On 22/10/13 13:09, Maxim Dounin wrote:
> Hello!
>
> On Mon, Oct 21, 2013 at 10:40:43PM +0100, Rob Stradling wrote:
<snip>
>> The following approach seems to work:
>>
>> #if OPENSSL_VERSION_NUMBER >= 0x10002000L
>> // OpenSSL 1.0.2 lets us do this properly
>> Call SSL_CTX_add1_chain_cert(ssl->ctx, x509)
>> #else
>> If (number of ssl_certificate directives > 1)
>> // Put this intermediate in the "trusted certificates store"
>> Call X509_STORE_add_cert(ssl->ctx->cert_store, x509)
>> Else
>> // This is what Nginx does currently
>> Call SSL_CTX_add_extra_chain_cert(ssl->ctx, x509)
>> End If
>> #endif
>
> An unwanted side effect would be that this will allow client
> certificate authentication to use certs from a server's
> certificate chain. Probably not something we want to happen.

Yes, that's a potentially unwanted side effect. But unfortunately,
AFAICT, putting the intermediates into the "trusted certificates store"
is the only way to implement this feature with OpenSSL <1.0.2.

Could you live with this side effect if the user had to explicitly
enable it? Like this...

#if OPENSSL_VERSION_NUMBER >= 0x10002000L
// OpenSSL 1.0.2 lets us do this properly
Call SSL_CTX_add1_chain_cert(ssl->ctx, x509)
#else
If (number of ssl_certificate directives > 1)
// Do nothing here.
// The user is expected to put the required intermediates
// into the ssl_trusted_certificate file. In doing so, they'll
// also be trusting those intermediates for verifying client
// certs - if that's unacceptable to them, then they'll have to
// stick to just 1 ssl_certificate directive or else upgrade to
// OpenSSL 1.0.2.
Else
// This is what Nginx does currently
Call SSL_CTX_add_extra_chain_cert(ssl->ctx, x509)
End If
#endif

>> (A side effect is that I'm seeing "OCSP_basic_verify:signer
>> certificate not found" from the stapling code in both cases where I
>> don't call SSL_CTX_add_extra_chain_cert() - another thing to look
>> into!)
>
> OCSP Stapling code uses certificate chain as available via
> SSL_CTX_get_extra_chain_certs() to look for issuer cert, see
> ngx_ssl_stapling_issuer().

Yep, I'd found that.

> Though certs from a trusted store should be used too.

ngx_event_openssl_stapling.c provides OCSP_basic_verify() with 3 places
to look for certs: inside the OCSP Response, the extra_chain_certs and
the "trusted certificate store".

OCSP_basic_verify() calls ocsp_find_signer() to locate the certificate
that signed the OCSP Response, but this function only looks in the first
2 of those 3 places. (There's a comment "/* Maybe lookup from store if
by subject name */", but no associated code).

This is a problem for OCSP Responses that are signed directly by the CA
certificate (rather than by a delegated OCSP Response Signing
Certificate). It currently works because that CA certificate is almost
certainly present in extra_chain_certs. But, to support RSA+DSA+ECC
certs signed by different intermediates, we already established that we
can't use extra_chain_certs.

To workaround this, I think the only option would be to pass to
OCSP_basic_verify() a different STACK_OF(X509) that includes all of the
extra_chain_certs plus whatever other CA certificates that Nginx can lay
its hands on!

--
Rob Stradling
Senior Research & Development Scientist
COMODO - Creating Trust Online

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