Strong cryptography for kids.

1. Secret Encoder Ring
Cut out the two letter rings. Make a small hole in the middle of each and
fix them together. Now you're ready to encode your secrets!
The Caesar Cipher
Roman Emperor Julius Caesar (100BC
- 44BC) was famous for his secret
messages.
Caesar's secret key was x. Every time
he wanted to send an a, he'd send x
and every time he wanted to send b
he'd send y. To use Caesar's cipher, set x on the inner
wheel to a on the outer wheel (where the dot is).
To hide the name of his favourite city (r o m e), encode each letter by
finding it on the outside wheel then reading off the letter on the inside
wheel.

2. r becomes o, o becomes l, m becomes j, e becomes b
To decrypt Casesar's secret message, o l j b, find each letter on the inside
wheel and read the letter from the outside wheel.
o becomes r, l becomes o, j becomes m, b becomes e.
Give it a go
Caesar has fallen in love, but who with?
z i b l m x q o x
Brutus Attacks
Caesar's cipher is not very good. Imagine if Brutus got one of Caesar's
hidden messages. Even though it's a jumble of letters, Brutus would:
Know how long the message is, 3 letters for y e s and 2 letters for n o
would be a big clue.
Know that once he's cracked that e is b, he can reveal all of the letter
e in the message!
One Time Pad
In 1882, Frank Miller, a banker, wrote about a perfectly
secure way to send messages over the telegraph.
Instead of picking just one secret key like Caesar, Miller
changed his key for every letter being sent. To do this, he
used a book of secret keys (just a list of letters). By
sending some extra letters on the end of his message from
the book, he could also hide the true length of his
message.
So long as each key is random and only ever used once, his system is still
impossible to break today.