this, each character read from the source file is substituted by a corresponding predetermined character and this character is written to the target file. 

For example, if character ‘A’ is read from the source file, and if we have decided that every ‘A’ is to be substituted by ‘!’, then a ‘!’ would be written to the target file in place of every ‘A’ Similarly, every ‘B’ would be substituted by ‘5’ and so on.

Ans:

File (g)1.txt

My name is Barry Allen and I am the fastest man alive.

#include<stdio.h>
#include<conio.h>
#include<ctype.h>

void encode(FILE*, FILE*);
void decode(FILE*, FILE*);
void display(FILE*);

int main()
{
	FILE *sf, *df;
	/*Source File*/
	sf = fopen("File (g)1.txt", "r");
	/*Destination File*/
	df = fopen("File (g)2.txt", "w");

	/*Displaying decoded file before encoding*/
	display(sf);
	
	/*Encoding file*/
	encode(sf, df);
	/*Displaying encoded file after encoding*/
	display(df);
	
	/*Decoding file*/
	decode(df, sf);
	/*Displaying file after decoding*/
	display(sf);

	_getch();
	return 0;
}

void encode(FILE *sf, FILE *df)
{
	char ch;
	while (1)
	{
		ch = fgetc(sf);
		if (ch == EOF)
			break;
		if (isalpha(ch) || ch == ' ' || ch == '\n')
		{
			switch (ch)
			{
			case '\n':
				fputc('@', df);
				break;
			case 'a':
				fputc('0', df);
			case 'A':
				fputc('!', df);
				break;
			case 'b':
				fputc('0', df);
			case 'B':
				fputc('#', df);
				break;
			case 'c':
				fputc('0', df);
			case 'C':
				fputc('$', df);
				break;
			case 'd':
				fputc('0', df);
			case 'D':
				fputc('%', df);
				break;
			case 'e':
				fputc('0', df);
			case 'E':
				fputc('&', df);
				break;
			case 'f':
				fputc('0', df);
			case 'F':
				fputc('*', df);
				break;
			case 'g':
				fputc('0', df);
			case 'G':
				fputc('(', df);
				break;
			case 'h':
				fputc('0', df);
			case 'H':
				fputc(')', df);
				break;
			case 'i':
				fputc('0', df);
			case 'I':
				putc('+', df);
				break;
			case 'j':
				fputc('0', df);
			case 'J':
				fputc('/', df);
				break;
			case 'k':
				fputc('0', df);
			case 'K':
				fputc('{', df);
				break;
			case 'l':
				fputc('0', df);
			case 'L':
				fputc('}', df);
				break;
			case 'm':
				fputc('0', df);
			case'M':
				fputc(';', df);
				break;
			case 'n':
				fputc('0', df);
			case 'N':
				fputc(':', df);
				break;
			case 'o':
				fputc('0', df);
			case 'O':
				fputc('[', df);
				break;
			case 'p':
				fputc('0', df);
			case 'P':
				fputc(']', df);
				break;
			case 'q':
				fputc('0', df);
			case 'Q':
				fputc('"', df);
				break;
			case 'r':
				fputc('0', df);
			case 'R':
				fputc('`', df);
				break;
			case 's':
				fputc('0', df);
			case 'S':
				fputc('\\', df);
				break;
			case 't':
				fputc('0', df);
			case 'T':
				fputc('|', df);
				break;
			case 'u':
				fputc('0', df);
			case 'U':
				fputc('=', df);
				break;
			case 'v':
				fputc('0', df);
			case 'V':
				fputc('-', df);
				break;
			case 'w':
				fputc('0', df);
			case 'W':
				fputc('_', df);
			case 'x':
				fputc('0', df);
			case 'X':
				fputc('<', df);
				break;
			case 'y':
				fputc('0', df);
			case 'Y':
				fputc('^', df);
				break;
			case 'z':
				fputc('0', df);
			case 'Z':
				fputc('?', df);
				break;
			case ' ':
				fputc('1', df);
				break;
			}
		}
		else
			fputc(ch, df);
	}
}

void decode(FILE *ef, FILE *df)
{
	char ch;
	while (1)
	{
		ch = fgetc(df);
		if (ch == EOF)
			break;
		switch (ch)
		{
		case '@':
			fputc('\n', ef);
			break;
		case '0':
			ch = fgetc(df);
			switch (ch)
			{
			case '!':
				fputc('a', ef);
				break;
			case '#':
				fputc('b', ef);
				break;
			case '$':
				fputc('c', ef);
				break;
			case '%':
				fputc('d', ef);
				break;
			case '&':
				fputc('e', ef);
				break;
			case '*':
				fputc('f', ef);
				break;
			case '(':
				fputc('g', ef);
				break;
			case ')':
				fputc('h', ef);
				break;
			case '+':
				fputc('i', ef);
				break;
			case '/':
				fputc('j', ef);
				break;
			case '{':
				fputc('k', ef);
				break;
			case'}':
				fputc('l', ef);
				break;
			case ';':
				fputc('m', ef);
				break;
			case ':':
				fputc('n', ef);
				break;
			case '[':
				fputc('o', ef);
				break;
			case ']':
				fputc('p', ef);
				break;
			case '"':
				fputc('q', ef);
				break;
			case '`':
				fputc('r', ef);
				break;
			case '\\':
				fputc('s', ef);
				break;
			case '|':
				fputc('t', ef);
				break;
			case '=':
				fputc('u', ef);
				break;
			case '-':
				fputc('v', ef);
				break;
			case '_':
				fputc('w', ef);
				break;
			case '<':
				fputc('x', ef);
				break;
			case '^':
				fputc('y', ef);
				break;
			case '?':
				fputc('z', ef);
				break;
			}
			break;
		case '!':
			fputc('A', ef);
			break;
		case '#':
			fputc('B', ef);
			break;
		case '$':
			fputc('C', ef);
			break;
		case '%':
			fputc('D', ef);
			break;
		case '&':
			fputc('E', ef);
			break;
		case '*':
			fputc('F', ef);
			break;
		case '(':
			fputc('G', ef);
			break;
		case ')':
			fputc('H', ef);
			break;
		case '+':
			putc('I', ef);
			break;
		case '/':
			fputc('J', ef);
			break;
		case '{':
			fputc('K', ef);
			break;
		case '}':
			fputc('L', ef);
			break;
		case';':
			fputc('M', ef);
			break;
		case ':':
			fputc('N', ef);
			break;
		case '[':
			fputc('O', ef);
			break;
		case ']':
			fputc('P', ef);
			break;
		case '"':
			fputc('Q', ef);
			break;
		case '`':
			fputc('R', ef);
			break;
		case '\\':
			fputc('S', ef);
			break;
		case '|':
			fputc('T', ef);
			break;
		case '=':
			fputc('U', ef);
			break;
		case '-':
			fputc('V', ef);
			break;
		case '_':
			fputc('W', ef);
		case '<':
			fputc('X', ef);
			break;
		case '^':
			fputc('Y', ef);
			break;
		case '?':
			fputc('Z', ef);
			break;
		case '1':
			fputc(' ', ef);
			break;
		default:
			fputc(ch, ef);
		}
	}
}

void display(FILE *fp)
{
	char str[50];
	while (fgets(str, 49, fp) != NULL)
		printf("%s", str);
	puts("\n\n");
	_getch();
}