Teach Your Kids To Code: A Parent-friendly Guide to Python Programming

Conditions (What If?) 95

s cret Messages e

Now that we understand how to use conditions, we’re going to

learn to encode and decode secret messages using a Caesar cipher.

A cipher is a secret code, or a way of changing messages to make

them harder to read. The Caesar cipher is named after Julius

Caesar, who is said to have liked sending private messages by

shifting letters in the alphabet:

SECRET MESSAGES ARE SO COOL! -> FRPERG ZRFFNTRF NER FB PBBY!

We can create a simple Caesar cipher by using an encoder ring

like the one shown in Figure 5-7. To create the encoded message,

decide on the key, or the number of letters you want to shift each let-

ter by. In the coded message and in Figure 5-7, each letter is being

shifted by a key value of 13 , meaning we take the letter we want

to encode and count 13 letters past it in

the alphabet to get our encoded letter.

An A becomes an N, a B becomes an O,

and so on.

We sometimes call this shift a

rotation because by the time we get to

M (which becomes Z), we’re at the end

of the alphabet. To be able to encode an

N, we wrap around to A again. O wraps

around to B, all the way to Z, which

becomes an M. Here’s an example of a

Caesar cipher lookup table for the key

value of 13 , where each letter is shifted

by 13 letters for encoding or decoding:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

N O P Q R S T U V W X Y Z->A B C D E F G H I J K L M

Notice a pattern? The letter A is encoded as an N, and N is

encoded as an A. We call this a symmetric cipher or symmetric code

because it has symmetry—it’s the same in both directions. We can

encode and decode messages using the same key of 13 because the

English alphabet has 26 letters, and the key value of 13 means that

we shift every letter exactly halfway around. You can try it with a

message of your own: HELLO -> URYYB -> HELLO.

ABC

D E F G H I J K

QPONML

R

S

T

U

V

W

X

YZ

CBA

ED

F

G

H

I

J

KL

MNOPQ

R

S

T

U

V

XW

ZY

Figure 5-7: A Caesar cipher