Chapter 4. Image encryption

As a storage administrator, you can set a secret key that is used to encrypt a specific RBD image. Image level encryption is handled internally by RBD clients.

Note

The krbd module does not support image level encryption.

Note

You can use external tools such as dm-crypt or QEMU to encrypt an RBD image.

4.1. Prerequisites

  • A running Red Hat Ceph Storage 5 cluster.
  • root level permissions.

4.2. Encryption format

RBD images are not encrypted by default. You can encrypt an RBD image by formatting to one of the supported encryption formats. The format operation persists the encryption metadata to the RBD image. The encryption metadata includes information such as the encryption format and version, cipher algorithm and mode specifications, as well as the information used to secure the encryption key.

The encryption key is protected by a user kept secret that is a passphrase, which is never stored as persistent data in the RBD image. The encryption format operation requires you to specify the encryption format, cipher algorithm, and mode specification as well as a passphrase. The encryption metadata is stored in the RBD image, currently as an encryption header that is written at the start of the raw image. This means that the effective image size of the encrypted image would be lower than the raw image size.

Note

Currently you can only encrypt flat RBD images. Clones of an encrypted RBD image are inherently encrypted using the same encryption profile and passphrase.

Note

Any data written to the RBD image before formatting might become unreadable, even though it might still occupy storage resources. RBD images with the journal feature enabled cannot be encrypted.

4.3. Encryption load

By default, all RBD APIs treat encrypted RBD images the same way as unencrypted RBD images. You can read or write raw data anywhere in the image. Writing raw data into the image might risk the integrity of the encryption format. For example, the raw data could override the encryption metadata located at the beginning of the image. To safely perform encrypted Input/Outout(I/O) or maintenance operations on the encrypted RBD image, an additional encryption load operation must be applied immediately after opening the image.

The encryption load operation requires you to specify the encryption format and a passphrase. All I/Os for the opened RBD image are encrypted or decrypted, for a cloned RBD image, this includes IOs for the parent images. The encryption key is stored in memory by the RBD client until the image is closed.

Note

Once the encryption is loaded on the RBD image, no other encryption load or format operation can be applied. Additionally, API calls for retrieving the RBD image size using the opened image context return the effective image size. The encryption is loaded automatically when mapping the RBD images as block devices through rbd-nbd.

4.4. Supported formats

Both Linux Unified Key Setup (LUKS) 1 and 2 are supported. The data layout is fully compliant with the LUKS specification. External LUKS compatible tools such as dm-crypt or QEMU can safely perform encrypted Input/Outout (I/O) on encrypted RBD images. Additionally, you can import existing LUKS images created by external tools, by copying the raw LUKS data into the RBD image.

Currently, only Advanced Encryption Standards (AES) 128 and 256 encryption algorithms are supported. xts-plain64 is currently the only supported encryption mode.

To use the LUKS format, format the RBD image with the following command:

Note

You need to create a file named passphrase.txt and enter a passphrase. You can randomly generate the passphrase, which might contain NULL characters. If the passphrase ends with a newline character, it will be stripped off.

Syntax

rbd encryption format POOL_NAME/LUKS_IMAGE luks1|luks2 passphrase.txt

Example

[ceph: root@host01 /]# rbd encryption format pool1/luksimage1 luks1 passphrase.txt

Note

You can select either luks1 or luks encryption format.

The encryption format operation generates a LUKS header and writes it at the start of the RBD image. A single keyslot is appended to the header. The keyslot holds a randomly generated encryption key, and is protected by the passphrase read from the passphrase file. By default, AES-256 in xts-plain64 mode, which is the current recommended mode and the default for other LUKS tools, is used. Adding or removing additional passphrases is currently not supported natively, but can be achieved using LUKS tools such as cryptsetup. The LUKS header size can vary that is upto 136MiB in LUKS, but it is usually upto 16MiB, dependent on the version of libcryptsetup installed. For optimal performance, the encryption format will set the data offset to be aligned with the image object size. For example, expect a minimum overhead of 8MiB if using an image configured with an 8MiB object size.

In LUKS1, sectors, which are the minimal encryption units, are fixed at 512 bytes. LUKS2 supports larger sectors, and for better performance, the default sector size is set to the maximum of 4KiB. Writes which are either smaller than a sector, or are not aligned to a sector start, will trigger a guarded read-modify-write chain on the client, with a considerable latency penalty. A batch of such unaligned writes can lead to I/O races which will further deteriorate performance. Red Hat recommends to avoid using RBD encryption in cases where incoming writes cannot be guaranteed to be LUKS sector aligned.

To map a LUKS encrypted image, run the following command:

Syntax

rbd device map -t nbd -o encryption-format=luks1|luks2,encryption-passphrase-file=passphrase.txt POOL_NAME/LUKS_IMAGE

Example

[ceph: root@host01 /]# rbd device map -t nbd -o encryption-format=luks1,encryption-passphrase-file=passphrase.txt pool1/luksimage1

Note

You can select either luks1 or luks2 encryption format.

Note

For security reasons, both the encryption format and encryption load operations are CPU-intensive, and may take a few seconds to complete. For encrypted I/O, assuming AES-NI is enabled, a relative small microseconds latency might be added, as well as a small increase in CPU utilization.