1. Homework 8
Operating Systems CS 3430
Sarah Diesburg

2. Transactions in File Systems
Sometimes multiple small actions need to take place to implement a single file system action
E.g. moving a file involves deleting the file from an old directory and adding it to a new directory
If computer crashes in the middle of the actions, file system could be left in an inconsistent state
Transactions solve this problem.

3. Transactions in File Systems
Offer 3 properties:
Atomic: all operations either happen or they do not (no partial operations)
Serializable: transactions appear to happen one after the other
Durable: once a transaction happens, it is recoverable and can survive crashes
How it works:
Write what you are going to do to a log (journal) in a transaction before you do it
If computer crashes, replay what is in journal

4. Access Control List vs Capability List
Access Control List - Stores all permissions for all users with each object
Capability List – Stores all objects a process can touch
Access control list is easier for file systems
Easy to know who can access the file when doing permission checks on a per-file basis

5. Public Key Encryption
I send you a message: Encrypt(KEYsarah_private, “Homework is due tomorrow”)

6. Public Key Encryption
I send you a message: Encrypt(KEYsarah_private, “Homework is due tomorrow”)
This is authentication – everyone can read the message with my public key, but only I could have created it.
You know it came from me!

7. Public Key Encryption
You send me a message: Encrypt(KEYsarah_public, Encrypt(KEYmy_private, “I need another day.”))

8. Public Key Encryption
You send me a message: Encrypt(KEYsarah_public, Encrypt(KEYmy_private, “I need another day.”))
Both authentication and secrecy. Only you can create this message created with your private key, but only I can read it because the whole thing is encrypted with my public key.

9. Pentagon Traffic Analysis
Another example – Can tell that it must be close to a project deadline due to the number of connections to the class Linux server