Slice indice error when I try to pass a value

Question

This is a fiestel cipher and I am trying to decode the stored message, however when I run it i get an exception unhandled "slice indices must be integers or None or have an index method". I traced it to (L,R) = G(test), but I am not sure what is wrong with what is being passed in

from base64 import b64decode

(a,b,c,d) = (12258487803861853193714140190369268261202205225846869845245886114503678185370576657001537611759067404857011052836681912514135296185688334428452462064527761,\
         13346376869506979374836874046204370339910872071884888902215442517395171258258152757258082136597260296613932276350407476889729430724339742164666125768654387,\
         10707278048073703090234519728300006549070759592219984101161107932419267899872157175740069946671710753166980969556028191258252903890793549897483872362043507,\
         11330448751098045546614990567612096132290751751225546822371413732253437433478002901406535282761940023232557605690376224913895910240841519464567354770276551)

def F(x):
    return (a*x*x + b*x + c)%d

def G(message):
    n = len(message)
    L = message[0:(n/2)]
    R = message[(n/2):n]

    L = int(L.encode("hex"), 16)
    R = int(R.encode("hex"), 16)

    return (L,R)

def fiestel(L, R):
    rounds = 8
    for i in xrange(rounds):
        (L,R) = (R, L^F(R))

    L = hex(L).replace("0x", "").replace("L", "")
    R = hex(R).replace("0x", "").replace("L", "")

    return R+L


test = "1SvXEaXhywrBE6DRX9zomKxKbZGYu46Tj7Z+oNrX0SxGU253OmLKDLHoO+LaJT2W+lPyQkWBToiPbo7wNz2lSIrTRT8yxV6AovUQO3Hvob33/hVfYmpHiytVwQ/dPmx+IQi7w+rTYZGro58FauonXu4hjwCnRaVYhwdjAvbC7cA="
test = b64decode(test) 

(L,R) = G(test)
next_test = fiestel(L,R)
FLAG = next_test.decode("hex")

print(FLAG)


#1SvXEaXhywrBE6DRX9zomKxKbZGYu46Tj7Z+oNrX0SxGU253OmLKDLHoO+LaJT2W+lPyQkWBToiPbo7wNz2lSIrTRT8yxV6AovUQO3Hvob33/hVfYmpHiytVwQ/dPmx+IQi7w+rTYZGro58FauonXu4hjwCnRaVYhwdjAvbC7cA=

Can you post the full traceback of the error? – glibdud May 02 '19 at 19:42 — glibdud, May 02 '19 at 19:42

Cohan · Answer 1 · 2019-05-02T20:12:23.160

1

When n is an odd number, n/2 is obviously a float. In your case, len(n) = 128, so that's not the issue, but something to be aware of.

You are using simple division / which returns a float. Use integer division // which will return an int.

print(128/2)
#64.0
print(128//2)
#64

This will at least get you to through to the next line where you want to change L.encode("hex") into L.hex(). Then xrange() should be range() if you're using python3.x.

edited May 02 '19 at 20:12

answered May 02 '19 at 19:49

Cohan

4,384
2
22
40

That worked however now I get this exception unhandled with this line L = int(L.encode("hex"), 16), the message says 'hex' is not a text encoding; use codecs.encode() to handle arbitrary codecs, but it should be able to take it in – zavier May 02 '19 at 19:57
1

https://stackoverflow.com/questions/6624453/whats-the-correct-way-to-convert-bytes-to-a-hex-string-in-python-3 – Cohan May 02 '19 at 19:58

Slice indice error when I try to pass a value

1 Answers1