[Cryptography] Best/simplest document encryption

[Henry Baker]

Hi all:

Here's the most basic crypto question of all:

What is the best (most secure & easiest to
use) system for *non-crypto* people to use
who have different platforms?

I.e., there are *senders* and *receivers*:

Senders can encrypt & send from a number of
different platforms: Windows, MacOs, Linux,
iPhone, Android.  Document can be anything
from a text file to a (small) movie.

Receivers can decrypt & receive on the
same platforms.

I don't trust Chrome or Firefox to do the
encryption/decryption, but I'm happy to let
them do the transmission of the encrypted
document.

I would love to use an open source system
if one exists, and I'd love to be able to
do a *reproducible build* of such a system.

For this purpose, I'm primarily interested
in commercial secrets, but these secrets
could conceivably be worth > $1 million.

Obviously, I can't control what happens if
either the sender's or receiver's platform
is compromised, but I would like to force
the sender & receiver to actually type in
a password/passphrase that they can exchange
via a 2FA (e.g., a phone call).

For this particular application, a symmetric
key system might be adequate.

It might be a good thing if the encryption
program kept a history list of salted hashed
passphrases to make sure that the user never
used any of these again.