Wireshark · Wireshark-dev: Re: [Wireshark-dev] Embed SSL keylog file in pcap-ng

[Ben Higgins <ben@xxxxxxxxxxxx>]

On Fri, May 4, 2018 at 1:15 AM, Peter Wu <peter@xxxxxxxxxxxxx> wrote:

Hi Ben,

On Thu, May 03, 2018 at 04:13:33PM -0700, Ben Higgins wrote:
> We're pretty interested in embedding SSL key log information into pcap-ng
> to make it really convenient to open up a single file and get SSL/TLS
> sessions decrypted.
>
> I looked around and found a ticket and some wiki content related to this
> subject:
>
> - "use capture file comment to configure SSL dissector" is at
> https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=9616
> - https://wiki.wireshark.org/Development/PcapNg#Wishlist includes "SSL
> session keys" with a description and a link to the above ticket
> - and there's https://wiki.wireshark.org/DecryptionBlock -- what's
> described here is sounds really cool but in practice might be pretty tricky
> to implement

Looks like you have done your homework, that is pretty much the current
state :-)

> What I'd like to do is instead create a new pcap-ng block type that we can
> put SSL keylog file contents into verbatim. Then we can leverage existing
> code in Wireshark for parsing keylog files. I'd also rather keep this
> scoped to keylog files and not private keys (since private keys are longer
> term secrets and are more sensitive to deal with and everything's heading
> toward PFS anyway).
>
> Any thoughts on this proposal? If folks are open to this approach then we'd
> be interested in writing up a patch.

The TLS key log file is indeed sufficient for decryption. If people
still use RSA key files for some legacy configuration, then Wireshark
can currently already generate a key log file for you (File -> Export
SSL Session keys).

At the moment I am not sure how the pcapng process works, but having a
specification would probably be nice for other interested parties. While
Wireshark supports multiple key log formats, I guess that those from
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format
should be mentioned explicitly (except for RSA). All other formats might
work, but there will be no guarantees on the long term.

That's what I was thinking, rather than creating a new format for including TLS session secrets, we'd make use of the key log format as already specified. (Also, FWIW, I don't think there's necessarily any harm in including the RSA type, which doesn't actually include the private key, if Wireshark otherwise supports it, though it's not going to continue being relevant.)

 

The specification should also answer:
- Where in the pcapng file should the block be located? The information
  must be available before the TLS dissector is invoked.

Great question. I don't know the details of how pcap-ng is processed by Wireshark (e.g. if, say, an index of blocks is created up-front or not), so I'll take a look and reply back next week.

 

- If it can be anywhere, can there be multiple blocks?

I don't think there'd be any harm in having multiple SSL/TLS session key log blocks; the contents of each could be merged into a single table, probably.

 

--
Kind regards,
Peter Wu
https://lekensteyn.nl
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