User authentication - create PublicKeyCredentialSource from saved publicKey

[NicoP-S]

Hi,

i try to get the authentication running. Before I saved a DB record which holds the credentialId & public_key ( base64 encoded )
On authentication I try to create all necessary objects. But i fails on deserialization.

Error: Malformed UTF-8 characters, possibly incorrectly encoded

Does someone know what am I doing wrong ?

$data = $request->getBody()->getContents();
$publicKeyCredential = $serializer->deserialize($data, PublicKeyCredential::class, 'json');
if (!$publicKeyCredential->response instanceof AuthenticatorAssertionResponse) 
    return $response->withStatus(400);

$cred  = Credential::where('credential_id', base64_encode($publicKeyCredential->rawId))->where('user_id', $publicKeyCredential->response->userHandle)->first();
if (!$cred) return $response->withStatus(400);

// this one fails  <-----------
$publicKeyCredentialSource = $serializer->deserialize(base64_decode($cred->public_key), PublicKeyCredentialSource::class, 'json');

$csmFactory = new CeremonyStepManagerFactory();
$requestCSM = $csmFactory->requestCeremony();
$authenticatorAssertionResponseValidator = AuthenticatorAssertionResponseValidator::create($requestCSM);
$publicKeyCredentialSource = $authenticatorAssertionResponseValidator->check(
    $publicKeyCredentialSource,
    $publicKeyCredential->response,
    $publicKeyCredentialRequestOptions,
    $host
    null
);