how to resolve apt update error - GPG error NO_PUBKEY

W: GPG error: ftp://www.hk.debian.org unstable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F1D53D8C4F368D5D NO_PUBKEY 010908312D230C5F
W: You may want to run apt-get update to correct these problems

Solution:
$ gpg --keyserver wwwkeys.eu.pgp.net --recv-keys F1D53D8C4F368D5D
$ gpg --armor --export F1D53D8C4F368D5D | apt-key add -
$ gpg --keyserver wwwkeys.eu.pgp.net --recv-keys 010908312D230C5F
$ gpg --armor --export 010908312D230C5F | apt-key add -