General block cipher implementation

#Block cipher
#Feistel cipher
import sys
import os
from _blake2 import blake2b

def f(R, K):
    #Irreversible function
    #R is integer
    #K is character
    K = ord(K)
    size = len(bin(R)[2:])
    #must return of the same size (bits) as input
    var = (str(K)+str(R))
    return int(blake2b(var.encode()).hexdigest()[::-1],16)^int('1'*size,2)

def xor(x, y):
    return x^y


def Feistel_enc(L0, R0, K):
    for i in range(0,len(K)):
        L_next = R0
        R_next = xor(L0, f(R0, K[i]))
        L0 = L_next
        R0 = R_next
    return R0,L0

def Feistel_dec(R0, L0, K):
    "K is the same as enc"
    K = K[::-1]
    return Feistel_enc(R0, L0, K)

def split(string):
    half = len(string)//2
    return string[:half], string[half:]

char_adjust = 8

def message2int(message):
    fin = []
    for char in message:
        value = str(ord(char))
        adjust = char_adjust-len(value)
        fin.append(('0'*adjust)+value)
    fins = "".join(fin)
    fini = int(fins)
    return fini

def int2message(integer):
    s = str(integer)
    fin = []
    counter = 0
    finl = [s[i:i+char_adjust] for i in range(0, len(s), char_adjust)]
    if len(finl[-1])<char_adjust:
        s = (char_adjust-len(finl[-1]))*"0" + s
        finl = [s[i:i+char_adjust] for i in range(0, len(s), char_adjust)]
    finl = list(map(int,finl))
    fin = list(map(chr,finl))
    return "".join(fin)

def main(args):
    if 'enc' in args:
        enc = True
        dec=False
    elif 'dec' in args:
        dec = True
        enc=False
    else:
        dec=False
        enc=False

    K = args[1]
    if enc:
        message = " ".join(args[2:])
        m1,m2 = split(message)
        m1i = message2int(m1)
        m2i = message2int(m2)
        l,r = Feistel_enc(m1i,m2i,K)
        #cipher = int2message(int(str(l)+str(r)))
        cipher = str(l)+str(r)
        return cipher
        print("Ciphertext:\n\n{}".format(cipher))
    elif dec:
        cipher = args[2]
        c1,c2 = split(cipher)
        #c1i = message2int(c1)
        #c2i = message2int(c2)
        c1i = int(c1)
        c2i = int(c2)
        l,r = Feistel_dec(c1i,c2i,K)
        plain = int2message(l)+int2message(r)
        #plain = int2message(int(str(l)+str(r)))
        return plain
        print("Plaintext:\n\n{}".format(plain))
    else:
        print("Something went wrong. Got\a\n{}".format(args))
        input()

def helping():
    print("Please use command line syntax:")
    print("""
fc enc KEY FILEPATH
    Encryption
fc dec KEY FILEPATH
    Decryption

Files must be plaintext. No binary data.""")

if __name__ == "__main__":
    args = sys.argv
    if len(args)<3:
        helping()
        sys.exit()
    #print(args)
    args = args[1:]
    if args[0]=="enc":
        ext = ".enc"
    elif args[0]=="dec":
        ext = ".dec"
    output = args[2]+ext
    try:
        with open(args[2], "r") as file:
            text = file.read()
    except OSError:
        print("Failed to open file: ", args[2])
        sys.exit()

    args[2] = text
    result = main(args)

    if output[-8:-4]==".enc":
        output = output[:-8]+output[-4:]
    elif output[-8:-4]==".dec":
        output = output[:-8]+output[-4:]

    try:
        with open(output, "w+") as out:
            out.write(result)
    except OSError:
        print("Failed to write to file: ", output)
        sys.exit()
    print("Executed successfully.")
    sys.exit()