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ACTION-87: Certificate errors and warnings displayed by Opera

From: Yngve N. Pettersen (Developer Opera Software ASA) <yngve@opera.com>
Date: Fri, 09 Feb 2007 01:13:27 +0100
To: "public-wsc-wg@w3.org" <public-wsc-wg@w3.org>
Message-ID: <op.tngf8pp6qrq7tp@nimisha.oslo.opera.com>

Certificate errors and warnings displayed by Opera.

Opera displays a number of warnings (which the user may continue past)  
about certificate problems, as well as errors (which the user may not  


   - Mismatch between servername (from URL) and the hostname(s) listed in  
the certificate.
   - Unknown certificate authority (without root in the chain)
   - Unknown root certificate (user may install the root)
   - Expired (or not yet valid) certificates

In addition Opera displays a warning with a certificate when the server  
has selected weak encryption methods, protocols or short public keys (less  
than 900 bits RSA/DSA/DH)

In all these cases, if the user choose to continue past the warning, the  
security level of the site is reduced to 1 (weak)

If more than one warning is necessary the most severe is identified by the  
dialog, and the rest are listed in a list in the dialog.

These problems can be resolved for the rest of the session, but unless  
they can be solved by installing certificates in the database (or on the  
server) the warning will be display next time Opera has been reset.

The user can also specify a preference for a certificate in the root store  
that makes Opera warn whenever a certificate is part of a certificate's  
chain. This is the default whenever a certificate is installed by  
downloading (but not when installing from a unknown root dialog).


   Primarily various certificate validation errors

     - unknown algorithms
     - unable to decode data in the certificate

These are usually identfied as "Invalid, or non-verifiable certificate."  
(TLS code 40, Bad Certificate)

A separate case exists when the signature of the certificate cannot be  
verified, it has an invalid purposeflags, or similar problems, this is  
specifically indicated by the error message

"The signatures of this certificate could not be verified. While this can  
be caused by the issuer using the wrong method to sign the certificate, it  
can also be caused by attempts to modify or fake the certificate."

We've seen a couple of cases where "homebrewed" certificates have  
triggered this error because the site's certificate looks like a  
selfsigned certificate (subject and issuer is the same, no Authorty key  
identifier), but the certificate is actually signed by the key of another  
certificate with the same name as used in the site certificate.

The actual full chain looks like this

  Site certificate     {subject, subject, PK1, Sig(PK2, hash(certdata1))}
  Signer certificate   {subject, subject, PK2, Sig(PK2, hash(certdata2))}

In this case Opera consider the site certificate selfsigned (because no  
Authority KeyIdentifier is specified) and cannot verify the certificate  
because the site's certificate did not sign the certificate (itself) for  
the site. AFAIK other browsers consider the certificate to have an unknown  
signer in such cases, and ask the user about the unknown authority.

There is also a fatal error about Revoked certificates,  and unknown  
certificate verification errors are reported as Transmission Error (TLS  
code 49, illegal parameter) because they will usually involve unknown  
parameters andvalues in the certificate.

The user can also, in addition to the certificate warning preference  
mentioned above, specify that all access to sites using a specific  
certificate in the root store is forbidden. This will be indicated by a  
error specifying the certificate is valid but access is forbidden.

Yngve N. Pettersen
Senior Developer                     Email: yngve@opera.com
Opera Software ASA                   http://www.opera.com/
Phone:  +47 24 16 42 60              Fax:    +47 24 16 40 01
Received on Friday, 9 February 2007 00:13:45 UTC

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