What are cryptography?

Cryptography is the art of securing communication, transforming readable data into something unreadable to protect it from prying eyes. Let’s break down some key concepts and ciphers, then peek into reverse engineering.

Caesar Cipher: One of the simplest encryption methods, it shifts each letter in the plaintext by a fixed number (e.g., shift “A” by 3 to get “D”). Easy to crack with frequency analysis, but it’s a historical stepping stone.

Substitution Cipher: A broader category where each letter is replaced by another based on a key. Unlike Caesar’s fixed shift, the mapping can be random (e.g., A→K, B→P). Still vulnerable to statistical attacks.

RSA: A public-key giant, RSA relies on the math of large prime numbers. Your public key encrypts, but only the private key decrypts. It’s foundational for secure data exchange, like HTTPS.

AES (Advanced Encryption Standard): A symmetric-key algorithm, meaning the same key encrypts and decrypts. Fast, secure, and widely used (e.g., in VPNs), it comes in 128, 192, or 256-bit flavors.

MD5: A hash function, not encryption. It turns data into a fixed 128-bit “fingerprint.” Great for integrity checks, but broken for security—collisions (different inputs, same hash) are too easy to find.

Base64: Not encryption, just encoding. It converts binary data (like images) into an ASCII string, using 64 characters. Handy for transmission, but anyone can decode it.

Hex (Hexadecimal): Another encoding, representing bytes as two-digit base-16 numbers (0-9, A-F). Think “FF” for 255. It’s common in debugging and data representation.

Reverse Engineering in Crypto: Ever wonder how hackers break this stuff? Reverse engineering involves dissecting code or systems to uncover their secrets. In cryptography, it’s about cracking weak implementations or finding keys. Tools like IDA Pro and Ghidra decompile binaries, while Wireshark sniffs network traffic for encrypted packets. Techniques include brute-forcing weak keys, exploiting side-channel leaks (e.g., timing attacks), or analyzing memory dumps with ** Volatility**. A poorly implemented AES can fall to a clever reverse engineer faster than you’d think.

Cryptography evolves with math and tech, but so do the tools to unravel it. Stay curious—and secure!

1. Static Analysis

• What: Examine the program without running it.
• Tools:
• IDA Pro: A top-tier disassembler showing assembly code and program structure.
• Ghidra: Free, open-source alternative from the NSA, equally powerful.
• Process: Open the binary, analyze assembly code, and locate key functions like main or crypto routines (e.g., AES encryption calls). Look for hardcoded keys or logic patterns.

2. Dynamic Analysis

• What: Run the program and watch it in action.
• Tools:
• GDB: A debugger for Linux, great for stepping through code.
• OllyDbg: Windows-friendly, with a GUI for real-time debugging.
• Process: Execute the program, set breakpoints, and inspect memory or registers. Modify values (e.g., flip a conditional jump) to alter execution flow or bypass checks.

3. Decompilation

• What: Convert compiled code back to a high-level language.
• Tools:
• Jadx: Decompiles Android APKs to Java source.
• JD-GUI: Handles Java bytecode, revealing readable code.
• Process: For Java or .NET binaries, restore logic to near-original form. Useful when assembly is too low-level.

Problem-Solving Approach

1. Find the Entry Point: Locate main or the program’s starting function using disassemblers.
2. Identify Key Logic: Trace the core functionality—how is a cryptographic “Flag” generated or stored?
3. Dynamic Debugging: Run it, observe behavior, and tweak memory/registers to expose secrets.

Real-World Example: Cracking challenge.exe

• Challenge: Extract a Flag from challenge.exe.
• Steps:

1. Static Analysis: Open in IDA Pro, find main. Spot a function encrypting a string into memory.
2. Code Logic: The Flag is XORed with a key (e.g., “secret123”) and stored at a specific address.
3. Dynamic Debugging: Use GDB, set a breakpoint after encryption, and inspect memory. Decrypt the Flag by reversing the XOR operation.

• Result: Flag revealed—say, FLAG{reversed!}.

Tools in Action

• IDA Pro/Ghidra: Disassemble and map program flow.
• GDB/OllyDbg: Step through execution, manipulate runtime.
• Wireshark: Sniff network traffic for crypto handshakes.
• Volatility: Dump memory to find keys or plaintext.

Cryptography builds walls; reverse engineering finds the cracks. Weak implementations—like a hardcoded RSA key or sloppy AES usage—fall fast to these methods. Stay sharp, and secure your code!