--- /dev/null
+======================
+ Image Encryption
+======================
+
+.. index:: Ceph Block Device; 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.
+
+.. note::
+ The ``krbd`` kernel module does not support encryption at this time.
+
+.. note::
+ 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.
+
+Encryption Format
+=================
+
+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.
+
+.. note::
+ 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.
+
+.. note::
+ Any data written to the image prior to its format may become unreadable,
+ though it may still occupy storage resources.
+
+.. note::
+ Images with the `journal feature`_ enabled cannot be formatted and encrypted
+ by RBD clients.
+
+Encryption Load
+=================
+
+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.
+
+.. note::
+ Once encryption has been loaded, no other encryption load / format
+ operations can be applied to the context of the opened image.
+
+.. note::
+ Once encryption has been loaded, API calls for retrieving the image size
+ using the opened image context will return the effective image size.
+
+.. note::
+ Encryption load can be automatically applied when mounting RBD images as
+ block devices via `rbd-nbd`_.
+
+Supported Formats
+=================
+
+LUKS
+~~~~~~~
+
+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.
+
+.. note::
+ The LUKS formats are supported on Linux-based systems only.
+
+.. note::
+ 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`.
+
+.. note::
+ 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 (upto 136MiB in LUKS2), but is usually upto
+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.
+
+.. _journal feature: ../rbd-mirroring/#enable-image-journaling-feature
+.. _Supported Formats: #supported-formats
+.. _rbd-nbd: ../../man/8/rbd-nbd
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