1 =====================================
2 Filesystem-level encryption (fscrypt)
3 =====================================
8 fscrypt is a library which filesystems can hook into to support
9 transparent encryption of files and directories.
11 Note: "fscrypt" in this document refers to the kernel-level portion,
12 implemented in ``fs/crypto/``, as opposed to the userspace tool
13 `fscrypt <https://github.com/google/fscrypt>`_. This document only
14 covers the kernel-level portion. For command-line examples of how to
15 use encryption, see the documentation for the userspace tool `fscrypt
16 <https://github.com/google/fscrypt>`_. Also, it is recommended to use
17 the fscrypt userspace tool, or other existing userspace tools such as
18 `fscryptctl <https://github.com/google/fscryptctl>`_ or `Android's key
20 <https://source.android.com/security/encryption/file-based>`_, over
21 using the kernel's API directly. Using existing tools reduces the
22 chance of introducing your own security bugs. (Nevertheless, for
23 completeness this documentation covers the kernel's API anyway.)
25 Unlike dm-crypt, fscrypt operates at the filesystem level rather than
26 at the block device level. This allows it to encrypt different files
27 with different keys and to have unencrypted files on the same
28 filesystem. This is useful for multi-user systems where each user's
29 data-at-rest needs to be cryptographically isolated from the others.
30 However, except for filenames, fscrypt does not encrypt filesystem
33 Unlike eCryptfs, which is a stacked filesystem, fscrypt is integrated
34 directly into supported filesystems --- currently ext4, F2FS, and
35 UBIFS. This allows encrypted files to be read and written without
36 caching both the decrypted and encrypted pages in the pagecache,
37 thereby nearly halving the memory used and bringing it in line with
38 unencrypted files. Similarly, half as many dentries and inodes are
39 needed. eCryptfs also limits encrypted filenames to 143 bytes,
40 causing application compatibility issues; fscrypt allows the full 255
41 bytes (NAME_MAX). Finally, unlike eCryptfs, the fscrypt API can be
42 used by unprivileged users, with no need to mount anything.
44 fscrypt does not support encrypting files in-place. Instead, it
45 supports marking an empty directory as encrypted. Then, after
46 userspace provides the key, all regular files, directories, and
47 symbolic links created in that directory tree are transparently
56 Provided that userspace chooses a strong encryption key, fscrypt
57 protects the confidentiality of file contents and filenames in the
58 event of a single point-in-time permanent offline compromise of the
59 block device content. fscrypt does not protect the confidentiality of
60 non-filename metadata, e.g. file sizes, file permissions, file
61 timestamps, and extended attributes. Also, the existence and location
62 of holes (unallocated blocks which logically contain all zeroes) in
63 files is not protected.
65 fscrypt is not guaranteed to protect confidentiality or authenticity
66 if an attacker is able to manipulate the filesystem offline prior to
67 an authorized user later accessing the filesystem.
72 fscrypt (and storage encryption in general) can only provide limited
73 protection, if any at all, against online attacks. In detail:
78 fscrypt is only resistant to side-channel attacks, such as timing or
79 electromagnetic attacks, to the extent that the underlying Linux
80 Cryptographic API algorithms are. If a vulnerable algorithm is used,
81 such as a table-based implementation of AES, it may be possible for an
82 attacker to mount a side channel attack against the online system.
83 Side channel attacks may also be mounted against applications
84 consuming decrypted data.
86 Unauthorized file access
87 ~~~~~~~~~~~~~~~~~~~~~~~~
89 After an encryption key has been added, fscrypt does not hide the
90 plaintext file contents or filenames from other users on the same
91 system. Instead, existing access control mechanisms such as file mode
92 bits, POSIX ACLs, LSMs, or namespaces should be used for this purpose.
94 (For the reasoning behind this, understand that while the key is
95 added, the confidentiality of the data, from the perspective of the
96 system itself, is *not* protected by the mathematical properties of
97 encryption but rather only by the correctness of the kernel.
98 Therefore, any encryption-specific access control checks would merely
99 be enforced by kernel *code* and therefore would be largely redundant
100 with the wide variety of access control mechanisms already available.)
102 Kernel memory compromise
103 ~~~~~~~~~~~~~~~~~~~~~~~~
105 An attacker who compromises the system enough to read from arbitrary
106 memory, e.g. by mounting a physical attack or by exploiting a kernel
107 security vulnerability, can compromise all encryption keys that are
110 However, fscrypt allows encryption keys to be removed from the kernel,
111 which may protect them from later compromise.
113 In more detail, the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl (or the
114 FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl) can wipe a master
115 encryption key from kernel memory. If it does so, it will also try to
116 evict all cached inodes which had been "unlocked" using the key,
117 thereby wiping their per-file keys and making them once again appear
118 "locked", i.e. in ciphertext or encrypted form.
120 However, these ioctls have some limitations:
122 - Per-file keys for in-use files will *not* be removed or wiped.
123 Therefore, for maximum effect, userspace should close the relevant
124 encrypted files and directories before removing a master key, as
125 well as kill any processes whose working directory is in an affected
128 - The kernel cannot magically wipe copies of the master key(s) that
129 userspace might have as well. Therefore, userspace must wipe all
130 copies of the master key(s) it makes as well; normally this should
131 be done immediately after FS_IOC_ADD_ENCRYPTION_KEY, without waiting
132 for FS_IOC_REMOVE_ENCRYPTION_KEY. Naturally, the same also applies
133 to all higher levels in the key hierarchy. Userspace should also
134 follow other security precautions such as mlock()ing memory
135 containing keys to prevent it from being swapped out.
137 - In general, decrypted contents and filenames in the kernel VFS
138 caches are freed but not wiped. Therefore, portions thereof may be
139 recoverable from freed memory, even after the corresponding key(s)
140 were wiped. To partially solve this, you can set
141 CONFIG_PAGE_POISONING=y in your kernel config and add page_poison=1
142 to your kernel command line. However, this has a performance cost.
144 - Secret keys might still exist in CPU registers, in crypto
145 accelerator hardware (if used by the crypto API to implement any of
146 the algorithms), or in other places not explicitly considered here.
148 Limitations of v1 policies
149 ~~~~~~~~~~~~~~~~~~~~~~~~~~
151 v1 encryption policies have some weaknesses with respect to online
154 - There is no verification that the provided master key is correct.
155 Therefore, a malicious user can temporarily associate the wrong key
156 with another user's encrypted files to which they have read-only
157 access. Because of filesystem caching, the wrong key will then be
158 used by the other user's accesses to those files, even if the other
159 user has the correct key in their own keyring. This violates the
160 meaning of "read-only access".
162 - A compromise of a per-file key also compromises the master key from
163 which it was derived.
165 - Non-root users cannot securely remove encryption keys.
167 All the above problems are fixed with v2 encryption policies. For
168 this reason among others, it is recommended to use v2 encryption
169 policies on all new encrypted directories.
177 Each encrypted directory tree is protected by a *master key*. Master
178 keys can be up to 64 bytes long, and must be at least as long as the
179 greater of the key length needed by the contents and filenames
180 encryption modes being used. For example, if AES-256-XTS is used for
181 contents encryption, the master key must be 64 bytes (512 bits). Note
182 that the XTS mode is defined to require a key twice as long as that
183 required by the underlying block cipher.
185 To "unlock" an encrypted directory tree, userspace must provide the
186 appropriate master key. There can be any number of master keys, each
187 of which protects any number of directory trees on any number of
190 Master keys must be real cryptographic keys, i.e. indistinguishable
191 from random bytestrings of the same length. This implies that users
192 **must not** directly use a password as a master key, zero-pad a
193 shorter key, or repeat a shorter key. Security cannot be guaranteed
194 if userspace makes any such error, as the cryptographic proofs and
195 analysis would no longer apply.
197 Instead, users should generate master keys either using a
198 cryptographically secure random number generator, or by using a KDF
199 (Key Derivation Function). The kernel does not do any key stretching;
200 therefore, if userspace derives the key from a low-entropy secret such
201 as a passphrase, it is critical that a KDF designed for this purpose
202 be used, such as scrypt, PBKDF2, or Argon2.
204 Key derivation function
205 -----------------------
207 With one exception, fscrypt never uses the master key(s) for
208 encryption directly. Instead, they are only used as input to a KDF
209 (Key Derivation Function) to derive the actual keys.
211 The KDF used for a particular master key differs depending on whether
212 the key is used for v1 encryption policies or for v2 encryption
213 policies. Users **must not** use the same key for both v1 and v2
214 encryption policies. (No real-world attack is currently known on this
215 specific case of key reuse, but its security cannot be guaranteed
216 since the cryptographic proofs and analysis would no longer apply.)
218 For v1 encryption policies, the KDF only supports deriving per-file
219 encryption keys. It works by encrypting the master key with
220 AES-128-ECB, using the file's 16-byte nonce as the AES key. The
221 resulting ciphertext is used as the derived key. If the ciphertext is
222 longer than needed, then it is truncated to the needed length.
224 For v2 encryption policies, the KDF is HKDF-SHA512. The master key is
225 passed as the "input keying material", no salt is used, and a distinct
226 "application-specific information string" is used for each distinct
227 key to be derived. For example, when a per-file encryption key is
228 derived, the application-specific information string is the file's
229 nonce prefixed with "fscrypt\\0" and a context byte. Different
230 context bytes are used for other types of derived keys.
232 HKDF-SHA512 is preferred to the original AES-128-ECB based KDF because
233 HKDF is more flexible, is nonreversible, and evenly distributes
234 entropy from the master key. HKDF is also standardized and widely
235 used by other software, whereas the AES-128-ECB based KDF is ad-hoc.
240 Since each master key can protect many files, it is necessary to
241 "tweak" the encryption of each file so that the same plaintext in two
242 files doesn't map to the same ciphertext, or vice versa. In most
243 cases, fscrypt does this by deriving per-file keys. When a new
244 encrypted inode (regular file, directory, or symlink) is created,
245 fscrypt randomly generates a 16-byte nonce and stores it in the
246 inode's encryption xattr. Then, it uses a KDF (as described in `Key
247 derivation function`_) to derive the file's key from the master key
250 Key derivation was chosen over key wrapping because wrapped keys would
251 require larger xattrs which would be less likely to fit in-line in the
252 filesystem's inode table, and there didn't appear to be any
253 significant advantages to key wrapping. In particular, currently
254 there is no requirement to support unlocking a file with multiple
255 alternative master keys or to support rotating master keys. Instead,
256 the master keys may be wrapped in userspace, e.g. as is done by the
257 `fscrypt <https://github.com/google/fscrypt>`_ tool.
262 The Adiantum encryption mode (see `Encryption modes and usage`_) is
263 suitable for both contents and filenames encryption, and it accepts
264 long IVs --- long enough to hold both an 8-byte logical block number
265 and a 16-byte per-file nonce. Also, the overhead of each Adiantum key
266 is greater than that of an AES-256-XTS key.
268 Therefore, to improve performance and save memory, for Adiantum a
269 "direct key" configuration is supported. When the user has enabled
270 this by setting FSCRYPT_POLICY_FLAG_DIRECT_KEY in the fscrypt policy,
271 per-file keys are not used. Instead, whenever any data (contents or
272 filenames) is encrypted, the file's 16-byte nonce is included in the
275 - For v1 encryption policies, the encryption is done directly with the
276 master key. Because of this, users **must not** use the same master
277 key for any other purpose, even for other v1 policies.
279 - For v2 encryption policies, the encryption is done with a per-mode
280 key derived using the KDF. Users may use the same master key for
281 other v2 encryption policies.
283 IV_INO_LBLK_64 policies
284 -----------------------
286 When FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64 is set in the fscrypt policy,
287 the encryption keys are derived from the master key, encryption mode
288 number, and filesystem UUID. This normally results in all files
289 protected by the same master key sharing a single contents encryption
290 key and a single filenames encryption key. To still encrypt different
291 files' data differently, inode numbers are included in the IVs.
292 Consequently, shrinking the filesystem may not be allowed.
294 This format is optimized for use with inline encryption hardware
295 compliant with the UFS or eMMC standards, which support only 64 IV
296 bits per I/O request and may have only a small number of keyslots.
301 For master keys used for v2 encryption policies, a unique 16-byte "key
302 identifier" is also derived using the KDF. This value is stored in
303 the clear, since it is needed to reliably identify the key itself.
305 Encryption modes and usage
306 ==========================
308 fscrypt allows one encryption mode to be specified for file contents
309 and one encryption mode to be specified for filenames. Different
310 directory trees are permitted to use different encryption modes.
311 Currently, the following pairs of encryption modes are supported:
313 - AES-256-XTS for contents and AES-256-CTS-CBC for filenames
314 - AES-128-CBC for contents and AES-128-CTS-CBC for filenames
315 - Adiantum for both contents and filenames
317 If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.
319 AES-128-CBC was added only for low-powered embedded devices with
320 crypto accelerators such as CAAM or CESA that do not support XTS. To
321 use AES-128-CBC, CONFIG_CRYPTO_ESSIV and CONFIG_CRYPTO_SHA256 (or
322 another SHA-256 implementation) must be enabled so that ESSIV can be
325 Adiantum is a (primarily) stream cipher-based mode that is fast even
326 on CPUs without dedicated crypto instructions. It's also a true
327 wide-block mode, unlike XTS. It can also eliminate the need to derive
328 per-file keys. However, it depends on the security of two primitives,
329 XChaCha12 and AES-256, rather than just one. See the paper
330 "Adiantum: length-preserving encryption for entry-level processors"
331 (https://eprint.iacr.org/2018/720.pdf) for more details. To use
332 Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled. Also, fast
333 implementations of ChaCha and NHPoly1305 should be enabled, e.g.
334 CONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM.
336 New encryption modes can be added relatively easily, without changes
337 to individual filesystems. However, authenticated encryption (AE)
338 modes are not currently supported because of the difficulty of dealing
339 with ciphertext expansion.
344 For file contents, each filesystem block is encrypted independently.
345 Starting from Linux kernel 5.5, encryption of filesystems with block
346 size less than system's page size is supported.
348 Each block's IV is set to the logical block number within the file as
349 a little endian number, except that:
351 - With CBC mode encryption, ESSIV is also used. Specifically, each IV
352 is encrypted with AES-256 where the AES-256 key is the SHA-256 hash
353 of the file's data encryption key.
355 - With `DIRECT_KEY policies`_, the file's nonce is appended to the IV.
356 Currently this is only allowed with the Adiantum encryption mode.
358 - With `IV_INO_LBLK_64 policies`_, the logical block number is limited
359 to 32 bits and is placed in bits 0-31 of the IV. The inode number
360 (which is also limited to 32 bits) is placed in bits 32-63.
362 Note that because file logical block numbers are included in the IVs,
363 filesystems must enforce that blocks are never shifted around within
364 encrypted files, e.g. via "collapse range" or "insert range".
369 For filenames, each full filename is encrypted at once. Because of
370 the requirements to retain support for efficient directory lookups and
371 filenames of up to 255 bytes, the same IV is used for every filename
374 However, each encrypted directory still uses a unique key, or
375 alternatively has the file's nonce (for `DIRECT_KEY policies`_) or
376 inode number (for `IV_INO_LBLK_64 policies`_) included in the IVs.
377 Thus, IV reuse is limited to within a single directory.
379 With CTS-CBC, the IV reuse means that when the plaintext filenames
380 share a common prefix at least as long as the cipher block size (16
381 bytes for AES), the corresponding encrypted filenames will also share
382 a common prefix. This is undesirable. Adiantum does not have this
383 weakness, as it is a wide-block encryption mode.
385 All supported filenames encryption modes accept any plaintext length
386 >= 16 bytes; cipher block alignment is not required. However,
387 filenames shorter than 16 bytes are NUL-padded to 16 bytes before
388 being encrypted. In addition, to reduce leakage of filename lengths
389 via their ciphertexts, all filenames are NUL-padded to the next 4, 8,
390 16, or 32-byte boundary (configurable). 32 is recommended since this
391 provides the best confidentiality, at the cost of making directory
392 entries consume slightly more space. Note that since NUL (``\0``) is
393 not otherwise a valid character in filenames, the padding will never
394 produce duplicate plaintexts.
396 Symbolic link targets are considered a type of filename and are
397 encrypted in the same way as filenames in directory entries, except
398 that IV reuse is not a problem as each symlink has its own inode.
403 Setting an encryption policy
404 ----------------------------
406 FS_IOC_SET_ENCRYPTION_POLICY
407 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
409 The FS_IOC_SET_ENCRYPTION_POLICY ioctl sets an encryption policy on an
410 empty directory or verifies that a directory or regular file already
411 has the specified encryption policy. It takes in a pointer to a
412 :c:type:`struct fscrypt_policy_v1` or a :c:type:`struct
413 fscrypt_policy_v2`, defined as follows::
415 #define FSCRYPT_POLICY_V1 0
416 #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8
417 struct fscrypt_policy_v1 {
419 __u8 contents_encryption_mode;
420 __u8 filenames_encryption_mode;
422 __u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
424 #define fscrypt_policy fscrypt_policy_v1
426 #define FSCRYPT_POLICY_V2 2
427 #define FSCRYPT_KEY_IDENTIFIER_SIZE 16
428 struct fscrypt_policy_v2 {
430 __u8 contents_encryption_mode;
431 __u8 filenames_encryption_mode;
434 __u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
437 This structure must be initialized as follows:
439 - ``version`` must be FSCRYPT_POLICY_V1 (0) if the struct is
440 :c:type:`fscrypt_policy_v1` or FSCRYPT_POLICY_V2 (2) if the struct
441 is :c:type:`fscrypt_policy_v2`. (Note: we refer to the original
442 policy version as "v1", though its version code is really 0.) For
443 new encrypted directories, use v2 policies.
445 - ``contents_encryption_mode`` and ``filenames_encryption_mode`` must
446 be set to constants from ``<linux/fscrypt.h>`` which identify the
447 encryption modes to use. If unsure, use FSCRYPT_MODE_AES_256_XTS
448 (1) for ``contents_encryption_mode`` and FSCRYPT_MODE_AES_256_CTS
449 (4) for ``filenames_encryption_mode``.
451 - ``flags`` contains optional flags from ``<linux/fscrypt.h>``:
453 - FSCRYPT_POLICY_FLAGS_PAD_*: The amount of NUL padding to use when
454 encrypting filenames. If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32
456 - FSCRYPT_POLICY_FLAG_DIRECT_KEY: See `DIRECT_KEY policies`_.
457 - FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64: See `IV_INO_LBLK_64
458 policies`_. This is mutually exclusive with DIRECT_KEY and is not
459 supported on v1 policies.
461 - For v2 encryption policies, ``__reserved`` must be zeroed.
463 - For v1 encryption policies, ``master_key_descriptor`` specifies how
464 to find the master key in a keyring; see `Adding keys`_. It is up
465 to userspace to choose a unique ``master_key_descriptor`` for each
466 master key. The e4crypt and fscrypt tools use the first 8 bytes of
467 ``SHA-512(SHA-512(master_key))``, but this particular scheme is not
468 required. Also, the master key need not be in the keyring yet when
469 FS_IOC_SET_ENCRYPTION_POLICY is executed. However, it must be added
470 before any files can be created in the encrypted directory.
472 For v2 encryption policies, ``master_key_descriptor`` has been
473 replaced with ``master_key_identifier``, which is longer and cannot
474 be arbitrarily chosen. Instead, the key must first be added using
475 `FS_IOC_ADD_ENCRYPTION_KEY`_. Then, the ``key_spec.u.identifier``
476 the kernel returned in the :c:type:`struct fscrypt_add_key_arg` must
477 be used as the ``master_key_identifier`` in the :c:type:`struct
480 If the file is not yet encrypted, then FS_IOC_SET_ENCRYPTION_POLICY
481 verifies that the file is an empty directory. If so, the specified
482 encryption policy is assigned to the directory, turning it into an
483 encrypted directory. After that, and after providing the
484 corresponding master key as described in `Adding keys`_, all regular
485 files, directories (recursively), and symlinks created in the
486 directory will be encrypted, inheriting the same encryption policy.
487 The filenames in the directory's entries will be encrypted as well.
489 Alternatively, if the file is already encrypted, then
490 FS_IOC_SET_ENCRYPTION_POLICY validates that the specified encryption
491 policy exactly matches the actual one. If they match, then the ioctl
492 returns 0. Otherwise, it fails with EEXIST. This works on both
493 regular files and directories, including nonempty directories.
495 When a v2 encryption policy is assigned to a directory, it is also
496 required that either the specified key has been added by the current
497 user or that the caller has CAP_FOWNER in the initial user namespace.
498 (This is needed to prevent a user from encrypting their data with
499 another user's key.) The key must remain added while
500 FS_IOC_SET_ENCRYPTION_POLICY is executing. However, if the new
501 encrypted directory does not need to be accessed immediately, then the
502 key can be removed right away afterwards.
504 Note that the ext4 filesystem does not allow the root directory to be
505 encrypted, even if it is empty. Users who want to encrypt an entire
506 filesystem with one key should consider using dm-crypt instead.
508 FS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors:
510 - ``EACCES``: the file is not owned by the process's uid, nor does the
511 process have the CAP_FOWNER capability in a namespace with the file
513 - ``EEXIST``: the file is already encrypted with an encryption policy
514 different from the one specified
515 - ``EINVAL``: an invalid encryption policy was specified (invalid
516 version, mode(s), or flags; or reserved bits were set)
517 - ``ENOKEY``: a v2 encryption policy was specified, but the key with
518 the specified ``master_key_identifier`` has not been added, nor does
519 the process have the CAP_FOWNER capability in the initial user
521 - ``ENOTDIR``: the file is unencrypted and is a regular file, not a
523 - ``ENOTEMPTY``: the file is unencrypted and is a nonempty directory
524 - ``ENOTTY``: this type of filesystem does not implement encryption
525 - ``EOPNOTSUPP``: the kernel was not configured with encryption
526 support for filesystems, or the filesystem superblock has not
527 had encryption enabled on it. (For example, to use encryption on an
528 ext4 filesystem, CONFIG_FS_ENCRYPTION must be enabled in the
529 kernel config, and the superblock must have had the "encrypt"
530 feature flag enabled using ``tune2fs -O encrypt`` or ``mkfs.ext4 -O
532 - ``EPERM``: this directory may not be encrypted, e.g. because it is
533 the root directory of an ext4 filesystem
534 - ``EROFS``: the filesystem is readonly
536 Getting an encryption policy
537 ----------------------------
539 Two ioctls are available to get a file's encryption policy:
541 - `FS_IOC_GET_ENCRYPTION_POLICY_EX`_
542 - `FS_IOC_GET_ENCRYPTION_POLICY`_
544 The extended (_EX) version of the ioctl is more general and is
545 recommended to use when possible. However, on older kernels only the
546 original ioctl is available. Applications should try the extended
547 version, and if it fails with ENOTTY fall back to the original
550 FS_IOC_GET_ENCRYPTION_POLICY_EX
551 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
553 The FS_IOC_GET_ENCRYPTION_POLICY_EX ioctl retrieves the encryption
554 policy, if any, for a directory or regular file. No additional
555 permissions are required beyond the ability to open the file. It
556 takes in a pointer to a :c:type:`struct fscrypt_get_policy_ex_arg`,
559 struct fscrypt_get_policy_ex_arg {
560 __u64 policy_size; /* input/output */
563 struct fscrypt_policy_v1 v1;
564 struct fscrypt_policy_v2 v2;
565 } policy; /* output */
568 The caller must initialize ``policy_size`` to the size available for
569 the policy struct, i.e. ``sizeof(arg.policy)``.
571 On success, the policy struct is returned in ``policy``, and its
572 actual size is returned in ``policy_size``. ``policy.version`` should
573 be checked to determine the version of policy returned. Note that the
574 version code for the "v1" policy is actually 0 (FSCRYPT_POLICY_V1).
576 FS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors:
578 - ``EINVAL``: the file is encrypted, but it uses an unrecognized
579 encryption policy version
580 - ``ENODATA``: the file is not encrypted
581 - ``ENOTTY``: this type of filesystem does not implement encryption,
582 or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX
583 (try FS_IOC_GET_ENCRYPTION_POLICY instead)
584 - ``EOPNOTSUPP``: the kernel was not configured with encryption
585 support for this filesystem, or the filesystem superblock has not
586 had encryption enabled on it
587 - ``EOVERFLOW``: the file is encrypted and uses a recognized
588 encryption policy version, but the policy struct does not fit into
591 Note: if you only need to know whether a file is encrypted or not, on
592 most filesystems it is also possible to use the FS_IOC_GETFLAGS ioctl
593 and check for FS_ENCRYPT_FL, or to use the statx() system call and
594 check for STATX_ATTR_ENCRYPTED in stx_attributes.
596 FS_IOC_GET_ENCRYPTION_POLICY
597 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
599 The FS_IOC_GET_ENCRYPTION_POLICY ioctl can also retrieve the
600 encryption policy, if any, for a directory or regular file. However,
601 unlike `FS_IOC_GET_ENCRYPTION_POLICY_EX`_,
602 FS_IOC_GET_ENCRYPTION_POLICY only supports the original policy
603 version. It takes in a pointer directly to a :c:type:`struct
604 fscrypt_policy_v1` rather than a :c:type:`struct
605 fscrypt_get_policy_ex_arg`.
607 The error codes for FS_IOC_GET_ENCRYPTION_POLICY are the same as those
608 for FS_IOC_GET_ENCRYPTION_POLICY_EX, except that
609 FS_IOC_GET_ENCRYPTION_POLICY also returns ``EINVAL`` if the file is
610 encrypted using a newer encryption policy version.
612 Getting the per-filesystem salt
613 -------------------------------
615 Some filesystems, such as ext4 and F2FS, also support the deprecated
616 ioctl FS_IOC_GET_ENCRYPTION_PWSALT. This ioctl retrieves a randomly
617 generated 16-byte value stored in the filesystem superblock. This
618 value is intended to used as a salt when deriving an encryption key
619 from a passphrase or other low-entropy user credential.
621 FS_IOC_GET_ENCRYPTION_PWSALT is deprecated. Instead, prefer to
622 generate and manage any needed salt(s) in userspace.
627 FS_IOC_ADD_ENCRYPTION_KEY
628 ~~~~~~~~~~~~~~~~~~~~~~~~~
630 The FS_IOC_ADD_ENCRYPTION_KEY ioctl adds a master encryption key to
631 the filesystem, making all files on the filesystem which were
632 encrypted using that key appear "unlocked", i.e. in plaintext form.
633 It can be executed on any file or directory on the target filesystem,
634 but using the filesystem's root directory is recommended. It takes in
635 a pointer to a :c:type:`struct fscrypt_add_key_arg`, defined as
638 struct fscrypt_add_key_arg {
639 struct fscrypt_key_specifier key_spec;
645 #define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR 1
646 #define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER 2
648 struct fscrypt_key_specifier {
649 __u32 type; /* one of FSCRYPT_KEY_SPEC_TYPE_* */
652 __u8 __reserved[32]; /* reserve some extra space */
653 __u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
654 __u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
658 :c:type:`struct fscrypt_add_key_arg` must be zeroed, then initialized
661 - If the key is being added for use by v1 encryption policies, then
662 ``key_spec.type`` must contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, and
663 ``key_spec.u.descriptor`` must contain the descriptor of the key
664 being added, corresponding to the value in the
665 ``master_key_descriptor`` field of :c:type:`struct
666 fscrypt_policy_v1`. To add this type of key, the calling process
667 must have the CAP_SYS_ADMIN capability in the initial user
670 Alternatively, if the key is being added for use by v2 encryption
671 policies, then ``key_spec.type`` must contain
672 FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, and ``key_spec.u.identifier`` is
673 an *output* field which the kernel fills in with a cryptographic
674 hash of the key. To add this type of key, the calling process does
675 not need any privileges. However, the number of keys that can be
676 added is limited by the user's quota for the keyrings service (see
677 ``Documentation/security/keys/core.rst``).
679 - ``raw_size`` must be the size of the ``raw`` key provided, in bytes.
681 - ``raw`` is a variable-length field which must contain the actual
682 key, ``raw_size`` bytes long.
684 For v2 policy keys, the kernel keeps track of which user (identified
685 by effective user ID) added the key, and only allows the key to be
686 removed by that user --- or by "root", if they use
687 `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_.
689 However, if another user has added the key, it may be desirable to
690 prevent that other user from unexpectedly removing it. Therefore,
691 FS_IOC_ADD_ENCRYPTION_KEY may also be used to add a v2 policy key
692 *again*, even if it's already added by other user(s). In this case,
693 FS_IOC_ADD_ENCRYPTION_KEY will just install a claim to the key for the
694 current user, rather than actually add the key again (but the raw key
695 must still be provided, as a proof of knowledge).
697 FS_IOC_ADD_ENCRYPTION_KEY returns 0 if either the key or a claim to
698 the key was either added or already exists.
700 FS_IOC_ADD_ENCRYPTION_KEY can fail with the following errors:
702 - ``EACCES``: FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR was specified, but the
703 caller does not have the CAP_SYS_ADMIN capability in the initial
705 - ``EDQUOT``: the key quota for this user would be exceeded by adding
707 - ``EINVAL``: invalid key size or key specifier type, or reserved bits
709 - ``ENOTTY``: this type of filesystem does not implement encryption
710 - ``EOPNOTSUPP``: the kernel was not configured with encryption
711 support for this filesystem, or the filesystem superblock has not
712 had encryption enabled on it
717 For v1 encryption policies, a master encryption key can also be
718 provided by adding it to a process-subscribed keyring, e.g. to a
719 session keyring, or to a user keyring if the user keyring is linked
720 into the session keyring.
722 This method is deprecated (and not supported for v2 encryption
723 policies) for several reasons. First, it cannot be used in
724 combination with FS_IOC_REMOVE_ENCRYPTION_KEY (see `Removing keys`_),
725 so for removing a key a workaround such as keyctl_unlink() in
726 combination with ``sync; echo 2 > /proc/sys/vm/drop_caches`` would
727 have to be used. Second, it doesn't match the fact that the
728 locked/unlocked status of encrypted files (i.e. whether they appear to
729 be in plaintext form or in ciphertext form) is global. This mismatch
730 has caused much confusion as well as real problems when processes
731 running under different UIDs, such as a ``sudo`` command, need to
732 access encrypted files.
734 Nevertheless, to add a key to one of the process-subscribed keyrings,
735 the add_key() system call can be used (see:
736 ``Documentation/security/keys/core.rst``). The key type must be
737 "logon"; keys of this type are kept in kernel memory and cannot be
738 read back by userspace. The key description must be "fscrypt:"
739 followed by the 16-character lower case hex representation of the
740 ``master_key_descriptor`` that was set in the encryption policy. The
741 key payload must conform to the following structure::
743 #define FSCRYPT_MAX_KEY_SIZE 64
747 __u8 raw[FSCRYPT_MAX_KEY_SIZE];
751 ``mode`` is ignored; just set it to 0. The actual key is provided in
752 ``raw`` with ``size`` indicating its size in bytes. That is, the
753 bytes ``raw[0..size-1]`` (inclusive) are the actual key.
755 The key description prefix "fscrypt:" may alternatively be replaced
756 with a filesystem-specific prefix such as "ext4:". However, the
757 filesystem-specific prefixes are deprecated and should not be used in
763 Two ioctls are available for removing a key that was added by
764 `FS_IOC_ADD_ENCRYPTION_KEY`_:
766 - `FS_IOC_REMOVE_ENCRYPTION_KEY`_
767 - `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_
769 These two ioctls differ only in cases where v2 policy keys are added
770 or removed by non-root users.
772 These ioctls don't work on keys that were added via the legacy
773 process-subscribed keyrings mechanism.
775 Before using these ioctls, read the `Kernel memory compromise`_
776 section for a discussion of the security goals and limitations of
779 FS_IOC_REMOVE_ENCRYPTION_KEY
780 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
782 The FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master
783 encryption key from the filesystem, and possibly removes the key
784 itself. It can be executed on any file or directory on the target
785 filesystem, but using the filesystem's root directory is recommended.
786 It takes in a pointer to a :c:type:`struct fscrypt_remove_key_arg`,
789 struct fscrypt_remove_key_arg {
790 struct fscrypt_key_specifier key_spec;
791 #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY 0x00000001
792 #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS 0x00000002
793 __u32 removal_status_flags; /* output */
797 This structure must be zeroed, then initialized as follows:
799 - The key to remove is specified by ``key_spec``:
801 - To remove a key used by v1 encryption policies, set
802 ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
803 in ``key_spec.u.descriptor``. To remove this type of key, the
804 calling process must have the CAP_SYS_ADMIN capability in the
805 initial user namespace.
807 - To remove a key used by v2 encryption policies, set
808 ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
809 in ``key_spec.u.identifier``.
811 For v2 policy keys, this ioctl is usable by non-root users. However,
812 to make this possible, it actually just removes the current user's
813 claim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY.
814 Only after all claims are removed is the key really removed.
816 For example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000,
817 then the key will be "claimed" by uid 1000, and
818 FS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000. Or, if
819 both uids 1000 and 2000 added the key, then for each uid
820 FS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim. Only
821 once *both* are removed is the key really removed. (Think of it like
822 unlinking a file that may have hard links.)
824 If FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also
825 try to "lock" all files that had been unlocked with the key. It won't
826 lock files that are still in-use, so this ioctl is expected to be used
827 in cooperation with userspace ensuring that none of the files are
828 still open. However, if necessary, this ioctl can be executed again
829 later to retry locking any remaining files.
831 FS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed
832 (but may still have files remaining to be locked), the user's claim to
833 the key was removed, or the key was already removed but had files
834 remaining to be the locked so the ioctl retried locking them. In any
835 of these cases, ``removal_status_flags`` is filled in with the
836 following informational status flags:
838 - ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s)
839 are still in-use. Not guaranteed to be set in the case where only
840 the user's claim to the key was removed.
841 - ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the
842 user's claim to the key was removed, not the key itself
844 FS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors:
846 - ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type
847 was specified, but the caller does not have the CAP_SYS_ADMIN
848 capability in the initial user namespace
849 - ``EINVAL``: invalid key specifier type, or reserved bits were set
850 - ``ENOKEY``: the key object was not found at all, i.e. it was never
851 added in the first place or was already fully removed including all
852 files locked; or, the user does not have a claim to the key (but
854 - ``ENOTTY``: this type of filesystem does not implement encryption
855 - ``EOPNOTSUPP``: the kernel was not configured with encryption
856 support for this filesystem, or the filesystem superblock has not
857 had encryption enabled on it
859 FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS
860 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
862 FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as
863 `FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the
864 ALL_USERS version of the ioctl will remove all users' claims to the
865 key, not just the current user's. I.e., the key itself will always be
866 removed, no matter how many users have added it. This difference is
867 only meaningful if non-root users are adding and removing keys.
869 Because of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires
870 "root", namely the CAP_SYS_ADMIN capability in the initial user
871 namespace. Otherwise it will fail with EACCES.
876 FS_IOC_GET_ENCRYPTION_KEY_STATUS
877 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
879 The FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl retrieves the status of a
880 master encryption key. It can be executed on any file or directory on
881 the target filesystem, but using the filesystem's root directory is
882 recommended. It takes in a pointer to a :c:type:`struct
883 fscrypt_get_key_status_arg`, defined as follows::
885 struct fscrypt_get_key_status_arg {
887 struct fscrypt_key_specifier key_spec;
891 #define FSCRYPT_KEY_STATUS_ABSENT 1
892 #define FSCRYPT_KEY_STATUS_PRESENT 2
893 #define FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED 3
895 #define FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF 0x00000001
898 __u32 __out_reserved[13];
901 The caller must zero all input fields, then fill in ``key_spec``:
903 - To get the status of a key for v1 encryption policies, set
904 ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
905 in ``key_spec.u.descriptor``.
907 - To get the status of a key for v2 encryption policies, set
908 ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
909 in ``key_spec.u.identifier``.
911 On success, 0 is returned and the kernel fills in the output fields:
913 - ``status`` indicates whether the key is absent, present, or
914 incompletely removed. Incompletely removed means that the master
915 secret has been removed, but some files are still in use; i.e.,
916 `FS_IOC_REMOVE_ENCRYPTION_KEY`_ returned 0 but set the informational
917 status flag FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY.
919 - ``status_flags`` can contain the following flags:
921 - ``FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF`` indicates that the key
922 has added by the current user. This is only set for keys
923 identified by ``identifier`` rather than by ``descriptor``.
925 - ``user_count`` specifies the number of users who have added the key.
926 This is only set for keys identified by ``identifier`` rather than
929 FS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors:
931 - ``EINVAL``: invalid key specifier type, or reserved bits were set
932 - ``ENOTTY``: this type of filesystem does not implement encryption
933 - ``EOPNOTSUPP``: the kernel was not configured with encryption
934 support for this filesystem, or the filesystem superblock has not
935 had encryption enabled on it
937 Among other use cases, FS_IOC_GET_ENCRYPTION_KEY_STATUS can be useful
938 for determining whether the key for a given encrypted directory needs
939 to be added before prompting the user for the passphrase needed to
942 FS_IOC_GET_ENCRYPTION_KEY_STATUS can only get the status of keys in
943 the filesystem-level keyring, i.e. the keyring managed by
944 `FS_IOC_ADD_ENCRYPTION_KEY`_ and `FS_IOC_REMOVE_ENCRYPTION_KEY`_. It
945 cannot get the status of a key that has only been added for use by v1
946 encryption policies using the legacy mechanism involving
947 process-subscribed keyrings.
955 With the encryption key, encrypted regular files, directories, and
956 symlinks behave very similarly to their unencrypted counterparts ---
957 after all, the encryption is intended to be transparent. However,
958 astute users may notice some differences in behavior:
960 - Unencrypted files, or files encrypted with a different encryption
961 policy (i.e. different key, modes, or flags), cannot be renamed or
962 linked into an encrypted directory; see `Encryption policy
963 enforcement`_. Attempts to do so will fail with EXDEV. However,
964 encrypted files can be renamed within an encrypted directory, or
965 into an unencrypted directory.
967 Note: "moving" an unencrypted file into an encrypted directory, e.g.
968 with the `mv` program, is implemented in userspace by a copy
969 followed by a delete. Be aware that the original unencrypted data
970 may remain recoverable from free space on the disk; prefer to keep
971 all files encrypted from the very beginning. The `shred` program
972 may be used to overwrite the source files but isn't guaranteed to be
973 effective on all filesystems and storage devices.
975 - Direct I/O is not supported on encrypted files. Attempts to use
976 direct I/O on such files will fall back to buffered I/O.
978 - The fallocate operations FALLOC_FL_COLLAPSE_RANGE,
979 FALLOC_FL_INSERT_RANGE, and FALLOC_FL_ZERO_RANGE are not supported
980 on encrypted files and will fail with EOPNOTSUPP.
982 - Online defragmentation of encrypted files is not supported. The
983 EXT4_IOC_MOVE_EXT and F2FS_IOC_MOVE_RANGE ioctls will fail with
986 - The ext4 filesystem does not support data journaling with encrypted
987 regular files. It will fall back to ordered data mode instead.
989 - DAX (Direct Access) is not supported on encrypted files.
991 - The st_size of an encrypted symlink will not necessarily give the
992 length of the symlink target as required by POSIX. It will actually
993 give the length of the ciphertext, which will be slightly longer
994 than the plaintext due to NUL-padding and an extra 2-byte overhead.
996 - The maximum length of an encrypted symlink is 2 bytes shorter than
997 the maximum length of an unencrypted symlink. For example, on an
998 EXT4 filesystem with a 4K block size, unencrypted symlinks can be up
999 to 4095 bytes long, while encrypted symlinks can only be up to 4093
1000 bytes long (both lengths excluding the terminating null).
1002 Note that mmap *is* supported. This is possible because the pagecache
1003 for an encrypted file contains the plaintext, not the ciphertext.
1008 Some filesystem operations may be performed on encrypted regular
1009 files, directories, and symlinks even before their encryption key has
1010 been added, or after their encryption key has been removed:
1012 - File metadata may be read, e.g. using stat().
1014 - Directories may be listed, in which case the filenames will be
1015 listed in an encoded form derived from their ciphertext. The
1016 current encoding algorithm is described in `Filename hashing and
1017 encoding`_. The algorithm is subject to change, but it is
1018 guaranteed that the presented filenames will be no longer than
1019 NAME_MAX bytes, will not contain the ``/`` or ``\0`` characters, and
1020 will uniquely identify directory entries.
1022 The ``.`` and ``..`` directory entries are special. They are always
1023 present and are not encrypted or encoded.
1025 - Files may be deleted. That is, nondirectory files may be deleted
1026 with unlink() as usual, and empty directories may be deleted with
1027 rmdir() as usual. Therefore, ``rm`` and ``rm -r`` will work as
1030 - Symlink targets may be read and followed, but they will be presented
1031 in encrypted form, similar to filenames in directories. Hence, they
1032 are unlikely to point to anywhere useful.
1034 Without the key, regular files cannot be opened or truncated.
1035 Attempts to do so will fail with ENOKEY. This implies that any
1036 regular file operations that require a file descriptor, such as
1037 read(), write(), mmap(), fallocate(), and ioctl(), are also forbidden.
1039 Also without the key, files of any type (including directories) cannot
1040 be created or linked into an encrypted directory, nor can a name in an
1041 encrypted directory be the source or target of a rename, nor can an
1042 O_TMPFILE temporary file be created in an encrypted directory. All
1043 such operations will fail with ENOKEY.
1045 It is not currently possible to backup and restore encrypted files
1046 without the encryption key. This would require special APIs which
1047 have not yet been implemented.
1049 Encryption policy enforcement
1050 =============================
1052 After an encryption policy has been set on a directory, all regular
1053 files, directories, and symbolic links created in that directory
1054 (recursively) will inherit that encryption policy. Special files ---
1055 that is, named pipes, device nodes, and UNIX domain sockets --- will
1058 Except for those special files, it is forbidden to have unencrypted
1059 files, or files encrypted with a different encryption policy, in an
1060 encrypted directory tree. Attempts to link or rename such a file into
1061 an encrypted directory will fail with EXDEV. This is also enforced
1062 during ->lookup() to provide limited protection against offline
1063 attacks that try to disable or downgrade encryption in known locations
1064 where applications may later write sensitive data. It is recommended
1065 that systems implementing a form of "verified boot" take advantage of
1066 this by validating all top-level encryption policies prior to access.
1068 Implementation details
1069 ======================
1074 An encryption policy is represented on-disk by a :c:type:`struct
1075 fscrypt_context_v1` or a :c:type:`struct fscrypt_context_v2`. It is
1076 up to individual filesystems to decide where to store it, but normally
1077 it would be stored in a hidden extended attribute. It should *not* be
1078 exposed by the xattr-related system calls such as getxattr() and
1079 setxattr() because of the special semantics of the encryption xattr.
1080 (In particular, there would be much confusion if an encryption policy
1081 were to be added to or removed from anything other than an empty
1082 directory.) These structs are defined as follows::
1084 #define FS_KEY_DERIVATION_NONCE_SIZE 16
1086 #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8
1087 struct fscrypt_context_v1 {
1089 u8 contents_encryption_mode;
1090 u8 filenames_encryption_mode;
1092 u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
1093 u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE];
1096 #define FSCRYPT_KEY_IDENTIFIER_SIZE 16
1097 struct fscrypt_context_v2 {
1099 u8 contents_encryption_mode;
1100 u8 filenames_encryption_mode;
1103 u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
1104 u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE];
1107 The context structs contain the same information as the corresponding
1108 policy structs (see `Setting an encryption policy`_), except that the
1109 context structs also contain a nonce. The nonce is randomly generated
1110 by the kernel and is used as KDF input or as a tweak to cause
1111 different files to be encrypted differently; see `Per-file keys`_ and
1112 `DIRECT_KEY policies`_.
1117 For the read path (->readpage()) of regular files, filesystems can
1118 read the ciphertext into the page cache and decrypt it in-place. The
1119 page lock must be held until decryption has finished, to prevent the
1120 page from becoming visible to userspace prematurely.
1122 For the write path (->writepage()) of regular files, filesystems
1123 cannot encrypt data in-place in the page cache, since the cached
1124 plaintext must be preserved. Instead, filesystems must encrypt into a
1125 temporary buffer or "bounce page", then write out the temporary
1126 buffer. Some filesystems, such as UBIFS, already use temporary
1127 buffers regardless of encryption. Other filesystems, such as ext4 and
1128 F2FS, have to allocate bounce pages specially for encryption.
1130 Filename hashing and encoding
1131 -----------------------------
1133 Modern filesystems accelerate directory lookups by using indexed
1134 directories. An indexed directory is organized as a tree keyed by
1135 filename hashes. When a ->lookup() is requested, the filesystem
1136 normally hashes the filename being looked up so that it can quickly
1137 find the corresponding directory entry, if any.
1139 With encryption, lookups must be supported and efficient both with and
1140 without the encryption key. Clearly, it would not work to hash the
1141 plaintext filenames, since the plaintext filenames are unavailable
1142 without the key. (Hashing the plaintext filenames would also make it
1143 impossible for the filesystem's fsck tool to optimize encrypted
1144 directories.) Instead, filesystems hash the ciphertext filenames,
1145 i.e. the bytes actually stored on-disk in the directory entries. When
1146 asked to do a ->lookup() with the key, the filesystem just encrypts
1147 the user-supplied name to get the ciphertext.
1149 Lookups without the key are more complicated. The raw ciphertext may
1150 contain the ``\0`` and ``/`` characters, which are illegal in
1151 filenames. Therefore, readdir() must base64-encode the ciphertext for
1152 presentation. For most filenames, this works fine; on ->lookup(), the
1153 filesystem just base64-decodes the user-supplied name to get back to
1156 However, for very long filenames, base64 encoding would cause the
1157 filename length to exceed NAME_MAX. To prevent this, readdir()
1158 actually presents long filenames in an abbreviated form which encodes
1159 a strong "hash" of the ciphertext filename, along with the optional
1160 filesystem-specific hash(es) needed for directory lookups. This
1161 allows the filesystem to still, with a high degree of confidence, map
1162 the filename given in ->lookup() back to a particular directory entry
1163 that was previously listed by readdir(). See :c:type:`struct
1164 fscrypt_digested_name` in the source for more details.
1166 Note that the precise way that filenames are presented to userspace
1167 without the key is subject to change in the future. It is only meant
1168 as a way to temporarily present valid filenames so that commands like
1169 ``rm -r`` work as expected on encrypted directories.
1174 To test fscrypt, use xfstests, which is Linux's de facto standard
1175 filesystem test suite. First, run all the tests in the "encrypt"
1176 group on the relevant filesystem(s). For example, to test ext4 and
1177 f2fs encryption using `kvm-xfstests
1178 <https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
1180 kvm-xfstests -c ext4,f2fs -g encrypt
1182 UBIFS encryption can also be tested this way, but it should be done in
1183 a separate command, and it takes some time for kvm-xfstests to set up
1184 emulated UBI volumes::
1186 kvm-xfstests -c ubifs -g encrypt
1188 No tests should fail. However, tests that use non-default encryption
1189 modes (e.g. generic/549 and generic/550) will be skipped if the needed
1190 algorithms were not built into the kernel's crypto API. Also, tests
1191 that access the raw block device (e.g. generic/399, generic/548,
1192 generic/549, generic/550) will be skipped on UBIFS.
1194 Besides running the "encrypt" group tests, for ext4 and f2fs it's also
1195 possible to run most xfstests with the "test_dummy_encryption" mount
1196 option. This option causes all new files to be automatically
1197 encrypted with a dummy key, without having to make any API calls.
1198 This tests the encrypted I/O paths more thoroughly. To do this with
1199 kvm-xfstests, use the "encrypt" filesystem configuration::
1201 kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto
1203 Because this runs many more tests than "-g encrypt" does, it takes
1204 much longer to run; so also consider using `gce-xfstests
1205 <https://github.com/tytso/xfstests-bld/blob/master/Documentation/gce-xfstests.md>`_
1206 instead of kvm-xfstests::
1208 gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto