Byte Stuffing/Unstuffing in Python

Question

I want to write a small script in python which does byte stuffing/unstuffing, since I have no idea how to do any of it Im just going for stuffing first.

So, the goal is simple, there is a function with three inputs, something like this: ByteStuffing(flagbyte, escapebyte, frame) and output with flag bytes in the beginning and end, and stuffed frame in between.

so lets say my flag byte is Z, escape byte is A and frame LEONARDO

ByteStuffing(Z,A,LEONARDO) = ZLEONAARDOZ

p.s. escape byte also escapes flag characters in frame, not just escape bytes itself.

unstuffing is just reverse function.

now, i am really not familiar with programming, just getting started so I am really not very creative when it comes to coding, that's why I want to start with this small 'projects', can anybody give me ideas how to get started with this problem?

p.s Python version 2.7.12

EDIT: from Keerthana Prabhakaran's answer i ended up with this.

def applyByteStuffing(flagbyte, escapebyte, payload):

flagbyte = ("Z")
escapebyte = ("A")


x = payload.replace (escapebyte, escapebyte*2)
y = x.replace (flagbyte, escapebyte+flagbyte)
print (flagbyte + y + flagbyte)
return;

if i try to test it like this:

def test():


assert applyByteStuffing("Z", "A", "TAZZA") == "ZTAAAZAZAAZ"

if __name__ == '__main__':

test()

I am getting assertion error. has anyone idea where my assertion error is happening?

just trying out trying out this: applyByteStuffing("Z" , "A" , "TAZZA")

gives me the correct output: ZTAAAZAZAAZ

@NickA I have no idea what is bit packing. I know byte stuffing and bit stuffing and I mean byte stuffing, not bit — Leonardo
– Leonardo, Commented May 22, 2017 at 14:29

Keerthana Prabhakaran · Accepted Answer · 2017-05-22 16:43:28Z

1

You can use str.replace() for this purpose. Replace the escapebyte with another string with the duplicate, and stuff the resulting string with the flagbyte!

>>> flagbyte + frame.replace(escapebyte,escapebyte*2)+ flagbyte
'ZLEONAARDOZ'

For the updated question, you need to return something from applyByteStuffing so that the return value can be asserted against the string on RHS.

def applyByteStuffing(flagbyte, escapebyte, payload):
    x = payload.replace (escapebyte, escapebyte*2)
    y = x.replace (flagbyte, escapebyte+flagbyte)
    return flagbyte + y + flagbyte #return value

And then assert!

assert applyByteStuffing('Z','A','LEONARDO') == 'ZLEONAARDOZ'

edited May 22, 2017 at 16:43

answered May 22, 2017 at 14:32

Keerthana Prabhakaran

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2 Comments

Leonardo Over a year ago

thanks for the idea, i edited the question before, saying that i also need to escape flag bytes. so i expanded your solution a little bit and it works overall. I will edit my question now with new problem :)

Keerthana Prabhakaran Over a year ago

Glad that it helped! I've updated my answer for the assertion error!

Aslam Mohammad · Accepted Answer · 2018-01-18 14:07:18Z

1

bits=[1,0,0,1,1,1,1,1,0,1,1,0]
stuffed=[]
count=0
for i in range(len(bits)):
    if bits[i]==1:
        count=count+1
        stuffed.append(bits[i])
    elif bits[i]!=1:
        count=0
        stuffed.append(bits[i])
    if count==5:
        stuffed.insert(i+1,0)
print stuffed
#Take any input the code works perfectly. #Tested

answered Jan 18, 2018 at 14:07

Aslam Mohammad

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