openssh unix dev - Feb 2016 - Let PAM know about accepted pubkey?

Hi,

first of: my familiarity with OpenSSH/Pam code-base is very limited.. 
Please excuse me if some of this does not make any sense or seems stupid!

I'm investigating if it is possible for a PAM module to find out which 
public key was accepted (when 'AuthenticationMethods 
publickey,keyboard-interactive' is used). From my digging in the source, 
it seems it is currently not.

Would it be possible to provide this information? Maybe using 
do_pam_putenv()? Would there be any security implications of doing this?

The reason I'm asking is that I'm looking into using 2FA (i.e. pubkey + 
pam_google_authenticator) for *some* of my SSH keys. For example, a SSH 
privkey on my trusted box can login without verification code, but 
another SSH privkey on my not-as-trusted box requires a verification 
code to login.

My initial thought was to implemented it through a pubkey whitelist in 
the specific PAM module, but other solutions might also be possible. 
Maybe a PAM module configured as "sufficient", which looks in a pubkey
whitelist and accepts immediately, skipping any further PAM modules 
would work.


Thanks for any feedback!

Regards
Johan (who is not on the list and appreciates a direct CC!)

On Thu, 18 Feb 2016, Johan Str?m wrote:
> Hi,
> 
> first of: my familiarity with OpenSSH/Pam code-base is very limited..
Please
> excuse me if some of this does not make any sense or seems stupid!
> 
> I'm investigating if it is possible for a PAM module to find out which
public
> key was accepted (when 'AuthenticationMethods
publickey,keyboard-interactive'
> is used). From my digging in the source, it seems it is currently not.
> 
> Would it be possible to provide this information? Maybe using
do_pam_putenv()?
> Would there be any security implications of doing this?
There is an open bug about this. I do hope to make the public keys used during
authentication available in the environment (and to PAM), but it won't
happen
until 7.3 at the earliest.

-d

On 19/02/16 00:27, Damien Miller wrote:
> On Thu, 18 Feb 2016, Johan Str?m wrote:
>
>> Hi,
>>
>> first of: my familiarity with OpenSSH/Pam code-base is very limited..
Please
>> excuse me if some of this does not make any sense or seems stupid!
>>
>> I'm investigating if it is possible for a PAM module to find out
which public
>> key was accepted (when 'AuthenticationMethods
publickey,keyboard-interactive'
>> is used). From my digging in the source, it seems it is currently not.
>>
>> Would it be possible to provide this information? Maybe using
do_pam_putenv()?
>> Would there be any security implications of doing this?
> There is an open bug about this. I do hope to make the public keys used
during
> authentication available in the environment (and to PAM), but it won't
happen
> until 7.3 at the earliest.
Ah, great! I also got an off-list message from Damien, the author of
said bug:

https://bugzilla.mindrot.org/show_bug.cgi?id=2408

I've added my +1 on the ticket to express my interest, hoping it
attracts some interest!