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Semester 1/Computer Systems Internals & Linux/Week 5/Week 5 Computer Systems.md

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+## Lecture 1 (12:00) - Data Communication Security
+
+### Normal Communication
+
+Alice => Message => Bob
+
+- We assume:
+	- A dedicated transmission system
+	- Good will, Co-operation, Competence among participants
+These are not valid assumptions.
+
+### Security
+
+- Human problem caused by deliberate exploitation of weakness in the transmission system.
+	- Bad passwords, spoofing / phishing, config errors
+	- Bad procedures, backdoors, overcomplexity
+	- Can solve some problems, manage some, and detect some, but cannot fix everything.
+- Security threats are generic to all forms of communication, not specifically computers or digital devices. Understanding them is vital to understanding IT.
+
+### Password Security
+
+- Password file must always be assumed to be readable
+- Contains encrypted passwords.
+- "Dictionary Attacks"
+	- Pre-encode a dictionary of words (and combination of words). Then simply look up any encoded passwords.
+- "Cracker" Programs / "Brute force attacks"
+
+### Encryption
+
+- Scramble the message in some way so that it is only meaningful to Alice and Bob.
+	- Alice takes message and applies some algorithm to each of the letters to generate an encrypted message.
+	- Bob applies the reverse algorithm to regenerate the original message
+- The aim is that Eve cant do one (or both) of these things
+	- If she cant decrypt, she cant eavesdrop
+	- If she cant encrypt, she cant masquerade.
+- The more keys, the longer it takes to break
+
+#### Caesar-shift
+
+- Each letter is replaced by another letter, k positions later in the alphabet
+- If k=3:
+	- A becomes D
+	- B becomes E
+	- C becomes F
+	- …
+	- "ATTACK AT TEN" becomes
+		- DWWDFN DW WHQ
+- plaintext - ATTACK AT TEN
+- ciphertext - DWWDFN DW WHQ
+- Encryption method - Substitution
+- Encryption Key - 3
+- Decryption Key - 3
+- Problem is how to share the key