Encryption 101 - Purdue University

Encryption 101

Beyond the Secret Decoder Ring

Sam Wagstaff

Computer Sciences and CERIAS

What is a secret decoder ring?

Popular 1930s to 1990s

Little Orphan Annie radio show

The image shown is from 1936

Also in breakfast cereal boxes

Another secret decoder ring

Captain Midnight and the Space Cadets,

radio and television

Also in breakfast cereal boxes

Most were badges, none was a ring

Plaintext or clear text ¨C you can understand it

Ciphertext ¨C looks like gibberish, but is equivalent to plaintext

Encipher or encrypt ¨C convert plaintext to ciphertext using a key

Decipher or decrypt ¨C convert ciphertext to plaintext using a key

The key for the secret decoder is the position of the alphabet circles.

The inner circle is the alphabet for plaintext.

The outer circle is the alphabet for ciphertext.

Example:

plaintext:

ciphertext:

DR IN K

GULQN

A secret decoder does ¡°clock

arithmetic,¡± in fact, arithmetic

modulo 26.

ABCDEFGHI J K L M N O P Q R S T U V W X Y Z

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Encipher by adding 3 to a plaintext letter¡¯s value:

T + 3 = W, H + 3 = K, E + 3 = H,

so THE becomes WKH.

Decipher by subtracting 3 from a ciphertext letter¡¯s

value.

This cipher is also called a Caesar cipher.

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download