Cryptography

The steps to encrypt “Hello. How are you?” with paper and pencil. Plaintext Letter      Plaintext Number    +   Key    Result    Subtract26?    Result    Ciphertext Letter H                            7                              +   13       = 20                               = 20                20 = U E                             4                              +   13      = 17                               = 17                17 = R L                             11                           +    13      = 24                               = 24                24 = Y L                             11                           +    13      = 24                               = 24                24 = Y O                            14                            +   13      = 27            - 26             = 1                  1 = B H                           7                              +    13      = 20                               = 20                 20 = U O         

The cipher wheel and St. Cyr slide are nice tools to do encryption and decryption with the Caesar cipher. But we can implement the Caesar cipher with just pencil and paper. Write out the letters of the alphabet from A to Z with the numbers from 0 to 25 under each letter. 0 goes underneath the A, 1 goes under the B, and so on until 25 is under Z. (There are 26 letters in the alphabet, but our numbers only go up to 25 because we started at 0, not 1.) It will end up looking something like this: A     B     C     D     E     F     G     H     I     J     K     L     M    0     1      2      3      4     5      6      7     8    9     10    11    12 N     O     P     Q     R     S     T     U     V     W     X     Y     Z    13   14    15   16    17    18    19   20    21    22    23    24    25 With the above letters-to-numbers code, we can use numbers to represent letters. This is a very powerful concept, because math uses numbers. Now we have a way to do math on letters.

Before we learn how to program computers to do encryption and decryption for us, let’s learn how to do it ourselves with simple paper tools. It is easy to turn the understandable English text (which is called the plaintext) into the gibberish text that hides a secret code (called the ciphertext). A cipher is a set of rules for converting between plaintext and ciphertext. These rules often use a secret key. We will learn several different ciphers in this book. Let’s learn a cipher called the Caesar cipher. This cipher was used by Julius Caesar two thousand years ago. The good news is that it is simple and easy to learn. The bad news is that because it is so simple, it is also easy for a cryptanalyst to break it. But we can use it as a simple learning exercise. To convert plaintext to ciphertext using the Caesar cipher, we will create something called a cipher wheel (also called a cipher disk). You can either photocopy the cipher wheel that appears in this book,  Cut out the two circle