#include <stdio.h>
#include <stdlib.h>
 
int main() {
    FILE *source, *target;
    char sourceFile[100], targetFile[100];
    char ch;
    int key;
 
    // Step 1: Input filenames and decryption key
    printf("Enter encrypted file name: ");
    scanf("%s", sourceFile);
 
    printf("Enter output (decrypted) file name: ");
    scanf("%s", targetFile);
 
    printf("Enter decryption key (same key used for encryption): ");
    scanf("%d", &key);
 
    // Step 2: Open files
    source = fopen(sourceFile, "r");
    if (source == NULL) {
        printf("Unable to open encrypted file!\n");
        return 0;
    }
 
    target = fopen(targetFile, "w");
    if (target == NULL) {
        printf("Unable to create output file!\n");
        fclose(source);
        return 0;
    }
 
    // Step 3: Decrypt file
    while ((ch = fgetc(source)) != EOF) {
        fputc(ch - key, target); // reverse the encryption
    }
 
    fclose(source);
    fclose(target);
 
    printf("File '%s' decrypted successfully to '%s' using key %d.\n", sourceFile, targetFile, key);
    return 0;
}