TL;DR

I recently discovered that OpenSSL has an enc sub-command.

So… if you have a shared password/secret, you can leverage it to do some encryption and decription.

Let’s assume that you set the password in the environment variable PASS:

$ PASS="$(printf >&2 'enter password> '; read x; printf '%s' "$x")"

The enc sub-command, by default, does the encryption, like this:

$ cleartext='53cr3t5!'
$ printf '%s' "$cleartext" \
    | openssl enc -base64 -aes256 -pass pass:"$PASS"
U2FsdGVkX18jFNoH2NtxHrdN8pnGKUMuk8XTnW4QBgM=

To decrypt, use the same command but add option -d:

$ cyphertext='U2FsdGVkX18jFNoH2NtxHrdN8pnGKUMuk8XTnW4QBgM='
$ printf '%s\n' "$cyphertext" \
    | openssl enc -d -base64 -aes256 -pass pass:"$PASS"
53cr3t5

It works!

One thing to note: the printf for feeding the $cyphertext to the decrypting process MUST end with a newline!