What Does a Post-Quantum Migration Cost a Smart-City Ledger? Measured Overheads of ML-DSA Identities on Hyperledger Fabric

Smart-city ledgers must keep alerts and model provenance verifiable for decades, yet Hyperledger Fabric’s identities use the elliptic-curve digital signature algorithm (ECDSA) by default, which quantum computers could forge. On a post-quantum fork of Fabric 3.1.4, we issued every identity ML-DSA-44, -65 or -87 (FIPS 204) or hybrid ECDSA/ML-DSA-44 certificates and ran an alert-anchoring workload ten times at 10–100 clients. A transaction grows from 5.5 kB to 32.8, 44.2 and 59.5 kB; about 72% of it is four PEM-encoded certificates, and 22% is signatures. At 20 clients throughput falls from 170 to 134, 101 and 79 transactions per second. Ten ML-DSA-87 transactions exceed the test network’s 512 KB block-size preference; raising it to 1 MB doubled throughput at 10 clients and cut 99th-percentile latency from 2.08 s to 0.16 s. Rebuilt block files suggest that binary (DER) identities would save about 20% of a post-quantum transaction, and identity references about 73%. Client connections already negotiate hybrid X25519MLKEM768 key exchange, but peers and the orderer still authenticate with ECDSA keys. At 10 alerts per second, ledger growth per peer rises from 1.9 TB to 10.6–19.0 TB per year: the cost lies mainly in how Fabric stores identities.