Validate a US ABA routing number before initiating a transfer: the 3-7-1 checksum, the public structure rules (9 digits; assigned prefixes 00-12 primary, 21-32 thrift, 61-72 electronic, 80 traveler's cheque) and the implied Federal Reserve district. Checksum and structure only — no bank-name directory (the Fed's is redistribution-restricted) and no proof of assignment. ?attest=1 → Ed25519-signed.

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