LP #1413044: Decrypting backup with the wrong key make the backup unusable and unrecoverable

**Reported in Launchpad by Jaime Sicam last update 26-03-2015 11:11:45

Tested on Xtrabackup 2.1.9

Perform backup with encryption:
innobackupex --defaults-file=/home/user/sandboxes/rsandbox_mysql-5_5_39/master/my.sandbox.cnf --encrypt=AES256 --encrypt-key="A1EDC73815467C083B0869508406637E" --host=127.0.0.1 --port=24394 --user=root --password=msandbox /backups/

Decrypt backup with wrong key:
innobackupex --decrypt=AES256 --encrypt-key="A1EDC73815467C083B0869508406637L" /backups/2015-01-21_10-27-04/

Apply logs:
150121 10:30:39 innobackupex: Starting ibbackup with command: xtrabackup_55 --defaults-file="/backups/2015-01-21_10-27-04/backup-my.cnf" --defaults-group="mysqld" --prepare --target-dir=/backups/2015-01-21_10-27-04 --tmpdir=/tmp

xtrabackup_55 version 2.1.9 for Percona Server 5.5.35 Linux (x86_64) (revision id: 746)
xtrabackup: cd to /backups/2015-01-21_10-27-04
xtrabackup: error: xtrabackup_read_metadata()
xtrabackup: This target seems not to have correct metadata...
xtrabackup: notice: xtrabackup_logfile was already used to '--prepare'.
xtrabackup: using the following InnoDB configuration for recovery:
xtrabackup: innodb_data_home_dir = ./
xtrabackup: innodb_data_file_path = ibdata1:10M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 2
xtrabackup: innodb_log_file_size = 268435456
xtrabackup: using the following InnoDB configuration for recovery:
xtrabackup: innodb_data_home_dir = ./
xtrabackup: innodb_data_file_path = ibdata1:10M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 2
xtrabackup: innodb_log_file_size = 268435456
xtrabackup: Starting InnoDB instance for recovery.
xtrabackup: Using 104857600 bytes for buffer pool (set by --use-memory parameter)
150121 10:30:39 InnoDB: The InnoDB memory heap is disabled
150121 10:30:39 InnoDB: Mutexes and rw_locks use GCC atomic builtins
150121 10:30:39 InnoDB: Compressed tables use zlib 1.2.3
150121 10:30:39 InnoDB: Initializing buffer pool, size = 100.0M
150121 10:30:39 InnoDB: Completed initialization of buffer pool
InnoDB: Error: checksum mismatch in data file ./ibdata1
150121 10:30:39 InnoDB: Could not open or create data files.
150121 10:30:39 InnoDB: If you tried to add new data files, and it failed here,
150121 10:30:39 InnoDB: you should now edit innodb_data_file_path in my.cnf back
150121 10:30:39 InnoDB: to what it was, and remove the new ibdata files InnoDB created
150121 10:30:39 InnoDB: in this failed attempt. InnoDB only wrote those files full of
150121 10:30:39 InnoDB: zeros, but did not yet use them in any way. But be careful: do not
150121 10:30:39 InnoDB: remove old data files which contain your precious data!
xtrabackup: innodb_init(): Error occured.
innobackupex: Error:
innobackupex: ibbackup failed at /usr/bin/innobackupex line 2560.

Since *.xbcrypt get deleted after decryption, you cannot recover the backup with the correct key.