Pyfhel subtraction - encryption

Question

I am trying to use Pyfhel library to perform some operations on encrypted integer list. But while performing subtraction operation, when negative values are expected, I am getting a different value.
"""Import all the packages useful for the Demo.
#-Pyfhel is useful to generate keys, encrypt and decrypt.
#-PyPtxt is useful to tranform the input vectors into plain text objects that could be encrypted.
#-PyCtxt is useful to tranform the plain text object in PyCtxt object that are encrypted (Cypher texts). PyCtxt can be add, multiply etc with homeomorphic operations."""
from Pyfhel import Pyfhel
from PyPtxt import PyPtxt
from PyCtxt import PyCtxt
"""Other imports useful for the demo."""
from itertools import izip
import itertools
from operator import sub
import numpy as np
import matplotlib.pyplot as plt
import sys
import argparse
import copy
import datetime
import os
#Instantiate a Pyfhel object called HE.
HE = Pyfhel()
print("******Generation of the keys for encryption******")
#Create the Key Generator parameters.
KEYGEN_PARAMS={ "p":257, "r":1,
"d":1, "c":2,
"sec":80, "w":64,
"L":10, "m":-1,
"R":3, "s":0,
"gens":[], "ords":[]}
"""Print the Key Generator parameters to let the user knows how his vectors will be encrypted."""
print(" Running KeyGen with params:")
print(KEYGEN_PARAMS)
"""Generate the keys that will be use to encrypted the vectors. The generation of the keys uses the Key Generator parameters. Then print a message to inform the user that the key generation has been completed."""
HE.keyGen(KEYGEN_PARAMS)
print(" KeyGen completed")
var1 = HE.encrypt(PyPtxt([130], HE))
var2 = HE.encrypt(PyPtxt([10], HE))
xyz = var2 - var1
abc = HE.decrypt(xyz)
abc[0][0] # 137
print(abc[0][0] - 257) # output: -120
As in the code above, I noticed that if I subtract the value of 'p' used while generating keys, I get the expected output, but that's not of much help, especially when the difference is more than 257.
Can anyone please let me know if this is the expected behavior or what could be done to obtain expected output?
Thanks!
(Couldn't add the related tags but it's around Homomorphic Encryption using Pyfhel library, Python implementation of HElib)

Answer 1

I can confirm you that this is the expected behaviour. All the operations you do are modulo p, therefore encrypted values will always stay in the interval [0, p-1]. Note that, even though Pyfhel supports substraction, it does not support negative numbers in its current implementation.
Nevertheless, a major upgrade is on the way, which will solve this issue. Stay tuned!

Answer 2

print(abc[0][0] - p) yields the expected output because we are working modulo p.