Getting "Segmentation Fault" when trying to write to a file

Question

I'm trying to write to a file and then store the content of that file in a variable, here's my code:

#include <stdio.h>
#include <sys/stat.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>

int main(){
    char *input;
    char *line;
    char *sn;
    char *serial_number;
    char* key = "Serial Number";
    printf("Password:\n");
    scanf("%s", input);
    if (!strcmp(input, "mypassword")){

        // Write a serial number to a file
        printf("Serial Number:\n");
        scanf("%s", sn);
        FILE *f = fopen("/tmp/file.txt", "w");
        if (f == NULL) {
            printf("Error opening file!\n");
            exit(1);
        }
        fprintf(f,"Serial Number: %s", sn);
        fclose(f);

        // Store the serial number from a file in a variable
        FILE *file = fopen("/tmp/file.txt", "r");
        while (fgets(line, sizeof line, file) != NULL) /* read a line from a file */ {
            //fprintf(stdout, "%s", line); //print the file contents on stdout.
            if (strncmp(line, key, strlen(key)) == 0) {
                char* value = strchr(line, ':');
                value += 2;
                serial_number = strdup(value);
                break;   // once the key has been found we can stop reading
            }
        }
        fclose(file);
        printf("Your hardware serial number is: %s\n", serial_number);
    }
    else {
        printf("Wrong password\n");
    }
    return 0;
}

When I execute that script, it gives me Segmentation Fault after I put in my password. Let's say my serial number is 1866203214226041 so /tmp/file.txt should contain:

Serial Number: 1866203214226041

And the variable serial_number should be: 1866203214226041 How should I fix it ?