Cryptography with python

Question

I am new in this field, Would you help me ? Each byte in my cipher text (encrypt3) is interpret as an integer between 0 to 255.For each byte Plaintext=Ciphertext -100 (mod 256). In the following code I have error: newbyte=c.to_bytes(1,byteorder=sys.byteorder,signed=False) OverflowError: can't convert negative int to unsigned Any help would be appreciated.

import sys
f=open("encrypt3.dat","rb")
s=f.read(100)
d=bytearray(s)
print (len(d))
strnew=''
newbyte=b''
for c in d :
    c=c-100 % 256
    newbyte=c.to_bytes(1,byteorder=sys.byteorder,signed=True)
    strnew=strnew + newbyte.decode('latin1')

print(strnew)

You are in effect using a byte-based Caesar Cipher. To move on from a Caesar Cipher, have a look at Vigenère or perhaps RC4 for your next cipher exploration. You did well to go for a byte-based implementation rather than an alphabet based implementation. — rossum
– rossum, Commented Dec 13, 2016 at 11:49

r3mainer · Accepted Answer · 2016-12-13 01:42:50Z

0

The modulus operator (%) is processed before the subtraction operator (-), so your statement c=c-100 % 256 is equivalent to to c=c-(100 % 256), or just c=c-100 (because 100 % 256 == 100). As a result, your code will crash whenever it encounters a character with an ASCII code less than 100 (='d').

You can fix this by changing c=c-100 % 256 to c=(c-100) % 256.

Read up on operator precedence in Python 3.

answered Dec 13, 2016 at 1:42

r3mainer

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