How to Encrypt a File on OS Xby K.C. Winslow
OS X includes a utility called FileVault in the Security menu of system preferences. This utility encrypts all of the contents of the home folder and decrypts them as you use them. This utility does not work to encrypt a single file or folder of files. For that task, you can use the included Disk Utility program to make a password-protected disk image of the file. A disk image is an exact digital copy of a file, folder or hard disk.
Open the "Applications" folder, double-click "Utilities" and double-click "Disk Utility."
Click "New Image." Enter a name for the encrypted file, and select the save destination for the file.
Select the size of the disk image file with the "Size" menu. The size must be greater than the size of the file being encrypted.
Change the "Format," if desired. The default format is fine for most uses.
Select the "Image Format." Apple recommends "read/write disk image" if you are not sure which to choose.
Choose 128-bit AES or 256-bit AES as the encryption standard. Both allow you to secure the image with a password. 128-bit is recommended by Apple; 256-bit offers greater security, but is slower.
Click "Create." Enter a password in the pop-up window. Deselect the option to "Remember password (add to keychain)" if you do not want the computer to remember the password.
Go to the encrypted disk image in the Finder. Drag a file or folder of files to the disk image and drop them.
Click the eject icon next to the disk image to unmount it from the desktop. The files are encrypted and require the password to be accessed.