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packages/shared/src/crypto/keys.ts

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+import { getCryptoBackend } from './backend.js';
+
+export interface X25519KeyPair {
+  publicKey: Uint8Array; // 32 bytes
+  privateKey: Uint8Array; // 32 bytes
+}
+
+export async function generateX25519KeyPair(): Promise<X25519KeyPair> {
+  const backend = getCryptoBackend();
+  const kp = backend.generateKeyPair();
+  return { publicKey: kp.publicKey, privateKey: kp.privateKey };
+}
+
+// ---- base64 helpers (used to persist private keys client-side) ----------
+// Use browser-native btoa/atob. Both are available in Tauri webview and RN
+// (Expo polyfills atob/btoa). No external dep needed.
+
+export async function toBase64(bytes: Uint8Array): Promise<string> {
+  let binary = '';
+  for (let i = 0; i < bytes.length; i++) {
+    binary += String.fromCharCode(bytes[i]!);
+  }
+  return btoa(binary);
+}
+
+export async function fromBase64(input: string): Promise<Uint8Array> {
+  const binary = atob(input);
+  const out = new Uint8Array(binary.length);
+  for (let i = 0; i < binary.length; i++) {
+    out[i] = binary.charCodeAt(i);
+  }
+  return out;
+}
+
+// Wipe a key best-effort. JS can't truly guarantee zeroization but
+// overwriting the buffer removes the value from live references.
+export function wipe(bytes: Uint8Array): void {
+  for (let i = 0; i < bytes.length; i++) bytes[i] = 0;
+}