std/crypto — cryptographic hash primitives (leaves over Zig's std.crypto). The primitives are deliberately thin: they do the cryptographic work (which is not expressible in Koru and has no business being), while the algorithm that USES them — e.g. AoC day04's unbounded search for the lowest n — stays in Koru as a label fold. md5-check is the per-candidate leaf; the search is the program.

// Does md5(key ++ decimal(n)) begin with `zeros` hexadecimal-zero nibbles?
//   found n   — yes: n is the answer.
//   miss (n+1) — no: the caller's search should try the next candidate.
// The +1 lives here so the Koru label fold just threads the returned value.
~pub tor md5-check { key: string, n: i64, zeros: i64 }
| found i64
| miss i64