Text Encrypt / Decrypt

Four ciphers here, and 2 of them are the same cipher wearing different clothes. ROT13 is Caesar with the shift nailed to 13, which is exactly half of 26 and therefore its own inverse. Atbash maps A to Z and B to Y, which is also its own inverse. Text Encrypt hides the encode and decode toggle for both of those and labels the pane Output (self-inverse), because offering a decode button for an operation that is identical to encoding is just a way to confuse people.

Caesar keeps a slider from 1 to 25, marked at 13 so the ROT13 equivalence is visible. Vigenère takes a keyword, forced to letters and uppercased as you type, and shifts each letter of your text by the alphabet position of the next keyword letter. Non-letters never consume a keyword letter, so punctuation and spacing do not knock the key out of step.

Every cipher leaves digits, punctuation and whitespace untouched, which keeps the shape of the message intact and is also the reason these are breakable. A card at the foot describes the selected cipher and restates the current shift or keyword. All of it runs in the page.

What Each Cipher Does to a Letter

• Caesar adds the shift to every letter, wrapping past Z. At a shift of 3, Attack at dawn becomes Dwwdfn dw gdzq. Decoding subtracts the same amount, so shift 3 and shift 23 are the same key in opposite directions.
• ROT13 is fixed at 13, which is why running it twice returns the original. It exists to hide a spoiler from casual reading, not from anyone who is looking.
• Atbash reflects the alphabet, so Meet me at the old bridge becomes Nvvg nv zg gsv low yirwtv. Notice that old becomes low, an ordinary word, which is the sort of coincidence that makes Atbash fun and useless.
• Vigenère cycles the keyword across the letters. With the default SECRET, Attack at dawn becomes Sxvrgd sx frag, and the repeated a in attack encrypts to two different letters, which is the whole reason it resisted analysis for 300 years.
• The swap arrow beside Copy moves the output into the input and flips the mode, which turns a round trip into 1 click rather than a copy and a paste.

When Not to Use a Classical Cipher

• Caesar has 25 possible keys. Trying all of them by hand takes a couple of minutes, and a script does it instantly, so the shift value is not a secret in any meaningful sense.
• Atbash and ROT13 have no key at all. Recognising which one was used is the entire attack.
• Vigenère falls to frequency analysis once the keyword length is guessed, and the Kasiski examination has been recovering that length from repeated ciphertext fragments since 1863. A 6-letter keyword over a paragraph is not a hard problem.
• Letter frequencies survive every cipher here, and so do word lengths, capitalisation and punctuation, because only the 26 letters are transformed. The shape of the message is visible even when the letters are not.
• For anything that actually matters, use a modern authenticated cipher through a maintained library. The tool says as much on screen: These are classical ciphers for learning/fun — not secure for real-world data protection.