Hashes

Hashes are one‑way functions that transform data into fixed‑length strings. They are widely used for password storage, integrity verification, and digital signatures, but in CTFs, they often protect flags or credentials.

Unlike encryption, hashing is non‑reversible. To retrieve the original input, attackers must use bruteforce, dictionary attacks, or precomputed lookup tables.

CTF challenges commonly include hash strings from algorithms such as MD5, SHA‑1, and SHA‑256. These can often be cracked using wordlists (rockyou.txt, SecLists) with tools like John the Ripper, Hashcat, or online databases such as CrackStation.

Typical goals in a hash challenge:

• Identify the hash algorithm (length and structure give hints)
• Lookup or crack the hash value
• Verify the output against flag patterns or password formats

# Using John the Ripper
john -wordlist=rockyou.txt hash.txt

# Using Hashcat
hashcat -a 0 -m 0 hash.txt rockyou.txt

A length extension attack exploits how hashes such as MD5, SHA‑1, and SHA‑256 process input data in blocks (the Merkle–Damgård construction).
If a system computes a message authentication code as hash(secret + message), and you know the hash output and the length of secret, you can append extra data and forge a valid hash for secret + message + extra without knowing the secret.

1. The hash algorithm processes data in fixed‑size blocks (e.g. 512 bits for MD5/SHA‑1).
2. When the final hash is produced, internal state depends only on the number of bits hashed so far.
3. An attacker can continue the hash calculation from this intermediate state by injecting data as though more blocks followed.
4. The prepended “secret” is never known but its length must be guessed.

This attack does not work if the application uses HMAC (HMAC(secret, message)), which protects against length extension.

A web service might validate a query string with a hash:

/verify?user=guest&sig=6d5f807e23db210bc254a28be2d6759a0f5f5d99

where sig = MD5(secret || data)

You can forge a different valid signature for user=guest&admin=true by performing a length extension attack.

hashpump implements this automatically.

HashPump -s 6d5f807e23db210bc254a28be2d6759a0f5f5d99 --data count=10&lat=37.351&user_id=1&long=-119.827&waffle=eggo -a &waffle=liege -k 14

Output:

0e41270260895979317fff3898ab85668953aaa2 count=10&lat=37.351&user_id=1&long=-119.827&waffle=eggo\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02(&waffle=liege

Use the new hash and payload in place of the original to bypass security checks.