refactor: export shield signature message as a standalone constant

src/notes/index.ts

@@ -7,7 +7,7 @@ export { Memo } from './memo.js'
 export type { NoteParams } from './note.js'
 export { Note } from './note.js'
 export type { ShieldNoteCreateParams, ShieldNoteParams } from './shield-note.js'
-export { ShieldNote } from './shield-note.js'
+export { SHIELD_PRIVATE_KEY_SIGNATURE_MESSAGE, ShieldNote } from './shield-note.js'
 export type { ShieldRequest } from './shield-request.js'
 export { buildShieldRequest } from './shield-request.js'
 export type { UnshieldNoteParams } from './unshield-note.js'

src/notes/shield-note.ts

@@ -33,6 +33,15 @@ type ShieldNoteCreateParams = {
 /** Maximum note value representable in the contract's uint120 preimage field. */
 const UINT_120_MAX = 2n ** 120n - 1n
 
+/**
+ * Message a public wallet signs to derive a shield private key.
+ *
+ * DO NOT MODIFY. The shield private key is derived from a signature over this
+ * exact string, so any change yields a different key and shields created under
+ * the previous message can no longer be decrypted.
+ */
+const SHIELD_PRIVATE_KEY_SIGNATURE_MESSAGE = 'RAILGUN_SHIELD'
+
 /**
  * Represents a Shield note for converting public assets into private RAILGUN notes.
  * This is the entry point for funds entering the privacy system via on-chain shield transactions.
@@ -282,4 +291,4 @@ class ShieldNote extends Note {
 }
 
 export type { ShieldNoteParams, ShieldNoteCreateParams }
-export { ShieldNote }
+export { SHIELD_PRIVATE_KEY_SIGNATURE_MESSAGE, ShieldNote }

src/wallet/railgun-wallet.ts

@@ -63,16 +63,6 @@ export class RailgunWallet {
     this.initializeKeyPairs()
   }
 
-  /**
-   * Retrieves the constant signature message used for shield private key operations.
-   * This message is a predefined string that should not be modified.
-   * @returns The constant signature message 'RAILGUN_SHIELD'.
-   */
-  static getShieldPrivateKeySignatureMessage () {
-    // DO NOT MODIFY THIS CONSTANT.
-    return 'RAILGUN_SHIELD'
-  }
-
   /**
    * Initializes the key pairs and related cryptographic keys for the wallet node.
    * This method retrieves the spending key pair, nullifying key, and viewing key pair

test/wallet-node.test.ts

@@ -5,6 +5,7 @@ import { stringify } from '@railgun-reloaded/0zk-addresses'
 import { hexToBytes } from '@railgun-reloaded/bytes'
 
 import { initializeCryptographyLibs } from '../src/keys.js'
+import { SHIELD_PRIVATE_KEY_SIGNATURE_MESSAGE } from '../src/notes/shield-note.js'
 import { RailgunWallet } from '../src/wallet/railgun-wallet.js'
 import { WalletNode } from '../src/wallet/wallet-node.js'
 
@@ -205,10 +206,9 @@ test('railgun-wallet - deterministic key derivation', async () => {
     'master public key should match expected value')
 })
 
-test('railgun-wallet - RailgunWallet.getShieldPrivateKeySignatureMessage', () => {
-  const message = RailgunWallet.getShieldPrivateKeySignatureMessage()
-
-  assert.equal(message, 'RAILGUN_SHIELD', 'should return correct constant message')
+test('shield-note - SHIELD_PRIVATE_KEY_SIGNATURE_MESSAGE', () => {
+  assert.equal(SHIELD_PRIVATE_KEY_SIGNATURE_MESSAGE, 'RAILGUN_SHIELD',
+    'should return correct constant message')
 })
 
 test('railgun-wallet - Different mnemonics generate different wallets', async () => {