fdkey 0.1.1

FDKEY verification primitives for MCP servers and HTTP backends — gate AI-agent access behind LLM-only puzzles.
Documentation

fdkey

FDKEY verification primitives (Rust). Gate AI-agent access to your tools / API behind LLM-only puzzles. Companion to the TypeScript and Python SDKs at https://github.com/fdkey/sdks.

What this crate ships

The Rust MCP server ecosystem is still consolidating across multiple community SDKs (rmcp, mcp-server-rs, tower-mcp, etc.) and there is no single canonical Anthropic-blessed Rust MCP SDK to wrap. This crate intentionally exposes primitives so you can plug FDKEY into whichever MCP server library you use — or your plain HTTP service:

  • Verifier — bundles JwtVerifier + VpsClient. The canonical entry point.
  • JwtVerifier — Ed25519 JWT verify against the cached /.well-known/fdkey.json.
  • VpsClient — POST /v1/challenge and POST /v1/submit to api.fdkey.com.
  • WellKnownClient — HashMap<kid, DecodingKey> cached for 1 hour, refresh on miss.
  • guard::{can_call, mark_verified, consume_policy} — pure per-session policy logic, identical to the TypeScript and Python SDKs.

The wire shape (challenge / submit JSON, JWT claims) matches the other SDKs exactly — the FDKEY VPS doesn't know which language called it.

Install

cargo add fdkey

Get an API key at app.fdkey.com.

Usage — verify a Bearer JWT in any HTTP service

use fdkey::{Verifier, FdkeyConfig, jwt::extract_bearer};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let cfg = FdkeyConfig {
        api_key: std::env::var("FDKEY_API_KEY")?,
        vps_url: Some("https://api.fdkey.com".into()),
        ..Default::default()
    };
    let verifier = Verifier::new(&cfg)?;

    // Inside your HTTP handler, given an Authorization header value:
    let auth = Some("Bearer eyJ...");
    if let Some(token) = extract_bearer(auth) {
        let claims = verifier.verify_token(token).await?;
        // `score` is a 0..1 float (today effectively binary 1.0/0.0;
        //  reserved for graduated capability scoring later).
        println!("score={}, tier={}", claims.score, claims.tier);
    }
    Ok(())
}

Usage — fetch + submit a challenge programmatically

use fdkey::{Verifier, FdkeyConfig};
use fdkey::vps::ChallengeMeta;

# async fn _example() -> Result<(), fdkey::FdkeyError> {
let cfg = FdkeyConfig { api_key: "fdk_...".into(), ..Default::default() };
let verifier = Verifier::new(&cfg)?;

let challenge = verifier.vps.fetch_challenge(&ChallengeMeta::default()).await?;
// hand the puzzles to your agent caller, collect their answers...
let answers = serde_json::json!({ "type1": [], "type3": {} });
let result = verifier.vps.submit_answers(&challenge.challenge_id, &answers).await?;
if result.verified {
    let jwt = result.jwt.expect("jwt on verified");
    let claims = verifier.verify_token(&jwt).await?;
    println!("score={}", claims.score);
}
# Ok(())
# }

Per-session policy gating

The crate exposes the same three policy variants the TS and Python SDKs support — pick the one that fits each tool / route:

use fdkey::Policy;

Policy::EachCall                           // verify on every invocation (irreversible actions)
Policy::OncePerSession                     // verify once per connection (signup-style flows)
Policy::EveryMinutes { minutes: 15 }       // verification valid for N minutes after solve;
                                           // does NOT extend on subsequent calls

Plug the per-session state machine the TS / Python SDKs use into your MCP server's tool-call dispatch:

use fdkey::{guard::*, Policy, SessionState};

let mut session = SessionState::default();
let policy = Policy::EachCall;

// On every tool call:
if can_call(&policy, "sensitive", &session) {
    // run the tool
    consume_policy(&policy, &mut session);
} else {
    // return fdkey_verification_required
}

// When the FDKEY submit step succeeds:
mark_verified(&mut session);

Configuration reference

FdkeyConfig {
    api_key: "fdk_...".into(),         // required
    vps_url: None,                      // defaults to https://api.fdkey.com
    difficulty: Difficulty::Medium,     // Easy | Medium | Hard
    on_fail: FailMode::Block,           // Block | Allow (puzzle failed)
    on_vps_error: FailMode::Allow,      // Block | Allow — see below
    protect: HashMap::new(),            // tool name -> Policy (your dispatch reads this)
    tags: None,                         // forwarded to FDKEY for analytics
}

Failure-mode defaults

on_vps_error: FailMode::Allow is the default — if the FDKEY scoring service is unreachable, your dispatch should fall through to the unprotected handler rather than rejecting traffic. We chose this so an FDKEY outage doesn't brick integrator workflows (e.g. if we shut down the service or api.fdkey.com is unreachable). FDKEY is verification, not gating — your service should still serve traffic when ours is down. Set on_vps_error: FailMode::Block if you'd rather reject unverified callers during an outage. The crate exposes this as a config field; the actual fail-open behavior is up to your dispatch implementation (see "Per-session policy gating" above).

Security notes — integrator obligations

Because this crate ships primitives rather than a single framework wrapper, three protections that the TypeScript and Python sibling SDKs enforce in code are YOUR responsibility in Rust. Skipping any of them leaves your service open to abuse the wire format already protects against.

1. NEVER return the JWT to the agent

The example in "Usage — fetch + submit a challenge programmatically" above does:

let jwt = result.jwt.expect("jwt on verified");
let claims = verifier.verify_token(&jwt).await?;

After that line, the JWT is a server-side verification artifact — discard it. Persist { verified_at, score: claims.score, tier: claims.tier } in your session store; surface only that to the agent. If you echo the JWT back in your HTTP response or in a tool result, you've handed the agent a bearer token it can replay against any other FDKEY-protected service within the JWT's lifetime (~5 min default).

The TypeScript reference at @fdkey/http implements the same flow correctly — its session-mediated design is the canonical pattern. Mirror it.

2. Use UUIDs (or other non-reusable identifiers) for session keys

Don't key your session store on raw pointer addresses, Box::into_raw casts, or any other identity that can be reused after a session is dropped. CPython has the same problem (see the Python SDK's _SessionKeyTracker for the parallel mitigation); in Rust the risk is smaller (the borrow checker prevents most aliasing) but pointer reuse is still a real foot-gun for raw-pointer-based session maps.

Generate a fresh UUID per session (uuid::Uuid::new_v4()) and store the mapping in a structure that guarantees no two live sessions can share the same key.

3. Bound your session store with TTL + LRU eviction

A naive HashMap<SessionId, SessionState> grows forever as agents connect and drift away. On a long-lived multi-tenant server this is a memory leak you'll only notice in production.

The TypeScript sibling ships InMemorySessionStore at mcp-integration/sdks/http/src/session-store.ts with a 1-hour idle TTL and a 10 000-entry hard cap. Port the pattern to Rust — sweep on access, drop the oldest LRU entry on insert when at the cap, no background timer needed. ~40 lines.

4. JWT aud is not validated by the SDK

The audience claim binds the JWT to the integrator's vps_users.id, which the SDK doesn't know at verify time. The VPS already binds aud to the API key that requested the challenge — defense in depth — but in principle, a JWT issued for one FDKEY-protected service could be replayed against a different one within the JWT lifetime (~5 min default). Keep the JWT lifetime short on the VPS side if your threat model includes cross-integrator replay.

Links

License

MIT — see LICENSE.