Secure apt feature has been added to 0.5 and experimental repositories.  
Although apt-get complains about the key is missing, you could still be able to 
add the packages from voyage apt repository with the key.

To add the public key to apt key rings and get rid of the error messages:

# wget http://www.voyage.hk/dists/experimental/archive.key -O - | apt-key add -

Icon  Name                    Last modified      Size  Description
[PARENTDIR] Parent Directory - [   ] InRelease 2019-02-21 07:00 2.8K [   ] Packages 2019-02-21 07:00 21K [   ] Packages.gz 2019-02-21 07:00 5.1K [TXT] Readme.txt 2008-06-03 09:27 370 [   ] Release 2019-02-21 07:00 1.9K [   ] Release.gpg 2019-02-21 07:00 833 [   ] Release.gpg.new 2017-02-15 15:28 181 [   ] Sources 2019-02-21 07:00 1.2K [   ] Sources.gz 2019-02-21 07:00 698 [DIR] aircrack-ng/ 2011-02-08 14:28 - [DIR] alsa-driver/ 2011-06-30 01:34 - [DIR] alsa-lib/ 2011-10-12 17:03 - [DIR] alsa-tools/ 2011-10-12 17:03 - [DIR] alsa-utils/ 2011-10-12 17:03 - [DIR] alsa/ 2012-09-21 09:31 - [   ] archive.key 2019-02-21 07:00 3.0K [DIR] asterisk-addons/ 2010-10-15 18:18 - [DIR] asterisk-core-sounds/ 2011-10-25 11:38 - [DIR] asterisk-extra-sounds/ 2011-06-30 01:34 - [DIR] asterisk-gui/ 2011-10-25 11:38 - [DIR] asterisk-moh-opsound/ 2010-10-15 18:30 - [DIR] asterisk/ 2012-05-24 00:46 - [DIR] atl1e/ 2009-07-08 01:44 - [DIR] atl2/ 2009-07-08 01:44 - [DIR] audiofile/ 2014-11-19 23:54 - [DIR] aufs/ 2011-09-12 17:11 - [DIR] awds/ 2010-05-27 13:16 - [DIR] batctl/ 2011-06-30 01:34 - [DIR] batman/ 2012-09-21 09:31 - [DIR] batmand/ 2012-09-21 09:31 - [DIR] bcm43xx-fwcutter/ 2006-06-01 22:24 - [TXT] build.sh 2017-03-01 15:17 654 [   ] build.sh.old 2017-02-15 16:05 642 [DIR] busybox/ 2006-11-15 23:07 - [DIR] casper/ 2006-10-07 00:42 - [DIR] crda/ 2014-11-19 23:54 - [DIR] dahdi-firmware/ 2012-05-24 00:46 - [DIR] dahdi-linux/ 2012-05-24 00:46 - [DIR] dahdi-tools/ 2012-05-24 00:46 - [DIR] dahdi/ 2014-11-19 23:54 - [DIR] et131x/ 2009-07-08 01:44 - [DIR] firmware-nonfree/ 2017-01-19 10:05 - [DIR] frottle/ 2006-06-01 22:24 - [DIR] fxload/ 2010-10-15 18:23 - [DIR] glib1.2/ 2011-06-30 01:34 - [DIR] haserl/ 2006-11-15 23:07 - [DIR] hostapd/ 2011-10-25 11:38 - [DIR] ieee80211/ 2006-06-01 22:39 - [DIR] ieee80211softmac/ 2006-06-01 22:39 - [DIR] initramfs-tools/ 2013-06-20 10:05 - [DIR] ipw2100/ 2006-06-01 22:39 - [DIR] ipw2200/ 2006-06-01 22:39 - [DIR] iw/ 2011-10-25 11:38 - [DIR] lame/ 2012-05-24 00:46 - [DIR] lcr/ 2011-02-08 14:28 - [DIR] libav/ 2017-01-19 10:05 - [DIR] libcue/ 2010-02-22 12:13 - [DIR] libffado/ 2011-10-25 11:38 - [DIR] libgea2/ 2008-06-23 01:44 - [DIR] libgea3/ 2010-05-27 13:16 - [DIR] libnl3/ 2011-10-25 11:38 - [DIR] libpri/ 2012-05-24 00:46 - [DIR] libresample/ 2010-10-15 18:26 - [DIR] libss7/ 2010-10-15 18:17 - [DIR] linux-firmware/ 2014-11-19 23:54 - [DIR] linux/ 2017-05-29 09:49 - [DIR] lirc/ 2011-09-12 17:11 - [DIR] lzma/ 2009-07-08 01:44 - [DIR] madwifi-ifupdown/ 2007-01-28 23:43 - [DIR] madwifi-ng/ 2006-11-15 23:07 - [DIR] madwifi-tools/ 2011-02-08 14:28 - [DIR] madwifi/ 2011-09-12 17:11 - [TXT] maintainers.sh 2005-03-17 10:23 108 [DIR] minidlna/ 2012-05-24 00:46 - [DIR] misdn-kernel/ 2011-02-08 14:28 - [DIR] misdn-user/ 2011-02-08 14:28 - [DIR] misdn/ 2011-10-11 01:44 - [DIR] misdnv2user/ 2011-02-08 14:28 - [DIR] mpd/ 2017-01-19 10:05 - [DIR] ncurses/ 2011-10-12 17:02 - [DIR] ndiswrapper/ 2011-09-12 17:11 - [DIR] nginx/ 2012-05-24 00:46 - [DIR] nocatauth/ 2006-06-01 22:24 - [DIR] nocatsplash/ 2011-06-30 01:34 - [DIR] olsrd/ 2010-05-27 13:16 - [DIR] openr2/ 2012-05-24 00:46 - [DIR] openssl/ 2010-02-22 12:13 - [DIR] openswan/ 2008-07-13 12:11 - [DIR] p910nd/ 2007-07-23 14:23 - [DIR] parprouted/ 2006-06-01 22:24 - [TXT] promote.sh 2009-01-08 13:05 241 [DIR] rt2400/ 2006-06-01 22:39 - [DIR] rt2500/ 2006-06-01 22:39 - [DIR] rt2860/ 2009-07-08 01:44 - [DIR] shairport/ 2015-05-04 11:40 - [DIR] snd/ 2017-05-29 09:49 - [DIR] spandsp/ 2012-05-24 00:46 - [DIR] squashfs-tools/ 2010-02-22 12:13 - [DIR] squashfs/ 2010-02-22 12:13 - [DIR] sysvinit/ 2011-10-11 14:59 - [DIR] usbmount/ 2017-03-06 11:48 - [DIR] voyage-util/ 2017-01-19 10:05 - [DIR] wavemon/ 2010-05-27 13:16 - [DIR] wifidog/ 2006-06-01 22:24 - [DIR] wireless-regdb/ 2014-10-20 11:25 - [DIR] wireless-tools/ 2008-07-13 12:11 - [DIR] wpa/ 2014-11-19 23:54 - [DIR] wpasupplicant/ 2011-10-25 11:38 - [DIR] x264/ 2012-05-24 00:46 - [DIR] xvidcore/ 2012-05-24 00:46 - [DIR] zaptel/ 2009-07-08 01:44 -
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