  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.
