Lecture 7 of Computing Security 1 Course for 4th clas

Lecture 7 overview:
?Classical Encryption Techniques
? Transposition Techniques
• Message Reversal Method
• Columns Transposition) Method
? Substitution Techniques
• Caesar Cipher & Multiplication Cipher
• Vigenere Cipher
• Beale Cipher
• PlayFair Cipher
?Classical Ciphers Cryptanalysis
18. Classical Encryption Techniques
Classical (Traditional or Conventional) ciphers have been used since ancient Egypt. Different methods and techniques are used in order to increase security level of such information. Some facts about classical cryptosystems:
• All of these systems are based on symmetric key encryption scheme.
• The only security service these systems provide is confidentiality of information.
• Unlike modern systems which are digital and treat data as binary numbers, the earlier systems worked on alphabets as basic element.
• Most of classical methods and techniques are based on the idea that each natural language has its own distribution characteristics.
However, there are two schemes or types of classical cryptosystems:
1- Transposition(Permutation) Techniques.
2- Substitution Techniques.
Computing Security 1 Lecture 6,7
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18.1 Transposition Techniques
Transposition ciphers rearrange characters according to some scheme (procedure). There are many transposition procedures; some of these have different methods. However, we will explain the following methods only:
? Message Reversal Method
This procedure requires that the plain text to be written backwards to produce a cipher text.
Example:
Plain text : COMPUTER SCIENCE
Cipher text : ECNEICS RETUPMOC
? Columns Transposition Method:
This method belongs to Geometric Patterns Scheme that base on divided the message to number of line has the columns of equal length (i.e., each line has number of letters change dynamic base on key) by writing the plaintext vertically:
Example: let, the plain text: COMPUTER AND DATA SECURITY, and Key = 4 1 5 3 2
Cipher text: PCUMO ATNRE TDAAD USRCE XIXYT
18.2 Substitution Techniques
Four basic types of classical cryptosystems based on Substitution are:
? Simple Substitution (e.g. Caesar Cipher and Decimation Cipher)
? Polyalphabetic Substitution (e.g. Vigenere Cipher).
? Homophonic Substitution (e.g. Beale Cipher).
? Polygram Substitution (PlayFair Cipher).
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