Attributes in MSP policy as strings

[tilenmarc]

In this PR we change the way the attributes are given for ABE scheme FAME. In the previous version attributes were ints, while in this version they are strings. In particular:

• in FAME they can be arbitrary stings, that do not include words AND, OR or characters ( and ).
• in the other ABE scheme GPSW they still must be ints (since at the scheme initialization the user must give L such that all the attributes are in the interval [0, L)). We allow that in GPSW attributes can be given as []int or as []string, for example attribs = []int{1, 2, 3} or attribs = []string{'1', '2', '3'}.

[tilenmarc]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
==============================
- abe/fame_test.go  8
- abe/gpsw.go  5
- abe/policy.go  1

Clones added
============
- abe/gpsw.go  5

See the complete overview on Codacy