crypto ctf hints
spoiler-free hints for cryptography challenges.
RSA · AES · XOR · hash cracking · classical ciphers · elliptic curves · padding oracles
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.
faq
how do I tell which RSA attack applies?+
read the parameters. a small public exponent (e=3) with a short message means you can cube-root the ciphertext, no factoring. the same message under a shared modulus with different e means a common-modulus attack. a small private exponent d means Wiener’s attack via continued fractions. very close prime factors mean a Fermat factorisation.
what tools should I have ready for crypto challenges?+
CyberChef for quick decoding and transforms, RsaCtfTool for automated RSA attacks, hashcat or john for cracking hashes, pycryptodome for scripting crypto in Python, and SageMath for number theory and elliptic-curve work.
is frequency analysis still worth trying?+
yes, for classical ciphers. single-byte XOR and Caesar fall to brute force plus letter-frequency scoring. Vigenere needs the key length first — find it with the index of coincidence — then run frequency analysis on each key position separately.