Introduction

1. Introduction • Conventional encryption, also referred to symmetric encryption, secret key, or single key encryption. • An encryption scheme has five ingredients: • Plaintext: this is the original message • Encryption Algorithm: performs various transformation and substitution on the plaintext

2. Continue… • Secret Key: The exact substitution and transformations performs by the algorithm depends on the Key • Cipher Text: this is the scrambled message produced as output. • Decryption Algorithm: this is the encryption algorithm run in reverse

3. Simplified Model of conventional Encryption

4. Essential Elements of a Conventional Encryption • Source for a message which produces a message in plaintext. M elements could be a 26 capital letters or nowadays the binary alphabet {0,1} • A key of the form: is generated at the source and is delivered to the destination by the mean of secure channel.

5. Continue … • The cipher text generated by this equation: • The intended receiver, in possession of the key is able to invert the transformation by this equation:

6. Cryptography Generally classified along three independent Dimensions: • The type of operations used for transforming plain text to cipher text (substitution, and transposition {permutation, and matrix}) • The number of keys used • Symmetric (single key) • Public key or asymmetric (two keys) • The way in which the plaintext is processed • Block (cipher one block at a time) • Stream (cipher the input continuously)

7. Cryptanalysis • The process of attempting to discover the plain text or key is known as cryptanalysis. • Brute–Force approach: knowing the algorithm and cipher text and trying all possible keys. • Chosen Plaintext: if the opponent knows the placement of certain key words in the header of file

8. Average Time Required for Key Exchange • An encryption scheme is computationally secure if : • The cost of breaking the cipher text exceeds the value of the encrypted information. • The time required to break the cipher exceeds the life time of the information.

9. Classical Encryption Techniques One useful classification is: • Substitution Technique: the letters of plaintext are replaced by other letters or by numbers, examples are (Caesar cipher and Mono-alphabetic ciphers, Hill cipher and Polyalphabetic cipher). • Transposition techniques: performing some permutation on the plaintext letters such as Rail Fence algorithm and rotor machines. (go to the net and find information about them)

10. Caesar Cipher Technique • Each letter of the alphabet is replaced by the letter stands three places further down the alphabet: • Note that the alphabet is wrapped around, so that the letter following Z is A

11. Continue… • Then have Caesar cipher as: • C = E(p) = (p + k) mod (26) • p = D(C) = (C – k) mod (26) • The important characteristics of this technique: • The encryption and decryption algorithms are known. • There are only 25 key to try which is far from security. • The language of the plaintext is known

12. Brute-Force Cryptanalysis Caesar Cipher