Blind Oracle

Zero-Knowledge Range Proof Oracle on Stellar Soroban

Blind Oracle: any smart contract can now ask 'is this number in range?' without ever seeing the number. Prove that private data is within a range, without revealing the data itself. Built for Stellar Hacks: Real-World ZK.

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The Problem

Smart contracts need to make decisions based on private data — but publishing raw data on-chain destroys privacy.

"Is BTC above $95,000 right now?" "Is this user's credit score between 600 and 850?" "Does this wallet hold enough collateral?"

The naive answer (publish the number) is unacceptable. Blind Oracle solves this with zero-knowledge proofs.

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What Blind Oracle Does

Private (stays with prover)       Public (on-chain)
────────────────────────          ──────────────────────────
price  = $97,500         ──ZK──▶  range: [$95,000 – $200,000]
nonce  = secret                   in_range: true ✅
                                  commitment: 0x8484...
                                  nullifier:  0x2060...

One Groth16 proof. One Stellar transaction. Market resolves. Price never revealed.

Powered by: Groth16 · BN254 · Circom 2.0 · Soroban · Poseidon

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Live Demo

export ALICE_SECRET="your-stellar-secret-key"

# Interactive range proof demo
bash demo.sh

# Prediction market demo
node --experimental-vm-modules prediction-market-demo.mjs

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Prediction Market Demo Results

All resolved on Stellar testnet — price never revealed:

Market	Question	Price	Range	Result	TX
BTC-001	BTC above $95k?	HIDDEN	$95k–$200k	YES ✅	view
BTC-002	BTC above $95k?	HIDDEN	$95k–$200k	NO ❌	view
ETH-001	ETH between $3.5k–$5k?	HIDDEN	$3.5k–$5k	YES ✅	view

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On-Chain Performance

Metric	Value
CPU Instructions	33,176,348 / 100,000,000 (33.2%)
Min Resource Fee	37,472 stroops = 0.0037 XLM
Proof Size	256 bytes (constant)
Curve	BN254 — 2× cheaper than BLS12-381

BN254 is natively supported by Soroban and runs at roughly half the instruction cost of BLS12-381, making Blind Oracle one of the most gas-efficient ZK verifiers on Stellar.

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Deployed Contract

Network	Contract ID
Stellar Testnet	CBXMLDKAE45OIUEOODGFMKZMFE5SA3CSR7NXW7TILBUQBVHEGNCLDQVH

View on Stellar Expert →

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How It Works

┌─────────────────────────────────────────────────────────────┐
│                     PROVER (Off-Chain)                      │
│                                                             │
│  Private: data_value, nonce                                 │
│  Public:  range_min, range_max  ← set by market maker       │
│                                                             │
│  [Circom Circuit] → [SnarkJS Groth16] → proof.bin (256B)    │
└──────────────────────────────┬──────────────────────────────┘
                               │ submit
                               ▼
┌─────────────────────────────────────────────────────────────┐
│                  STELLAR SOROBAN (On-Chain)                 │
│                                                             │
│  verify(proof_bytes, pub_signals_bytes)                     │
│    → BN254 pairing check (33.2M instructions)               │
│    → return in_range: true / false                          │
│                                                             │
│  Market resolves. Price never stored anywhere.              │
└─────────────────────────────────────────────────────────────┘

Three Guarantees Per Proof

Guarantee	Mechanism
range_min ≤ data_value ≤ range_max	GreaterEqThan + LessEqThan (252-bit)
commitment = Poseidon(data_value, nonce)	Binding — prover cannot change value after commit
nullifier = Poseidon(data_value, commitment)	Replay protection — same data = same nullifier

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Contract Interface

// One-time: store the Groth16 verification key
set_vk(vk_bytes: Bytes) → Result<(), VerifierError>

// Per proof: verify range membership
verify(proof_bytes: Bytes, pub_signals_bytes: Bytes) → Result<bool, VerifierError>
// Returns true  → proof valid AND data is within range
// Returns false → proof valid but data is outside range
// Returns Error → malformed input

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Use Cases

Private Prediction Markets

Price oracles prove a feed value is within a market's target range without ever publishing the exact price. Markets resolve trustlessly with no data leakage.

KYC / Identity

Prove age ≥ 18, credit score in range, or accreditation status — without storing personal data on-chain.

DeFi Collateral

Prove collateral exceeds a threshold for undercollateralized lending without exposing portfolio composition.

Compliance Oracles

Prove transaction values stay within regulatory limits without revealing individual amounts to the protocol.

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ZK Circuit

File: circuits/blind_oracle_v2.circom

Private inputs : data_value, nonce
Public inputs  : range_min, range_max   ← verifier sets these
Public outputs : commitment, in_range, nullifier

Constraints    : 1,806
Curve          : BN254
Proof system   : Groth16

The range bounds are public inputs — not hardcoded. One deployed circuit and one deployed contract serve any range, any asset, any use case.

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All Test Results

Scenario	Data	Range	Result	TX
Credit Score	750	600–850	✅ in_range	view
KYC Age	25	18–65	✅ in_range	view
DeFi Collateral	15,000	10k–50k	✅ in_range	view
Credit Score FAIL	500	600–850	❌ out of range	view
BTC Market YES	HIDDEN	$95k–$200k	✅ YES	view
BTC Market NO	HIDDEN	$95k–$200k	❌ NO	view
ETH Market YES	HIDDEN	$3.5k–$5k	✅ YES	view

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Quick Start

# Install dependencies
npm install

# 1. Create input
cat > circuits/my_input.json << EOF
{
  "data_value": "97500",
  "nonce": "314159",
  "range_min": "95000",
  "range_max": "200000"
}
EOF

# 2. Generate witness
node circuits/blind_oracle_v2_js/generate_witness.js \
  circuits/blind_oracle_v2_js/blind_oracle_v2.wasm \
  circuits/my_input.json \
  circuits/my_witness.wtns

# 3. Generate proof
npx snarkjs groth16 prove \
  circuits/circuit_v2_final.zkey \
  circuits/my_witness.wtns \
  circuits/my_proof.json \
  circuits/my_public.json

# 4. Encode + verify on-chain
node encode-proof-v2.mjs
export ALICE_SECRET="S..."
node --experimental-vm-modules test-all-scenarios.mjs

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Tech Stack

Layer	Technology
ZK Proof System	Groth16 (SnarkJS 0.7)
ZK Circuit	Circom 2.0 + circomlib
Elliptic Curve	BN254 (Barreto-Naehrig)
Hash Function	Poseidon
Smart Contract	Rust → WASM (Soroban SDK v22)
Blockchain	Stellar Soroban Testnet
SDK	@stellar/stellar-sdk v16
Trusted Setup	Powers of Tau (2^16)

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Documentation

File	Description
ARCHITECTURE.md	System design, data flow, binary formats
WHITE_PAPER.md	Cryptographic construction, security analysis
GUIDE.md	Developer guide, troubleshooting
DEPLOYMENTS.md	All deployments and TX hashes

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Security Notes

• Trusted Setup: Single-contributor ceremony. Production requires multi-party MPC.
• Testnet Only: Not audited for mainnet.
• Nullifier Registry: On-chain nullifier tracking not yet implemented (replay protection is off-chain).
• Reserve Inputs: ZK proves computation correctness, not input truthfulness.

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License

MIT