apt-get source linux-image-$(uname -r)
Reading package lists... Done
Picking 'linux-signed-hwe' as source package instead of 'linux-image-4.15.0-42-generic'
Need to get 12.1 kB of source archives.
Get:1 http://archive.ubuntu.com/ubuntu xenial-updates/main linux-signed-hwe 4.15.0-42.45~16.04.1 (dsc) [1'824 B]
Get:2 http://archive.ubuntu.com/ubuntu xenial-updates/main linux-signed-hwe 4.15.0-42.45~16.04.1 (tar) [10.2 kB]
Fetched 12.1 kB in 0s (169 kB/s)
gpgv: Signature made Mon 19 Nov 2018 13:56:49 CET using RSA key ID FBFD0D3E
gpgv: Can't check signature: public key not found
dpkg-source: warning: failed to verify signature on ./linux-signed-hwe_4.15.0-42.45~16.04.1.dsc
dpkg-source: info: extracting linux-signed-hwe in linux-signed-hwe-4.15.0
dpkg-source: info: unpacking linux-signed-hwe_4.15.0-42.45~16.04.1.tar.xz

apt-key list
/etc/apt/trusted.gpg
--------------------
pub   1024D/437D05B5 2004-09-12
uid                  Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
sub   2048g/79164387 2004-09-12

pub   4096R/C0B21F32 2012-05-11
uid                  Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>

pub   4096R/EFE21092 2012-05-11
uid                  Ubuntu CD Image Automatic Signing Key (2012) <cdimage@ubuntu.com>

pub   1024D/FBB75451 2004-12-30
uid                  Ubuntu CD Image Automatic Signing Key <cdimage@ubuntu.com>

pub   2048R/FBFD0D3E 2016-12-12
uid                  Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
sub   2048R/C69B8715 2016-12-12

cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial

uname -a
Linux XXXXXX 4.15.0-42-generic #45~16.04.1-Ubuntu SMP Mon Nov 19 13:02:27 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux