Caesar Cipher
One of the oldest codes: shift every letter by the same amount. First spin the wheel to make a message, then crack one without knowing the shift.
Reading the wheel: A → D、B → E
A simplified teaching model. It says nothing about real-world security.
How to play
In "Make a code", change the shift to spin the wheel and turn the text below into a coded message. Switch to "Decode" to turn a coded message back. You can type your own text. "Spin automatically" turns the wheel one step at a time. In "Crack a code", you get a message with an unknown shift. Move the shift until the chart bars line up with the marks for ordinary text (black ticks), and the message becomes readable. "Let the computer solve it" tries every shift for you.
Learn more
The Caesar cipher is said to have been used by Julius Caesar: every letter is moved the same number of steps. There are only a few possible shifts, so trying them all breaks it. A smarter way is frequency analysis. In ordinary writing, some letters appear often and others rarely. Shifting the letters moves this pattern sideways without changing its shape, so the position of the peaks reveals the shift. For Japanese, the wheel has 46 plain hiragana: dakuten and handakuten marks are dropped, small kana (like っ and ゃ) become full-size, and katakana becomes hiragana, so a solved message loses those marks. The "ordinary text" guide for Japanese is a rough count taken from this page's own sample sentences; for English it uses the widely published letter frequencies. On a very short message the guide can miss. This is a simplified teaching model and has nothing to do with the security of modern encryption.