Read Part 1 first! That lab was already super cool, but this one is even better imo 😎
Lab 2: Authorization & Trust
This lab is all about writing an automated theorem prover for a limited set of theorems to prove. As a part of that, you get to learn the answers to fun questions like
What Does It Mean To Prove Something?
That is, how can we show something like $(A \rightarrow B) \land (B \rightarrow C) \rightarrow (A \rightarrow C)$ is true?
What Does It Mean For Something To Be True?
Great question! Is $A$ true? Why not?
The answer is, it depends on your interpretation of $A$. And I mean that word “interpretation” quite literally, that’s the vocabulary we use for “the assignment of truth values to variables.
There is an interpretation where $A$ is true: $\{A: \top\}$, and an interpretation where $A$ is false: $\{A: \bot\}$.
So to be more precise, when we are asking if some logical formula is true, we are really asking whether it is true in all interpretations. Here’s some symbols:
$\models P \leftrightarrow \forall I. I \models P$
So Back To The Main Question: Proofs
So now we have this idea of interpretations, and truthiness from the standard boolean connectives $(\land, \lor, \rightarrow, \lnot)$, and need some way of showing that a formula will hold under any interpretation. Not only that, the way we do this needs to be in some format that a computer can quickly recognize and manipulate, so text is right out^{1}.
The solution we use is proof transformation rules in a tree structure. A single rule might go something like this:
$\frac{\Gamma,A \vdash B}{\Gamma \vdash A \rightarrow B, \Delta}(\rightarrow R)$
The more complex rule is at the bottom, and gets broken up into simpler rules as you go up the tree. More explanations of symbols:
 $\vdash$: If everything on the left side is true, then at least one thing on the right right is true. Like a big implication but more flexible for our needs.
 $\Gamma,\Delta$: Leftover clauses on the right and left hand sides of the $\vdash$ to signify which rule we are applying to and when we can carry things over^{2}.
To finish a proof, you need to “close out” every single branch, signified by $\star$. Here are some rules that do this:
$\frac{\star}{\Gamma \vdash \top, \Delta}(\top) \quad\quad \frac{\star}{\Gamma, \bot \vdash \Delta}(\bot) \quad\quad \frac{\star}{\Gamma,P \vdash P,\Delta}(id)$
These types of proofs (in the right format) can be easily checked by a computer, since all it as to do is, for each proof rule, check that it was applied properly. Then, since you know all the proof rules are valid, the entire proof must be valid!
Unfortunately, I can’t write out an example, since doing simple ones of those is a homework problem early in the class, but u get the gist right? Moving on!
What Are We Trying To Prove, Exactly?
So imagine you are a smart door lock. You have some notion of “who is allowed to access this door” and “who is allowed to delegate access to this door”. These are can be encoded as affirmation logic^{3} principles like:
A  B 

door says canopen(jack)  door says forall p . (jack says canopen(p) ) implies canopen(p) 
These statements have a close tie to cryptography! A “says” clause is like a signed statement, and proving the identity of the signer requires knowing about Certificate Authorities, and it all gets pretty complicated, but! There is a reason for all of it and it can be automated very naturally.
The lab is then to present a series of virtual doors with the appropriate signed statements and a proof using those statements to show that you can enter the door. The proof goals are as follows:
 Given a signed statement you can enter the door, construct enough of the rest of the proof (with CAs and such) to get the “says” statement
 Given a delegation of authority, open the door
 Given a very broad transitive delegation policy, open the door
 Exploit the door’s certificate checking code to open a door you logically shouldn’t be able to
Sounds simple right?? :)
I really enjoy this lab because it combines together a lot of disparate topics and they all still work together well, and it really shows the power of computers to do Logic, very quickly, and it feels like magic.
That’s pretty much all I can say without spoiling the rest of the lab, if this still sounds interesting to you I think you should read it and try it out! It is free! I really appreciate that this course is just,, publicly available to anyone who wants to learn it (even if the lectures really bring out the best in the subject but i digress).
That’s about it, thanks for reading!!
Footnotes

Yes, not even ChatGPT can reliably do stuff like this 😔 ↩

As an exercise, convince yourself that all the proof rules I’ve shown are true! ↩

Working with these “says” clauses is a little tricky and has a lot of subtlety to it. If you’re interested in learning more, I highly recommend reading the 15316 course notes. They are available for free online and are really comprehensive!! ↩