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What is Cross-signing?

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.

Updated

One user key signs a certificate for each device. An unsigned device stays unpublished.

Encryption

Cross-signing, explained

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.

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. A second device receives the user key from the encrypted key backup, opened with the recovery phrase, and signs its own certificate with it. Enclessa states its current limitation openly: a device that skips that step stays unpublished until it is restored from settings, and a person who has lost the phrase starts a new identity, which every contact is shown as a change of key.