aba

Overview ¶

Package aba decodes and validates an ABA routing transit number (RTN) — the 9-digit code that identifies a US bank or credit union in ACH and wire transfers. It is the US-domestic counterpart to internal/iban (the international account): a routing number turns up in ACH-fraud / business-email-compromise lures, leaked direct-deposit forms, and check images, so decoding one off a paste tells an operator whether the number is internally consistent and which Federal Reserve district issued it.

Wrap-vs-native judgement ¶

Native. An RTN is a fixed digit-field layout — a 4-digit Federal
Reserve routing symbol, a 4-digit ABA institution identifier, and 1
check digit — guarded by a weighted modulus-10 checksum. Validating
it is integer arithmetic plus a small fixed range classification; no
runtime dependency or hand-transcribed lookup beyond the 12 statutory
Federal Reserve districts is justified. stdlib only, no new go.mod
dep.

Verifiable / no confidently-wrong output ¶

The modulus-10 checksum is the verification anchor — it is recomputed
and compared, and a mismatch is reported as checksum_valid=false with
a note (a mistyped RTN), never asserted as a definitively fake bank.
The Federal Reserve district and the institution type are derived
from the leading two digits by the published numbering ranges (not a
guess); a prefix outside those ranges leaves the district undetermined
rather than inventing one. The institution identifier is surfaced raw
— it maps to a specific bank only via the proprietary American Bankers
Association registry, which is not embedded.