An Identity Based Strong Bi-Designated Verifier (t, n) Threshold Proxy   Signature Scheme
  Proxy signature schemes have been invented to delegate signing rights. The paper proposes a new concept of Identify Based Strong Bi-Designated Verifier threshold proxy signature (ID-SBDVTPS) schemes. Such scheme enables an original signer to delegate the signature authority to a group of 'n' proxy signers with the condition that 't' or more proxy signers can cooperatively sign messages on behalf of the original signer and the signatures can only be verified by any two designated verifiers and that they cannot convince anyone else of this fact. 
Sunder Lal and Vandani Verma