A note on parallel executions of restrictive blind issuing protocols for secret-key certificates

The receiver in a restrictive blind certificate issuing protocol can blind the issued public key and the certificate but not a certain blinding-invariant predicate of the secret key. Recently a generally applicable technique was described for designing restrictive blind issuing protocols for a certain type of secret-key certificates, and it was shown that the resulting issuing protocols should not be run in parallel since that would enable an attack in which completely blinded triples can be retrieved. To allow the signer in highly demanding applications to run the restrictive blind certificate issuing protocol in parallel mode, a simple technique for modifying the issuing protocols was proposed. In this note it is shown that the proposed modification technique does not overcome the parallel attack problem.