Need handler string encoder error

[dark-lbp]

Python Version 2.7.16

String encoder error will break fuzzing progress.
Running this script will raise UnicodeDecodeError: 'ascii' codec can't decode byte 0xfe in position 11: ordinal not in range(128)

from kitty.model import *
from kitty.interfaces import WebInterface
from kitty.fuzzers import ServerFuzzer
from kitty.model import GraphModel
from katnip.targets.file import FileTarget


t1 = Template(name='str_encoder_test', fields=[
    String(name='bString', value='hello_kitty', encoder=StrEncodeEncoder('utf_16_le'), max_size=254 / 2)
]
                       )

target = FileTarget('FileTarget', './tmp', 'fuzzed', 'bin')

model = GraphModel()
model.connect(t1)

fuzzer = ServerFuzzer()
fuzzer.set_interface(WebInterface(port=26001))
fuzzer.set_model(model)
fuzzer.set_target(target)
fuzzer.set_range(1, 10)
fuzzer.start()

This problem can be reproduced with the code show below .

from kitty.model import *

test = String(name='bString', value='hello_kitty', encoder=StrEncodeEncoder('utf_16_le'), max_size=254 / 2)
test.mutate()
test.mutate()
test.mutate()
print(test.render())

[dfiloni]

To fix this issue I changed py2_str_encoder_func function in kitty/model/low_level/encoder.py as follows:

def py2_str_encoder_func(encoding):
    if encoding not in _py2_str_encoder_funcs_cache:
        _py2_str_encoder_funcs_cache[encoding] = lambda x: strToUtf8(x).encode(encoding)
    return _py2_str_encoder_funcs_cache[encoding]

So x is encoded in utf8 first and then it's encoded with the requested encoding (the decoding is already performed by Python in the encode() function)

Honestly I'm not sure that's a valid solution, I've executed few/poorly tests and only with utf_16_le and utf_16_be encodings.