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I recently had a need to create a password protected folder on my Mac.  Did a little digging and found DiskUtility to be the answer.

1. Open DiskUtility at ~/Applications/Utilities/Disk Utility
2. File > New > Disk Image From Folder
3. Select a folder and click the Image button
4. Image Format -> read/write (if you want to be able to modify the contents)
5. Encryption -> 128-bit AES
6. Enter and confirm password
7. Password protected folder is created

This was great.  My encrypted folder is now mounted at /Volumes/name_of_folder when I open the *.dmg file.  Except that I needed to add files to the folder periodically and the disk image I had created was bound by the original size, give or take a few bytes.

So start over.  Did a little more digging and found sparse images. Sparse Image

1. Open DiskUtility at ~/Applications/Utilities/Disk Utility
2. Click New Image icon, making sure that nothing is selected on the left hand side
3. Enter a name in the Save As field.  This will be the filename of the sparseimage file
4. Enter a name in the Name field.  This is the name of the disk image that will be opened
5. Choose a maximum size for the disk image
6. Choose 128-bit AES encyrption
7. Choose sparse disk image under Image format
8. Confirm password
9. Create your sparseimage file

Done.  Now click on the *.sparseimage and your disk image is mounted as /Volumes/name_of_disk_image.  Add, delete and modify to your heart’s content.