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POAES encrypted File transfer over TCP socket; padding issue
 

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  1. POAES encrypted File transfer over TCP socket; padding issue
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    <p>I'm trying to do a file transfer using encrypted TCP socket with AES 256.</p> <ul> <li>If I transfer a file without encryption, it works fine.</li> <li>If I send small commands (such as 'ipconfig') to the client or server, the encryption works fine.</li> <li><p>No matter the file size is, I keep receiving the following error message:</p> <pre><code>DecodeAES = lambda c, e: c.decrypt(base64.b64decode(e)).rstrip(PADDING) File "/usr/lib/python2.6/base64.py", line 76, in b64decode raise TypeError(msg) TypeError: Incorrect padding </code></pre></li> </ul> <p>My function for encoding and decoding is the following (the <code>hat</code> variable is the message):</p> <pre><code>def AESENC(hat,typ): BLOCK_SIZE = 32 PADDING = '{' pad = lambda s: s + (BLOCK_SIZE - len(s) % BLOCK_SIZE) * PADDING EncodeAES = lambda c, s: base64.b64encode(c.encrypt(pad(s))) DecodeAES = lambda c, e: c.decrypt(base64.b64decode(e)).rstrip(PADDING) secret = '01234567890123456789012345678912' IV = 'wir&amp;/&gt;H54mgd9a";' cipher = AES.new(secret,AES.MODE_CFB,IV) if typ == 0: encoded = EncodeAES(cipher, hat) return encoded else: decoded = DecodeAES(cipher, hat) return decoded </code></pre> <p><strong>Client side</strong></p> <pre><code>if os.path.exists(df): print ' found the file ' f = open(df, 'rb') packet = f.read(1024) while packet != '': s.send(AESENC(packet,0)) s.send( AESENC('123XXX',0) ) s.send('123XXX') f.close() </code></pre> <p><strong>Server side</strong></p> <pre><code>f = open('/root/Desktop/Transfer.mp3','wb') while True: bits = AESENC ( conn.recv(1024) , 1 ) while (bits): f.write(bits) bits = AESENC ( conn.recv(1024) , 1 ) if bits.endswith('123XXX'): print '[+] Transfer completed ' break f.close() break return </code></pre> <p>Does anybody know how to fix this?</p>
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    1. COOh hey, more crypto code with vulnerabilities. You know that you should never reuse an initialization vector, right? And you know that encryption does not provide authentication?
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      1. USDietrich Epp
    2. COokay but how we can explain that the only the file transfer is not working ! , i mean when i execute a command in the CMD shell on the server side i can see it's encrypted in wireshark and it's being encrypted and decrypted fine on both side using same function
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      1. USHussam
    3. COHow are you sending & receiving the data? It looks like the base64 decoding step is failing, and in my experience this padding error means that you have an incorrect data length. Perhaps your data is being truncated somehow. Can you compare the data that is sent and received?
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      1. USWhatang
 

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