Use appointment-specific pre-shared ECDH key (?) #40

Currently the provider app shares a single public ECDH key with appointments for users to employ when sending encrypted data. To increase secrecy we should ideally make this key appointment-specific. Currently this is not done as it would require robust synchronization of appointment ECDH keys between different provider apps.

In general we should think about how we can improve the key agreement protocol using e.g. a double or triple Diffie Hellman key agreement schemes following e.g. the Signal protocol.

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