Salted, Slow, and Recomputable

UserStore stores SHA256(password) directly — fast, unsalted, reversible by lookup tables. Replace it with the standard shape using the provided KdfShape primitive (PBKDF2-SHA256, 32 bytes): ``csharp public static class Solution { public static string HashPassword(string password, byte[] salt); // hex, 100_000 iterations public static bool Verify(string password, byte[] salt, string storedHex); public static void RequireSalt(byte[] salt); // throws ArgumentException when salt is null or shorter than 16 bytes } ` Rules: - HashPassword calls RequireSalt first, then derives via KdfShape.Derive(password, salt, 100_000) and returns the lowercase hex — the test computes the same reference and compares exactly, - Verify treats a null or too-short salt as a failed verification (false) — it must never throw for bad inputs; otherwise it recomputes with the stored salt and returns whether the hex matches, - wrong password → false`, never an exception.