Lab 2—Channels and Shared Memory


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ECE 459: Programming for Performance
Lab 2—Channels and Shared Memory1

Learning Objectives:
• Become familiar with message-passing for communicating between threads; and,
• Become familiar with the use of shared memory.
The core content for this lab is in Lecture 3, though there is somewhat-relevant content up to
Lectures 16 and 17. You can also see a tutorial video from 2021 at
In this lab, you’ll be cracking JWT signatures. JWT stands for JSON Web Token, which is (from
the Wikipedia page) “an Internet standard for creating data with optional signature and/or
optional encryption whose payload holds JSON that asserts some number of claims”. Basically,
it’s a string consisting of a header, a payload and a signature. The three parts are separated
by dots, and each of the three parts is encoded in base 64. Base 64 is an encoding that takes
arbitrary binary data and converts it to a text format, and it’s typically used in URLs. Many web
applications use JWTs for user authentication.
Here is a typical JWT:
• header = “eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ”;
• payload = “eyJsb2dnZWRJbkFzIjoiYWRtaW4iLCJpYXQiOjE0MjI3Nzk2Mzh9”;
• signature = “gzSraSYS8EXBxLN_oWnFSRgCzcmJmMjLiuyu5CSpyHI”.
The signature is used to verify that the header and payload have not been tampered with. The
header and the payload are combined with a value called the “secret”, and then hashed using
HMAC-SHA256 to produce the signature. Specifically, the signature defined as:
HMAC-SHA256(base64UrlEncode(header) + “.” + base64UrlEncode(payload), secret)
1v0, 28Jan22
The “secret” is what prevents malicious actors from generating fake JWT. Your task is to write
a program that brute forces a JWT’s secret.
There are three positional command-line arguments to your program. The first is the token,
the second is the maximum possible length of the secret (it could be shorter), and the third is
the alphabet (i.e. the set of characters that might be used in the secret).
You need to extract the signature portion from the token, and then use a brute-force approach
to find the secret. You do this by exploring all possible values for the secret until one is found
that produces the correct signature. The possible values for the secret are constrained by the
key length and the alphabet. In a real-world situation, the problem is intractable because the
alphabet is larger and the key length is longer (at least 32 characters). The examples used in
this lab are chosen to have a short key length (four or five characters) and an alphabet consisting
of the lowercase letters plus the digits 0 through 9.
Please don’t try to use this code on UW hardware to crack anything other than some test data
for this exercise. The test data is sufficiently small that you can crack it in a few minutes; if it’s
taking much longer than that, something is wrong.
We provide starter code that consists of a single-threaded implementation in Instructions for running it can be found in the README file. Your job is to modify the code to improve
its performance by using multiple threads.
In some cases, this is straightforward since the tasks are largely independent of each other.
However, in this case things are more complex since the threads need to be able to communicate
with each other. In particular, you want the threads to end once a solution has been found rather
than exploring the entire solution space.
The two approaches you will be using are message passing and shared memory. The files and are initially just copies of, and you’re expected to modify them to use message-passing and inter-thread communication respectively.
You’re free to implement things as you see fit. You may want to look at the Crossbeam crate,
since it will do a lot of the work for you. In particular, you can use crossbeam::scope for shared
memory. Just don’t use channels. You can use other crates as long as you don’t trivialize the
For message passing, you are expected to use channels. There are multiple ways to do this, but
we recommend that you start by looking at the unbounded channels provided by Crossbeam.
Note that you should rely only on channels. You should not (and should not need to) make
direct use of mutexes, atomics, or other shared state primitives. The channels do all this work
for you.
By the same token (so to speak. . .), in the shared memory version you should not (and should
not need to) use channels.
Public service announcement. Please be sure to clean up after yourself and not leave longrunning tasks hanging around and hogging all the CPUs.
The general principle is that correct solutions earn full marks. However, it is your responsibility to demonstrate to the TA that your solution is correct. Well-designed, clean solutions are
therefore more likely to be recognized as correct.
Solutions that do not compile will earn at most 39% of the available marks for that part. Segfaulting or otherwise crashing solutions earn at most 49% .
The mark breakdown is as follows:
• Message-passing solution (40 marks).
• Shared-memory solution (40 marks).
• Written report (20 marks):
– 8 marks for discussion of the message-passing solution;
– 8 marks for discussion of the shared-memory solution; and
– 4 marks for clarity.
Do I have to do the impossible? No. We will always test your code with solvable test cases.
Is there a performance target? No specific target, though faster is better. We’re just marking
your use of the required idioms (message-passing and shared memory). We’d be sad if your
parallel solution was slower than the sequential code, but we wouldn’t take it out on you.
What do you mean by shared memory? Our definition of shared memory means that you
have a mutable variable which is accessed by multiple threads protected by locks and are not
using the Rust channels. What you do with this mutable variable is up to you. (You can also
use lock-free data structures).
Do I have to stop immediately once I find a solution? A small delay is fine, but you do need
to communicate the stopping condition to all threads.
Valgrind/Helgrind complain at me. Yeah, it happens that sometimes it’s not your fault (e.g.
library code). Obviously, in this class, you are only responsible for things that are your fault.
How many threads? You are not required to support variable numbers of threads (though
it’s not a bad idea to do that experiment). The recommended value is the number of CPUs on
the machine (value returned by num_cpus::get()).
How many possible solutions exist? The number of possible solutions will fit in a u64.
How big is the alphabet? It can be smaller than the number of CPUs.
Do we care about file layout (e.g. modules)? No, go ahead and modularize if you want, as
long as it compiles and runs using the default commands.
What am I required to put in the report? Just a clear explanation of your implementation.
We do appreciate benchmark results, though. If there’s any explaining to do for the TAs about
why you know that Helgrind reported errors are not a real problem, then that’s sensible in that
it might prevent having to debate correctness with a TA.
I’m still worried about Helgrind errors. To make it clear: if you are sure that the Helgrind
errors are from library code, they’re not a problem. tokio semaphores and Crossbeam channels
are known to make Helgrind upset. Also, it’s safe to share Crossbeam channels by reference
across threads (that’s why the compiler allows it), so you don’t need to worry about channelrelated race conditions specifically.

Lab 2—Channels and Shared Memory
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