[Openswan Users] X.509 key usage

[Jacco de Leeuw]

Andreas Steffen wrote:

> 4.1.3.13. ExtendedKeyUsage
> 
>     No ExtendedKeyUsage usages are defined specifically for IPsec, so if
>     this extension is present and marked critical, use of this
>     certificate for IPsec MUST be treated as an error unless the
>     extension contains the anyExtendedKeyUsage keyPurposeID, which
>     asserts that the certificate can be used for any purpose.
>     Implementations MAY ignore this extension if it is marked non-
>     critical. Implementations MUST NOT generate this extension in
>     certificates which are being used for IPsec.

The L2TP/IPsec client included with Windows 2000/XP uses ExtendedKeyUsage
for distinguishing between certificates issued to client and servers,
respectively. The "Client Authentication" EKU has a value of 1.3.6.1.5.5.7.3.2
for instance. If a client certificate does not contain this EKU, an attacker
could use this client certificate to pose as a 'server' to other clients.

> Gregor, in your opinion, what would be the gain in terms of
> increased security by heeding the keyUsage flags? Which would be the
> most important checks you'd like to see implemented in *swan?

Perhaps certificates presented in L2TP/IPsec connections could be checked
for the "Client Authentication" EKU, but it's probably not that important.
If ever a stolen server certificate is presented as a client certificate
you have bigger problems :-)

Jacco
-- 
Jacco de Leeuw                         mailto:jacco2 at dds.nl
Zaandam, The Netherlands           http://www.jacco2.dds.nl