Miscellaneous Cryptography

cryptointermediate

Index of miscellaneous crypto pages - XOR and PRNG attacks.

cryptomiscxorprngindex

Miscellaneous Cryptography

This section covers crypto techniques that don't fit neatly into classical or modern categories - XOR-based ciphers and pseudo-random number generator weaknesses.

Pages in This Subsection

Page	Difficulty	Key Technique
XOR Cryptography	Beginner	Single-byte, multi-byte, known-plaintext
PRNG Attacks	Advanced	MT19937 cloning, LCG cracking

When to use each technique

XOR

Single-byte key: brute all 256 values; score plaintexts with English letter frequency.
Repeating key: normalized Hamming distance → guess key length → solve each position as single-byte XOR.
Known plaintext (e.g. flag{): XOR known bytes with ciphertext to recover key bytes.

PRNG

Python random (MT19937): 624 consecutive 32-bit outputs recover state - tools like randcrack.
Java Random: often brute-forceable when only part of the output is hidden.
Time-based seed: try Unix timestamps in a plausible window.
Linear congruential generator (LCG): enough outputs to solve for multipliers and modulus ( $a$ , $c$ , $m$ ).

Last updated on Aug 1, 2025

Hash Length Extension Attack

Exploit MD5 and SHA-1/SHA-256 Merkle-Damgård construction to forge signatures without knowing the secret.

XOR Cryptography

Exploit XOR cipher weaknesses: single-byte key, multi-byte repeating key, and known plaintext attacks.

On this page

Miscellaneous Cryptography Pages in This Subsection When to use each technique XOR PRNG