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diff --git a/Documentation/device-mapper/dm-crypt.rst b/Documentation/device-mapper/dm-crypt.rst new file mode 100644 index 000000000000..8f4a3f889d43 --- /dev/null +++ b/Documentation/device-mapper/dm-crypt.rst @@ -0,0 +1,173 @@ +======== +dm-crypt +======== + +Device-Mapper's "crypt" target provides transparent encryption of block devices +using the kernel crypto API. + +For a more detailed description of supported parameters see: +https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt + +Parameters:: + + <cipher> <key> <iv_offset> <device path> \ + <offset> [<#opt_params> <opt_params>] + +<cipher> + Encryption cipher, encryption mode and Initial Vector (IV) generator. + + The cipher specifications format is:: + + cipher[:keycount]-chainmode-ivmode[:ivopts] + + Examples:: + + aes-cbc-essiv:sha256 + aes-xts-plain64 + serpent-xts-plain64 + + Cipher format also supports direct specification with kernel crypt API + format (selected by capi: prefix). The IV specification is the same + as for the first format type. + This format is mainly used for specification of authenticated modes. + + The crypto API cipher specifications format is:: + + capi:cipher_api_spec-ivmode[:ivopts] + + Examples:: + + capi:cbc(aes)-essiv:sha256 + capi:xts(aes)-plain64 + + Examples of authenticated modes:: + + capi:gcm(aes)-random + capi:authenc(hmac(sha256),xts(aes))-random + capi:rfc7539(chacha20,poly1305)-random + + The /proc/crypto contains a list of curently loaded crypto modes. + +<key> + Key used for encryption. It is encoded either as a hexadecimal number + or it can be passed as <key_string> prefixed with single colon + character (':') for keys residing in kernel keyring service. + You can only use key sizes that are valid for the selected cipher + in combination with the selected iv mode. + Note that for some iv modes the key string can contain additional + keys (for example IV seed) so the key contains more parts concatenated + into a single string. + +<key_string> + The kernel keyring key is identified by string in following format: + <key_size>:<key_type>:<key_description>. + +<key_size> + The encryption key size in bytes. The kernel key payload size must match + the value passed in <key_size>. + +<key_type> + Either 'logon' or 'user' kernel key type. + +<key_description> + The kernel keyring key description crypt target should look for + when loading key of <key_type>. + +<keycount> + Multi-key compatibility mode. You can define <keycount> keys and + then sectors are encrypted according to their offsets (sector 0 uses key0; + sector 1 uses key1 etc.). <keycount> must be a power of two. + +<iv_offset> + The IV offset is a sector count that is added to the sector number + before creating the IV. + +<device path> + This is the device that is going to be used as backend and contains the + encrypted data. You can specify it as a path like /dev/xxx or a device + number <major>:<minor>. + +<offset> + Starting sector within the device where the encrypted data begins. + +<#opt_params> + Number of optional parameters. If there are no optional parameters, + the optional paramaters section can be skipped or #opt_params can be zero. + Otherwise #opt_params is the number of following arguments. + + Example of optional parameters section: + 3 allow_discards same_cpu_crypt submit_from_crypt_cpus + +allow_discards + Block discard requests (a.k.a. TRIM) are passed through the crypt device. + The default is to ignore discard requests. + + WARNING: Assess the specific security risks carefully before enabling this + option. For example, allowing discards on encrypted devices may lead to + the leak of information about the ciphertext device (filesystem type, + used space etc.) if the discarded blocks can be located easily on the + device later. + +same_cpu_crypt + Perform encryption using the same cpu that IO was submitted on. + The default is to use an unbound workqueue so that encryption work + is automatically balanced between available CPUs. + +submit_from_crypt_cpus + Disable offloading writes to a separate thread after encryption. + There are some situations where offloading write bios from the + encryption threads to a single thread degrades performance + significantly. The default is to offload write bios to the same + thread because it benefits CFQ to have writes submitted using the + same context. + +integrity:<bytes>:<type> + The device requires additional <bytes> metadata per-sector stored + in per-bio integrity structure. This metadata must by provided + by underlying dm-integrity target. + + The <type> can be "none" if metadata is used only for persistent IV. + + For Authenticated Encryption with Additional Data (AEAD) + the <type> is "aead". An AEAD mode additionally calculates and verifies + integrity for the encrypted device. The additional space is then + used for storing authentication tag (and persistent IV if needed). + +sector_size:<bytes> + Use <bytes> as the encryption unit instead of 512 bytes sectors. + This option can be in range 512 - 4096 bytes and must be power of two. + Virtual device will announce this size as a minimal IO and logical sector. + +iv_large_sectors + IV generators will use sector number counted in <sector_size> units + instead of default 512 bytes sectors. + + For example, if <sector_size> is 4096 bytes, plain64 IV for the second + sector will be 8 (without flag) and 1 if iv_large_sectors is present. + The <iv_offset> must be multiple of <sector_size> (in 512 bytes units) + if this flag is specified. + +Example scripts +=============== +LUKS (Linux Unified Key Setup) is now the preferred way to set up disk +encryption with dm-crypt using the 'cryptsetup' utility, see +https://gitlab.com/cryptsetup/cryptsetup + +:: + + #!/bin/sh + # Create a crypt device using dmsetup + dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0" + +:: + + #!/bin/sh + # Create a crypt device using dmsetup when encryption key is stored in keyring service + dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0" + +:: + + #!/bin/sh + # Create a crypt device using cryptsetup and LUKS header with default cipher + cryptsetup luksFormat $1 + cryptsetup luksOpen $1 crypt1 |