Taikun Logo


Ceph Encryption


Starting with the Pacific release, image-level encryption can be handled internally by RBD clients. This means you can set a secret key that will be used to encrypt a specific RBD image. This page describes the scope of the RBD encryption feature.


The krbd kernel module does not support encryption at this time.


External tools (e.g. dm-crypt, QEMU) can be used as well to encrypt an RBD image, and the feature set and limitation set for that use may be different than described here.


By default, RBD images are not encrypted. To encrypt an RBD image, it needs to be formatted to one of the supported encryption formats. The format operation persists encryption metadata to the image. The encryption metadata usually includes information such as the encryption format and version, cipher algorithm and mode specification, as well as information used to secure the encryption key. The encryption key itself is protected by a user-kept secret (usually a passphrase), which is never persisted. The basic encryption format operation will require specifying the encryption format and a secret.

Some of the encryption metadata may be stored as part of the image data, typically an encryption header will be written to the beginning of the raw image data. This means that the effective image size of the encrypted image may be lower than the raw image size. See the Supported Formats section for more details.


Currently only flat images (i.e. not cloned) can be formatted. Clones of an encrypted image are inherently encrypted using the same format and secret.


Any data written to the image prior to its format may become unreadable, though it may still occupy storage resources.


Images with the journal feature enabled cannot be formatted and encrypted by RBD clients.


Formatting an image is a necessary pre-requisite for enabling encryption. However, formatted images will still be treated as raw unencrypted images by all of the RBD APIs. In particular, an encrypted RBD image can be opened by the same APIs as any other image, and raw unencrypted data can be read / written. Such raw IOs may risk the integrity of the encryption format, for example by overriding encryption metadata located at the beginning of the image.

In order to safely perform encrypted IO on the formatted image, an additional encryption load operation should be applied after opening the image. The encryption load operation requires supplying the encryption format and a secret for unlocking the encryption key. Following a successful encryption load operation, all IOs for the opened image will be encrypted / decrypted. For a cloned image, this includes IOs for ancestor images as well. The encryption key will be stored in-memory by the RBD client until the image is closed.


Once encryption has been loaded, no other encryption load / format operations can be applied to the context of the opened image.


Once encryption has been loaded, API calls for retrieving the image size using the opened image context will return the effective image size.


Encryption load can be automatically applied when mounting RBD images as block devices via rbd-nbd.



Both LUKS1 and LUKS2 are supported. The data layout is fully compliant with the LUKS specification. Thus, images formatted by RBD can be loaded using external LUKS-supporting tools such as dm-crypt or QEMU. Furthermore, existing LUKS data, created outside of RBD, can be imported (by copying the raw LUKS data into the image) and loaded by RBD encryption.


The LUKS formats are supported on Linux-based systems only.


Currently, only AES-128 and AES-256 encryption algorithms are supported. Additionally, xts-plain64 is currently the only supported encryption mode.

To use the LUKS format, start by formatting the image:

$ rbd encryption format {pool-name}/{image-name} {luks1|luks2} {passphrase-file} [–cipher-alg {aes-128 | aes-256}]

The encryption format operation generates a LUKS header and writes it to the beginning of the image. The header is appended with a single keyslot holding a randomly-generated encryption key, and is protected by the passphrase read from passphrase-file.


If the content of passphrase-file ends with a newline character, it will be stripped off.

By default, AES-256 in xts-plain64 mode (which is the current recommended mode, and the usual default for other tools) will be used. The format operation allows selecting AES-128 as well. Adding / removing passphrases is currently not supported by RBD, but can be applied to the raw RBD data using compatible tools such as cryptsetup.

The LUKS header size can vary (up to 136MiB in LUKS2), but is usually up to 16MiB, depending 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 imageconfigured 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 we set the default sector size 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 IO races which will further deteriorate performance. Thus it is advisable to avoid using RBD encryption in cases where incoming writes cannot be guaranteed to be sector-aligned.

To mount a LUKS-encrypted image run:

$ rbd -p {pool-name} device map -t nbd -o encryption-format={luks1|luks2},encryption-passphrase-file={passphrase-file}

Note that for security reasons, both the encryption format and encryption load operations are CPU-intensive, and may take a few seconds to complete. For the encryption operations of actual image IO, assuming AES-NI is enabled, a relative small microseconds latency should be added, as well as a small increase in CPU utilization.

Was this article helpful?