New Method Validates COBOL Migration by Preserving the Bugs

A paper proposing the “Locksmith Loop” attacks the real blocker in mainframe modernisation: legacy systems have no test suite, no specification and production data that cannot be copied into a test environment. The method synthesises test inputs iteratively while executing both the COBOL source and the migrated Java on commodity hardware, then compares behaviour. Its validation target is equivalence with the legacy implementation including that implementation’s bugs — which is correct, because after thirty years the quirks have been absorbed by every downstream consumer. Adjacent work reports 93% conversion accuracy against 75% manual, but separately notes complete functional correctness could not be verified in practice. Generation is handled; verification is the bottleneck.