The field signed_data in CDR.singed_data.signed_values is described with:

The format of the content depends on the EncodingMethod field.

In fact the format OCMF contains the signature in its file definition. In case of OCMF we would have different options to comply with OCMF, all feel odd:

1. Send whole data twice: escaped JSON in plain_data and base64 encoded JSON in signed_data
2. Manipulate the OCMF file: Strip signature from OCMF to fill plain_data as above and send the complete signed data with signature in base64 encoding as above
3. Split the OCMF at the pipe characters: OCMF|<payload data section>|<signature section> then send payload data in plain_data and signature section in signed_data

All options contradict with the semantics of the OCPI specification.