XOR calculator (repeating key)

How it works

Exclusive or, written $\oplus$, gives 1 when two bits differ and 0 when they agree. On bytes it works on all 8 bits in parallel: $\mathtt{48} \oplus \mathtt{6B} = \mathtt{23}$, because 01001000 and 01101011 differ in three places. The key property is that XOR undoes itself: $(d \oplus k) \oplus k = d$, so the same operation encrypts and decrypts.

With a key as long as the message, truly random and never reused, XOR is the one-time pad (Vernam cipher), which is perfectly secure: every plaintext of the same length is equally likely. Stream ciphers such as ChaCha20 and AES in counter mode produce a long pseudo-random key stream and XOR it with the data.

A short key repeated over the data is a different story. It is a Vigenère cipher on bytes, broken by finding the key length (for instance with the Hamming distance between blocks) and then each key byte on its own by frequency analysis.