Introduction to Cryptology

Fall 2006

Definitions

Digital encryption techniques are used to protect data in two ways: to maintain privacy and to prove integrity [1]

Cryptography comes from the Greek words Kryptos, meaning hidden, Graphen, meaning to write.

Cryptography is the art of communicating confidentially through an insecure channel

Cryptanalysis is the art of deciphering those communications when one is not the legitimate receiver.

Cryptology is the union of these two domains. Cryptology is actually the study of codes and ciphers.

Definitions

Plaintext the original message that is to be changed into a secret form

Cipher, Cryptosystem, Encryption Method is a procedure for converting the plaintext message into encrypted message – ciphertext

The word cipher originates from the Hebrew word "Saphar," meaning "to number."

Decryption - the process that conducted by the intended receiver to recover original message (plaintext) from the encrypted message (ciphertext)

Key - keeps the message secret

Encryption-Decryption

Plaintext Ciphertext

Encryption Method

Encryption key

Ciphertext Plaintext

Decryption method

Decryption Key

Symmetric and Asymmetric Ciphers

Symmetric Cipher – knowledge of encryption key is equivalent to knowledge of the decryption key, or the same key is used for encryption and decryption.

Asymmetric Cipher (Public-Key Cryptosystem) the knowledge of encryption key give little information about the decryption key and vice versa

Early History of Cryptology

Polybius’ Checkboard

205 – 123 B.C.

Plaintext POLYBIUS produces the ciphertext

3534315412244543

1 2 3 4 5

1 A B C D E

2 F G H IJ K

3 L M N O P

4 Q R S T U

5 V W X Y Z

Julius Caesar Cipher

Caesar enciphered messages by replacing each letter by the third letter after that one:

A D, B E, C F, D G, …Z C Plaintext “ET TU, BRUTE?'' becomes the ciphertext “HW

WX, EUXWH?'' Ciphertext may be deciphered by replacing each letter by

the third previous letter. We think of each letter as corresponding to a number from 0

to 25 To encipher, we map numbers according to

C ( P + 3 )(mod 26) To decipher, we map numbers according to

P (C – 3) (mod 26)