why is it a risk to clone a LUKS container?

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http://askubuntu.com – I recently installed Ubuntu onto an encrypted partition using the LUKS+LVM combination that the 'alternate' installer offers. Now I want to create backups of my HD. From the LUKS FAQ: 6.15 Can I clone a LUKS container? You can, but it breaks security, because the cloned container has the same header and hence the same master key. You cannot change the master key on a LUKS container, even if you change the passphrase(s), the master key stays the same. That means whoever has access to one of the clones can decrypt them all, completely bypassing the passphrases. How can the exis (HowTos)