W: GPG error: http://deb.debian.org/debian stretch-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010
W: The repository 'http://deb.debian.org/debian stretch-updates 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.
W: GPG error: http://deb.debian.org/debian stretch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010 NO_PUBKEY EF0F382A1A7B6500
W: The repository 'http://deb.debian.org/debian stretch 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: http://security.debian.org stretch/updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY 8B48AD6246925553
W: The repository 'http://security.debian.org stretch/updates 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.
W: GPG error: http://security.debian.org/debian-security stretch/updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY 8B48AD6246925553
W: The repository 'http://security.debian.org/debian-security stretch/updates 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.

gpg --keyserver pgpkeys.mit.edu --recv-key  AED4B06F473041FA                                                         58.4s
gpg: key AED4B06F473041FA: public key "Debian Archive Automatic Signing Key (6.0/squeeze) <ftpmaster@debian.org>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1

gpg -a --export AED4B06F473041FA | sudo apt-key add -                                                                53.2s
[sudo] password for sathish:
/usr/bin/apt-key: 267: /usr/bin/apt-key: find: not found
/usr/bin/apt-key: 267: /usr/bin/apt-key: find: not found
/usr/bin/apt-key: 267: /usr/bin/apt-key: find: not found
OK