Re: [TLS] AIA cert fetching seen as harmful

[Nelson B Bolyard <nelson at bolyard.com>]

Mike wrote, On 2008-04-10 20:20:
>>> This could be made safe with some help from PKIX (if X.509 doesn't
>>> already support it -- I haven't read RFC 3280 or -bis in a while).
>>> If root certificates listed constraints on what constitutes a valid
>>> URL for retrieving issued certificates, then a server could scan
>>> the combined list from each trusted root to determine if it is safe
>>> to fetch a client certificate.
>> Are you all aware of this paper, now making a stir?
>>
>>     https://www.cynops.de/techzone/http_over_x509.html
>>
>> It claims that fetching CA certs from URLs found in AIA extensions in certs
>> that have not yet been validated is a vulnerability.  At least one browser
>> organization known to me agrees.
> 
> What I suggested is that the information about which URL's are safe
> for the client certificate URL extension could be embedded in the
> -root- certificate, which you trust.  When a client sends you a URL
> in place of a certificate, you would compare it to the information
> in each of your root certificates.  If the URL matches one of them,
> you know it's safe to retrieve it; otherwise you best not. 

Please look at a diagram of the US Federal Bridge CA PKI, or the equivalent
bridged PKI in Japan or South Korea, and tell us how your proposal would
work in those environments.

Regards,
/Nelson
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