Re: key usage - key encipherment or data encipherment

[wpolk@xxxxxxxx]

Simon,

Your interpretation matches the PKIX interpretation.  Since the RSA key is 
used to encrypt the AES key, it is key encipherment not data encipherment.  
The AES key enciphers the data.

This is a recurring problem with applications.  In general, policies used by 
most PKIs (including the U.S. government) forbid setting the data encipherment 
bit.  There are a number of security problems taht can arise if an application 
actuially uses the RSA key to encipher the datat directly.  It is almost 
always a better choice to encrypt data under a good symmetric algorithm and 
encipher the key using the RSA algorithm.  Unfortunatley, a number of 
applications have been implemnnted using your vendor's interpretations.

This presents you with an uncomfortable choice - set data encipherment to 
enable your apps, but open a security vulnerability, or make the security 
purist choice and break your app.  If your vendor is cooperative, that will 
make your choice easier.

Tim Polk


Quoting Simon McMahon <Simon_McMahon@xxxxxxxxxxxxxxxxx>:

> 
> Hi,
>  
> I have had a recent interoperability issue with a application vendor that
> didn't like the key-usage attributes in a cert from a CA vendor's
> certificate. Signing certs work fine, it was an encryption cert that failed.
>  
> CA sets key-usage = "key encipherment".
> Application wants to encrypt some XML data so looks for key-usage = "data
> encipherment". Reason - because XML is data, not a key.
> 
> I believe the application vendor is wrong and I explained that the RSA key
> actually encrypts an AES key so it doesn't directly encrypt the data but they
> want an official "pkix" ruling based on the standard so can someone please
> refer me to a statement in the standard that clears this up.
>  
> Thanks,
>  
> Simon McMahon.
>  
>  
> 
> Simon McMahon
>  
> Work: (07) 31311420
> Mobile: (043) 2294180
> 
> 
> 
> Simon McMahon
> 
> Work: (07) 31311420
> Mobile: (043) 2294180
> 
> 
> 
> 
> 
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