  Hyperledger Fabric is a prominent and flexible solution for building
permissioned distributed ledger platforms. Access control and identity
management relies on a Membership Service Provider (MSP) whose cryptographic
interface only handles standard PKI methods for authentication: RSA and ECDSA
classical signatures. Also, MSP-issued credentials may use only one signature
scheme, tying the credential-related functions to classical single-signature
primitives. RSA and ECDSA are vulnerable to quantum attacks, with an ongoing
post-quantum standardization process to identify quantum-safe drop-in
replacements. In this paper, we propose a redesign of Fabric's
credential-management procedures and related specifications in order to
incorporate hybrid digital signatures, protecting against both classical and
quantum attacks using one classical and one quantum-safe signature. We create
PQFabric, an implementation of Fabric with hybrid signatures that integrates
with the Open Quantum Safe (OQS) library. Our implementation offers complete
crypto-agility, with the ability to perform live migration to a hybrid
quantum-safe blockchain and select any existing OQS signature algorithm for
each node. We perform comparative benchmarks of PQFabric with each of the NIST
candidates and alternates, revealing that long public keys and signatures lead
to an increase in hashing time that is sometimes comparable to the time spent
signing or verifying messages itself. This is a new and potentially significant
issue in the migration of blockchains to post-quantum signatures.
