Enable HTTPS (Hypertext Transfer Protocol Secure) for package managers using libapt-pkg library.
You can easily identify that HTTPS transport is missing during regular package management operations as error messages are very clear.
$ sudo apt-get update
Get:1 http://security.debian.org stretch/updates InRelease [63.0 kB] Ign:2 http://deb.debian.org/debian stretch InRelease Hit:3 http://deb.debian.org/debian stretch Release Get:4 http://security.debian.org stretch/updates/main amd64 Packages [269 kB] Get:5 http://security.debian.org stretch/updates/main Translation-en [118 kB] Fetched 450 kB in 0s (961 kB/s) Reading package lists... Done E: The method driver /usr/lib/apt/methods/https could not be found. N: Is the package apt-transport-https installed? E: Failed to fetch https://dl.bintray.com/rabbitmq/debian/dists/stretch/InRelease E: Some index files failed to download. They have been ignored, or old ones used instead.
$ sudo apt-get install apt-transport-https
Reading package lists... Done Building dependency tree Reading state information... Done The following additional packages will be installed: libcurl3-gnutls The following NEW packages will be installed: apt-transport-https libcurl3-gnutls 0 upgraded, 2 newly installed, 0 to remove and 6 not upgraded. Need to get 460 kB of archives. After this operation, 873 kB of additional disk space will be used. Do you want to continue? [Y/n] Get:1 http://deb.debian.org/debian stretch/main amd64 libcurl3-gnutls amd64 7.52.1-5+deb9u3 [289 kB] Get:2 http://deb.debian.org/debian stretch/main amd64 apt-transport-https amd64 1.4.8 [171 kB] Fetched 460 kB in 0s (9505 kB/s) debconf: delaying package configuration, since apt-utils is not installed Selecting previously unselected package libcurl3-gnutls:amd64. (Reading database ... 17075 files and directories currently installed.) Preparing to unpack .../libcurl3-gnutls_7.52.1-5+deb9u3_amd64.deb ... Unpacking libcurl3-gnutls:amd64 (7.52.1-5+deb9u3) ... Selecting previously unselected package apt-transport-https. Preparing to unpack .../apt-transport-https_1.4.8_amd64.deb ... Unpacking apt-transport-https (1.4.8) ... Setting up libcurl3-gnutls:amd64 (7.52.1-5+deb9u3) ... Processing triggers for libc-bin (2.24-11+deb9u1) ... Setting up apt-transport-https (1.4.8) ...
Now you can use HTTPS protocol without any problems.
$ sudo apt-get update
Hit:1 http://security.debian.org stretch/updates InRelease Ign:2 http://deb.debian.org/debian stretch InRelease Hit:3 http://deb.debian.org/debian stretch Release Ign:4 https://dl.bintray.com/rabbitmq/debian stretch InRelease Hit:6 https://dl.bintray.com/rabbitmq/debian stretch Release Reading package lists... Done