wire_transfer.c

/* Welcome freedom fighter.  The liberation front has gained access to the
 * source code for a wire transfer program used by FIRST NATIONAL CYBER, a
 * bank used by the alliance for oppression and we need you to find a
 * vulnerability in this program and stea... er liberate the funds from an
 * operative named prince_john.
 *
 * The bank programmers have attempted to put in a number of defensive checks
 * to prevent abuse of the bank wire network, and we cannot modify the
 * program source code without risking detection but we suspect that deadlines
 * and corporate training programs have left them without the motivation 
 * required to think through each possibility clearly.
 *
 * It is imperative that you find a way to drain the account completely
 * otherwise they may be able to withdraw the remaining funds and go
 * underground before we can finish tracking them in the physical world.
 *
 * Good luck and as always, should you be detected your actions will be
 * blamed on the Chinese Government.
 *
 *        ~~~0xDEADFEED~~~
 *
 * To build and run the wire transfer program:
 *
 * $ cc -o wire_transfer wire_transfer.c
 * $ ./wire_transfer
 *
 */

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

#define MAXLEN 255

typedef struct Account {
  int balance;
  char owner[MAXLEN];
} Account;

Account *init_account(const char *name, int balance);
bool read_transfer_amount(unsigned int *amount);
bool wiretransfer(Account *source, Account *dest, int amount);
void display_balance(Account *account);
void exit_if(bool condition, const char *message);
void validate_heist(Account *target);

int main(int argc, const char *argv[])
{
  unsigned int transfer_amount;
  char input[MAXLEN];
  char *dash;

  printf("----- WELCOME TO FIRST CYBER BANK -----\n");
  printf("username: robin_hood\n");
  printf("password: *****************\n");
  printf("Logged In Successfully!\n");
  printf("account> wiretransfer prince_john\n");
  printf("----- BEGIN WIRE TRANSFER SETUP -----\n");

  Account *yours = init_account("robin_hood", 10000);
  exit_if(yours == NULL, "Problem allocating memory");

  Account *theirs = init_account("prince_john", 500000);
  exit_if(theirs == NULL, "Problem allocating memory");

  display_balance(yours);
  display_balance(theirs);

  while (true) {
    printf("Amount to transfer from your account to theirs: ");

    if (read_transfer_amount(&transfer_amount)) {
      break;
    }

    printf("Invalid transfer amount!\n");
  }

  printf("----- ATTEMPTING WIRE TRANSFER -----\n");
  printf("Wire transfer status: ");
  printf("%s\n", wiretransfer(yours, theirs, transfer_amount) ? "SUCCESS" : "FAILURE");

  display_balance(yours);
  display_balance(theirs);
  validate_heist(theirs);

  free(yours);
  free(theirs);

  return 0;
}

Account *init_account(const char *name, int balance)
{
  Account *account = malloc(sizeof(Account));

  if (account != NULL) {
    strncpy(account->owner, name, sizeof(account->owner) - 1);
    account->balance = balance;
  }

  return account;
}

bool read_transfer_amount(unsigned int *amount)
{
  char input[MAXLEN];
  char *dash;

  fgets(input, sizeof(input), stdin);

  while ((dash = strnstr(input, "-", sizeof(input))) != NULL) {
    *dash = '0';
  }

  *amount = strtoul(input, NULL, 10);
  return amount > 0;
}

bool wiretransfer(Account *source, Account *dest, int amount)
{
  if (source == NULL || dest == NULL) return false;
  if (source->balance - amount < 0) return false;
  if (dest->balance + amount < 0) return false;

  source->balance -= amount;
  dest->balance += amount;
  return true;
}

void display_balance(Account *account)
{
  if (account == NULL) return;
  printf("%s has $%d in their account.\n", account->owner, account->balance);
}

void exit_if(bool condition, const char *message)
{
  if (condition) {
    perror(message);
    exit(EXIT_FAILURE);
  }
}

void validate_heist(Account *target)
{
  if (target->balance == 0) {
    printf("Target funds liberated: You win ;)\n");
  } else {
    printf("Target still has operating funds, try again.\n");
  }
}