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I have to check the possibility of case mentioned in the title. An average Linux could be booted up into root's terminal directly without authentication by modifying kernel boot parameters.

I'd like to know, how is it possible (if any) on a LUKS encrypted device? How should I modify parameters without any other Live CD?

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Please add whether the attacker knows, or does not know, the passphrase for the encryption. – Hendrik Brummermann Dec 20 '12 at 22:31
Okay, so when a new user came or his/her computer replaced, he/she should know encryption passphrase, otherwise ha/she cannot start his computer :) Anyway, I found the option, so thanks for all contributions! – sh4d0w Jan 5 '13 at 9:22
If you know the decryption passphrase, this sounds like a better question for Unix.StackExchange or SuperUser.SE or Ubuntu.SE. (I agree with Hendrik that you should edit the question to specify whether you know the passphrase.) – D.W. Feb 3 '13 at 8:18

If the root partition is encrypted, then the only way to run its /bin/bash is to decrypt that file. In fact, if the partition is encrypted, the only way to even know whether there is a file called /bin/bash is to decrypt directories on the partition. With a LUKS encrypted device, you can't even know whether there's a filesystem there. So no, it is not possible to boot into a system where the root filesystem is encrypted. (Unless you know or guess the key, of course.)

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If I have physical access to the machine, I can boot it with my own OS from a live CD or similar. Then yes, I am 'root' and can access the network card and other peripherals as normal.

I cannot however access the encrypted storage device. I cannot access any information on that device, such as private keys and so forth. So I cannot completely masquerade as that machine's owner.

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It looks like I was not absolutely accurate. I mean to get root access without any other Live CD - I will modify the question. Anyway, thanks for your answer. – sh4d0w Dec 19 '12 at 7:33

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