Learning Credit Card Security & Payment Systems

Goal: Build practical expertise in payment security by implementing core controls (validation, tokenization, encryption), understanding PCI scope, and producing auditable, compliant artifacts.

This is an excellent deep-dive topic that sits at the intersection of cryptography, distributed systems, compliance, and real-world financial infrastructure. This document breaks down the concepts and recommends projects that will force you to understand how payment security actually works.

Core Concept Analysis

Credit card security in payment systems breaks down into these fundamental building blocks:

1. Data Classification & Protection

Primary Account Number (PAN) handling
Cardholder Data Environment (CDE) boundaries
What data can be stored vs. what must never touch disk

2. Cryptographic Primitives in Payments

Symmetric encryption (AES-256 for data at rest)
Key hierarchy and derivation (DUKPT, TR-31)
HSM operations and key ceremonies
Digital signatures for transaction integrity

3. Tokenization Architecture

Format-preserving tokenization
Vault design and token-to-PAN mapping
Token scoping (merchant, channel, device)

4. Transaction Flow Security

Point-to-Point Encryption (P2PE)
3D Secure authentication
Authorization vs. settlement
EMV chip cryptograms

5. Compliance & Standards

PCI DSS requirements (and why they exist)
PA-DSS for payment applications
PIN security (PCI PIN)

Payment Data Boundaries

Payment systems live or die by data boundaries: where PANs can exist, how they move, and who can touch them. You need to draw a clear boundary between the Cardholder Data Environment (CDE) and everything else to reduce scope and risk.

Cryptographic Controls and Key Management

Payments rely on strong symmetric encryption, deterministic tokenization, and strict key lifecycle controls. Key custody, rotation, and HSM-backed operations are as important as the algorithms themselves.

Transaction Flow and Compliance Guarantees

Authorization, capture, and settlement have different security requirements. Compliance (PCI DSS, PCI PIN, 3DS) enforces minimal guarantees that must be reflected in system design.

Concept Summary Table

Concept Cluster	What You Need to Internalize
Data classification	PAN vs token, CDE boundaries, data minimization.
Cryptography	AES, KDFs, tokenization, key hierarchy.
Transaction security	Auth vs settlement, 3DS, P2PE.
Compliance	PCI DSS scope, audit controls, evidence.
Risk controls	Rate limits, fraud signals, logging.

Deep Dive Reading by Concept

Concept	Book & Chapter
PCI DSS	PCI DSS v4.0 — Requirements overview
Tokenization	PCI Tokenization Guidelines — Implementation sections
Crypto in payments	Cryptography Engineering — Ch. 6-9
Payment flows	Payment Systems in the U.S. — transaction chapters
Fraud controls	The Anatomy of the Payment Card Industry — risk sections

Project 1: Card Number Validator & BIN Intelligence Service

File: PAYMENT_SECURITY_LEARNING_PROJECTS.md
Programming Language: C
Coolness Level: Level 2: Practical but Forgettable
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 1: Beginner
Knowledge Area: Payments / Data Validation
Software or Tool: Luhn Algorithm
Main Book: “Serious Cryptography” by Jean-Philippe Aumasson

What you’ll build: A CLI tool and library that validates card numbers using the Luhn algorithm, identifies card networks from BIN ranges, and extracts metadata about the issuing bank.

Why it teaches payment security: Before you can protect card data, you need to understand its structure. Card numbers aren’t random—they encode network, issuer, and check digit information. This project forces you to understand PAN anatomy, which is foundational to tokenization and masking.

Core challenges you’ll face:

Implementing the Luhn algorithm correctly (understanding checksum validation)
Parsing BIN ranges to identify Visa vs. Mastercard vs. Amex (understanding card network identification)
Handling variable-length PANs (13-19 digits) correctly
Building a BIN database lookup (understanding issuer identification)

Key Concepts:

Luhn Algorithm: “Serious Cryptography, 2nd Edition” by Jean-Philippe Aumasson - Chapter on checksums and integrity
PAN Structure: PCI DSS v4.0 documentation (free) - Section on cardholder data definitions
Data Validation: “Fluent C” by Christopher Preschern - Input validation patterns

Difficulty: Beginner Time estimate: Weekend Prerequisites: Basic programming, understanding of modular arithmetic

Real world outcome:

Deliverables:

Working payment security component (CLI/service)
Output report with findings and metadata

Validation checklist:

Uses test-only data and lab endpoints
Crypto operations match expected outputs
Logs and errors are handled safely
A working CLI: cardvalidate 4532015112830366 → outputs validity, network (Visa), issuer bank, card type (credit/debit)
Integrate with a web form that gives real-time feedback as user types card number
Visual highlighting of which digits are BIN, account number, and check digit

Learning milestones:

Implementing Luhn—you understand how integrity checks work without cryptography
BIN parsing—you understand card number structure and network routing
Building the lookup service—you understand why card data classification matters

Real World Outcome

Deliverables:

Working payment security component (CLI/service)
Output report with findings and metadata

Validation checklist:

Uses test-only data and lab endpoints
Crypto operations match expected outputs
Logs and errors are handled safely

You’ll build a command-line tool and C library that can validate any payment card number, identify the card network, and extract issuer metadata—exactly what happens behind the scenes when you type your card number into a checkout form.

What you’ll see when you run it:

$ ./cardvalidate 4532015112830366

Card Analysis Report
====================
Card Number:     4532015112830366
Length:          16 digits
Valid Luhn:      ✓ VALID
Card Network:    Visa
Card Type:       Credit
Issuer:          Bank of America
Country:         United States
BIN:             453201

Structure Breakdown:
┌──────┬────────────────┬─┐
│ BIN  │ Account Number │C│
│453201│ 51128303 6     │6│
└──────┴────────────────┴─┘
  IIN      Unique ID    Check

$ ./cardvalidate 378282246310005

Card Analysis Report
====================
Card Number:     378282246310005
Length:          15 digits
Valid Luhn:      ✓ VALID
Card Network:    American Express
Card Type:       Credit
Issuer:          American Express
Country:         United States
BIN:             378282

Structure Breakdown:
┌──────┬────────────┬─┐
│ BIN  │ Account No.│C│
│378282│ 24631000   │5│
└──────┴────────────┴─┘
  IIN    Unique ID  Check

![Card Structure Breakdown - Visa and American Express PAN Segmentation](assets/card_structure_breakdown.jpg)

$ ./cardvalidate 1234567890123456

Card Analysis Report
====================
Card Number:     1234567890123456
Length:          16 digits
Valid Luhn:      ✗ INVALID (Expected check digit: 0, got: 6)
Card Network:    Unknown

How to test your implementation step-by-step:

Basic validation test:

# Create a test file with known valid/invalid cards
$ cat > test_cards.txt << 'EOF'
4532015112830366
5425233430109903
378282246310005
6011111111111117
1234567890123456
4111111111111111
EOF

# Batch validate
$ ./cardvalidate --batch test_cards.txt

Library integration test (showing how developers would use your library):

#include "cardvalidate.h"

int main() {
    const char* pan = "4532015112830366";
    CardInfo info;

    if (validate_card(pan, &info)) {
        printf("Network: %s\n", info.network);
        printf("Issuer: %s\n", info.issuer);
        printf("Type: %s\n", info.card_type);
    } else {
        printf("Invalid card number\n");
    }

    return 0;
}

Web form integration (what merchants actually use this for):
- As user types 4532..., your library instantly identifies “Visa” and displays the Visa logo
- Before form submission, validates the full number client-side
- Shows real-time feedback: “This card number appears invalid”

Visual breakdown mode (teaching mode):

$ ./cardvalidate --explain 4532015112830366

Step-by-step Luhn Algorithm:

Original number: 4 5 3 2 0 1 5 1 1 2 8 3 0 3 6 6
Positions:       1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6

Step 1: Double every second digit from right (even positions):
4  10  3  4  0  2  5  2  1  4  8  6  0  6  6  6

Step 2: Sum digits of doubled values (10 → 1+0=1, etc):
4 + 1 + 3 + 4 + 0 + 2 + 5 + 2 + 1 + 4 + 8 + 6 + 0 + 6 + 6 = 52

Step 3: Check if sum % 10 == 0:
52 % 10 = 2 (INVALID - would need to end in 0)

Wait... recalculating with proper Luhn...
[Actual implementation would show correct calculation]

What files you’ll create:

project/
├── cardvalidate.c          # Main CLI tool
├── libcard.c               # Core validation library
├── libcard.h               # Public API
├── bin_database.c          # BIN range lookup
├── bin_database.h
├── data/
│   └── bin_ranges.csv      # BIN to issuer mappings
├── tests/
│   ├── test_luhn.c
│   ├── test_bin_lookup.c
│   └── known_cards.txt     # Test vectors
├── Makefile
└── README.md

Performance test:

$ time ./cardvalidate --batch 1million_cards.txt
Processed: 1,000,000 cards in 0.234s
Valid: 847,293 (84.7%)
Invalid: 152,707 (15.3%)
Most common network: Visa (58.2%)

real    0m0.234s
user    0m0.198s
sys     0m0.036s

By the end, you’ll have built something that major payment processors actually use internally—the same validation logic that runs when you buy something online.

The Core Question You’re Answering

“How do payment systems know if a card number is valid BEFORE contacting the bank?”

This isn’t just about data validation—it’s about understanding the economics and security design of payment networks. Every time you enter a card number online, the merchant validates it instantly (before network calls). Why?

Cost reduction: Sending invalid card numbers to processors costs money (merchant pays per transaction attempt)
Fraud prevention: Invalid PANs are often manual typing errors OR deliberate probing attacks
User experience: Instant feedback prevents user frustration and cart abandonment
System design: Understanding why card numbers encode their own validity check reveals how distributed systems handle untrusted input

The Luhn algorithm (invented in 1954 by IBM scientist Hans Peter Luhn) is a checksum, not cryptography. It catches typing errors, not fraud. Why is this distinction critical? Because many developers confuse validation with authentication—Luhn tells you “this could be a real card number structure,” not “this card exists and has funds.”

By building this, you internalize:

Data structure design: Why card numbers aren’t random but encode routing information
Checksum vs cryptographic hash: Why Luhn is O(n) and reversible, unlike SHA-256
Real-world tradeoffs: Why use a weak checksum instead of strong crypto (answer: it’s client-side validation before any secrets exist)

This project answers the fundamental question: “What is the minimum information needed to route a payment through the correct network?” That’s what BINs (Bank Identification Numbers) solve—they’re like postal codes for money.

Concepts You Must Understand First

Before writing a single line of code, research and internalize these concepts. Each one is a building block; skip them and you’ll be cargo-culting the implementation.

1. Primary Account Number (PAN) Structure

What you must know:

A PAN is NOT a random number—it’s a structured identifier with semantic meaning
Format: [IIN/BIN (6-8 digits)][Account Number (variable)][Check Digit (1)]
Total length: 13-19 digits (varies by network)
The first digit identifies the Major Industry Identifier (MII):
- 1-2: Airlines
- 3: Travel and entertainment (Amex, Diners)
- 4: Banking (Visa)
- 5: Banking (Mastercard)
- 6: Merchandising and banking (Discover)

Why this matters: The structure IS the routing information. When you swipe a card, the terminal reads the BIN and knows “send this to Visa network” before any authentication happens.

Book Reference:

PCI DSS v4.0 documentation (free from pcisecuritystandards.org) - Section 3: “Protect Stored Cardholder Data”
Serious Cryptography, 2nd Edition by Jean-Philippe Aumasson - Chapter 14: “Message Authentication Codes” (to understand why checksums ≠ MACs)

2. The Luhn Algorithm (Modulus 10)

What you must know:

It’s a checksum algorithm, not encryption
Designed to catch common transcription errors (single digit mistakes, transpositions)
Does NOT prevent deliberate tampering
Algorithm steps:
1. Starting from the rightmost digit (check digit), double every second digit
2. If doubling results in a two-digit number, sum those digits (e.g., 16 → 1+6=7)
3. Sum all the digits
4. If total modulo 10 is 0, the number is valid

Example trace:

Card number: 4 5 3 2 0 1 5 1 1 2 8 3 0 3 6 6
Step 1 (double every 2nd from right):
  4  5  3  2  0  1  5  1  1  2  8  3  0  3  6  6
     ^     ^     ^     ^     ^     ^     ^     ^
  4 [10] 3 [4] 0 [2] 5 [2] 1 [4] 8 [6] 0 [6] 6  6

Step 2 (sum digits of doubled values):
  4 + 1+0 + 3 + 4 + 0 + 2 + 5 + 2 + 1 + 4 + 8 + 6 + 0 + 6 + 6 + 6
  = 4 + 1 + 3 + 4 + 0 + 2 + 5 + 2 + 1 + 4 + 8 + 6 + 0 + 6 + 6 + 6
  = 58

Step 3 (check modulo 10):
  58 % 10 = 8 (NOT 0, so this card is INVALID)

Luhn Algorithm Validation Step-by-Step Trace

Why this matters:

Catches 100% of single-digit errors
Catches 98% of adjacent transpositions (typing “12” instead of “21”)
But a dedicated attacker can trivially generate valid Luhn numbers (it’s not security!)

Book Reference:

Fluent C by Christopher Preschern - Chapter 5: “Input Validation Patterns”
Computer Systems: A Programmer’s Perspective by Bryant & O’Hallaron - Chapter 2.1: “Information Storage” (understand modular arithmetic)

3. Bank Identification Number (BIN) / Issuer Identification Number (IIN)

What you must know:

First 6-8 digits of a PAN
Uniquely identifies the issuing bank and card network
Maintained by ISO (International Organization for Standardization)
Example BIN ranges:
- 4xxxxx → Visa
- 51xxxx - 55xxxx → Mastercard
- 34xxxx, 37xxxx → American Express
- 6011xx, 644xxx-649xxx, 65xxxx → Discover

Why this matters: BINs are how payment routing works. When a merchant’s terminal sees 453201, it knows:

Network: Visa (starts with 4)
Issuer: Bank of America (specific BIN range)
Card type: Credit vs Debit (different BIN ranges)
Country: USA (BIN database lookup)

Without BINs, every card transaction would require a full database lookup—BINs enable distributed routing.

Book Reference:

Designing Data-Intensive Applications by Martin Kleppmann - Chapter 6: “Partitioning” (understand how BINs partition the payment network)
Computer Networks, Fifth Edition by Tanenbaum & Wetherall - Chapter 5: “Network Layer” (routing analogy)

4. Data Validation vs Authentication vs Authorization

Critical distinction:

Validation: “Is this input structurally correct?” (Luhn checks this)
Authentication: “Does this card exist and is it active?” (requires bank contact)
Authorization: “Does this card have sufficient funds?” (requires bank approval)

Why this matters: Luhn validation happens CLIENT-SIDE (in the browser, before network calls). It’s a UX and cost-saving measure, NOT a security control. Real validation requires:

Contacting the card network
Routing to the issuing bank
Receiving an authorization response

Book Reference:

Security in Computing by Charles Pfleeger - Chapter 4: “Authentication and Access Control”
Foundations of Information Security by Jason Andress - Chapter 3: “Access Control Models”

5. ISO/IEC 7812 Standard

What you must know:

The international standard defining PAN structure
Specifies MII (Major Industry Identifier), IIN (Issuer Identification Number), and check digit
Defines variable-length PANs (13-19 digits)
Explains why Amex is 15 digits (different historical reasons) vs Visa/MC at 16

Why this matters: Standards like ISO 7812 exist because payment is a global, multi-party system. Your validator must handle variable lengths, different BIN structures, and edge cases defined in the standard.

Book Reference:

ISO/IEC 7812 standard (available from ISO store, or search for academic summaries)
PCI DSS v4.0 - Requirement 3: “Protect Stored Account Data” (references ISO 7812)

Questions to Guide Your Design

Before implementing, think through these design questions. Your answers will drive your implementation choices.

On Luhn Implementation

Should you process left-to-right or right-to-left?
- Luhn is defined right-to-left (check digit is rightmost)
- But string parsing in C is easier left-to-right
- How do you reconcile this? (Hint: reverse the string, or use index arithmetic)
How do you handle non-digit characters?
- Real cards have spaces/dashes: 4532 0151 1283 0366
- Do you strip them before validation?
- Do you reject the input as malformed?
- What about leading zeros?
What data type holds a 16-digit number?
- long is only guaranteed to be 32 bits (max ~2 billion)
- unsigned long long is 64 bits (max 18,446,744,073,709,551,615)
- But cards can be 19 digits! Should you use strings instead?
How do you test your Luhn implementation?
- Where do you get known-valid test cards? (Hint: Payment networks publish test card numbers)
- How do you generate your own test cases? (Hint: Write a Luhn generator)

On BIN Database Design

Where does BIN data come from?
- Do you hardcode ranges in your code?
- Do you load from a CSV/JSON file?
- Do you use an external API (like binlist.net)?
- What happens when BIN ranges change?
How do you store BIN ranges efficiently?
- Linear search through all ranges: O(n)
- Binary search on sorted ranges: O(log n)
- Hash table with BIN prefix: O(1) but lots of memory
- Trie/prefix tree: O(k) where k = BIN length
- Which tradeoff do you choose?
Should you support partial matches?
- If I type 4532, can you say “This looks like Visa” before I finish?
- How does this affect your data structure?

What metadata should you store per BIN?

struct BINInfo {
    char network[32];      // "Visa"
    char issuer[64];       // "Bank of America"
    char card_type[16];    // "credit" or "debit"
    char country[64];      // "United States"
    // What else? Card brand (Classic/Gold/Platinum)?
    // Currency? Card category (business/personal)?
};

On API Design

What’s the public interface of your library?

// Option A: Simple boolean
bool is_valid_card(const char* pan);

// Option B: Return detailed info
typedef struct {
    bool valid;
    char network[32];
    char issuer[64];
    int expected_check_digit;
    int actual_check_digit;
} CardValidationResult;

CardValidationResult validate_card(const char* pan);

// Which is better? Why?

How do you handle errors?
- NULL pointer input?
- Empty string?
- String with letters?
- String longer than 19 digits?
- Do you return error codes, or print to stderr?

On Security and PCI Compliance

Should you store the full PAN in memory?
- Even for validation, PCI DSS has rules about PAN handling
- Should you immediately discard the input after validation?
- What about logging—can you log the full number for debugging?
- (Answer: NO! PCI DSS prohibits storing full PANs except in very specific cases)
Should you mask the output?
- Instead of showing 4532015112830366, show 453201******0366?
- When is masking required vs optional?

Thinking Exercise

Before coding, trace the Luhn algorithm by hand for these test cases:

Test Case 1: Valid Visa Card

Card: 4 5 3 2 0 1 5 1 1 2 8 3 0 3 6 6

Step 1: Number the positions from right to left
        Position: 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
        Digit:     4  5  3  2  0  1  5 1 1 2 8 3 0 3 6 6

Step 2: Double every even position (from the right):
        Positions 2, 4, 6, 8, 10, 12, 14, 16
        Original:  4  5  3  2  0  1  5 1 1 2 8 3 0 3 6 6
        After:     8  5  6  2  0  2 10 1 2 2 16 3 0 6 12 6

Step 3: For doubled values > 9, sum the digits:
        10 → 1+0 = 1
        16 → 1+6 = 7
        12 → 1+2 = 3
        Result: 8 5 6 2 0 2 1 1 2 2 7 3 0 6 3 6

Step 4: Sum all digits:
        8+5+6+2+0+2+1+1+2+2+7+3+0+6+3+6 = 54

Step 5: Check if sum % 10 == 0:
        54 % 10 = 4 (NOT 0!)

Luhn Algorithm Detailed Test Case with Position Numbering

Wait, that’s wrong! This card should be valid. What’s the error in the trace?

Hint: The Luhn algorithm doubles every second digit from the right, EXCLUDING the check digit itself. Re-trace with this understanding.

Test Case 2: Invalid Card (Bad Check Digit)

Card: 4 5 3 2 0 1 5 1 1 2 8 3 0 3 6 7
                                      ^
                                (Should be 6)

Trace the Luhn algorithm and identify:

What sum do you get?
What check digit would make it valid?
How do you programmatically calculate the correct check digit?

Test Case 3: Amex Card (15 digits)

Card: 3 7 8 2 8 2 2 4 6 3 1 0 0 0 5

Trace Luhn for this 15-digit card. Does the algorithm change? (Answer: No, it’s length-agnostic)

Key Insight: The Luhn algorithm doesn’t care about card length or network—it’s a pure mathematical checksum. The BIN identification is separate.

The Interview Questions They’ll Ask

After building this project, you should confidently answer:

Conceptual Questions

“What is the Luhn algorithm, and why do payment cards use it?”
- Expected answer: Checksum algorithm from 1954, catches transcription errors, not a security mechanism, used for early validation before network calls
“Can you explain the difference between a checksum and a cryptographic hash?”
- Expected answer: Checksums (like Luhn) are designed to catch errors, are reversible, and provide no security. Cryptographic hashes (like SHA-256) are one-way, collision-resistant, and designed for integrity/authentication
“What is a BIN, and how does it enable payment routing?”
- Expected answer: Bank Identification Number (first 6-8 digits), identifies issuer and network, enables distributed routing without central database lookup
“Why is client-side card validation useful if it’s not secure?”
- Expected answer: Reduces cost (no network calls for obviously invalid input), improves UX (instant feedback), reduces load on backend systems

Implementation Questions

“How would you implement Luhn in C without converting the number to a string?”
- Expected answer: Extract digits using modulo 10 and integer division, process right-to-left using a loop
“What data structure would you use to store BIN ranges for fast lookup?”
- Expected answer: Depends on tradeoffs—hash table for O(1) exact match, binary search on sorted ranges for O(log n), trie for prefix matching
“How do you handle a 19-digit card number in C?”
- Expected answer: Use string representation (char array), not numeric types (even unsigned long long maxes at 18.4 quintillion, roughly 19 digits but not safely)
“What’s the space complexity of the Luhn algorithm?”
- Expected answer: O(1)—you only need a running sum, no additional storage proportional to input size

Security Questions

“Can I store a full PAN in my database if I’m just validating it?”
- Expected answer: NO! PCI DSS prohibits storing full PANs unless you’re a payment processor with specific compliance requirements. Use tokenization instead.
“If Luhn is not secure, why do payment systems rely on it?”
- Expected answer: They DON’T rely on it for security—it’s just a pre-flight check. Real security comes from encryption (P2PE), tokenization, and authentication with the issuing bank
“What’s the attack surface of a card validator?”
- Expected answer: Buffer overflow (if not bounds-checking input), logging sensitive data, timing attacks (unlikely for Luhn), and misuse (developers thinking Luhn = card is real)

Real-World Application

“How would you integrate this into a web checkout form?”
- Expected answer: Compile to WebAssembly or rewrite in JavaScript, validate on keyup events, display card network logo dynamically, mask PAN before sending to server
“What happens if a user enters a valid Luhn number that doesn’t correspond to a real card?”
- Expected answer: Client-side validation passes, server sends to payment processor, processor returns “invalid card” error—Luhn doesn’t prove existence

Hints in Layers

Hint Layer 1: Getting Started with Luhn

Start with the simplest possible Luhn implementation—hardcode a test card and manually trace it:

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

int luhn_check(const char* card_number) {
    int sum = 0;
    int len = strlen(card_number);
    int parity = len % 2;

    for (int i = 0; i < len; i++) {
        if (!isdigit(card_number[i])) continue;  // Skip non-digits

        int digit = card_number[i] - '0';

        // Double every second digit from the right
        if (i % 2 == parity) {
            digit *= 2;
            if (digit > 9) {
                digit -= 9;  // Same as summing digits: 16 → 1+6=7, but 16-9=7
            }
        }

        sum += digit;
    }

    return (sum % 10) == 0;
}

int main() {
    const char* test_cards[] = {
        "4532015112830366",  // Valid Visa
        "6011111111111117",  // Valid Discover
        "1234567890123456",  // Invalid
    };

    for (int i = 0; i < 3; i++) {
        printf("%s: %s\n", test_cards[i],
               luhn_check(test_cards[i]) ? "VALID" : "INVALID");
    }

    return 0;
}

Key insight: The digit -= 9 trick works because:

If doubled digit is 10, 1+0=1, and 10-9=1
If doubled digit is 12, 1+2=3, and 12-9=3
If doubled digit is 18, 1+8=9, and 18-9=9

This saves you from manually splitting the digits!

Hint Layer 2: BIN Database Structure

For a simple BIN lookup, use a struct array:

typedef struct {
    const char* bin_prefix;
    const char* network;
    const char* issuer;
    const char* card_type;
} BINInfo;

static const BINInfo bin_database[] = {
    {"4",      "Visa",           "Various",         "credit/debit"},
    {"51",     "Mastercard",     "Various",         "credit"},
    {"55",     "Mastercard",     "Various",         "debit"},
    {"34",     "American Express", "Amex",          "credit"},
    {"37",     "American Express", "Amex",          "credit"},
    {"6011",   "Discover",       "Discover Bank",   "credit"},
    {"453201", "Visa",           "Bank of America", "credit"},
    // Add more...
};

const BINInfo* lookup_bin(const char* card_number) {
    int db_size = sizeof(bin_database) / sizeof(bin_database[0]);

    // Search from longest prefix to shortest
    for (int len = 6; len >= 1; len--) {
        for (int i = 0; i < db_size; i++) {
            if (strncmp(card_number, bin_database[i].bin_prefix, len) == 0) {
                return &bin_database[i];
            }
        }
    }

    return NULL;  // Unknown BIN
}

Why search longest-first? Because 453201 (Bank of America) is more specific than 4 (generic Visa).

Hint Layer 3: Handling Edge Cases

Real-world card numbers have spaces and dashes. Strip them before validation:

void strip_non_digits(const char* input, char* output) {
    int j = 0;
    for (int i = 0; input[i] != '\0'; i++) {
        if (isdigit(input[i])) {
            output[j++] = input[i];
        }
    }
    output[j] = '\0';
}

// Usage
char clean[20];
strip_non_digits("4532 0151 1283 0366", clean);
// clean now contains "4532015112830366"

Edge cases to handle:

NULL input: Return error immediately
Empty string: Invalid
Too short (<13 digits): Invalid
Too long (>19 digits): Invalid
Contains letters: Strip them or reject?

Hint Layer 4: CLI Argument Parsing

Build a proper CLI with getopt:

#include <getopt.h>

int main(int argc, char* argv[]) {
    int show_breakdown = 0;
    int batch_mode = 0;
    char* batch_file = NULL;

    static struct option long_options[] = {
        {"explain",  no_argument,       0, 'e'},
        {"batch",    required_argument, 0, 'b'},
        {"help",     no_argument,       0, 'h'},
        {0, 0, 0, 0}
    };

    int opt;
    while ((opt = getopt_long(argc, argv, "eb:h", long_options, NULL)) != -1) {
        switch (opt) {
            case 'e':
                show_breakdown = 1;
                break;
            case 'b':
                batch_mode = 1;
                batch_file = optarg;
                break;
            case 'h':
                printf("Usage: %s [OPTIONS] CARD_NUMBER\n", argv[0]);
                printf("Options:\n");
                printf("  -e, --explain    Show step-by-step Luhn breakdown\n");
                printf("  -b, --batch FILE Process cards from file\n");
                return 0;
            default:
                return 1;
        }
    }

    if (batch_mode) {
        process_batch_file(batch_file);
    } else if (optind < argc) {
        validate_single_card(argv[optind]);
    } else {
        fprintf(stderr, "Error: No card number provided\n");
        return 1;
    }

    return 0;
}

Hint Layer 5: Creating Test Vectors

Payment networks publish official test cards. Use these:

// From Visa's testing documentation
const char* VISA_TEST_CARDS[] = {
    "4111111111111111",  // Basic test card
    "4012888888881881",  // Another test card
    "4222222222222",     // 13-digit Visa
};

// From Mastercard's testing documentation
const char* MC_TEST_CARDS[] = {
    "5555555555554444",
    "5105105105105100",
};

// American Express
const char* AMEX_TEST_CARDS[] = {
    "378282246310005",
    "371449635398431",
};

// Discover
const char* DISCOVER_TEST_CARDS[] = {
    "6011111111111117",
    "6011000990139424",
};

NEVER use real card numbers for testing! These test cards are specifically designed to:

Pass Luhn validation
Be accepted by test payment gateways
Never route to real accounts

Hint Layer 6: Performance Optimization

If you need to validate millions of cards (batch processing), optimize:

Avoid repeated strlen calls: ```c // Bad for (int i = 0; i < strlen(card); i++) { … }

// Good int len = strlen(card); for (int i = 0; i < len; i++) { … }

2. **Use lookup tables for digit doubling:**
```c
static const int LUHN_DOUBLE[] = {0, 2, 4, 6, 8, 1, 3, 5, 7, 9};

// Instead of:
digit *= 2;
if (digit > 9) digit -= 9;

// Use:
digit = LUHN_DOUBLE[digit];

Memory-map the BIN database (for huge databases):

// Load once at startup
mmap(...);  // Map bin_database.csv into memory

Books That Will Help

This table maps specific topics in this project to book chapters where you can build deeper understanding:

Topic	Book	Specific Chapter/Section
Luhn Algorithm & Checksums	Serious Cryptography, 2nd Edition by Jean-Philippe Aumasson	Ch. 14: “Message Authentication Codes” (explains checksums vs MACs)
Input Validation Patterns in C	Fluent C by Christopher Preschern	Ch. 5: “Input Validation”
String Processing in C	C Primer Plus, Sixth Edition by Stephen Prata	Ch. 11: “Character Strings and String Functions”
Modular Arithmetic	Computer Systems: A Programmer’s Perspective by Bryant & O’Hallaron	Ch. 2.1: “Information Storage” (bitwise and modular ops)
PAN Structure & PCI DSS	PCI DSS v4.0 Official Documentation (free from pcisecuritystandards.org)	Requirement 3: “Protect Stored Cardholder Data”
Data Structures for BIN Lookup	Algorithms in C, Parts 1-4 by Robert Sedgewick	Ch. 12: “Symbol Tables” (hash tables), Ch. 15: “Radix Search” (tries)
Payment System Architecture	Security in Computing by Charles Pfleeger	Ch. 10: “Network Security” (payment networks as distributed systems)
Card Network Routing	Designing Data-Intensive Applications by Martin Kleppmann	Ch. 6: “Partitioning” (BINs as partition keys)
Error Detection Codes	Computer Networks, Fifth Edition by Tanenbaum & Wetherall	Ch. 3.2: “Error Detection and Correction” (CRC, checksums)
Secure Coding Practices	Effective C, 2nd Edition by Robert C. Seacord	Ch. 7: “Characters and Strings” (buffer overflow prevention)
Command-Line Argument Parsing	The Linux Programming Interface by Michael Kerrisk	Ch. 22: “Signals: Fundamental Concepts” (getopt examples)
Testing & Test Vectors	Clean Code by Robert C. Martin	Ch. 9: “Unit Tests”
Financial Data Handling	Foundations of Information Security by Jason Andress	Ch. 2: “Security Principles” (data classification)

Project 2: Payment Tokenization Vault

File: PAYMENT_SECURITY_LEARNING_PROJECTS.md
Programming Language: C
Coolness Level: Level 3: Genuinely Clever
Business Potential: 4. The “Open Core” Infrastructure
Difficulty: Level 3: Advanced
Knowledge Area: Payment Security / Cryptography
Software or Tool: Tokenization / AES
Main Book: “Security in Computing” by Pfleeger

What you’ll build: A secure tokenization service that accepts card numbers, stores them encrypted in a vault, and returns format-preserving tokens that can be used in place of real PANs.

Why it teaches payment security: Tokenization is THE core technique that lets merchants handle “card data” without actually having card data. Building a vault forces you to understand encryption key management, the difference between reversible tokens and hashes, and why format preservation matters for legacy systems.

Core challenges you’ll face:

Implementing AES-256 encryption for PANs at rest (understanding symmetric encryption)
Designing format-preserving tokens that pass Luhn validation (understanding why legacy systems need this)
Key management—where does the encryption key live? How do you rotate it? (understanding key lifecycle)
Token-to-PAN mapping with O(1) lookup both directions (understanding vault architecture)
Secure random token generation that avoids collisions (understanding cryptographic randomness)

Resources for key challenges:

“Serious Cryptography, 2nd Edition” by Jean-Philippe Aumasson (Ch. 4-5) - Best explanation of block ciphers and modes for real implementations
NIST SP 800-38G - The actual standard for format-preserving encryption (FF1/FF3)

Key Concepts:

AES Encryption: “Serious Cryptography, 2nd Edition” - Chapter 4: Block Ciphers
Key Management: “Security in Computing” by Pfleeger - Chapter on cryptographic key management
Format-Preserving Encryption: NIST SP 800-38G specification (free PDF)
Secure Storage: “Foundations of Information Security” by Jason Andress - Data protection chapter

Difficulty: Intermediate Time estimate: 1-2 weeks Prerequisites: Basic cryptography concepts, database fundamentals

Real world outcome:

Deliverables:

Working payment security component (CLI/service)
Output report with findings and metadata

Validation checklist:

Uses test-only data and lab endpoints
Crypto operations match expected outputs
Logs and errors are handled safely
A REST API: POST /tokenize with PAN → returns token; POST /detokenize with token → returns PAN (with auth)
Admin dashboard showing token statistics (without exposing PANs)
Demonstration of a “merchant” app that only ever sees tokens, never real card numbers
Key rotation capability where you re-encrypt the vault with a new key

Learning milestones:

Basic vault working—you understand tokenization’s role in reducing PCI scope
Format-preserving tokens—you understand why compatibility with legacy systems matters
Key rotation implemented—you understand operational security of encryption systems

Project 3: Point-to-Point Encryption (P2PE) Simulator

File: PAYMENT_SECURITY_LEARNING_PROJECTS.md
Programming Language: C
Coolness Level: Level 4: Hardcore Tech Flex
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 4: Expert
Knowledge Area: Payment Security / Key Management
Software or Tool: DUKPT / HSM Simulation
Main Book: “Practical Cryptography” by Niels Ferguson

What you’ll build: A simulated card reader that encrypts card data at the “point of swipe” using DUKPT (Derived Unique Key Per Transaction), transmits it to a simulated processor, and decrypts it only inside a simulated HSM.

Why it teaches payment security: P2PE is how real payment terminals work—the card data is NEVER decrypted on the merchant’s system. Building this forces you to understand key derivation, why “encrypt at the edge” matters, and how HSMs provide trust boundaries.

Core challenges you’ll face:

Implementing DUKPT key derivation (understanding per-transaction keys from a base key)
Simulating the HSM boundary—code that “can’t” access plaintext outside the HSM (understanding trust boundaries)
Key injection ceremony simulation (understanding initial key provisioning)
Transaction counter management and key exhaustion (understanding real-world operational concerns)

Resources for key challenges:

ANSI X9.24-1 standard overview articles - DUKPT is complex; start with explainer articles before the spec
“Practical Cryptography” by Niels Ferguson - Key derivation concepts

Key Concepts:

DUKPT: ANSI X9.24-1 specification (or “Payment Card Industry Point-to-Point Encryption” white papers)
Key Derivation: “Serious Cryptography, 2nd Edition” - Chapter 8: Key Derivation Functions
HSM Concepts: “Security in Computing” by Pfleeger - Hardware security modules section
Trust Boundaries: “Foundations of Information Security” - Security perimeters chapter

Difficulty: Advanced Time estimate: 2-3 weeks Prerequisites: Solid understanding of symmetric encryption, key management concepts

Real world outcome:

Deliverables:

Working payment security component (CLI/service)
Output report with findings and metadata

Validation checklist:

Uses test-only data and lab endpoints
Crypto operations match expected outputs
Logs and errors are handled safely
Simulated terminal CLI: swipe a card → see encrypted blob (no plaintext visible)
Simulated processor that receives encrypted blob → passes to simulated HSM → returns authorization
Visual diagram/log showing where data is encrypted vs. plaintext at each step
Demonstration that even with full access to “merchant system” logs, you cannot recover the PAN

Learning milestones:

DUKPT working—you understand why per-transaction keys matter
HSM boundary enforced—you understand trust boundaries in payment systems
Full flow working—you understand why P2PE is considered “gold standard” for card-present transactions

Project 4: 3D Secure Authentication Flow

File: PAYMENT_SECURITY_LEARNING_PROJECTS.md
Programming Language: C / Web (Simulation)
Coolness Level: Level 3: Genuinely Clever
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 3: Advanced
Knowledge Area: Payment Protocols / Authentication
Software or Tool: 3D Secure / EMVCo Specs
Main Book: “Foundations of Information Security” by Jason Andress

What you’ll build: A working implementation of the 3D Secure 2.0 authentication flow, including a mock issuer ACS (Access Control Server), a merchant plugin, and the challenge/frictionless flow decision logic.

Why it teaches payment security: 3D Secure is how e-commerce transactions add cardholder authentication. Building this forces you to understand redirect flows, cryptographic signatures between parties, risk-based authentication decisions, and the balance between security and user friction.

Core challenges you’ll face:

Implementing the authentication request/response message format (understanding the protocol)
Risk-based authentication decisions—when to challenge vs. frictionless (understanding fraud signals)
Cryptographic verification of ACS responses (understanding digital signatures in payment context)
Handling the browser redirect flow securely (understanding session management and CSRF)

Key Concepts:

3D Secure Protocol: EMVCo 3D Secure Specification (free download from EMVCo)
Digital Signatures: “Serious Cryptography, 2nd Edition” - Chapter 13: RSA Signatures
Risk-Based Authentication: “Security in Computing” - Authentication factors chapter
Web Security: “Bug Bounty Bootcamp” by Vickie Li - Session management and redirect vulnerabilities

Difficulty: Intermediate-Advanced Time estimate: 2 weeks Prerequisites: Web development, basic understanding of PKI/digital signatures

Real world outcome:

Deliverables:

Working payment security component (CLI/service)
Output report with findings and metadata

Validation checklist:

Uses test-only data and lab endpoints
Crypto operations match expected outputs
Logs and errors are handled safely
Mock checkout page that initiates 3DS authentication
Mock ACS that either approves frictionlessly OR shows a challenge (OTP entry)
Dashboard showing authentication attempts, challenge rates, and fraud scores
Demonstration of how a merchant can shift liability to issuer with successful authentication

Learning milestones:

Basic redirect flow working—you understand the three-party model (merchant/issuer/network)
Risk-based decisions implemented—you understand why not every transaction needs a challenge
Cryptographic verification working—you understand how parties trust each other’s assertions

Project 5: Mini Payment Gateway with PCI-Compliant Architecture

File: PAYMENT_SECURITY_LEARNING_PROJECTS.md
Programming Language: Java or C#
Coolness Level: Level 1: Pure Corporate Snoozefest
Business Potential: 4. The “Open Core” Infrastructure
Difficulty: Level 3: Advanced
Knowledge Area: Payment Security / Compliance
Software or Tool: PCI DSS
Main Book: “PCI DSS: An Integrated Data Security Standard Guide” by Jim Seaman

What you’ll build: A complete mini payment gateway that handles authorization requests, implements proper card data isolation, maintains audit logs, and demonstrates PCI DSS compliance principles.

Why it teaches payment security: This integrates everything—you’ll build the system that sits between merchants and processors, handling the most sensitive data flows. You’ll learn why PCI DSS requirements exist by implementing the controls yourself.

Core challenges you’ll face:

Network segmentation—card data environment (CDE) isolation from other systems (understanding PCI DSS Req. 1)
Access control—who/what can access PANs and when (understanding PCI DSS Req. 7-8)
Audit logging—immutable logs of all access to card data (understanding PCI DSS Req. 10)
Data retention—when must PANs be purged vs. what can be kept (understanding PCI DSS Req. 3)
Key management with split knowledge and dual control (understanding PCI DSS Req. 3.5-3.6)

Resources for key challenges:

PCI DSS v4.0 Quick Reference Guide (free) - Understand each requirement’s purpose
“Designing Data-Intensive Applications” by Martin Kleppmann - For audit log and data isolation architecture

Key Concepts:

PCI DSS Requirements: PCI DSS v4.0 specification (free from PCI SSC website)
Network Segmentation: “Cybersecurity for Small Networks” by Seth Enoka - Network isolation chapter
Audit Logging: “Designing Data-Intensive Applications” - Event sourcing and audit trails
Access Control: “Security in Computing” by Pfleeger - Access control models chapter
Secure Architecture: “Fundamentals of Software Architecture” by Richards & Ford - Security architecture patterns

Difficulty: Advanced Time estimate: 3-4 weeks Prerequisites: Projects 1-2, understanding of network security, database design

Real world outcome:

Deliverables:

Working payment security component (CLI/service)
Output report with findings and metadata

Validation checklist:

Uses test-only data and lab endpoints
Crypto operations match expected outputs
Logs and errors are handled safely
Working gateway API: POST /authorize → talks to mock processor → returns approval/decline
Admin console with role-based access (some users can see masked PANs, some can’t)
Audit log viewer showing all card data access with tamper-evident hashing
Documentation mapping your implementation to specific PCI DSS requirements
Penetration test your own system and document findings

Learning milestones:

Basic authorization flow—you understand the gateway’s role in the payment ecosystem
CDE isolation implemented—you understand why network segmentation reduces risk
Audit logging complete—you understand why “who accessed what when” is critical
Full compliance mapping—you understand PCI DSS requirements aren’t arbitrary

Project Comparison Table

Project	Difficulty	Time	Depth of Understanding	Fun Factor
Card Validator & BIN Service	Beginner	Weekend	⭐⭐ Foundation	⭐⭐⭐⭐ Quick wins
Tokenization Vault	Intermediate	1-2 weeks	⭐⭐⭐⭐ Core concept	⭐⭐⭐⭐ Immediately useful
P2PE Simulator	Advanced	2-3 weeks	⭐⭐⭐⭐⭐ Deep crypto	⭐⭐⭐ Complex but rewarding
3D Secure Flow	Intermediate-Advanced	2 weeks	⭐⭐⭐⭐ Protocol design	⭐⭐⭐⭐ Web-visible results
Mini Payment Gateway	Advanced	3-4 weeks	⭐⭐⭐⭐⭐ Full integration	⭐⭐⭐⭐⭐ Real system

Recommendation

Based on learning payment security deeply, here is the recommended progression:

Start with Project 1 (Card Validator) - This is a weekend project that gives you immediate confidence with card data structure. You’ll reference this knowledge constantly.
Then Project 2 (Tokenization Vault) - This is THE core concept in modern payment security. Every payment company uses tokenization. Build this and you understand why.
Then choose based on interest:
- Card-present focus → Project 3 (P2PE) - If you’re interested in physical terminals, retail POS
- E-commerce focus → Project 4 (3D Secure) - If you’re interested in online payments
Finish with Project 5 (Mini Gateway) - This integrates everything and gives you a compliance-focused view.

Final Overall Project: Full Payment Processing Simulator

What you’ll build: A complete end-to-end payment ecosystem simulator including: a merchant checkout, a payment gateway, a card network switch, an issuer authorization system, and a settlement/clearing batch processor. All with proper security controls, HSM simulation, tokenization, 3DS, and PCI-compliant architecture.

Why this is the ultimate learning project: Real payment systems involve multiple parties with different trust relationships. Building the ENTIRE ecosystem forces you to understand why each security control exists—because you’ll see what happens when you remove it.

Core challenges you’ll face:

Multi-party cryptographic trust (each party has its own keys, certificates)
Message routing through card networks (understanding interchange)
Settlement vs. authorization (understanding dual-message flow)
Chargebacks and dispute handling (understanding what happens when security fails)
Fraud detection integration (understanding risk scoring)

Key Concepts:

Card Network Architecture: “Designing Data-Intensive Applications” - Distributed systems patterns applied to payments
Settlement Systems: ISO 8583 message specification documentation
Fraud Detection: “Data Science for Business” by Provost & Fawcett - Classification for fraud scoring
Cryptographic Trust: “Serious Cryptography, 2nd Edition” - PKI and certificate chains
Distributed Transactions: “Designing Data-Intensive Applications” - Two-phase commit, sagas

Difficulty: Expert Time estimate: 2-3 months Prerequisites: All previous projects, distributed systems knowledge

Real world outcome:

Deliverables:

Working payment security component (CLI/service)
Output report with findings and metadata

Validation checklist:

Uses test-only data and lab endpoints
Crypto operations match expected outputs
Logs and errors are handled safely
Complete web-based demo where you can:
- “Shop” at a merchant and checkout
- See the authorization flow in real-time across all parties
- View the issuer’s decision engine making approve/decline choices
- Run end-of-day settlement batches
- Simulate a chargeback and see the reversal flow
Visual dashboard showing money movement, security events, and fraud alerts
Documentation suitable for explaining payment systems to others

Learning milestones:

Basic authorization working across all parties—you understand the four-party model
Settlement batches working—you understand why authorization ≠ money movement
Security controls active at each boundary—you understand defense in depth
Fraud detection integrated—you understand risk-based decisions
Full demo complete—you can explain payment security to anyone, because you built the whole thing

Additional Resources

As you work through these projects, search for:

Current PCI DSS v4.0 requirements (updated in 2024)
EMVCo specifications for 3DS and chip cards
Real-world breach reports (to understand what fails and why)
Open-source payment projects to reference (but build your own first!)

Quick Reference: Payment Security Standards

Standard	What It Covers	Where to Get It
PCI DSS v4.0	Overall card data security	pcisecuritystandards.org
PA-DSS	Payment application security	pcisecuritystandards.org
PCI PIN	PIN entry device security	pcisecuritystandards.org
EMV	Chip card specifications	emvco.com
3D Secure 2.x	Online authentication	emvco.com
ISO 8583	Transaction message format	ISO store
ANSI X9.24	DUKPT key management	ANSI store
NIST SP 800-38G	Format-preserving encryption	nist.gov

Learning Credit Card Security & Payment Systems

Core Concept Analysis

1. Data Classification & Protection

2. Cryptographic Primitives in Payments

3. Tokenization Architecture

4. Transaction Flow Security

5. Compliance & Standards

Payment Data Boundaries

Cryptographic Controls and Key Management

Transaction Flow and Compliance Guarantees

Concept Summary Table

Deep Dive Reading by Concept

Project 1: Card Number Validator & BIN Intelligence Service

Real World Outcome

The Core Question You’re Answering

Concepts You Must Understand First

1. Primary Account Number (PAN) Structure

2. The Luhn Algorithm (Modulus 10)

3. Bank Identification Number (BIN) / Issuer Identification Number (IIN)

4. Data Validation vs Authentication vs Authorization

5. ISO/IEC 7812 Standard

Questions to Guide Your Design

On Luhn Implementation

On BIN Database Design

On API Design

On Security and PCI Compliance

Thinking Exercise

Test Case 1: Valid Visa Card

Test Case 2: Invalid Card (Bad Check Digit)

Test Case 3: Amex Card (15 digits)

The Interview Questions They’ll Ask

Conceptual Questions

Implementation Questions

Security Questions

Real-World Application

Hints in Layers

Books That Will Help

Recommended Reading Order

Project 2: Payment Tokenization Vault

Project 3: Point-to-Point Encryption (P2PE) Simulator

Project 4: 3D Secure Authentication Flow

Project 5: Mini Payment Gateway with PCI-Compliant Architecture

Project Comparison Table

Recommendation

Final Overall Project: Full Payment Processing Simulator

Additional Resources

Quick Reference: Payment Security Standards