[openssl-dev] OpenSSL and certain PEM formats

[Sean Leonard]

On 12/19/2014 5:47 AM, Daniel Kahn Gillmor wrote:
> On 12/18/2014 04:42 AM, Kurt Roeckx wrote:
>> On Wed, Dec 17, 2014 at 08:34:52PM +0100, Erwann Abalea wrote:
>>> Le 17/12/2014 20:17, Viktor Dukhovni a écrit :
>>>> On Wed, Dec 17, 2014 at 10:56:34AM -0800, Sean Leonard wrote:
>>>>
>>>>> For reference for the group (in case you didn't take a look at the draft),
>>>>> the draft documents the following labels:
>>>>>
>>>>> CERTIFICATE
>>>>> ...
>>>> Perhaps also "TRUSTED CERTIFICATE"?
>>>>
>>>>      crypto/pem/pem.h:#define PEM_STRING_X509_TRUSTED        "TRUSTED CERTIFICATE"
>>> It's specific to OpenSSL.
>> And it would be useful if it wasn't.
> It might be useful, but getting the semantics right of what "TRUSTED
> CERTIFICATE" actually means is a non-trivial task.  I'm not convinced
> that OpenSSL's interpretation of it is particularly useful in many
> common contexts.
>
> Does OpenSSL have documented someplace exactly what it means to have a
> "TRUSTED CERTIFICATE"?
>
> For example, say we're talking about a certificate that i am willing to
> accept for the peer foo.example.  If i mark it TRUSTED and it has
> another SubjectAltName of bar.example, will OpenSSL subsequently accept
> it for bar.example as well?

I actually wrote something up in an e-mail to the IESG a few days 
ago.........


The headers that OpenSSL recognizes in various portions include those 
found in:
/crypto/pem/pem.h

[SNIP]


There is also a "TRUSTED CERTIFICATE" label that OpenSSL uses...I 
believe this is a vendor-specific extension but now that I am spelunking 
through the source code I see that it could be abused. Relevant source 
code/comments say:
https://www.openssl.org/docs/apps/x509.html

A*trusted certificate*is an ordinary certificate which has several 
additional pieces of information attached to it such as the permitted 
and prohibited uses of the certificate and an "alias".

Normally when a certificate is being verified at least one certificate 
must be "trusted". By default a trusted certificate must be stored 
locally and must be a root CA: any certificate chain ending in this CA 
is then usable for any purpose.

Trust settings currently are only used with a root CA. They allow a 
finer control over the purposes the root CA can be used for. For example 
a CA may be trusted for SSL client but not SSL server use.

See the description of the*verify*utility for more information on the 
meaning of trust settings.

Future versions of OpenSSL will recognize trust settings on any 
certificate: not just root CAs.



   *) Extensive changes to support certificate auxiliary information.
      This involves the use of X509_CERT_AUX structure and X509_AUX
      functions. An X509_AUX function such as PEM_read_X509_AUX()
      can still read in a certificate file in the usual way but it
      will also read in any additional "auxiliary information". By
      doing things this way a fair degree of compatibility can be
      retained: existing certificates can have this information added
      using the new 'x509' options.

      Current auxiliary information includes an "alias" and some trust
      settings. The trust settings will ultimately be used in enhanced
      certificate chain verification routines: currently a certificate
      can only be trusted if it is self signed and then it is trusted
      for all purposes.
      [Steve Henson]

/* X509_CERT_AUX routines. These are used to encode additional
  * user modifiable data about a certificate. This data is
  * appended to the X509 encoding when the *_X509_AUX routines
  * are used. This means that the "traditional" X509 routines
  * will simply ignore the extra data.
  */

ASN1_SEQUENCE(X509_CERT_AUX) = {
     ASN1_SEQUENCE_OF_OPT(X509_CERT_AUX, trust, ASN1_OBJECT),
     ASN1_IMP_SEQUENCE_OF_OPT(X509_CERT_AUX, reject, ASN1_OBJECT, 0),
     ASN1_OPT(X509_CERT_AUX, alias, ASN1_UTF8STRING),
     ASN1_OPT(X509_CERT_AUX, keyid, ASN1_OCTET_STRING),
     ASN1_IMP_SEQUENCE_OF_OPT(X509_CERT_AUX, other, X509_ALGOR, 1)
} ASN1_SEQUENCE_END(X509_CERT_AUX)