Utilities
Six small tools that do one job each, and they all run on your own device. Nothing you paste is sent anywhere, because there is nowhere to send it: these pages are static files and the work happens in the browser.
Why there is a lesson with every tool
A toolbox teaches you to press buttons. That is useful, but it stops being useful the moment you need something slightly different, or you are on a machine where the tool is not installed, or you want to know whether the answer you got is right.
So each utility here has a companion lesson that builds the same thing from scratch in Termux, using commands you already have. Read the tool to see what it should do. Read the lesson to be able to do it without the tool. The second one is the skill.
Generates from your device’s own random source, reports the real entropy in bits rather than a made-up strength bar, and offers a passphrase mode built on the EFF word list.
Lesson Build a password generator
Converts between the representations people confuse with encryption, byte-accurate for anything that is not ASCII, and says plainly which of them hide nothing.
Lesson Build an encoder and decoder
Computes real digests, checks a file against a published hash, and is blunt about which of these algorithms are broken and what each is still good for.
Lesson Build a hash and checksum tool
Decodes the header and payload, dates the expiry claims, flags alg:none, and verifies an HS or RS signature with a key you supply. It never calls a token verified without checking it.
Lesson Build a JWT inspector
Toggles the rwx bits, converts between 755 and rwxr-xr-x in both directions, handles setuid, setgid and sticky correctly, and explains what each bit lets someone do.
Lesson Build a permissions calculator
Works out what a prefix actually means, handles the /31 and /32 cases that catch people out, splits a network into subnets, and shows the binary so the mask stops being a mystery.
Lesson Build a subnet calculator
Learning these properly
The utilities pair with the later tracks in the course, where the same ideas come up as part of a bigger job: randomness in the cryptography track, hashing and encoding in security fundamentals, addresses in networking.