FIT2093 Lecture Notes - Lecture 4: Block Cipher, Substitution Cipher, Modulo Operation

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FIT2093 - W4 Notes
What we have seen so far:
2 levels of defence
Authentication to gain access to the system
Access Control (DAC by operating system and MAC by the
organisation) to have rightful operation on the information resources
such as files, programs, etc.
Third level --> to protect the contents of data resources
In the following discussions we will use the term 'messages' to mean
the contents of a data file or message exchanged between two
parties
!
Definition of Cryptography
The study of message secrecy (hidden writing)
Can also be used (if properly modified) for:
Message authentication/integrity
Derived from Greek words kryptos (hidden) and graphein (writing)
Original message: plain text (or just plaintext)
Scrambled text: cipher text (or just ciphertext)
Aim of cryptography is to generate efficiently the cipher text from
plain text and vice-versa if you know the secret scrambling
information (usually referred as the key)
!
Brief History of Cryptography
Traditionally cryptography is used to provide message confidentiality
Symmetric Key (the same key is used on both sides to encrypt and
decrypt
!
Basic terms, notations and structure of cryptography
Cryptology:
Cryptography: code designing
Cryptanalysis: code breaking
Cryptologist:
Encryption/encipherment: scrambling plaintext data into unintelligible
form to unauthorised parties
Decryption/decipherment: un-scrambling
!
Classical Cryptographic Techniques
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Document Summary

Access control (dac by operating system and mac by the organisation) to have rightful operation on the information resources such as les, programs, etc. Third level --> to protect the contents of data resources. In the following discussions we will use the term "messages" to mean the contents of a data le or message exchanged between two parties. The study of message secrecy (hidden writing: can also be used (if properly modi ed) for, message authentication/integrity, derived from greek words kryptos (hidden) and graphein (writing, original message: plain text (or just plaintext) Aim of cryptography is to generate ef ciently the cipher text from plain text and vice-versa if you know the secret scrambling information (usually referred as the key) Traditionally cryptography is used to provide message con dentiality. Symmetric key (the same key is used on both sides to encrypt and decrypt. Cryptology: cryptography: code designing, cryptanalysis: code breaking. Encryption/encipherment: scrambling plaintext data into unintelligible form to unauthorised parties.

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