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Win2k Security Advice Archives: Re: Windows 2000 Password Encry

Re: Windows 2000 Password Encryption


Subject: Re: Windows 2000 Password Encryption
From: Paul Leach (paulleEXCHANGE.MICROSOFT.COM)
Date: Thu Dec 30 1999 - 16:42:04 CST


> -----Original Message-----
> From: Maniac . [mailto:m_a_n_i_a_c_HOTMAIL.COM]
> Sent: Thursday, December 30, 1999 1:48 PM
>
> A LanMan hash is made by taking the user's password and
> converting it to all
> uppercase, padding or truncating it to 14 characters,
> splitting it into two
> seven character halves, encrypting each half with a known
> constant, and
> concatenating the results to form 16 bytes of encrypted hash.

Not quite correct. It uses each password half as a key with with to encrypt
a known constant. This is standard practice as a way use DES as a hash
function; Unix passwords have always been hashed that way. It works fine _on
each half_.

>
> This is done no matter what service pack you are running.
> The problem here
> is the fact that is used a KNOWN CONSTANT to encrypt the
> hashes. A program
> like L0phtcrack can use this knowledge to easily and quickly crack the
> passwords.

No, the problem is that DES in ECB mode is used to generate the hash, not
DES in CBC mode, and no IV is used. As a result, the 7 byte halves are
totally independently computed, and can be attacked totally idependently.

>
> I am assuming that W2K would also be vulnerable to this as it
> most likely
> uses LM Authentication.

No, it uses Kerberos by default. And it can be configured to use NT hashes,
or NTLMv2 hashes, only. See KB article Q147706.

>
> You can combat this by simply disabling LM Authentication.
> BUT, be warned,
> if you disable LM Authentication, 95,98,WFW and other older
> MS boxes will
> not be able to authenticate to your NT boxes. Non-NT MS
> boxes need LM to
> authenticate. NT does not.

Not quite true. The ability to accept authentication based on LM hashes is
independent from the ability to generate authentication based on LM hashes.
I.e., NT clients can disable LM authentication, while DCs can still allow
win9x and WFW clients to use it.

Once all your DCs run NT4 SP4 or later, you can enable NTLMv2 on all your NT
clients.

Once Windows 2000 is available, the "Directory Services Client" for Win9x
included on the Windows 2000 server CD supports NTLMv2 for Win9x.
See KB article Q239869.

>
> As far as your pause goes when typing in your password. I
> doubt it has
> anything to do with this and probably more to do with machine
> performance
> etc....

Yup. The password never makes it out of the dialog box until CR is hit.

Paul

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This archive was generated by hypermail 2b27 : Thu Dec 30 1999 - 17:09:31 CST