Index of hashing pages - hash functions, length extension attacks, and hash cracking.

Cryptographic Hashing

Cryptographic hashing is a one-way mathematical function that deterministically maps arbitrary-length inputs to fixed-size outputs (fingerprints), designed to be irreversible and collision-resistant. How it works: Hash functions process input in blocks with non-linear operations (mixing, rotation, addition); identical inputs always produce identical outputs; reversing a hash requires brute-forcing the input space since no algebraic inverse exists. Why it matters: Used for password storage, data integrity verification, and digital signatures; exploitable through weak algorithms (MD5, SHA-1 collisions), unsalted storage, or length extension attacks; CTF challenges often combine hashing vulnerabilities with weak randomness or predictable inputs.

Pages in This Subsection

Page	Difficulty	Key Technique
Hash Functions	Beginner	Understand MD5/SHA, magic hashes
Length Extension Attack	Intermediate	Forge MAC without knowing secret
Hash Cracking	Beginner	hashcat, john, online databases

Quick selection guide

Given a hash, recover the plaintext

Try CrackStation first (instant if the password is in a database).
hashcat -m 0 hash.txt rockyou.txt - MD5
hashcat -m 100 hash.txt rockyou.txt - SHA-1
hashcat -m 1400 hash.txt rockyou.txt - SHA-256

$\mathrm{MAC} = H(\mathrm{secret} \,\|\, \mathrm{message})$ - forge without the secret

Length extension (MD5, SHA-1, SHA-256 Merkle–Damgård): hashpumpy, hash_extender.
HMAC - not vulnerable to length extension in the same way.

PHP `==` and “magic” hashes

Try values whose hex digest starts with 0e and only digits (parsed as scientific notation → 0).
Example: md5("240610708") begins with 0e462097… → loose == can match other 0e… digests.

Need an MD5 collision (not a preimage)

Use known collision generators / precomputed pairs (e.g. research tooling such as cr.yp.to collision demos).

Is it HMAC?

If the challenge uses proper HMAC, assume no raw length-extension forgery on $H(\mathrm{secret} \,\|\, \mathrm{msg})$ .

Hash length reference

Fingerprint	Likely algorithm
32 hex chars	MD5
40 hex chars	SHA-1
56 hex chars	SHA-224
64 hex chars	SHA-256
96 hex chars	SHA-384
128 hex chars	SHA-512
`$2y$10$…` (~60)	bcrypt
`$6$…`	SHA-512 crypt
`$1$…`	MD5 crypt

Last updated on Aug 1, 2025

Cryptographic Hashing

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