Migrating a Kerberos Realm
Douglas E. Engert
deengert at anl.gov
Fri Nov 3 09:32:37 EST 2006
Ken Raeburn wrote:
> On Nov 2, 2006, at 17:48, Henry B. Hotz wrote:
>
>>OTOH, it sounds like a fun idea to me. Do the cryptosystem RFC's
>>specify the default salt?
>
>
> Actually, the default salt, derived from the realm and principal
> name, is specified in the main Kerberos protocol document, and is
> invariant across cryptosystems; the cryptosystem RFCs don't know
> anything about principal names or realm names. What gets done with
> the salt string is defined per cryptosystem, though.
>
> Using a fixed per-principal salt string allows an attacker to convert
> a standard password-cracking dictionary into a set of keys for a
> given principal, and try that set of keys repeatedly despite the user
> changing her password. If the salt string (which is supposed to be
> UTF-8 if I recall correctly) is randomized and long enough, then any
> key of N bits should be possible[*] even if the password is in the
> dictionary, and the attacker can only precompute his key list for a
> given salt string. So it's probably worth considering despite the
> bugs of one implementation.
>
But the salt is returned in the KRB_ERROR KRB5KDC_ERR_PREAUTH_REQUIRED(25)
message on the PA_ENCTYPE_INFO in clear text so just having a different salt
per principal should make it just as difficult for the attacker.
This feature came in handy while trying to figure out why Java < 1.6 did not
work correctly with pre-auth assuming it knew the salt. In a mixed case
principal name. AD is case insensitive but the salt is not.
> Ken
>
> [*] Assuming the cryptosystem actually uses the salt string, and
> incorporates it properly, of course. The RC4 cryptosystem, for
> example, does not use it, and thus the key is derived from the
> password alone, and a dictionary can be converted to keys that can be
> tried for any user in any realm.
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--
Douglas E. Engert <DEEngert at anl.gov>
Argonne National Laboratory
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