spoiler-free hints for cryptography challenges.

what crypto ctf challenges look like

crypto challenges give you ciphertext, a script that encrypted something, public keys, or a description of a custom cipher. the goal is usually to recover plaintext or a flag. challenges range from classical ciphers (Caesar, Vigenere) to modern broken implementations of RSA, AES, and elliptic curve crypto.

common crypto challenge types

• RSA — small public exponentif e=3 and the message is small, m³ < n so you can just cube-root the ciphertext. no factoring needed.
• RSA — common modulus attackif the same message is encrypted with the same n but different e values, you can recover m using the extended Euclidean algorithm.
• RSA — Wiener's attackif d is small relative to n, the private key can be recovered from the continued fraction expansion of e/n.
• AES ECB modeECB encrypts each block independently. identical plaintext blocks produce identical ciphertext blocks — look for patterns or use a chosen-plaintext attack.
• CBC padding oracleif the server reveals whether decryption padding is valid, you can decrypt or forge arbitrary ciphertext byte by byte.
• XOR ciphersingle-byte XOR: try all 256 keys and score by letter frequency. repeating-key XOR: find key length via index of coincidence, then solve each position separately.
• hash crackingMD5/SHA1 hashes of common strings can be looked up in rainbow tables. use hashcat or john for longer wordlist attacks.
• classical ciphersCaesar shift — brute force 26 rotations. Vigenere — use index of coincidence to find key length, then frequency analysis per position.

useful tools

CyberChef for quick transforms and decoding · RsaCtfTool for automated RSA attacks · hashcat / john for hash cracking · pycryptodome for scripting crypto in Python · SageMath for number theory and elliptic curve operations.