Signing some data using CRYPTO

[Alessio Vanni]

Hello,

I have a service which sends some data to a remote peer via CADET to a
well known port.  This data is preceded by a header with a bunch of
informations, including a signature to verify that nothing happened
while the data was being transmitted.  I thought I could use CRYPTO to
sign this data, but it's a bit unclear how it works.  The signature
should be performed using the keys of an ego.

The function to sign the data expects a "purpose", which is a structure
with two field: "size" and "purpose".  It's not clear where the data to
sign goes, but I'll guess it goes after the structure, similarily to how
messages created with `GNUNET_MQ_msg_extra' have extra space at the end.
The "purpose" field has a problem though: the documentation says the
value has to be one from "gnunet_signatures.h", but none of them are a
"generic signature" value, only something specific to a certain
situation.

There is also another small problem, because the data can have a size
that can fit into a uint64_t (when sent through CADET, the data is
fragmented as needed) and the "size" field in the purpose is a uint32_t,
but since the service is still being developed the uint64_t can become a
uint32_t if needed.

With all that said, how can I sign some data using CRYPTO?

Thanks,
A.V.