Here is a secret message (cypher-text)

11, 4, 3, 10.5, 4, 11.5

You have little chance of deciphering this!

But if I told you “add 3 and divide by 2” (i.e. the key is 3, 2), you have a chance…

The original message was ‘secret’

a=1, b=2, c=3, d=4, e=5, f=6, g=7, h=8, i=9, j=10, etc.

hence, the original plain text ‘secret’ becomes…

19, 5, 3, 18, 5, 20

add 3, it becomes…

22, 8, 6, 21, 8, 23

divide by 2, it becomes…

11, 4, 3, 10.5. 4. 11.5

The numbers 3, 2 are known as a symmetric key, because you use the same key to decrypt the message as was used to encrypt it.

Try This

Think of a word (keep it secret).

Encrypt the word using the same algorithm (add x, divide by y), where x, y is your symmetric key.

Give your cypher-text and your symmetric key to your partner

Decrypt your partner’s word