Specifically with regard to the secrecy of a voter's own ballot, is that a voter's privilege or does a voter have an obligation to keep his ballot secret?
The various states have provided mechanisms for a voter to maintain the secrecy of their ballot whether by voting in person or by mail. If a voter chooses to not employ those mechanisms, can the voter be penalized?
This issue is central to the recent Pennsylvania Supreme Court Ruling that if a voter chooses to not use the security envelope, but instead places their "naked" ballot in the provided mailing (outer) envelope, that voter will forfeit his vote(s). It can only be speculated if the voter intentionally or innocently neglected to use the security envelope, but in 2016 approximately 6% of PA Philadelphia voters submitted ballots in this way.
Some election officials have estimated that as many as 100,000 Pennsylvania ballots will be voided on this basis (the naked ballot basis).
I can not conceive of how a voter should be sanctioned if they were to show their ballot, for example, to their wife, children, even the next-door-neighbor. That would seem to be that voter's right and privilege.
On the other hand, sanctioning a voter for not exercising their privilege, gives the appearance of voter suppression.
EDITED TO ADD: As it relates to the obligation of a voter to protect their own voting choices this link addresses Ballot Selfies, wherein the voter not only takes a picture of their own ballot but publishes it! National Conference of Legislatures