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All parameters with potentially binary value are to be sent base64-encoded. That's also true for PEM files.
If the parameter is the integer representation of an EAP type, you can look up the number to use in the source (const INTEGER_...).
List of result codes
The HTTP POST will be answered with a "result" field, which is either "SUCCESS" or "ERROR". It is accompanied by a "details" field, which contains either the response details, or in the case of error, an additional "errorcode" and "description". The auhoritative reference for the list of error codes is on GitHub, https://github.com/GEANT/CAT/blob/master/web/lib/admin/API.php : the class constants API::ERROR_*
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