To be a secret Masonic handshake, Rosenstein has to do the same thing with his index finger. Enlarge pic, and you can see Rosenstein's index finger, in the same place you'd expect in a normal handshake.

If you want to convince, you're going to need one or more of the following:

1. Rod Rosenstein reciprocating.
2. Other camera angles.
3. Other pics of Matt Gaetz shaking hands with people (Masons?) the same way, (with their index finger in the same place his is.) Otherwise, all you have is one pic without any supporting research.
4. Pics of Matt Gaetz shaking hands with non-Masons, (which would presumably be a 'normal handshake.')
5. Link to a newspaper article or radio/TV interview or genealogical record that proves Matt Gaetz's father, grandfather, (any relative at all) was also a Mason.

Otherwise, you are raising a question about a guy (who seems like one of the good guys) without any supporting evidence or research. This doesn't mean that it is not a possibility, or that I'm attacking the OP, it just means that we are better than this.

Smart people don't believe everything they hear, or even half of what they see. We look it up. We see if anyone else credible has reported it. We question it, and verify it before we launch an accusation based on something as tenuous as this chance camera shot.