Cryptography
Cryptography is the mathematics of secure communication. CTF crypto challenges teach you how ciphers work by making you break weak ones. You'll start with classical pen-and-paper ciphers, move through modern encoding schemes and hash functions, and eventually attack real-world protocols like RSA when they're implemented incorrectly.
- 1
Classical Ciphers
Before modern cryptography there were substitution and transposition ciphers. A Caesar cipher shifts each letter by a fixed amount; a Vigenere cipher uses a repeating key. These are trivially broken with frequency analysis, but understanding them builds the intuition you need to attack anything harder.
- 2
Encoding and Hashing
Encoding (Base64, hex, URL encoding) is not encryption but it looks like it. Hash functions (MD5, SHA-1, bcrypt) are one-way transforms used to store passwords. You'll learn the difference between the two and practice cracking hash digests using wordlists and rainbow tables.
- 3
Custom Encryption Schemes
Rolling your own crypto is almost always a mistake. These challenges present home-grown encryption implementations with subtle flaws: simple XOR with a short key, Caesar-based schemes, and alphabet remapping. The skill is reading the code, understanding the transform, and inverting it.
- 4
RSA and Public-Key Cryptography
RSA is the most widely deployed public-key cryptosystem. When properly used it's secure, but small primes, reused parameters, and padding oracle attacks make weak RSA implementations breakable with basic number theory. These challenges introduce the math and show where real-world RSA goes wrong.