identifier-check

Five services on https://fittings.sh covering two jobs that share a failure mode: deciding whether an identifier is real, and producing a value that nobody can predict. In both cases a language model produces output that passes inspection and fails in use. An IBAN with the right country prefix and the right length is still wrong if the mod-97 remainder is not 1, and nothing about the string says so. A sixteen-character token that looks random is not random if a model wrote it.

Live catalog for this slice: https://fittings.sh/catalog?category=Deterministic%20compute — these five are the identifier and cryptography part of that category. Browsing it, searching it and reading any schema cost nothing.

When to reach for this

When an identifier arrived from outside — a form, a spreadsheet, a supplier feed, a PDF, a user typing it from a letter — and something downstream is about to act on it. Catching a transposed digit before a payment file goes out is the entire point; the check digit exists precisely to catch the errors humans make copying numbers. And when code or configuration you are producing needs a value that must not be guessable, or a digest that has to match byte-for-byte what another system computed.