Hi SSL gurus,
I'm running into a problem with the OpenSSL 0.9.5a
library which we're using in one of our products. When
it tries to verify a particular chain of certificates,
it seems to return the X509_v_ERR_INVALID_CA error for
what appears to me to be no good reason at all...
The chain in question consists of three certs, and
the verification is failing on the second one. I stepped
through the code to find out why, and I found the
following code in v3_purp.c:
static int check_purpose_ssl_client(X509_PURPOSE *xp, X509 *x, int ca)
{
if(xku_reject(x,XKU_SSL_CLIENT)) return 0;
if(ca) {
int ca_ret;
....
and shortly after:
static int check_purpose_ssl_server(X509_PURPOSE *xp, X509 *x, int ca)
{
if(xku_reject(x,XKU_SSL_SERVER|XKU_SGC)) return 0;
/* Otherwise same as SSL client for a CA */
if(ca) return check_purpose_ssl_client(xp, x, 1);
.....
When checking this certificate, it calls 'check_purpose_ssl_server',
and since 'ca' is non-zero (the certificate is used to sign another
certificate) this function hands off to 'check_purpose_ssl_client',
which then rejects the certificate because the XKU_SSL_CLIENT flag
is not set in the X509 structure. This flag only appears to be set
in the case that there's a 'NID_client_auth' ASN1 object in the
certificate's Extended Key Usage field.
So, is OpenSSL doing the wrong thing? Or is it my understanding
that's inaccurate?
I notice that this code has been changed in OpenSSL 0.9.6 - there is
a new 'check_ssl_ca' function which is called by both
'check_purpose_ssl_client' and 'check_purpose_ssl_server', and now the
code path from 'check_purpose_ssl_server' doesn't test for the
XKU_SSL_CLIENT flag.
So, what should we do?
- Should we upgrade to 0.9.6? This would be quite a bit of work
since we've made a few changes to get it to work in our
application, but would obviously be a good idea in the long
term.
- As a temporary measure, can we extract the new check_purpose_*
stuff from 0.9.6 and put it into our existing version? I tried
this fix and it solved the problem, but I'd rather not put it
into production code without some sort of reassurance that I
haven't missed any subtleties.
- Or should I give up & hand the whole thing over to someone who
actually knows what he's doing?
Thanks for your time,
Mike
--
Michael Playle, Software Engineer, ANT Ltd.
phone +44 1223 716418 - fax +44 1223 716401
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