[Q] Backup of disk encryption master key

I've been extensively searching on this, but I just can't find any answer, although I really can't image I'm the first one to come up with this.

I have a Google Nexus 4 (rooted, of course) and I'm using the native phone encryption.
There is some information that can be found, revealing that google uses the native linux dmsetup tool for this.

Now, just in case anything goes wrong, I'd like to have a backup of the encryption master-key,
that I could use to restore the dm-crypt header.

On a normal linux system, this would be easy, only one call to dmsetup and I'm done.

However, I just can't find any info to do this on android.
The UI doesn't offer this option (as is to be expected) and I just don't know how to do it by using adb.
I've opened a root shell session and tried some commands (dmsetup, cryptsetup, some calls to "vdc cryptfs", etc...), but I just don't find a way to do it.

Has someone already found a way and can share how it's done, any ideas?


Thanks in advance