Keys and Security

Platus implements a dual-key architecture where the ability to view transactions and the ability to spend funds are cryptographically isolated.

The scheme is built on re-randomizable stealth addresses, a mechanism that allows any third party to generate unlinkable payment addresses for a recipient without knowing their private keys or previous transaction history.

Key Architecture

Spending Key (sk)

The spending key is the root secret from which all other keys are derived.

Properties:

• Grants full control over funds
• Required to authorize transactions
• Never leaves client or transmitted on-chain

Spending Public Key (sPK)

From the spending key, the spending public key is derived as:

$$\text{sPK} = \text{SHA-512}(\text{sk})[0:32] \times G$$

Where:

• $G$ is the base point of the Baby Jubjub curve.

Viewing Key (vk)

The viewing key is deterministically derived from spending public key:

$$\mathrm{vk} = \text{Poseidon}(\mathrm{sPK}_x , \mathrm{sPK}_y , \mathrm{nonce})$$

Where:

• $sPK_x, sPK_y$ are the (x,y) coordinates of the spending public key.
• $\mathrm{nonce}$ starts at 1 and increments until $\mathrm{vk} < \mathbb{F}_r$.

Properties:

• Cannot authorize spending
• Cannot derive the spending key (one-way derivation)
• Combined with ML-KEM secret key for decryption

Identity key (ik)

The identity key is derived from the viewing key as:

$$\text{ik} = \text{vk} \times G$$

Properties:

• Cannot authorize spending or leak transaction history
• Used by third parties to generate stealth addresses

ML-KEM Public Key (mPK)

Derived from post quantum seed, where seed:

$$\mathrm{prk} = \mathrm{Poseidon}(\mathrm{sPK}_x, \mathrm{sPK}_y, \mathrm{domain\_separator})$$ $$\mathrm{seed} = \mathrm{Poseidon}(\mathrm{prk}, 0) \,\|\, \mathrm{Poseidon}(\mathrm{prk}, 1)$$

Properties:

• Used to generate shared secret key for encryption
• Provides post-quantum security

Hybrid Public Key (hPK)

Combination of identity key and ML-KEM public key:

$$\mathrm{hPK} = (\mathrm{ik}, \mathrm{mPK})$$

Properties:

• Can be shared publicly to receive payments
• Used to encrypt notes with hybrid post-quantum security
• Used to generate stealth addresses for the user
• Combines classical ECDH with post-quantum ML-KEM-1024

App Key (ak)

These are protocol-specific keys derived as:

$$\mathrm{ak} = \mathrm{Keccak256}(sk , \mathrm{protocol\_identifier})$$

where appIdentifier can be domain, ENS name, or IPFS hash identifying the app.