I'm having a similar problem.
I want to send information to a PHP via AJAX POST and I want to encrypt the parameters I'm sending in the query string.
I'm being able to encrypt using the cryptography plugin, but I cant find a tool using AES-GCM outside Construct 3 that will decrypt it.
I read the manual and the addition Ashley posted:
Bytes Description
0-1 Reserved (must be 0)
1-17 Salt (16 bytes)
17-29 Initialization vector (aka IV, 12 bytes)
29-33 Iterations (uint32, 4 bytes)
33+ Encrypted data payload
However it is of no much use as there is no way to define these parameters beforehand and after the encrypted string is generated, all the metadata is already encrypted except for the password.
Is there any way I can encrypt in C3 and decrypt serverside defining only password?