Trying to send string variable via Python socket

Question

I'm in a CTF competition and I'm stuck on a challenge where I have to retrieve a string from a socket, reverse it and get it back. The string changes too fast to do it manually. I'm able to get the string and reverse it but am failing at sending it back. I'm pretty sure I'm either trying to do something that's not possible or am just too inexperienced at Python/sockets/etc. to kung fu my way through.

Here's my code:

import socket

aliensocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

aliensocket.connect(('localhost', 10000))
  
aliensocket.send('GET_KEY'.encode())
key = aliensocket.recv(1024)

truncKey = str(key)[2:16]
revKey = truncKey[::-1]

print(truncKey)
print(revKey)

aliensocket.send(bytes(revKey.encode('UTF-8')))
print(aliensocket.recv(1024))

aliensocket.close()

And here is the output:

F9SIJINIK4DF7M
M7FD4KINIJIS9F
b'Server expects key to unlock or GET_KEY to retrieve the reversed key'

@JohnGordon ah, great question - when i capture the key initially it comes prefaced with "b'*" in the recv data. So I need to "clean" it before reversing it. I'm sure there's a more elegant way to do it... — Jodi Rehlander, Sep 13 '22 at 19:31
Are you sure `b'` is actually part of the key? I would expect that's just how it prints, not that it's actually part of the key. — John Gordon, Sep 13 '22 at 19:32
Try adding `print(key[0])` to see if the first letter really is `b`. — John Gordon, Sep 13 '22 at 19:34
@JohnGordon that is correct, the truncKey var is removing the b' and then I reverse what's left in the key var. — Jodi Rehlander, Sep 13 '22 at 19:38
I would double-check that. There's no good reason for `b'` to actually be part of the key. Did you try my suggestion of a print statement? — John Gordon, Sep 13 '22 at 19:41
@JohnGordon spot on, key[0] was unexpected value so I played with the truncKey slice values and got my flag!!! thank you!!!!! — Jodi Rehlander, Sep 13 '22 at 19:41

score 1 · Accepted Answer · answered Sep 13 '22 at 22:03

1

key is received as a byte string. The b'' wrapped around it when printed just indicates it is a byte string. It is not part of the string. .encode() turns a Unicode string into a byte string, but you can just mark a string as a byte string by prefixing with b.

Just do:

aliensocket.send(b'GET_KEY')
key = aliensocket.recv(1024)
revKey = truncKey[::-1]
print(truncKey)  # or do truncKey.decode() if you don't want to see b''
print(revKey)
aliensocket.send(revKey)

answered Sep 13 '22 at 22:03

Mark Tolonen

166,664
26
169
251

This has the elegance for which I strive (and often miss, ha), with bonus education on byte strings. Saving for next time. – Jodi Rehlander Sep 14 '22 at 01:43

score -1 · Answer 2 · answered Sep 13 '22 at 19:57

-1

data  = ''
while True:
   chunk = aliensocket.recv(1)
   data +=chunk
   if not chunk:
        rev = data[::-1]
        aliensocket.sendall(rev)
        break

answered Sep 13 '22 at 19:57

cacoch

87
2
4

Trying to send string variable via Python socket

2 Answers2