Lecture3 of Information Security for Diploma 2018-2019

Lecture 3 Title : Classical Encryption
Lecture Outlines:
3.1 Symmetric Cipher Model
3.2 Cryptosystems and Cryptanalysis
3.3 Substitution Techniques
3.4 Transposition Techniques
Objectives:
After studying this lecture, you will be able to discuss
? Understand basic principle of symmetric cipher
? Encrypt and decrypt messages using simple substitution methods
? Understand the weakness of encryption methods
? Devise ways to strengthen the methods
? Devise cryptanalytic attacks on the methods
? Use transposition for encryption
? Understand the operation of rotor machines which is multiple encryption
? Appreciate the strength and simplicity of steganography for hiding messages
3.1 Symmetric Cipher Model
A symmetric encryption scheme has five items (Figure 3.1). They are:
1. Plaintext: This is the original intelligible message or data that is fed into the algorithm as input.
2. Encryption algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext.
3. Secret key: The secret key is also input to the encryption algorithm. The key is a value independent of the plaintext and of the algorithm. The algorithm will produce a different output depending on the specific key being used at the time. The exact substitutions and transformations performed by the algorithm depend on the key.
4. Cipher text: This is the scrambled message produced as output. It depends on the plaintext and the secret key. For a given message, two different keys will produce two different cipher texts. The cipher text is random stream of data and, as it stands, is unintelligible.
5. Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the cipher text and the secret key and produces the original plaintext.