Caesar Cipher

The Caesar cipher is the most famous substitution cipher in history — named, of course, after Julius Caesar, who reportedly used a shift of 3 to protect military messages (A becomes D, B becomes E, C becomes F, and so on). It is also the simplest: pick a number between 1 and 25, and shift every letter in the alphabet forward by that many positions, wrapping around from Z back to A. To decrypt, shift backward by the same amount. That is the entire algorithm, and it is why the Caesar cipher is the first thing taught in any cryptography or classical-cipher course — it demonstrates the core ideas of a key (the shift value), a substitution alphabet, and the brute-force vulnerability of a keyspace too small to resist enumeration, all in about three lines of code.\n\nDespite being trivially breakable (there are only 25 possible shifts; a computer can try all of them in less than a millisecond), the Caesar cipher holds an important place in the history of ideas. For centuries after the Roman Empire fell, it was considered genuinely secure, because the people intercepting Roman messages did not know that a substitution cipher was being used — the very concept of a cipher was the secret, not the key. Once the method is known, the cipher is broken. This pattern — security through obscurity, a method that works only as long as the method itself remains hidden — has recurred throughout the history of cryptography and computer security, and the Caesar cipher is the canonical example used to explain why it fails.\n\nThis tool lets you choose any shift from 1 to 25 (0 and 26 are the identity — no change — and 13 is the special case known as ROT13, which has its own dedicated tool in this collection because of its self-inverse property). You can toggle between Encrypt mode (shift forward) and Decrypt mode (shift backward) — decrypting with shift equals N is exactly the same as encrypting with shift equals 26 minus N. For example, decrypting a shift-3 message is the same as encrypting with shift 23. The tool also handles the edge case where users paste text that already contains both shifted and unshifted content — the output is always consistent across the entire input, applying the chosen shift to every applicable letter.