Who holds the key, and why it’s the whole game.

Last time I said the method is public and the key is the secret. This is the part where that stops being a nice idea and starts telling you which apps you can actually trust with your writing.

Any company that encrypts your notes has to make one decision before it writes a line of code. Does it keep a copy of the key, or not.

Everything you can and can’t trust about that company follows from the answer. It’s almost never on the pricing page.

A key is not a password

Worth separating those two, because they get used as if they were the same thing.

Your password is something you remember. A key is a very long number, so long that writing it out would fill a line or two of this page, and nobody memorises one.

They are connected, though. Plenty of systems take the password you typed and run it through a slow, deliberate grinding process to produce a key. Same password in, same key out, every time. The password’s job is to be the raw material. The grinding turns it into something that can do the actual work.

Which is also why a weak password is worse than it looks. It doesn’t just get guessed. It produces a guessable key.

Catchlight goes a different way, twelve ordinary English words generated on the phone the first time you open it, and those words derive the key. I’ll come back to why words rather than a password.

The locksmith who kept a copy

Picture a locksmith fitting your front door. He cuts you a key, you pay him, he leaves. What you don’t know is whether he dropped a second key in a drawer at the shop on the way home.

Both versions of that man are telling the truth about the lock. It’s a good lock. It’s exactly as strong as he said it was. The lock was never where the difference lived. The drawer is.

Encrypted software is the same shape. Two apps can use identical encryption, the same published, tested, hammered-on method, and one of them can read everything you write while the other can’t. Not because one has better maths. Because one kept a copy.

Where your key actually lives

There are only a few arrangements in practice, and it’s worth being able to spot which one you’re in.

Only you hold it. The key is made on your device, from something only you have, and nothing that could rebuild it ever leaves. The company stores cipher-text it can’t turn back. This is what people mean by zero-knowledge, and it’s rarer than the marketing suggests.

You hold one and so does the company. By far the most common setup, and the one that hides most comfortably behind the word “encrypted”. Your notes really are encrypted, on the disk and in the cloud, and the company can also open them whenever it has a reason to. A support request. A subpoena. A bad hire. An “improvement” to the product that needs to read your text.

The company holds it and you never touch one. Common in web apps where you sign in with an email and a password and everything happens on their servers. The encryption is real and it’s protecting their storage from other people, not protecting your notes from them.

Somebody holds a spare on your behalf. Recovery keys, escrow, a copy kept in another part of the same company. Sometimes optional and clearly explained. Sometimes on by default, which makes it the second arrangement disguised as the first.

Only the first one survives the question that matters, which is what happens when someone with power leans on that company?

The test that takes two minutes

Open the app you keep your private writing in and start the forgotten-password flow. Don’t finish it. Just watch what it offers you.

If it emails a link, lets you set a new password, and all your old notes are sitting there afterwards exactly as they were, then somebody other than you can open those notes. It can’t work any other way. Your old password produced the key that unlocked the old writing. You replaced the password and the writing came back regardless, so the key was never really yours to begin with.

If instead it warns you that recovering the account means losing the contents, or it asks for a recovery phrase you were told to write down on day one, that’s a system where you’re the one holding the key.

No policy reading, no cryptography, no marketing to parse. The recovery flow gives it away every time, because a company can only hand back what it’s able to open.

What “we can’t read your data” has to mean

It’s a strong claim and a checkable one. For it to be true rather than aspirational, a few things have to be arranged so that the company simply doesn’t have the option.

The key has to be made on your device rather than issued to you. If it arrived from a server, it existed on a server.

Nothing capable of rebuilding it can leave. Not the phrase, not the password, not a helpful backup copy stored “for your convenience”.

And the company has to have built for you losing it. Which is the uncomfortable bit, because it means the honest answer to “I’ve forgotten my phrase” is that the notes are gone.

If a company promises both that it can’t read your notes and that it can get you back in, one of those two is doing some very quiet work.

The cost, simply put

That’s the trade, and I’d rather write it down than dress it up.

Catchlight hands you twelve words the first time you open it and tells you to put them somewhere that isn’t the phone. Those words re-derive the key on any device, which is how a second phone picks up your writing without me being involved at any point. There’s no account, so there’s nothing at my end to reset. Lose the words and no support ticket fixes it, because I’ve got nothing to fix it with.

I use words rather than a password for a fairly boring reason. A password you invent tends to be one you’ve used somewhere before, and it has to survive being ground down into a key. Twelve random words carry far more randomness than anything you’d think up, and you can copy them onto a card without getting them wrong.

The words feel like an odd bit of ceremony for about thirty seconds. They are the entire reason nobody else can read your notes.

Why this matters for your notes

Hold any app’s privacy page against the single question. Not whether it’s encrypted, because everyone can say yes to that now and be telling the truth. Who holds the key?

I pulled that thread for one company in can Apple read your notes, and watched a whole country find out the hard way when the UK lost Advanced Data Protection. If you want the short version of the term itself, what zero-knowledge actually means covers it, the glossary keeps the definitions in one place, and the comparison pages line the apps up on this exact point. For the app most people already have, there’s Catchlight compared with Apple Notes.

The lock on the front door is rarely the issue, the drawer at the back of the shop often is.

Next time, three phrases that get used as if they were interchangeable, end-to-end, zero-knowledge and on-device. They promise different things and one of them is doing less work than you think.