Googling on this reveals a number of similar solutions, none of which have worked for me. e.g. https:||community.brave.com|t|missing-public-key-for-linux-updates/182721
# lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 9.13 (stretch)
Following https:||brave.com|linux| apt-get update returns:
W: GPG error: https:||brave-browser-apt-release.s3.brave.com stable InRelease: The following signatures couldn’t be verified because the pub
lic key is not available: NO_PUBKEY A8580BDC82D3DC6C
W: The repository ‘https:||brave-browser-apt-release.s3.brave.com stable 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.
# apt-key adv --keyserver keyserver.ubuntu.com --recv-keys
gpg: key 0BB75829C2D4E821: “Brave Software <firstname.lastname@example.org>” not changed
gpg: Total number processed: 1
gpg: unchanged: 1
seems fine, returns
[Substitude / for each |, above.]