BSV Message Encryption

Encrypt and decrypt messages between parties using @bsv/sdk.

Recommended: Use EncryptedMessage from @bsv/sdk

The @bsv/sdk provides EncryptedMessage for secure message encryption. This is the preferred approach - avoid rolling custom encryption implementations.

import { PrivateKey, EncryptedMessage, Utils } from '@bsv/sdk'

const sender = PrivateKey.fromRandom()
const recipient = PrivateKey.fromRandom()

// Encrypt: sender uses their private key + recipient's public key
const message = Utils.toArray('Secret message', 'utf8')
const encrypted = EncryptedMessage.encrypt(message, sender, recipient.toPublicKey())

// Decrypt: recipient uses their private key
const decrypted = EncryptedMessage.decrypt(encrypted, recipient)
const plaintext = Utils.toUTF8(decrypted)

Two Encryption Patterns

Pattern 1: Static-Static ECDH (Both Parties Have Keys)

Use when both parties have established keypairs (e.g., BAP identities, paymail addresses).

import { PrivateKey, EncryptedMessage, Utils } from '@bsv/sdk'

// Alice and Bob both have persistent keys
const alice = PrivateKey.fromWif('L1...')
const bob = PrivateKey.fromWif('K1...')

// Alice encrypts to Bob
const ciphertext = EncryptedMessage.encrypt(
  Utils.toArray('Hello Bob', 'utf8'),
  alice,
  bob.toPublicKey()
)

// Bob decrypts from Alice
const plaintext = EncryptedMessage.decrypt(ciphertext, bob)

Use cases:

• Peer-to-peer encrypted messaging
• Encrypted backups to your own key
• Communication between known identities

Pattern 2: ECIES-Style (Ephemeral Sender Key)

Use when sender doesn't have a persistent identity (e.g., browser-to-server).

import { PrivateKey, EncryptedMessage, Utils } from '@bsv/sdk'

// Server has persistent key, client generates ephemeral
const serverKey = PrivateKey.fromWif('L1...')
const ephemeralClient = PrivateKey.fromRandom()

// Client encrypts (ephemeral → server)
const encrypted = EncryptedMessage.encrypt(
  Utils.toArray('sensitive data', 'utf8'),
  ephemeralClient,
  serverKey.toPublicKey()
)

// Include ephemeral public key so server can decrypt
const payload = {
  ephemeralPub: ephemeralClient.toPublicKey().toString(),
  ciphertext: Utils.toHex(encrypted)
}

// Server decrypts using ephemeral pubkey
const clientPub = PublicKey.fromString(payload.ephemeralPub)
// Note: EncryptedMessage.decrypt needs the sender info embedded in ciphertext

Use cases:

• Browser-to-server encryption
• One-way secure channels
• Forward secrecy (fresh key per message)

How ECDH Key Agreement Works

Both parties derive the same shared secret:

Alice: alicePrivKey × bobPubKey = sharedPoint
Bob:   bobPrivKey × alicePubKey = sharedPoint

The shared point is then used to derive a symmetric key for AES-256-GCM encryption.

API Reference

EncryptedMessage.encrypt()

static encrypt(
  message: number[],      // Plaintext as byte array
  sender: PrivateKey,     // Sender's private key
  recipient: PublicKey    // Recipient's public key
): number[]               // Encrypted bytes

EncryptedMessage.decrypt()

static decrypt(
  ciphertext: number[],   // Encrypted bytes from encrypt()
  recipient: PrivateKey   // Recipient's private key
): number[]               // Decrypted plaintext bytes

Utility Functions

// String to bytes
Utils.toArray('hello', 'utf8')  // number[]

// Bytes to string
Utils.toUTF8([104, 101, 108, 108, 111])  // 'hello'

// Bytes to hex
Utils.toHex([1, 2, 3])  // '010203'

// Hex to bytes
Utils.toArray('010203', 'hex')  // [1, 2, 3]

Security Properties

• Authenticated encryption: AES-256-GCM provides confidentiality + integrity
• Key agreement: ECDH ensures only intended recipient can decrypt
• No key transmission: Private keys never leave their owner

Common Mistakes

Don't roll your own crypto

❌ Wrong: Implementing ECDH + AES manually

// Don't do this - use EncryptedMessage instead
const sharedSecret = myPrivKey.deriveSharedSecret(theirPubKey)
const aesKey = sha256(sharedSecret)
// ... manual AES encryption

✅ Correct: Use the SDK's built-in class

const encrypted = EncryptedMessage.encrypt(message, sender, recipient)

Don't confuse encryption with authentication

• Encryption (this skill): Hides message content
• Authentication (bitcoin-auth): Proves sender identity

For authenticated + encrypted communication, use both:

// Encrypt the payload
const encrypted = EncryptedMessage.encrypt(payload, sender, recipient)

// Sign the request (proves sender identity)
const authToken = getAuthToken({ privateKeyWif, requestPath, body })