crypttab and crypttabinitramfs

Crypttab

The four fields of /etc/crypttab are defined as follows:

• The first field contains the name of the resulting volume with decrypted data; its block device is set up below /dev/mapper/.
• The second field contains a path to the underlying block device or file, or a specification of a block device via "UUID=" followed by the device UUID.
• The third field specifies an absolute path to a file with the encryption key. Optionally, the path may be followed by “:” and an /etc/fstab style device specification (e.g. starting with "LABEL=" or similar); in which case the path is taken relative to the specified device’s file system root. If the field is not present or is "none" or "-", a key file named after the volume to unlock (i.e. the first column of the line), suffixed with .key is automatically loaded from the /etc/cryptsetup-keys.d/ and /run/cryptsetup-keys.d/ directories, if present. Otherwise, the password has to be manually entered during system boot. For swap encryption, /dev/urandom may be used as key file, resulting in a randomized key.
• If the specified key file path refers to an AF_UNIX stream socket in the file system, the key is acquired by connecting to the socket and reading it from the connection. This allows the implementation of a service to provide key information dynamically, at the moment when it is needed.

Crypttab.initramfs

• It is same as crypttab but it is used for mainly root partition to decrypt

td;lr

crypttab is for encrypted partitions other than root. For root use crypttab.initramfs.