For full disk encryption you need DSM >= 7.2 and you can either, store it locally (useless) or in a KMIP server. [0]

As a KMIP server you use:

  - Another Synology NAS with DSM >= 7.2
  - A KMIP compatible key server

Except for the demo implementation that Synology uses (PyKMIP), all the KMIP compatible servers I've found have licenses in the tens of thousands a year. So if anybody has any suggestions to substitute PyKMIP...

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  0: https://kb.synology.com/en-global/DSM/tutorial/Which_models_support_encrypted_volumes