Using Automator or AppleScript to encrypt/decrypt with OpenSSL
Actually, Automator is not a bad choice for this, as it allows you to combine AppleScript and shell scripting without actually having to mix them (which leads you straight to escaping hell, after a short stay in quoting purgatory) and pass values between them an orderly fashion. Also, besides a droplet application, Automator will allow you to create a Service with excellent integration into Finder:


Create a new Automator workflow. 


either select “Application” when prompted what type of workflow to create – that will get you a droplet that processes files and folders you send to it, or
select “Folder action” and set “Accepts selected” to “files and folders” – that will get you an item in the Finder Services and context menu of files an folders (all translations approximate, I’m on a German system).

Add a "Run AppleScript” action and edit its contents as follows:

on run {input, parameters}
    try
        tell application "System Events" to set thePassword to text returned of (display dialog "Please input your password for OpenSSL encryption" default answer "" with hidden answer)
    on error errorMessage number errorNumber
        if errorNumber is -128 then quit me -- user has canceled
        error errorMessage number errorNumber
    end try
    return (thePassword as list) &amp; input
end run


– this will prompt the user for the encryption password and pass it as the first argument to the next action.
Add a “Run Shell Script” action, setting it to get its input through arguments (not stdin, as is default). Make sure the shell is set to /bin/bash. Edit the script contents as follows:

[[ -n $1 ]] &amp;&amp; password="$1" &amp;&amp; shift || exit 0
for f in "$@"; do
    if [[ ${f##*.} = "encrypted" ]]; then
        fname="${f%.encrypted}"
        openssl enc -d -aes-256-cbc -salt -in "$f" -out "$fname".tar.gz -pass pass:$password || continue
        tar -xPf "$fname".tar.gz &amp;&amp; rm "$fname".tar.gz || continue
    else
        fname=$([[ -f $f ]] &amp;&amp; printf "${f%.*}" || printf "$f")
        tar -czPf "$fname".tar.gz "$f" || continue 
        openssl enc -aes-256-cbc -salt -in "$fname".tar.gz -out "$fname".encrypted  -pass pass:$password &amp;&amp; rm -f "$fname".tar.gz || continue
    fi
done


– this will decrypt and untar-gzip .encrypted files, tar-gzip and encrypt all other files and directories with AES 256-CBC encryption and the password given.


Caveat Empteor: error handling is primitive (basically, the for loop skips an iteration when it encounters an error), there is no logging and there is no failsafe against wrong password inputs (you might want to ask twice and compare the results, as the shell utility does). Disasters should not happen, though, as files are only deleted when the previous steps complete successfully.

Finally, you might want to investigate alternatives to prompting for a password – a passphrase file on a USB key, say (use -pass file:/Volumes/volname/passfile instead of -pass pass:$password, skip the Applescript step and remove the first line of the shell script), or storing your password in the OS X keychain and retrieving it programmatically (see this answer of mine on Stack Exchange for ways to do that).