[Interop] Cryptographic Secret Management between BPIs

[Therecanbeonlyone1969]

Agree on how cryptographic secrets between two BRI-1 and BRI-3 are communicated and managed:

• encryption keys for messages
  • Suggestion: Use keyAgreement property in DID doc of BPI Subjects used as stand-ins for the two Working Groups. This way the symmetric encryption/decryption key can be derived on the fly without having to be stored anywhere (using Diffie-Hellman)
• Lock Commitment:
  • Suggestion: Only needs to be validated but not really stored (do not need to worry about audit trail here).