Identifying the correct password-hashing algorithm is vital to brute-force a password. This is true in the case of Rainbow tables and classic brute-force attempts. It is also required if all that is needed is root access to the box itself, without the extra work of brute forcing.
As was mentioned earlier, all you need to do to achieve root login from a physical security perspective is
• Gain physical access to the box
• Obtain a Linux boot CD (BackTrack, Knoppix-STD, Arudius...)
• Get a selection of salted password hashes using the same salt
• Obtain a one-way hashing algorithm as the target system
• Use your ability to delete the password on the target system or copy and paste the password from a text file over the root password in the /etc/shadow file on the target system
As a side note, you may also want to copy the root password from the target system, so you can include it again at a later time (if required). This is particularly helpful in any kind of covert operation where the system needs to appear unmodified.
Theoretically, the password could be deleted from the target user entirely and the machine could be booted and logged in to with no password. However, many modern systems have password complexity requirements that can actually lock out users who specify passwords that do not meet the requirements and then try to log in using them. This situation only surfaces if password requirements are implemented on a system that has noncompliant passwords (particularly the root password) or if that password is manually changed by editing the /etc/shadow file. In either case, a password can still be recovered using the method described above and specifying a compliant password.
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