sudo apt-get update failed on AIY vision/voice kit with the folowing signature error:
W: GPG error: https://dl.google.com/aiyprojects/deb stable Release: The following signatures were invalid: EXPKEYSIG 6494C6D6997C215E Google Inc. (Linux Packages Signing Authority) <[email protected]>
W: The repository 'https://dl.google.com/aiyprojects/deb stable Release' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://www.linux-projects.org/listing/uv4l_repo/raspbian/stretch stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 978FA72DF442B950
W: The repository 'http://www.linux-projects.org/listing/uv4l_repo/raspbian/stretch stretch InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.