What is the Caesar cipher?
The Caesar cipher is one of the oldest and simplest encryption techniques. Named after Julius Caesar, who reportedly used it to communicate with his generals, it works by shifting each letter in the plaintext by a fixed number of positions in the alphabet. For example, with a shift of 3: A becomes D, B becomes E, C becomes F, and so on. When the alphabet wraps around, X becomes A, Y becomes B, Z becomes C. Non-letter characters (spaces, numbers, punctuation) are left unchanged.
How to encode and decode
To encode: type your plain text, set the desired shift (1–25), and select "Encode." The output will be your encoded message. To decode: paste the encoded text, set the same shift value that was used to encode it, and select "Decode." The output will be the original plain text. Alternatively, you can decode by selecting "Encode" and using the complementary shift (26 minus the original shift).
ROT13 and Caesar cipher
ROT13 is simply the Caesar cipher with a shift of 13. Because 13 + 13 = 26 (the alphabet length), ROT13 is self-inverse — encoding and decoding use the same operation. For any other shift value, you must use the opposite shift to decode. For example, text encoded with shift 5 must be decoded with shift 5 in "Decode" mode (or shift 21 in "Encode" mode, since 26 − 5 = 21).
Is the Caesar cipher secure?
No — the Caesar cipher is trivially breakable. There are only 25 possible non-trivial shifts, so an attacker can try all of them in seconds (brute force). It's also vulnerable to frequency analysis: in English, the most common letters are E, T, A, O, I, N. By finding the most common letter in the ciphertext and assuming it corresponds to E, the shift can often be deduced. The Caesar cipher is useful for educational purposes and light puzzles, but should never be used to protect sensitive information.