Drop the redundant copy in decodePublicKey

Buffer is a subclass of Uint8Array, so Buffer.from(encoded,
"base64url") already returns something that satisfies the
Uint8Array<ArrayBuffer> return type and the only downstream
caller (SimpleWebAuthn's verifyAuthenticationResponse) reads the
bytes either way.  The previous version allocated a fresh
ArrayBuffer and copied the buffer's contents into a plain
Uint8Array; nothing here benefits from that round trip.

The two unit tests that round-trip a key now compare via
Array.from() rather than toEqual() on the typed-array instance,
since toEqual checks the prototype as well and a Buffer doesn't
deep-equal a Uint8Array under that rule.

#487 (comment)

Assisted-by: Claude Code:claude-opus-4-7

Author: dahlia

src/passkey.test.ts

@@ -87,7 +87,7 @@ describe("encodePublicKey / decodePublicKey", () => {
     const bytes = new Uint8Array([0, 1, 2, 250, 251, 252, 253, 254, 255]);
     const encoded = encodePublicKey(bytes);
     expect(typeof encoded).toBe("string");
-    expect(decodePublicKey(encoded)).toEqual(bytes);
+    expect(Array.from(decodePublicKey(encoded))).toEqual(Array.from(bytes));
   });
 
   it("encodes to base64url (no +, /, or = padding)", () => {
@@ -99,7 +99,7 @@ describe("encodePublicKey / decodePublicKey", () => {
   it("decodes the same value when given a string with padding stripped", () => {
     const bytes = new Uint8Array([1, 2, 3]);
     const encoded = encodePublicKey(bytes);
-    expect(decodePublicKey(encoded)).toEqual(bytes);
+    expect(Array.from(decodePublicKey(encoded))).toEqual(Array.from(bytes));
   });
 });
 

src/passkey.ts

@@ -225,10 +225,7 @@ export function encodePublicKey(bytes: Uint8Array): string {
 
 /** Decode a base64url-encoded public-key blob back into bytes. */
 export function decodePublicKey(encoded: string): Uint8Array<ArrayBuffer> {
-  const buf = Buffer.from(encoded, "base64url");
-  const out = new Uint8Array(new ArrayBuffer(buf.byteLength));
-  out.set(buf);
-  return out;
+  return Buffer.from(encoded, "base64url");
 }
 
 const PLATFORM_LABELS: ReadonlyArray<{ pattern: RegExp; label: string }> = [