[Doc] Potentially confusing encryption documentation

[Raghavendra D Prabhu]

http://www.percona.com/doc/percona-xtrabackup/2.1/innobackupex/encrypted_backups_innobackupex.html#encrypted-backups-ibk suggest to

generate a key for aes256-cbc (which expects a 32-byte key) but discard it and use its iv instead.

This may be confusing for users since you don't need enc here (which also does message digest and several rounds over the password, and the Password provided on cli has no connection to iv generated) since a

    < /dev/urandom tr -dc _A-Z-a-z-0-9 | head -c${1:-32};echo;

or

    openssl rand -base64 32

will do.

(Because xtrabackup directly asks for a key rather than derive it from a password using a KDF (which is in itself a bug), it doesn't matter how you get a key, any random 32-char string will do).

[Alexey Kopytov (akopytov)]

OK, I think the example in the doc is correct. But if it looks ambiguous to someone, let's replace it with something more simple like "openssl rand -base64 32".

[Hrvoje Matijakovic (hrvojem)]

Fixed here: https://github.com/percona/percona-xtrabackup/pull/23 and https://github.com/percona/percona-xtrabackup/pull/24

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXB-929