XOR

A bitwise operation fundamental to many ciphers.

XOR is a bitwise operation that outputs one only when its two input bits differ, and it is foundational to symmetric cryptography because applying the same key twice returns the original data. Stream ciphers and the one-time pad rely on combining plaintext with a keystream using XOR. Its strength depends entirely on the key: a truly random key as long as the message is unbreakable, but reusing a short or predictable key makes the ciphertext trivial to recover.