Proprietary Certificates ( Extended Abstract )

Certificates play an essential role in public-key cryptography, and are likely to become a cornerstone of commerce-related applications. Traditional certificates, however, are not secure against certificate lending, i.e., a situation in which a certificate holder voluntarily shares with others the rights bestowed upon him through a certificate. This type of abuse is a concern in several types of applications, such as those related to digital rights management. In this paper, we introduce the notion of proprietary and collateral certificates. We present a scheme whereby one certificate, known as a proprietary certificate, may be linked to another, known as a collateral certificate. If the owner of the proprietary certificate shares the associated private key, then the private key of the collateral certificate is simultaneously divulged. Certificates in our scheme can be integrated easily into standard PKI models and work with both RSA and discrete-log-based keys (such as those for DSS). Our scheme leaks no significant information about private keys, and leaks only a small amount of information about certificate ownership. Thus, use of proprietary certificates still allows users to maintain multiple, unlinkable pseudonyms, and adds functionality without posing any threats to user privacy.