The libgcrypt library in Red Hat Enterprise Linux 6 has a flaw in the Whirlpool algorithm implementation.
The bug is fixed in the later version of the library included in Red Hat Enterprise Linux 7. LUKS containers
created by using the libgcrypt present in Red Hat Enterprise Linux 6 cannot be unlocked by using the one
present in Red Hat Enterprise Linux 7 unless the GCRYPT_WHIRLPOOL_BUG environmental variable is set.

For optional LUKS containers (not involved in the system boot) it is not a critical
obstacle and administrators can unlock such containers by using the environmental variable above.
The situation is different when you deal with mandatory volumes: 
In case the preupgrade script has detected the Whirlpool hash in one of the volumes
involved in the system boot (any volume listed in the /etc/crypttab configuration file),
it is considered to be a blocking event for the in-place upgrade to continue. We strongly
recommend to rehash the LUKS header using the cryptsetup-reencrypt package added to
Red Hat Enterprise Linux 6.6 and later.

A command for rehashing the LUKS header without changing the master key:
`# cryptsetup-reencrypt -h sha1 --keep-key <path_to_luks_device>`

Supply a device identified to be using the Whirlpool hash instead of the
pattern: <path_to_luks_device>.

The SHA-1 function is a recommended hash function for LUKS headers in Red Hat Enterprise Linux 6 and also in Red Hat Enterprise Linux 7.

We also recommend to rehash all optional volumes.
