+++ This bug was initially created as a clone of Bug #1716674 +++

Description of problem:
There is no option with kickstart to set the key size for a LUKS volume. Only when no "--cipher" option is set the key size defaults to 512 bits, otherwise the default LUKS option is used. Making it impossible to use 512 bits with any cipher other than aes-xts-plain64.

Furthermore the documentation about this is incomplete/incorrect.
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/security_guide/sec-encryption#sec-Using_LUKS_Disk_Encryption
Says: "The default key size for LUKS is 256 bits. The default key size for LUKS with Anaconda (XTS mode) is 512 bits."
However, when the "--cipher" option is set the key size fals back to the LUKS default. So for example when using serpent (XTS mode) the key size is 256 bits.

How reproducible:
Use a kickstart file to install RHEL and check the partition afterwards with cryptsetup luksDump

Steps to Reproduce:
part ... --encrypted --passphrase=temppass
part ... --encrypted --passphrase=temppass --cipher=aes-xts-plain64
part ... --encrypted --passphrase=temppass --cipher=serpent-xts-plain64
part ... --encrypted --passphrase=temppass --key-size=512
part ... --encrypted --passphrase=temppass --key-size=512 --cipher=serpent-xts-plain64

Expected results:
master key size: 512
master key size: 512
master key size: 512
master key size: 512
master key size: 512

Actual results:
master key size: 512
master key size: 256
master key size: 256
master key size: N/A
master key size: N/A

Additional info:
To make the documentation sound:
https://github.com/storaged-project/blivet/blob/rhel7-branch/blivet/formats/luks.py
This:
        if not self.exists and not self.cipher:
            self.cipher = "aes-xts-plain64"
            if not self.key_size:
                # default to the max (512 bits) for aes-xts
                self.key_size = 512
Should be changed to something like this:
        if not self.exists
            if not self.cipher:
                self.cipher = "aes-xts-plain64"
            if not self.key_size and "xts" in self.cipher:
                # default to the max (512 bits) fot xts
                self.key_size = 512