Re: (ITS#5361) cert verification failures with GnuTLS and DNS subjectAltName

[hyc@symas.com]

Steve Langasek wrote:
> On Sat, Feb 09, 2008 at 11:04:18PM -0800, Howard Chu wrote:
>> I cannot duplicate this error with GnuTLS 1.7.8 or 1.7.9. The altname
>> length that is returned just includes the non-NUL characters. Note that
>> all of libldap's TLS functionality was tested and working with GnuTLS
>> 1.7. What version are you using?
>
> Reproduced with GnuTLS 2.0.4 and GnuTLS 2.2.1.
>
>> It seems to me that if your version of GnuTLS is indeed behaving this way,
>> then it's a GnuTLS bug, since in C, the length of a string never includes
>> the trailing NUL.
>
> It's true that the /length/ of a string doesn't include the trailing NUL,
> but it does have to be included in the storage /size/ of a C string, and
> it's debatable which is intended here.

Since this is an ASN.1 structure, one would ordinarily not expect any NUL 
termination in the first place. And since other GnuTLS library functions are 
returning the raw data size, excluding any trailing NUL, the behavior you're 
seeing here is pretty suspicious.

> Given that one of the errors
> returned by gnutls_x509_crt_get_subject_alt_name() is
> GNUTLS_E_SHORT_MEMORY_BUFFER, it seems obvious to me that this should use
> semantics for storage size rather than string length, and the only question
> in my mind is whether the trailing NUL is included as part of the internal
> representation of the string.
>
> If this is a behavior change as you say, then I guess we need clarification
> from GnuTLS upstream about whether this is intentional.

That sounds like the best step for now. Just to be sure, how was the 
certificate created? Have you verified that libldap with OpenSSL accepts the 
certificate correctly? So far it sounds just as likely to me that your 
subjectAltName actually includes a trailing NUL in its data. ASN.1 structures 
don't use NUL-terminated strings here, the DER form requires definite lengths 
to be encoded up front.
-- 
   -- Howard Chu
   Chief Architect, Symas Corp.  http://www.symas.com
   Director, Highland Sun        http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP     http://www.openldap.org/project/