[NSE] HUGE ssl-enum-ciphers speed improvement

[Daniel Miller <bonsaiviking () gmail com>]

List,

I've found a way to vastly improve the speed and efficiency of the 
ssl-enum-ciphers by letting the server choose ciphers instead of trying 
every single one. First, the numbers, based on a scan of nmap.org, port 
443. Before: 858 Client Hello messages, 9.56s NSE time. After: 24 Client 
Hello messages, 3.07s NSE time.

Previously, the script took this algorithm: For each of the 4 protocol 
versions (SSLv3, TLSv1.0, TLSv1.1, TLSv1.2), start 213 threads, one for 
each Cipher Suite. When all threads for one protocol have finished, 
process them. If the protocol was not supported, discard results. If at 
least one Cipher Suite was supported, do the same thing for the 3 
compressors. Besides the primary improvement, this means 212 extra 
connections per unsupported protocol (since support for the protocol 
should be detectable in one connection).

The SSL protocol allows clients to specify up to 2^16-1 cipher suites in 
one Client Hello, and the server must choose one that it supports. I 
chose to reimplement the script to remove the one suite chosen by the 
server and retry. If the server supports 7 cipher suites, then the 8th 
Client Hello will be rejected, and the script can stop trying. To retain 
some of the multithreading benefits, I chose to implement each protocol 
in a separate thread, retaining a linear execution within each protocol. 
This lets me evaluate after each Hello whether to keep trying or to finish.

One oddity popped up with this method: https://nmap.org/ appears to 
support the DEFLATE compressor, but the existing ssl-enum-ciphers 
doesn't detect it. Wireshark clearly shows the server responding with 
the DEFLATE method chosen. A possible explanation is this line from RFC 
6101 (SSLv3): "All implementations must support CompressionMethod.null." 
If a Client Hello is sent with only DEFLATE or LZS, then it is not 
compliant, and is rejected (speculation!).

New script and patch attached. Testing appreciated!

Dan

Attachment: ssl-enum-ciphers.patch
Description:

Attachment: ssl-enum-ciphers.nse
Description:

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