Technical Explanation

Architecture Overview

PIVY uses a multi-layered cryptographic system for privacy-preserving payments which can be visually explained in the flow diagram below.

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Core Components

1. Meta Key Generation

Meta keys are deterministically derived from the user's wallet signature and PIN:

// Derive meta spend key
const spendSeed = sha512(
  Uint8Array.from([
    ...walletSignature,           // Unique per wallet
    ...pinBytes,                  // User's PIN
    ...new TextEncoder().encode("PIVY Spend Authority | Deterministic Derivation")
  ])
).slice(0, 32);

const metaSpendKeypair = Secp256k1Keypair.fromSecretKey(spendSeed);

Meta Keys Overview

PIVY uses two types of meta keys derived from your wallet + PIN.

Meta Viewing Keys

• Meta View Private Key (metaViewPriv): Decrypts payment details.
• Meta View Public Key (metaViewPub): Used by senders to encrypt data.

Meta Spending Keys

• Meta Spend Private Key (metaSpendPriv): Grants spending access and never exposed.
• Meta Spend Public Key (metaSpendPub): Used by senders to create stealth addresses.

Key Security and Flow

Private keys (metaSpendPriv, metaViewPriv) are the most critical piece of security and are NEVER EXPOSED: ❌ Never transmitted, logged, or shared with external services. ✅ Only used locally for cryptographic operations.

Public keys (metaSpendPub, metaViewPub) are shared and used by senders for payment routing and encryption. They cannot be used to spend your funds.

Usage Flow and Critical Security

Action	Keys Used	Security Note
Sending	Public Keys	Used for address generation.
Receiving/Spending	Private Keys	Used to detect and unlock funds.

The metaSpendPriv is your master key. It's never published, only used for key derivation, and is securely protected by PIN-based encryption in your local storage.

Key Properties

• Deterministic: Your meta keys are always the same (Wallet + PIN).
• Two-Factor: Control requires both your wallet and your PIN.
• Domain Separated: Keys are unique for different contexts.

2. Ephemeral Key Encryption

Payment details are encrypted using one-time ephemeral keys:

// ECDH key agreement
const shared = secp.getSharedSecret(ephPriv, metaViewPub, true);
const key = hkdf(sha256, shared.slice(1), salt, "ephemeral-key-encryption", 32);

// Encrypt with ChaCha20-Poly1305
const encrypted = chacha20poly1305(key, nonce).encrypt(plaintext);

Security Properties:

• Forward Secrecy: Each payment uses unique ephemeral key.
• Authenticated Encryption: ChaCha20-Poly1305 provides integrity.
• Key Derivation: HKDF ensures proper key material.

3. Stealth Address System

Each payment uses a unique, unlinkable address derived from meta keys:

// ECDH shared secret between ephemeral key and meta view key
const shared = secp.getSharedSecret(ephPriv, metaViewPub, true);
const tweak = sha256(shared.slice(1));

// Point arithmetic: StealthPub = MetaSpendPub + tweak * G
const stealthPoint = metaSpendPoint.add(secp.Point.BASE.multiply(tweakScalar));

Mathematical Foundation:

• Stealth Public Key: S = M + t * G (where M = meta spend pub, t = tweak).
• Stealth Private Key: s = m + t (where m = meta spend priv).
• Unlinkability: Each payment uses different ephemeral key → different stealth address.

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Payment Flow

1. Sending Payment

1. Generate Ephemeral Keypair

   const ephemeral = pivy.generateEphemeralKey();

2. Derive Stealth Address

   const stealthAddress = await pivy.deriveStealthPub(
     receiverMetaSpendPub,
     receiverMetaViewPub,
     ephemeral.privateKey
   );

3. Encrypt Payment Data

   const encryptedMemo = await pivy.encryptEphemeralPrivKey(
     ephemeral.privateKey,
     receiverMetaViewPub
   );

2. Receiving Payment

1. Detect Payment: System monitors blockchain for stealth address activity.

3. Withdrawing Payment

1. Derive Stealth Keypair:

   const stealthKP = await pivy.deriveStealthKeypair(
     metaSpendPriv,
     metaViewPriv,
     payment.ephemeralPubkey
   );

2. Spend Funds: Use derived keypair to sign withdraw transactions.

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Security Model

Cryptographic Primitives

Component	Algorithm	Purpose
Key Derivation	SHA-512 + domain separation	Meta key generation
Key Agreement	ECDH (secp256k1)	Shared secret generation
Encryption	ChaCha20-Poly1305	Ephemeral key encryption
Storage	AES-256-GCM + PBKDF2	Local key storage
Hashing	SHA-256	Tweak derivation

Attack Resistance

Attack Vector	Protection Mechanism
PIN Brute Force	6-digit PIN with rate limiting. The PIN alone does not grant access to user's fund, it needs to be combines with the user's wallet.
Wallet Compromise	PIN still required for meta keys. The wallet alone does not grant access to user's fund, it needs to be combines with the user's PIN.
Storage Theft	AES-256-GCM encryption.
Network Analysis	Unlinkable stealth addresses.
Transaction Correlation	Ephemeral keys prevent linking.

Security Guarantees

1. Unlinkability: Stealth addresses cannot be linked to user identity.
2. Forward Secrecy: Compromising one payment doesn't affect others.
3. Deterministic Recovery: Meta keys can be regenerated from wallet + PIN.
4. No Single Point of Failure: Both wallet and PIN required for access.

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Multi-Chain Implementation

Sui Network

• Curve: secp256k1
• Address Format: 0x-prefixed hex
• Transaction: Move smart contracts

Solana Network

• Curve: Ed25519
• Address Format: Base58
• Transaction: Anchor program calls

Both implementations follow the same cryptographic principles, but use chain-specific primitives, account generation formula, and transaction formats.