How PassCub protects your vault

What PassCub encrypts, who holds which keys, what the sync server can and can't see, and the things no password manager can protect you from.

• What's encrypted
• Unlocking
• Sync
• Adding a PC
• Recovery kit
• Browser extension
• Updates
• Limits
• Reporting a problem

What's encrypted

Everything about every item: its title, username, password, website addresses, notes, custom fields, tags and password history. Nothing is left readable to make searching or syncing easier. Your vault file lives on your PC and holds only sealed data.

Each item has its own key. Items are sealed with XChaCha20-Poly1305 in an envelope that commits to its key and is bound to where the item belongs, so a sealed item can't be swapped for another or moved somewhere else unnoticed. Sealed data is padded, so sizes give little away.

Unlocking

Opening your vault takes two things together: your master password and a key stored on that PC, which Windows protects for your Windows account. Neither works alone. That's what makes a stolen vault file worthless: there's no way to guess your password against it without the PC's key as well.

Your master password goes through Argon2id with 256 MiB of memory and 3 passes, which makes every guess slow and expensive even for someone with the PC's key. PassCub asks for at least 10 characters.

PassCub locks itself after 10 minutes without use, and whenever you lock Windows. While it's open, other programs running without administrator rights are blocked from reading its memory, and Windows crash reports leave out the memory where its keys live.

A copied password goes to the clipboard marked so that Windows keeps it out of clipboard history and cloud clipboard, and PassCub clears it after 30 seconds.

Sync

Sync is optional. When it's on, your PCs exchange changes through a relay. The relay stores sealed, padded data under an ID that looks random and says nothing about you, and passes it on to your other PCs. It never receives your name, email address, master password or keys, and it can't tell which items or sites the data is about.

Each change goes out as one sealed record, about a kilobyte, a second after you stop typing. Your other PCs hear about it right away and fetch it, so changes usually arrive within a second or two. A PC that has been offline for a long time catches up from one sealed snapshot plus recent changes.

The relay can't create or change anything, because it doesn't have the keys. Each PC's changes are also chained together with hashes, so if the relay dropped some from the middle or replayed old ones, your PCs would notice and say so. To deliver anything, the relay necessarily sees your IP address and when and how much you send; it doesn't log IP addresses. You can run your own relay instead.

Adding a PC

To add a PC, the one you already use shows a one-time code of 27 characters. You type it on the new PC, and both screens then show the same 6-digit safety code. You approve only if they match, so nobody in the middle, including the relay, can slip in a key of their own. The code works once and expires after 10 minutes.

Recovery kit

If you lose your master password or the PC itself, a recovery kit gets you back in. It splits a spare key into shares using Shamir's secret sharing, for example three shares where any two reopen the vault. Keep them in different places. Fewer shares than needed reveal nothing at all.

There is no other way back in. PassCub has no reset email and no master key, because nobody else holds a copy of your vault.

Browser extension

The extension for Chrome, Edge and Firefox is a popup and nothing else. It has no background process and doesn't watch the pages you visit. When you click it, it reads the address of the current tab and asks the PassCub app on your PC for logins saved for that exact site, down to the subdomain. It touches the page only when you click Fill or Save.

It stores nothing. It talks only to the PassCub app on the same PC, over a channel that only your Windows account can open, and the app checks that it really is talking to PassCub on the other end. You unlock PassCub in the app, so your master password never enters the browser.

Updates

PassCub checks for updates and installs one only if it's signed with PassCub's release key and the signature names the exact version being installed. The signing key lives on one PC and nowhere else. An update that fails the check is refused.

Limits

No password manager can protect you from everything. PassCub can't protect your vault from:

• Malware running on your PC while PassCub is unlocked. Whatever you can see, it can see.
• Someone who knows your master password and can sign in to your Windows account.
• Losing both your master password and your recovery kit. Nobody, including us, can get you back in.

And some protections aren't finished yet:

• PassCub hasn't had an independent security audit yet.
• Windows protects the PC's key for your Windows account. A program running as you could unseal it, though it would still need your master password. Keeping that key in the PC's security chip is planned.
• Removing a PC makes your other PCs ignore its changes, but until key rotation ships, a removed PC that kept a copy of the vault could still read new changes.
• Autofill fills login forms on the page itself, not inside embedded frames.
• The installer isn't code-signed yet, so Windows warns about it the first time. Updates are signed separately, as described above.

Reporting a problem

If you find a security problem, please email support@passcub.com with the details before telling anyone else, and we'll reply as soon as we can. The same address is in security.txt.