End-to-end vs zero-knowledge encryption: what each term promises

Both phrases appear on app store pages, and both sound reassuring. Neither has a legal definition, so each company can use them as it likes. Knowing what they were meant to say helps you tell a real promise from a slogan.

End-to-end encryption

The “ends” are the two devices at either end of a journey, for example your phone and your friend’s. Data is encrypted on the first and decrypted only on the second. Everything between, such as servers and networks, carries scrambled data it can’t read.

Compare this with encryption in transit, where data is protected on the road but the server at the other side can open it. Many services do this, and it keeps out someone on the café Wi-Fi, but not the service itself.

A simple picture: sending a sealed letter that only the recipient can open, rather than sending it to a post office that opens it, reads it and reseals it.

Zero-knowledge

This phrase describes what the service knows: nothing about your content. The service never holds your key, never sees your data in readable form, and so can’t hand it over, lose it in a breach in a readable state, or reset your password.

The cost is that a forgotten password can’t be recovered by support. That is why such apps give you a recovery phrase.

(There is another meaning in mathematics: a “zero-knowledge proof” lets you prove you know something without revealing it. That is a different technique, and most apps using the phrase don’t mean it.)

How they overlap

An app can be end-to-end encrypted and still know things about you, such as your account, contacts and file sizes. An app can claim zero-knowledge for your files while still learning who you are.

Question	End-to-end	Zero-knowledge
Focus	How data travels between devices	What the provider can know
Typical use	Messaging, sharing	Storage, backup, vaults
Provider can read content?	Not in transit	Never
Provider can see metadata?	Often yes	Depends on the app

Why the labels don’t settle it

Words on a page don’t tell you how keys are handled. Cases where the claim breaks down:

• The company keeps a copy of your key. Convenient for resets, but it means the company can open your data.
• The password is sent to the server, and the key is derived there.
• The backup is stored unencrypted, even though the live data is encrypted.
• Metadata is collected, such as file names, sizes, times or contacts.

Questions to ask an app

1. Where is my key made, and does it ever leave my device?
2. If I forget my password, can you get my data back? (If yes, you hold a way in.)
3. Is my backup encrypted before it is uploaded, with a key you don’t have?
4. Is the file format published, or has anyone outside the company reviewed it?
5. What do you still see, such as my email, purchases, file names and sizes?

How Silo fits

Silo has no account and no server of its own. Photos and files are encrypted on the phone and never leave it unless you choose to export or back them up. Backup, arriving with launch, is encrypted on the phone and goes to your own cloud storage, which Silo’s developer can’t open. The format is published at /format/ so the claims can be checked. These are zero-knowledge properties in the sense above, because there is nobody who knows your key.