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The REC's would be tokenized on a public blockchain ledger, while the utility bills would be placed on a permissioned Hyperledger Fabric utility emissions data channel. The REC's would follow standards such as those being developed by the Interwork Alliance.
The application would thus need to be able to interact with both public and permissioned ledgers. This would be done by holding keys to both ledgers in a common wallet, such as a Hardware Security Module (HSM) or Key Management Service (KMS).
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