Paxos From First Principles

Thu, 08 Jun 2023 00:00:00 +0000

Paxos is a fault-tolerant distributed consensus algorithm invented by Leslie Lamport. A fault-tolerant distributed consensus algorithms lets us for example run a database on multiple servers simultaneously to make the database resilient to hardware failures.

Dr. Lamport initially tried to prove that a fault-tolerant distributed consensus algorithm cannot exist and in the process discovered a very elegant solution the problem. Such solutions can be difficult to understand if the reader is not well versed in the problem. In this post, I’ll walk you through the problem from first principles to arm you with the knowledge that makes Paxos easy to understand.

iCloud Keychain Security

Sun, 24 Apr 2022 00:00:00 +0000

Apple claims that iCloud Keychain items are end-to-end encrypted, but recoverable with an Apple ID and device passcode even if all devices are lost. The official Apple support documents don’t explain well how this is possible, so I’ve dug in to resolve this seeming contradiction.

The short answer is that secrets stored in iCloud Keychain are end-to-end encrypted, albeit with a low entropy key derived from the user’s device passcode. Apple strengthens the device passcode by limiting the number of passcode tries it allows to recover iCloud Keychain items with hardware security modules (HSM) in their data centers according to their security manual. It’s irrational for iOS users to treat Apple as an adversary, so this leaves open the possibility of HSM compromise[^1] or an undisclosed law enforcement backdoor as a threat to users’ iCloud Keychain items. It’s up to each user to decide if these are acceptable risks.

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Paxos From First Principles

iCloud Keychain Security