ALL: Improve serialization of DRR

[jpaskhay]

Currently the DRR serialization is essentially JSON and includes base64-encoded data. We should investigate and implement a more compact serialization strategy to save on storage the DRK/DRR take up (namely the resulting encrypted data payload).

Likely less important for the SK and IK size as they don't have data payloads, but we could investigate there as well if it makes sense.