Homework 8 Operating Systems CS 3430 Sarah Diesburg

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.

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

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

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

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!

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

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.

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