Here is a re-submit of this code as a merge request with Mathieu's comments applied.
During the hackfest, we discussed having caps with the keys being an array of structures containing 2 buffers each (mki & key), and I tried implementing that, but it seems that our parser can't deal with an array containing structures as array members are comma-separated, and structures are just a comma-separated list too. So I suggest keeping this acceptable-not-amazing API with "key2=x, mki2=y, key3=z, mki3=a, etc..".
Closes #803 (closed)