Unfortunately PyCrypto accesses the internal representation of PyLong objects, which pypy cannot expose. This part of
PyCrypto (in _fastmath.c: mpzToLongObj and longObjToMPZ) has to be modified before pypy can compile this module.
Unfortunately the CPython API does not provide many alternatives. _PyLong_FromByteArray() could be used, but it requires
an additional copy and would be slower.
The best would be to invent a new CPython API: PyLong_GET_DIGITS() and PyLong_GET_SIZE(), which PyPy could implement like
the string object and allow a mutable buffer until the object exits the function.