Z3 String/Char xor?

问题

I'm working with Z3 in Python and am trying to figure out how to do String operations. In general, I've played around with z3.String as the object, doing things like str1 + str2 == 'hello world'. However, I have been unable to accomplish the following behavior:

solver.add(str1[1] ^ str1[2] == 12) # -- or --
solver.add(str1[1] ^ str1[2] == str2[1])

So basically add the constraint that character 1 xor character 2 equals 12. My understanding is that the string is defined as a sequence of 8-bit BitVectors under the hood, and BitVectors should be able to be xor'd.

Thanks!

回答1:

So far I don't expose ways to access characters with a function. You would have to define auxiliary functions and axioms that capture extraction. The operator [] extracts a sub-sequence, which is of length 1 if the index is within bounds.

Here is a way to access the elements:

from z3 import *

nth = Function('nth', StringSort(), IntSort(), BitVecSort(8))

k = Int('k')
str1, str2, s = Strings('str1 str2 s')

s = Solver()
s.add(ForAll([str1, k], Implies(And(0 <= k, k < Length(str1)), Unit(nth(str1, k)) == str1[k])))

s.add( ((nth(str1, 1)) ^ (nth(str2, 2))) == 12)

来源：https://stackoverflow.com/questions/35991948/z3-string-char-xor