project.c

#include<stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

#define ROT 13
#define MAXCHAR 1000


char *readfile(FILE *fp){
    char *str = malloc(sizeof(char) * 255);
    while (fgets(str, MAXCHAR, fp) != NULL){}

        return str;
}

char *encrypt(char *text,int rot) {
    int c, e;
    char *encrypted_text = malloc(sizeof(char) * strlen(text));
	int i;


    for ( i = 0; i < strlen(text); i++) {

        if (isupper(text[i])) {
            if ((e = text[i] + rot) <= 'Z')
                encrypted_text[i] = e;
            else {
                int tmp = e - 'Z' - 1;
                e = 65 + tmp;
                encrypted_text[i] = e;
            }
        } else if (islower(text[i])) {
            if ((e = text[i] + rot) <= 'z')
            	 encrypted_text[i] = e;
			else {
                int tmp = e - 'z' - 1;
                e = 97 + tmp;
                encrypted_text[i] = e;
            }
        } else
            encrypted_text[i] = text[i];
    }



    return  encrypted_text;
}

int main(int argc,char *argv[]) {
    FILE *plain_fp;
    FILE *encrypted_fp;

    int rot = 13;
    int mode = 1;

    char *encrypted_text;
    char *plain_text;

    if(argv[1] != NULL){
    	plain_fp = fopen(argv[1],"r");
	}else{
		printf("Invalid Argument : please input file name");
		return -1;
	}

    if(argv[2] != NULL){
        rot = atoi(argv[2]);
    }else {
    	printf("Invalid Argument : ROT number");
		return -1;
	}

	if(argv[3] != NULL){
        mode = atoi(argv[3]);
    }else {
        encrypted_fp = fopen("encypted.txt","w");
    }


	plain_text = readfile(plain_fp);
	encrypted_text = encrypt(plain_text,rot);

    if(mode == 2){
        printf("Decrypted text : %s",encrypted_text);
    }else{
        fprintf(encrypted_fp,"%s",encrypted_text);
    }

    return 0;
}