18.13. Encrypting Swap

Written by Christian Brüffer.

Like the encryption of disk partitions, encryption of swap space is used to protect sensitive information. Consider an application that deals with passwords. As long as these passwords stay in physical memory, they are not written to disk and will be cleared after a reboot. However, if FreeBSD starts swapping out memory pages to free space, the passwords may be written to the disk unencrypted. Encrypting swap space can be a solution for this scenario.

This section demonstrates how to configure an encrypted swap partition using gbde(8) or geli(8) encryption. It assumes a UFS file system where /dev/ada0s1b is the swap partition.

18.13.1. Configuring Encrypted Swap

Swap partitions are not encrypted by default and should be cleared of any sensitive data before continuing. To overwrite the current swap partition with random garbage, execute the following command:

# dd if=/dev/random of=/dev/ada0s1b bs=1m

To encrypt the swap partition using gbde(8), add the .bde suffix to the swap line in /etc/fstab:

# Device		Mountpoint	FStype	Options		Dump	Pass#
/dev/ada0s1b.bde	none		swap	sw		0	0

To instead encrypt the swap partition using geli(8), use the .eli suffix:

# Device		Mountpoint	FStype	Options		Dump	Pass#
/dev/ada0s1b.eli	none		swap	sw		0	0

By default, geli(8) uses the AES algorithm with a key length of 256 bits. These defaults can be altered in the options field in /etc/fstab. The possible flags are:

aalgo

Data integrity verification algorithm used to ensure that the encrypted data has not been tampered with. See geli(8) for a list of supported algorithms.

ealgo

Encryption algorithm used to protect the data. See geli(8) for a list of supported algorithms.

keylen

The length of the key used for the encryption algorithm. See geli(8) for the key lengths that are supported by each encryption algorithm.

sectorsize

The size of the blocks data is broken into before it is encrypted. Larger sector sizes increase performance at the cost of higher storage overhead. The recommended size is 4096 bytes.

This example configures an encryped swap partition using the Blowfish algorithm with a key length of 128 bits and a sectorsize of 4 kilobytes:

# Device		Mountpoint	FStype	Options				Dump	Pass#
/dev/ada0s1b.eli	none		swap	sw,ealgo=blowfish,keylen=128,sectorsize=4096	0	0

18.13.2. Encrypted Swap Verification

Once the system has rebooted, proper operation of the encrypted swap can be verified using swapinfo.

If gbde(8) is being used:

% swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/ada0s1b.bde   542720        0   542720     0%

If geli(8) is being used:

% swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/ada0s1b.eli   542720        0   542720     0%

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