Create PEM for Android Things: Invalid authentication key. Check the format.

[li2]

Finally, fixed this issue by almost one day:

Firstly, execute the follow command to create a pem file, I use EC in this command:

adb shell am start -e project_id <PROJECT_ID> -e cloud_region <REGION> -e registry_id <REGISTRY_ID> -e device_id <DEVICE_ID> [-e key_algorithm <one of RSA,EC>] com.example.androidthings.sensorhub/.SensorHubActivity

the key point is -e key_algorithm, which allows RSA and EC, and EC is actually ES256:

ES256 key pair using the Eliptic Curve algorithm
See Google Cloud Internet of Things Core - Creating Public/Private Key Pairs

Lastly, upload the pem to GCP console, and select ES256_X509:

This function in AuthKeyGenerator.class is actually for _X509

public void initialize() throws GeneralSecurityException, IOException {
    if (certificate instanceof X509Certificate) { // is true

private void writeCertificatePEM(OutputStream os) throws GeneralSecurityException, IOException {
    os.write("-----BEGIN CERTIFICATE-----\n".getBytes());
    os.write(Base64.encode(certificate.getEncoded(), Base64.DEFAULT));
    os.write("-----END CERTIFICATE-----\n".getBytes());
}

Done