Verify & Authenticate

[Kelly76]

I'm new to Linux but old to the world. I've struggled for nearly a week trying to apply the recommended commands to verify linuxmint-19.1-mate-64bit.iso until finding one YouTube clip that suggested the following code: "openssl sha256 *.iso" rather than "sha256sum -b *.iso". Finally, success ... I got a match. Now I'm working on authentication, for which I need to import a "key" from Ubuntu. But just as the recommended code for verification (in the guidelines on the LinuxMint website) (ie; "sha256sum -b *.iso") failed, I am now experiencing the same failed result using the recommended code for importing the "key" (ie; "gpg --keyserver keyserver.ubuntu.com --recv-key "27DE B156 44C6 B3CF 3BD7 D291 300F 846B A25B AE09" ". When I hit the Enter key, I get "-bash: gpg: command not found" - which is the message I got on my terminal when I used "sha256sum -b *.iso" .... & yes, I entered the correct path & file name in place of the asterisk.

Are my unrecognized commands a result of the fact that I'm doing this on a 2010 MacBook Pro?
Is there some program that I should import, like rEFIne, or Virtual Box, or Pendrivelinux, etc., that is not typically pre-loaded on a Mac that will mediate between Mac OS & bash?
Or, can somebody provide another command code for obtaining the "key" from Ubuntu?

[ajgringo619]

I feel for you Kelly76. I've been using/building/programming computers for 30 years, and for the life of me I cannot understand why there are so many threads regarding downloading/using a simple .ISO file. Every version of Linux/Windows/AIX/Solaris I've ever downloaded has never been corrupted...so why is it happening so much now? I can't believe I'm that lucky.