Let’s say you have screenshots of sensitive information on your desktop that you want to password protect. You know that images of tax, banking, or money transfer forms, etc., can easily be viewed by anyone with physical or remote access to your device, but you’re not sure how to secure them. Unfortunately, macOS Preview doesn’t support password-protecting files, but there are quick solutions that don’t require third-party software or downloading any programs that aren’t already installed on your Mac.
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The first method involves creating an encrypted virtual disk using Disk Utility. You can do this by gathering your screenshots (and documents) and placing them in a folder. Then, go to Finder > Applications > Disk Utility. In Disk Utility, go to File > New Image > Image from Folder and select the location of the folder you want to encrypt.
From here, you can name the virtual disk (via a DMG file), choose its location, encryption level (up to AES-256 encryption), and image format. For the latter, select Read/Write. This will allow you to add or remove files later. The Compressed, selected by default, compresses the contents to save space but is read-only.
You have successfully created an encrypted DMG file containing all your private contents. The downside of this method is that you will have to mount and unmount the virtual disk each time.
There is another solution that requires fewer clicks and can be done in seconds. By converting your graphic file to PDF, you can unlock a useful password feature hidden in its settings, turning your image into a password-encrypted PDF. This only takes a few seconds.
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Right-click on the image you want to password protect, go to “Open With” and select Preview. Most likely, Preview is already your default, so you can just double-click and it will open in Apple’s image viewer.
Then, name your file, choose its location on your device, and click More Details > Permissions to provide more options, including the ability to add a password to the file.
Check the “Require a Password to Open Document” box and enter a password. This screen will also require you to enter an owner password even if you don’t intend to share the file, and no permissions are selected. You can enter the same password here.
Click “Apply” and then click “Save” and a new encrypted PDF will appear in your saved location.
You can now delete the unprotected file and empty your trash. The next time you click on the newly created PDF to view its contents, you will be prompted to enter a password.
Unlike Disk Utility, which uses AES-256-bit, Preview uses AES-128 when encrypting PDFs with a password. This doesn't necessarily mean your files are any less secure. AES-128 produces a key size of 128 bits, still providing a huge number of possible key combinations, making any brute force attempt nearly impossible.
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Unless you're hosting highly sought-after government information, it's more than enough encryption against common threats. Overall, this method may seem a bit lengthy at first, but once you've done it a few times, you can create those locked PDFs quickly.
