CrossSigning

Crosssigning is a cryptographic practice used in distributed identity and messaging systems to establish trust between keys across devices and accounts. By having keys signed by trusted keys, users can verify that a given device or identity truly belongs to the claimed owner.

The central idea is to bind a public key to an identity by means of digital signatures. A trusted key signs other keys (for devices or users). If a recipient already trusts the signer, they can verify new keys without trusting the signer directly each time. This creates a flexible web of trust that propagates endorsements through the network.

Implementation varies by system. In practice, cross-signing often uses multiple keys per identity, including a master signing key and device keys. A signing key signs the keys of the user's devices; those device keys are then used to sign messages or establish trust with others. When a user adds a new device, a pre-existing trusted device can approve it by signing its key, enabling seamless onboarding.

In Matrix, cross-signing is used to enable cross-device verification. A user can sign their own devices and, in some configurations, sign other users' signing keys to attest to identities. This reduces the need to re-verify on every login and allows trust to remain even if a device is temporarily offline.

Security considerations include key compromise, rotation, and revocation. Cross-signing shifts some trust from centralized servers to the cryptographic integrity of the signing keys. Proper key management and revocation procedures are essential.