Cracking the Algorithm: Understanding CSC Calculation

The algorithm to calculate a payment card’s Card Security Code (CSC) is not publicly disclosed for security reasons. However, it is widely known that CSCs are generated using proprietary encryption algorithms within the issuing bank’s secure environment.

Here’s a general explanation of how the process works.

Types of CSCs

There are several security codes, but the main ones are Card Verification Value (CVV), used by Visa, and Card Verification Code (CVC), used by MasterCard.

There are also different versions, as shown below.

CVV1 or CVC1: Encoded in the card’s magnetic stripe and used for in-person transactions when the card is swiped.

CVV2 or CVC2: Printed on the back of the card and used for card-not-present transactions (e.g., online or phone orders).

The calculation process for both types is similar but may include slight variations in the data used.

Components of CSC Generation

Several pieces of information are required to calculate the CSC.

They are:

• Primary Account Number (PAN): The card number (usually 16 digits).
• Expiration Date: The card’s expiry date.
• Service Code: A 3-digit code that specifies the card’s functionality (e.g., chip-enabled, ATM-only). It is separate from and should not be confused with the Card Security Code. It defines how and where the card can be used.
• Secret Key: A cryptographic key that is securely stored and known only by the issuing bank.

Algorithm (High-Level Overview)

The CSC is calculated using a cryptographic algorithm such as Triple DES, HMAC or a similar secure hashing algorithm.

The general process is:

1. Data Concatenation
Concatenate the card data (PAN, expiration date, and service code) into a single string.

2. Cryptographic Processing
Encrypt or hash this concatenated data using the secret key and the bank’s cryptographic algorithm. Here the role of cryptographic randomness in preventing prediction or brute force attacks is crucial!

3. Output Truncation
Extract a specific number of digits (usually 3 for VISA and MasterCard, 4 for AMEX) from the resulting encrypted value. This becomes the CSC.

Generating a CSC in C

Here’s an example of implementing a basic simulation of a CSC generation process in C language. This example uses a simplified approach with cryptographic processing simulated via a hash function (e.g., SHA256) and truncation.

#include <stdio.h>
#include <string.h>
#include <openssl/sha.h>
#include <stdlib.h>

// Function to concatenate data safely
int concatenate_data(const char *pan, const char *expiry_date,
                     const char *service_code, char *output,
                     size_t output_size) {
  // Ensure there is enough space in the output buffer
  size_t required_size = strlen(pan) + strlen(expiry_date) +
                         strlen(service_code) + 1;
  if (required_size > output_size) {
    fprintf(stderr, "Output buffer too small for concatenated data.\n");
    return -1; // Indicate failure
  }

  snprintf(output, output_size, "%s%s%s", pan, expiry_date, service_code);
  return 0; // Indicate success
}

// Function to simulate cryptographic processing (SHA256)
void cryptographic_processing(const char *data,
                              unsigned char *hash_output) {
  if (data == NULL || hash_output == NULL) {
    fprintf(stderr, "Null pointer passed to cryptographic_processing.\n");
    exit(EXIT_FAILURE);
  }

  /* In a real-world scenario, Triple DES or HMAC
     might be used with a secret key. */
  SHA256((unsigned char *)data, strlen(data), hash_output);
}

// Function to truncate the cryptographic output for CSC
void truncate_output(const unsigned char *hash, char *csc_output, int length) {
  if (hash == NULL || csc_output == NULL || length <= 0) {
    fprintf(stderr, "Error: Invalid arguments to truncate_output.\n");
    exit(EXIT_FAILURE);
  }
  
  for (int i = 0; i < length; i++) {
    csc_output[i] = (hash[i] % 10) + '0'; // Convert to numeric character
  }
  csc_output[length] = '\0'; // Null-terminate the string
}

int main() {
  // Input data
  const char *pan = "1234567890123456"; // Primary Account Number
  const char *expiry_date = "1226";     // Expiration Date (MMYY)
  const char *service_code = "101";     // Service Code
  const int csc_length = 3;             // Length of CSC (typically 3 or 4)

  char concatenated_data[64];
  unsigned char hash_output[SHA256_DIGEST_LENGTH];
  char csc_output[csc_length + 1];

  // Step 1: Data Concatenation with bounds-checking
  if (concatenate_data(pan, expiry_date, service_code, concatenated_data, sizeof(concatenated_data)) != 0) {
    fprintf(stderr, "Failed to concatenate data. Exiting.\n");
    return EXIT_FAILURE;
  }
  printf("Concatenated Data: %s\n", concatenated_data);

  // Step 2: Cryptographic Processing
  cryptographic_processing(concatenated_data, hash_output);

  // Step 3: Output Truncation
  truncate_output(hash_output, csc_output, csc_length);

  // Output the generated CSC
  printf("Generated CSC: %s\n", csc_output);

  /* Example Output
     Concatenated Data: 12345678901234561226101
     Generated CSC: 228 */

  return EXIT_SUCCESS;
}

You need to install OpenSSL development libraries to compile the program. For example, on Debian-based distros:

sudo apt install libssl-dev
gcc -o generate_csc generate_csc.c -lcrypto

Why CSC Algorithms Are Not Public

The CSC calculation algorithm is kept confidential to protect against fraud.

Even if someone obtains the card number, expiration date, and service code, they cannot generate a valid CSC without access to the secret key stored in the issuing bank’s secure systems.

Therefore, the CSC cannot be reverse-engineered from the card data and above all, attempts to bypass CSC validation are illegal!

Furthermore, CSC is not stored in payment systems to enhance security. Even the card issuer recalculates the CSC using the stored secret key and compares it with the one provided each time.

This is why it is crucial to keep your CSC private!