FIT2093 Lecture Notes - Lecture 5: Substitution Cipher, Ciphertext, Plaintext
30 May 2018
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Lecture 4 - Fundamental Concepts of Cryptography
Cryptography: study of message secrecy
Aim of cryptography is to generate efficiently the cipher text from plain text and vice-versa if you
know the secret scrambling information
Terminology:
-Plaintext: original message
-Ciphertext: scrambled text
Principles: Heavily used in classical ciphers
-Substitution: systematically substitute letters or a group of letters with other letters or a group
of letters
-e.g. changing letter with another letter
-Transposition: rearrange the position of the letters in the message according to some rules
-Move each letter in the message by 1 position
-Permutation
Caesar Cipher - substitution cipher
-each letter is translated into the letter a fixed number of positions after it
Scramble Message Property:
1. Confusion: seeks to make the relationship between the statistics of the cipher text and the
value of the encryption key as complex as possible
- achieved by complex substitution, difficult to deduce the key
2. Diffusion: the statistical structure of the plaintext is dissipated in the long range statistics of the
cipher text
- scrambling should spread the information from the plain test so that each plain text digit
affects many cipher text digits
- achieved by complex transposition
3. Avalanche Effect: is a property of any encryption algorithm such that a small change in either
the plaintext or the key produces a significant change in the cipher text
- change in one bit of plaintext or one bit of the key should produce a change in many bits
4. Unconditional security & Computational security
-ideally, unconditional security so that no matte how much computer power, cipher cannot be
broken since there is insufficient information to uniquely determine the corresponding plaintext
-In practice, sufficient to have computational security given attacker’s limited computing
resources, the cipher cannot be broken
Block
-A long message can be broken down into equal sized blocks, except the last block
-Decryption Correctness: There is an efficient decryption algorithm D(K,C) that correctly decrypts
cipher texts produced by E(K,M), i.e.:
-For any (M,K), if C = E(K,M) then D(C,K) = M
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Aim of cryptography is to generate efficiently the cipher text from plain text and vice-versa if you know the secret scrambling information. Substitution: systematically substitute letters or a group of letters with other letters or a group. Transposition: rearrange the position of the letters in the message according to some rules of letters. Move each letter in the message by 1 position. Each letter is translated into the letter a fixed number of positions after it. Scramble message property: confusion: seeks to make the relationship between the statistics of the cipher text and the value of the encryption key as complex as possible. Achieved by complex substitution, difficult to deduce the key: diffusion: the statistical structure of the plaintext is dissipated in the long range statistics of the cipher text.
