1. Fun with Cryptography
The Science of Secrecy

2. 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 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
CAESAR CIPHER
Choose plain text
Choose a key in the range [1..25]
For each letter:
1. find the position of the letter
2. if encrypt, add key to the position
3. if decrypt, subtract key from position
4. if the result is negative, add 26
5. divide the result by by 26
6. find the remainder in the table
7. the character is above the remainder
Encrypt
Letter = ‘D’, Key = 24
Sum = % 26 = 1.
‘B’ is at position 1
Decrypt
Letter = ‘B’, Key = 24
1 – 24 = = 3. 3 % 26 = 3
‘D’ is at position 3
VIGNERE CIPHER
Choose a plain text
Choose a key word
Write the key word over the plain text
For each letter and corresponding key letter:
find the position of the letter in the key
find the position of the letter in the plaintext below the key
if encrypt, add what’s found in step 1 to what’s found in step 2
if decrypt, subtract what’s found in step 1 to what’s found in step 2
if the result is negative, add 26
divide the result by 26. Find the remainder in the table above
the character is above the remainder
Key = GOZAGS
Plain Text = COMPUTERSCIENCE G O Z A S C M P U T E R I N