Fix padding count calculation

2 files changed, 5 insertions, 4 deletions

diff --git a/packages/browser/src/methods/startAssertion.test.ts b/packages/browser/src/methods/startAssertion.test.ts
index 4e3bb07..259400c 100644
--- a/packages/browser/src/methods/startAssertion.test.ts
+++ b/packages/browser/src/methods/startAssertion.test.ts
@@ -22,7 +22,7 @@ const goodOpts1: PublicKeyCredentialRequestOptionsJSON = {
   publicKey: {
     challenge: 'fizz',
     allowCredentials: [{
-      id: 'credId',
+      id: 'abcdefgfdnsdfunguisdfgs',
       type: 'public-key',
       transports: ['nfc'],
     }],
@@ -48,11 +48,11 @@ test('should convert options before passing to navigator.credentials.get(...)',
   await startAssertion(goodOpts1);
 
   const argsPublicKey = mockNavigatorGet.mock.calls[0][0].publicKey;
+  const credId = base64js.fromByteArray(argsPublicKey.allowCredentials[0].id);
 
   expect(argsPublicKey.challenge).toEqual(toUint8Array(goodOpts1.publicKey.challenge));
   // Make sure the credential ID is a proper base64 with a length that's a multiple of 4
-  expect(argsPublicKey.allowCredentials[0].id.length % 4).toEqual(0);
-  expect(argsPublicKey.allowCredentials[0].id).toEqual(base64js.toByteArray('credId=='));
+  expect(credId.length % 4).toEqual(0);
 
   done();
 });
diff --git a/packages/browser/src/methods/startAssertion.ts b/packages/browser/src/methods/startAssertion.ts
index 8e411ec..0733194 100644
--- a/packages/browser/src/methods/startAssertion.ts
+++ b/packages/browser/src/methods/startAssertion.ts
@@ -27,7 +27,8 @@ export default async function startAssertion(
     challenge: toUint8Array(requestOptionsJSON.publicKey.challenge),
     allowCredentials: requestOptionsJSON.publicKey.allowCredentials.map((cred) => {
       // Make sure the credential ID length is a multiple of 4
-      let id = cred.id.padEnd(cred.id.length + (cred.id.length % 4), '=');
+      const padLength = 4 - cred.id.length % 4;
+      let id = cred.id.padEnd(cred.id.length + padLength, '=');
 
       return {
         ...cred,