zeno

Architecture

Zeno operates on a Proof-of-Work (PoW) model to verify human presence without intrusive challenges.

Workflow

1. Challenge Request:
   • Client requests a challenge from /api/challenge.
   • Server generates a random seed + Discriminant (2048-bit Class Group).
   • Server stores the challenge hash in R2 (zeno-challenges).
2. Solving (Client-Side):
   • Client receives the seed, discriminant, and difficulty parameters.
   • Phase 1 (Memory): Finds a 42-cycle in a Cuckatoo graph ($2^{18}$ nodes by default, ~32MB RAM).
   • Phase 2 (Time): Computes a VDF (Verifiable Delay Function) in the Class Group. This enforces a sequential time delay (~0.6s at VDF=100).
   • Results are sent to the server.
3. Redemption:
   • Client submits the solution to /api/redeem.
   • Server verifies the graph cycle and VDF proof ($O(1)$ fast).
   • Server issues a stateful Token (UUID) and stores it in R2 (zeno-tokens).
   • Challenge is deleted immediately to prevent replay.
4. Verification:
   • Your backend sends the token to /api/verify.
   • Payload:

     POST /api/verify
     {
       "site_key": "YOUR_SITE_KEY",
       "token": "TOKEN_FROM_CLIENT",
       "single": true
     }
     

     • single: true: Burn token (Strict Single-Use).
     • single: false: Keep token (Multi-use if token_reuse=true).
   • Zeno Server checks R2 for the token’s existence and expiry.
5. Deletion (Optional):
   • Your backend can call /api/delete to revoke a token early (e.g. user logout).

Components

• Rust Core (WASM): Handles the Cuckatoo cycle finding and VDF (Class Groups) computation.
• Worker: Cloudflare Worker managing the API, CORS, and R2.
• Storage: Cloudflare R2 for transient state (Challenges, Tokens, Bans).

Default Parameters

Parameter	Value	Effect
graph_bits	18	~32 MB RAM, ~0.9s cycle time
vdf	100	~0.6s sequential delay
Total	—	~0.9s solve, ~32MB RAM

Security Model

Memory Wall (Primary)

• Default: 32MB per solve instance
• Stops GPU farms: 24GB VRAM = only 750 parallel instances
• Stops ASICs: Cannot add RAM to custom silicon
• Stops bot farms: 1000 threads = 32GB RAM required

Time Wall (Secondary)

• VDF is inherently sequential
• Cannot be parallelized regardless of hardware
• Prevents pre-computation attacks

Verification Asymmetry

| Operation | Time | RAM | | :— | :— | :— | | Solve | ~0.9s | ~32MB | | Verify | ~1ms | ~1KB |

This 1000× asymmetry makes DoS attacks impractical.

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