hash-attack-techniques

A cryptanalysis playbook for attacking weaknesses in hash functions during CTF challenges and authorized security assessments. It documents length extension attacks against Merkle-Damgard constructions (MD5, SHA-1, SHA-256, SHA-512), collision generation for MD5 and SHA-1, HMAC timing side channels, birthday attacks, and proof-of-work solving. A recurring emphasis is knowing which attack applies where: length extension works against Merkle-Damgard hashes but not against HMAC, SHA-3/Keccak (a sponge construction), or BLAKE2, and the material carefully distinguishes identical-prefix collisions from chosen-prefix collisions.

The content is organized around a quick attack-selection table that maps a scenario to a technique and a tool. Length extension is covered with padding-calculation code, tool usage for HashPump and hash_extender, and a Python example using the hlextend library. Collision sections describe identical-prefix generation with fastcoll, chosen-prefix collisions with hashclash, UniColl single-block near-collisions, and real-world impacts such as rogue CA certificates and the 2017 SHAttered SHA-1 collision. Practical CTF tricks are included, such as PHP magic-hash comparisons (the '0e' type-juggling issue) and array-based bypasses of loose equality.

Reach for this material when a challenge or engagement involves a hash-based authentication scheme, a check that compares hashes, or a proof-of-work gate, and you need to decide whether the construction is exploitable. It also serves as a reference for the birthday bound, giving expected collision effort of roughly 2^(n/2) for an n-bit hash and noting how truncation lowers that cost. The routing section points to related skills for RSA signature weaknesses, symmetric-cipher key derivation, and classical cipher analysis when a hash weakness feeds a larger attack chain.