CRYPTOGRAPHY
Digital Diva
March 2012
Eastern Michigan University
Professor S. Haynes, shaynes@emich.edu
Definitions
Plaintext:
Ciphertext:
an unencrypted message
an encrypted message
Cipher: encrypt, ciphertext, encryption algorithm
Encryption: transforming plaintext into ciphertext so that the plaintext is kept a
secret; this usually requires use of a key.
Decryption:
key.
transforming ciphertext into plaintext; this usually requires use of a
Encode: transform something into something else. For example, English  Spanish,
letters to numbers (a, b, c, …, z)  (1, 2, 3, … 26).
Decode: undo an encoding (Spanish  English; (1, 2, 3, … , 26)  (a, b, c, … z) ).
Algorithm: A sequence of instructions to accomplish something. An algorithm is like
a recipe. A computer algorithm uses data (ingredients) and instructions to modify
the data.
Key: Something that is known to the sender and the receiver of a message and is
used to encrypt and decrypt messages. The key is the most important single thing to
protect the security (secrecy) of the message.
Secret key (aka private key, symmetric key, shared key): the sender and the receiver
have exactly the same key. The sender encrypts with the secret key, the receiver
decrypts with the same secret key.
Public key (aka asymmetric key): a key that comes in two parts: (1) public and (2)
private. The sender encrypts with the private part, the receiver decrypts with the
public part.
Shift cipher: To encrypt plaintext, shift each plaintext letter to the right by key steps.
To decrypt ciphertext, shift each ciphertext letter to the left by key steps.
Substitution cipher: Each plaintext letter is converted to a specific cipher text letter;
each cipher text letter is decrypted to the original plaintext letter.
Steganography: the secret message is hidden in another message.
Modular arithmetic: The rules for addition (subtraction) and multiplication
(division) in a list of consecutive integers like 0, 1, 2, 3. We do modular arithmetic
when using a 12 hour clock, a 7 day week, a 12 month year. Anything that does
wrap-around can be described using modular arithmetic. To do modular
arithmetic, you have to start with 0. So for a 12 hour clock: 0, 1, 2, … , 11; for English
letters: (a, b, c, …, z)  (0, 1, 2, … 25).
Modulus: the number of consecutive integers being used in modular arithmetic. For
letters in English, the modulus is 26 (mod 26), for hours on a 12-hour clock: mod 12,
for months in a year: mod 12, for days in a week: mod 7.
Shift cipher
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
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
Caesar Cipher: The Story
Shared key: 3
Encryption algorithm: Each plaintext letter is encrypted as the letter three spaces to
the right with wrap-around.
Decryption algorithm: Each ciphertext letter is encrypted as the letter three spaces
to the left with wrap-around.
Example Encryption
Plaintext:
Ciphertext:
lazy one
odcb rqh
Example Decryption
Ciphertext:
Plaintext:
tzlc
quiz
To implement this on the computer
- Encode plaintext character c as x, where x is an integer 0 – 25 (see above)
X = encode(p)
- Encrypt x using Caesar cipher algorithm
y = ( x + 3 ) mod 26
- Decode y to ciphertext character
c = decode(y)
Decryption algorithm: Encode and decode as above.
Decrypt
X = ( y – 3 ) mod 26
plaintext:
Encode:
take more quizzes
19 0 10 4 12 14 17 4 16 20 8 25 25 4 18
Encrypt:
Ciphertext:
24 3 13 7 15 17 20 7 19 23 11 2 2 7 21