Subject: [Cfrg] Using the same public/private key pair for encryption and signatures

From: "Narayanan, Vidya" <[email-masked]>
Date: Tue, 13 Mar 2007 12:42:07 -0700

Hi Tuomas, all,
RFC3972 provides the following guidance: 

"Moreover, because of the
   possibility of related-protocol attacks, the public key MUST be used
   only for signing, and it MUST NOT be used for encryption. "

The related protocol attacks described in the section (7.4) seem to be
addressed by the use of a "type tag" in any protocol that uses a CGA
signature. Beyond that though, there seems to be no explanation of the
downside of using the public key for encryption. Also, the guidance of
public key used for signing is a bit confusing - isn't it the private
key that would be used for signing? It appears that the concern is
around the use of a given private key for signing by one entity and the
corresponding public key for encryption by some other entity. 

If someone can explain the issues with doing the above, I'd appreciate
it. Also, are the issues limited to the CGA (RSA) public/private key
pair or do they generally apply to the use of any public/private key
pairs? 

Thanks,
Vidya

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