A system, such as an extension service, receives a first public key that is derivable based at least in part on a secret that is shared between at least a first device and a second device. The system, in an embodiment, derives a cryptographic key based at least in part on the first public key and transmits a second public key that enables another system to derive the cryptographic key. In an embodiment, the cryptographic key is a symmetric key and the system lacks access to a first private key that corresponds to the first public key.
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