Note Encryption

Axiomatic uses X25519 ECDH (Elliptic Curve Diffie-Hellman) for per-note encryption on the confidential ledger. Each note gets a unique encryption key, ensuring inter-sender privacy.

Encryption (Sender)

1. Retrieve recipient's X25519 public key from their stealth meta-address
2. Generate ephemeral X25519 keypair (r, R)
3. Compute shared secret: x25519(r, recipientX25519Pub)
4. Derive note key: HKDF-SHA256(sharedSecret, commitment, "axiomatic.note.ecdh.v1")
5. Encrypt note data with AES-256-GCM
6. Submit transaction with encryptedNote, ephemeralPubKey = R, scanTag

Decryption (Recipient)

1. Derive X25519 private key from IVK: HKDF(ivk, "axiomatic.x25519.v1")
2. Compute shared secret: x25519(x25519Private, ephemeralPubKey)
3. Derive note key: HKDF-SHA256(sharedSecret, commitment, "axiomatic.note.ecdh.v1")
4. Decrypt with AES-256-GCM

Inter-Sender Privacy

Because each sender generates a fresh ephemeral keypair per note:

• Sender A's notes produce different shared secrets than Sender B's
• Even if Sender A is compromised, they cannot decrypt Sender B's notes to the same recipient
• The ephemeral private key r is discarded after computing the shared secret

Key Derivation

IVK (Incoming Viewing Key)
  |
  |-- HKDF(ivk, "axiomatic.x25519.v1") --> X25519 private key
  |
  |-- getPublicKey(private) --> X25519 public key

The X25519 public key is published in the stealth meta-address. The private key exists only on the client and (encrypted) on the server for cron scanning.