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Glossary · Encryption

Cross-signing

All termsHow Enclessa secures data

Cross-signing is a scheme in which each person holds one identity key that signs certificates for their own devices, so that verifying a person once is enough to trust every device they own rather than verifying each device separately.

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What Cross-signing means

In a protocol where the device is the unit of membership, a person with a laptop, a phone and a desktop is three members. Without cross-signing, every contact would have to verify all three, and again whenever one is replaced. The number of verifications grows with the product of people and devices, which in practice means nobody verifies anything.

Cross-signing collapses this. A user key signs a certificate for each of that user’s devices; a contact verifies the user key once and can then check any device certificate against it. Trust moves from "I have checked this device" to "this device is vouched for by a key I have checked", which is the same shift a certificate authority makes, except that the authority is the person themselves.

The hard part of cross-signing is not the signatures but the key’s lifecycle: where the user key lives, how a second device gets the ability to sign, and what happens when every device is lost. Those questions are usually answered by an encrypted backup protected by a recovery phrase, which is why cross-signing and key backup arrive together.

How Enclessa uses it

Enclessa publishes a cross-signing key per user and a certificate per device, and a direct message verifies the other side’s certificate before the conversation opens. Enclessa’s current limitation is stated in `docs/PROGRESS.md` rather than hidden: a second device of the same user does not yet sign itself, because the user key stays on the device that minted it until the recovery backup has an interface. The encrypted key backup itself is built.

Where Cross-signing is specified

Related terms

Safety numbersA safety number is a short string derived from the public keys of both sides of an encrypted conversation, which the two people compare through a separate channel to confirm that no third party has interposed itself.Read the entryMLS (RFC 9420)MLS, or Messaging Layer Security, is the IETF standard published as RFC 9420 for end-to-end encrypted group messaging, designed so that adding or removing a member costs work proportional to the logarithm of the group size rather than to the group size itself.Read the entryEnd-to-end encryptionEnd-to-end encryption is a design in which a message is encrypted on the sending device and decrypted only on the receiving devices, so that the server carrying it holds no key that can open it.Read the entryPost-compromise securityPost-compromise security is the property that a conversation heals after a device is compromised, so that once the compromised device is removed and the group re-keys, the attacker who held its keys can no longer read new messages.Read the entry

Read further

The security page explains how Enclessa protects data and which certifications it does not hold. The trust centre covers the processing agreement, the subprocessors and the residency position. The FAQ answers the questions buyers ask most often.

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