Show HN: Credential Shield Protocol – Server-blind breach checking for passwords

We're building an open protocol that lets organizations continuously audit credentials against breach databases WITHOUT exposing passwords, full hashes, or query results to any server. The core approach: 1. Hash locally with Argon2 2. Send only first 4 chars of SHA-512 (32-bit prefix) to server 3. Server returns all prefix matches 4. Client verifies locally → server never learns outcome Current state: Protocol spec + threat model + PowerShell client library. We're pre-funding and actively seeking feedback before building the reference implementation. Try it: https://iamaxolotl-04.github.io/csp-2026/ https://iamaxolotl-04.github.io/csp-2026/ Spec: https://github.com/IAmAxolotl-04/csp-2026 https://github.com/IAmAxolotl-04/csp-2026 We'd love feedback on: - Where does this break in real deployments? - What operational constraints are we missing? - Would you use this in your environment? Happy to answer questions in the comments.