dumping access point certificate from wpa_supplicant

[Jouni Malinen]

On Wed, Oct 06, 2021 at 11:31:50PM +0100, Sam Mason wrote:
> I've just been forced to use 802.1X on a wired lan (PEAP with MS-CHAP
> v2) but the network operator isn't providing a certificate for their
> server.  I was hoping that validating the certificate might help
> prevent PITM attacks, but I'm not even sure if that makes sense for
> this protocol.
> Google pointed me to https://superuser.com/a/853602/171763 as a way to
> dump the certificate, and I've searched back through the archives here
> but couldn't find anything more recent.

wpa_supplicant sends out CTRL-EVENT-EAP-PEER-CERT events as control
interface events during EAP authentication that uses TLS. Those message
include a SHA256 hash of the certificate and full hexdump of the raw DER
encoded certificate. The entries with depth=0 are for the server
certificate and the higher depth values are for the CAs provided by the
server.

As an example, you would see something like this in wpa_cli when
connected to the wpa_supplicant control interface:

CTRL-EVENT-EAP-PEER-CERT depth=0 subject='/C=FI/O=w1.fi/CN=server.w1.fi' hash=5891bd91eaf977684e70d4376d1514621d18f09ab2020bea1ad293d59a6e8944
CTRL-EVENT-EAP-PEER-CERT depth=0 subject='/C=FI/O=w1.fi/CN=server.w1.fi' cert=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


If you want to trust only the specific certificate for future
connections, you can use that hash value in the network profile in the
following style:

ca_cert="hash://server/sha256/5891bd91eaf977684e70d4376d1514621d18f09ab2020bea1ad293d59a6e8944"

This is the way that can be used to implement trust-on-first-use type of
an policy which would sound like the use case you are describing here. A
bit more complete mechanism is defined in the Wi-Fi Alliance WPA3
specification (https://www.wi-fi.org/file/wpa3-specification) (search
for "TOD" in that document).

-- 
Jouni Malinen                                            PGP id EFC895FA