Swap encryption in FreeBSD is easy to configure. Depending on which version of FreeBSD is being used, different options are available and configuration can vary slightly. The gbde(8) or geli(8) encryption systems can be used for swap encryption. Both systems use the encswap rc.d script.
The previous section, Encrypting Disk Partitions, includes a short discussion on the different encryption systems.
Like the encryption of disk partitions, encryption of swap space is done to protect sensitive information. Imagine an application that e.g., deals with passwords. As long as these passwords stay in physical memory, all is well. However, if the operating system starts swapping out memory pages to free space for other applications, the passwords may be written to the disk platters unencrypted and easy to retrieve for an adversary. Encrypting swap space can be a solution for this scenario.
Note: For the remainder of this section, ad0s1b will be the swap partition.
Up to this point the swap has been unencrypted. It is possible that there are already passwords or other sensitive data on the disk platters in cleartext. To rectify this, the data on the swap partition should be overwritten with random garbage:
# dd if=/dev/random of=/dev/ad0s1b bs=1m
The .bde suffix should be added to the device in the respective /etc/fstab swap line:
# Device Mountpoint FStype Options Dump Pass# /dev/ad0s1b.bde none swap sw 0 0
# Device Mountpoint FStype Options Dump Pass# /dev/ad0s1b.eli none swap sw 0 0
geli(8) uses the AES algorithm with a key length of 128 bit by default.
Optionally, these defaults can be altered using the geli_swap_flags option in /etc/rc.conf. The following line tells the encswap rc.d script to create geli(8) swap partitions using the Blowfish algorithm with a key length of 128 bit, a sectorsize of 4 kilobytes and the “detach on last close” option set:
geli_swap_flags="-e blowfish -l 128 -s 4096 -d"
Please refer to the description of the onetime command in the geli(8) manual page for a list of possible options.
Once the system has been rebooted, proper operation of the encrypted swap can be verified using the swapinfo command.
If gbde(8) is being used:
% swapinfo Device 1K-blocks Used Avail Capacity /dev/ad0s1b.bde 542720 0 542720 0%
If geli(8) is being used:
% swapinfo Device 1K-blocks Used Avail Capacity /dev/ad0s1b.eli 542720 0 542720 0%