Subject: Certificates and Blind Signatures

From: David Lefranc <[email-masked]>
Date: Fri, 18 Nov 2005 17:24:37 +0100

Hi,

I would like to use a blind RSA signature algorithm, and I am currently 
analyzing the best way to use X509 certificates : I need to precise that 
the public key will be used with such an algorithm.

The easiest solution would be to create a new algorithmIdentifier 
identifying the Blind RSA signature algorithm so that I could use it in 
the field subjectPublicKeyInfo.
However, since blind RSA signature and classical signature algorithms 
are quite similar, I think a better solution would be the following :

1. Keep the algorithmIdentifier for the classical RSA signature 
algorithm in the field subjectPublicKeyInfo.
2. Do not use the Key Usage extension.
3. Use the Extended Key Usage extension, set it as "non critical", and 
create a new KeyPurposeId for "blind signature", (and also eventually 
precise DigitalSignature and NonRepudiation).

With this second solution, the system for blind RSA signatures that I 
develop SHOULD require such certificates, and the certificates could 
also be used in a classical way outside my system to perform classical  
RSA signatures.

Does it seem reasonable?

David