Write the alphabet, then write it again underneath but slid along by your key number. Every letter in the message swaps for the letter below it. A shift of 13 is special: encoding and decoding are the same job.
Caesar’s three-letter slide
The Roman writer Suetonius reported that Julius Caesar wrote his private letters with the alphabet shifted along by three, so A became D. It worked for a simple reason: most people who intercepted a message could not read at all, and those who could had never heard of such a trick. Two thousand years later, a shift cipher takes a code-breaker about a minute.
Print this page and you get a worksheet: how the cipher works, where it came from, and the puzzles to crack, with the machine and the menus left off.
Print this page as a worksheet Its sheet in the pack Try the challengesLonger, harder and written for grown-ups. Worth a look when this page runs out.