Scytale

Set how many letters fit round the rod, then type the message. That number is the key, and decoding needs the same one. Spaces are dropped, but every other character is kept as it is, because a scytale only moves letters around.

If you have cipher text and no key, try each number in turn. The right one reads as plain text straight away.

Wrap a strip of parchment round a rod in a spiral, and write the message along the rod, one row at a time. When the rod has turned all the way round, start the next row. Unwind the strip and the letters are scrambled, reading down the columns instead of along the rows.

With four turns round the rod, "I am hurt very badly help" fills four rows of five, IAMHU RTVER YBADL YHELP. Read down the columns and you get IRYYATBHMVAEHEDLURLP. Wikipedia uses the same example.

To read it, the receiver winds the strip round a rod of the same thickness and the rows line up again.

The scytale is linked with ancient Sparta. The earliest hint comes from the Greek poet Archilochus, in the 7th century BC, but a full description of how it worked doesn't appear until Plutarch, who lived from about AD 50 to 120.

Historians argue about what it was really for. Some think it carried messages in plain text and that Plutarch's account is a myth. Others suggest it was used to check that a message was genuine rather than to hide it.