In short. Zero-access encryption means a provider stores your data encrypted with a key it does not have. Your email sits on the provider’s servers, but only the key holder can read it. The main difference from end-to-end encryption: zero access is about how data is stored, not how it travels.
How it works
It rests on the same key pair as end-to-end encryption [1]:
- the public key is kept by the provider, and anyone can use it to encrypt a message for you;
- the private key is kept by the provider only in encrypted form, and only your device can decrypt it, for example with your password. The password itself never goes to the server.
When a message arrives, the server encrypts it with your public key right away and stores it encrypted. It can only be read on your device, where the private key is unlocked.
How it differs from end-to-end encryption
- End-to-end encryption protects a message along the whole way: it is encrypted on the sender’s device and decrypted only on the recipient’s. More in What is end-to-end encryption?
- Zero access protects what is stored. It also covers mail from regular providers that arrives without end-to-end encryption: the server encrypts it the moment it arrives.
A good email service combines both: mail between its users travels end-to-end encrypted, and everything stored, including mail from outside, is kept with zero access.
How it differs from regular disk encryption
Many providers say they encrypt data on their disks. That protects against stolen disks and outsiders, but the provider holds the keys to that encryption, so it can read your mail. With zero access, the provider has no key to what it stores. Privacy Guides sets a minimum requirement for email providers: they “must encrypt email account data at rest with asymmetric encryption, where only the user has the private keys needed to decrypt it” [2].
Where zero access stops
- The moment of arrival. A message from a regular email provider arrives unencrypted. At that moment the server sees it, and only then encrypts it. That is why a court can order a provider to keep a specific mailbox’s future incoming mail before it is encrypted. It happened in 2020, when a court in Cologne ordered the email service Tutanota to keep future unencrypted mail for one mailbox; mail already stored was not affected [3].
- Metadata. Addresses, time and size are needed for delivery. What of it a provider keeps depends on the provider.
- Webmail. Email in a browser loads its code from the server each time you sign in. Unless the provider offers a way to verify that code, you have to trust it has not been swapped.
- A lost password. If only you hold the key to what is stored, losing your password and every recovery method means nobody can open your old mail.
How to check that a provider really has zero access
- What happens to old mail after a password reset through support? If old messages stay readable without a recovery phrase, another device of yours or a passkey, the provider had the key to them.
- Does your password go to the server? With zero access, the private key is unlocked on your device, and the provider does not need your password.
- What is stored encrypted? Only message text, or also subjects, addresses and contacts?
- What does the provider say about mail from outside? An honest provider says plainly that it sees such mail as it arrives.
- Was there an independent audit? App code cannot show how the server stores data. An independent audit can.
Zero access and “zero knowledge”
Some providers call zero access “zero knowledge”. In cryptography, a zero-knowledge proof means something else: a way to prove a statement is true without revealing anything beyond that [4]. If a provider says “zero-knowledge”, it is worth asking what exactly it means.
Sources
- RFC 9580: OpenPGP. IETF, July 2024. https://www.rfc-editor.org/rfc/rfc9580
- Privacy Guides: Email Services, Criteria section (updated June 28, 2026). https://www.privacyguides.org/en/email/#criteria
- The Register, December 8, 2020. https://www.theregister.com/2020/12/08/tutanota_backdoor_court_order/
- NIST CSRC glossary: zero-knowledge proof (NISTIR 8301). https://csrc.nist.gov/glossary/term/zero_knowledge_proof