Hashes, MACs & KDFs

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Common CTF patterns

• "Signature" is actually hash(secret || message) → length extension.
• Unsalted password hashes → trivial cracking / lookup.
• Confusing hash with MAC (hash != authentication).

Hash length extension attack

Technique

You can often exploit this if a server computes a "signature" like:

sig = HASH(secret || message)

and uses a Merkle–Damgård hash (classic examples: MD5, SHA-1, SHA-256).

If you know:

• message
• sig
• hash function
• (or can brute-force) len(secret)

Then you can compute a valid signature for:

message || padding || appended_data

without knowing the secret.

Important limitation: HMAC is not affected

Length extension attacks apply to constructions like HASH(secret || message) for Merkle–Damgård hashes. They do not apply to HMAC (e.g., HMAC-SHA256), which is specifically designed to avoid this class of problem.

Tools

• hash_extender:
  {{#ref}}
  https://github.com/iagox86/hash_extender
  {{#endref}}
• hashpump:
  {{#ref}}
  https://github.com/bwall/HashPump
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Good explanation

{{#ref}}
https://blog.skullsecurity.org/2012/everything-you-need-to-know-about-hash-length-extension-attacks
{{#endref}}

Password hashing and cracking

First questions

• Is it salted? (look for salt$hash formats)
• Is it a fast hash (MD5/SHA1/SHA256) or a slow KDF (bcrypt/scrypt/argon2/PBKDF2)?
• Do you have a format hint (hashcat mode / John format)?

Practical workflow

1. Identify the hash:
2. hashid <hash>
3. hashcat --example-hashes | rg -n "<pattern>"
4. If unsalted and common: try online DBs and identification tooling from the crypto workflow section.
5. Otherwise crack:
6. hashcat -m <mode> -a 0 hashes.txt wordlist.txt
7. john --wordlist=wordlist.txt --format=<fmt> hashes.txt

Common mistakes you can exploit

• Same password reused across users → crack one, pivot.
• Truncated hashes / custom transforms → normalize and retry.
• Weak KDF parameters (e.g., low PBKDF2 iterations) → still crackable.

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