  Permissioned Blockchain has become quite popular with enterprises forming
consortium since it prioritizes trust over privacy. One of the popular
platforms for distributed ledger solution, Hyperledger Fabric, requires a
transaction to be endorsed or approved by a group of special members known as
endorsers before undergoing validation. To endorse a transaction, an endorser
mentions its identity along with the signature so that it can be verified
later. However, for certain transactions, difference in opinion may exist among
endorsers. Disclosing the identity of an endorser may lead to conflict within
the consortium. In such cases, an endorsement policy which not only allows an
endorser to support a transaction discreetly, but at the same time takes into
account the decision of the majority is preferred. Thus we propose an Anonymous
Endorsement System which uses a threshold endorsement policy in order to
address the issue. All these factors motivated us to design a new ring
signature scheme, called Fabric' Constant-Sized Linkable Ring Signature
(FCsLRS) with Transaction-Oriented linkability for hiding identity of the
endorsers. We have implemented the signature scheme in Golang and analyzed its
security and performance by varying the RSA (Rivest-Shamir-Adleman) modulus
size. Feasibility of implementation is supported by experimental analysis.
Signature and tag generation time is quite fast and remains constant
irrespective of change in message length or endorsement set size for a given
RSA modulus value, assuming all the endorsers generates their signature in
parallel. Lastly, we also discuss the integration of the scheme on v1.2
Hyperledger Fabric.
