In order to add authentication features to [this CP-ABE scheme](http://dl.acm.org/citation.cfm?id=1264219), I have tried to combine it with the [ABS scheme proposed by Maji et. al](http://link.springer.com/chapter/10.1007%2F978-3-642-19074-2_24). 

I want to use the sign() and verify() algorithms of ABS scheme. But the problem is that the two scheme support different access policy structures; CP-ABE uses access trees to define access policy whereas the ABS uses monotone span program (MSP). 

What changes are necessary for the two access policy structures to work together? For example, can I translate an MSP access policy to an access tree?