[579] in Kerberos
Re: password checking
daemon@TELECOM.MIT.EDU (Jennifer Steiner)
Mon Jan 9 18:13:23 1989
To: Jerome H. Saltzer <Saltzer@ATHENA.MIT.EDU>
Cc: sms-dev@ATHENA.MIT.EDU, kerberos@ATHENA.MIT.EDU
In-Reply-To: Your message of Sun, 08 Jan 89 22:11:27 -0500.
From: Jennifer Steiner <steiner@ATHENA.MIT.EDU>
Bill's arguments against central (Kerberos) password
quality checking assume that the central checking of
the key must be done against an ascii string. Is this
necessarily the case? The string-to-key'd version can
be checked against a string-to-key'd database.
I would argue against central checking for other reasons,
though. Two separate issues have been brought up: 1) is
the (ascii) password easily guessed and 2) is the resulting
(string-to-key'd) version a good DES key. I would submit
that the client should check for the first, and Kerberos
should worry about the second. Also, the key should always
be given to Kerberos in the DES form (either a string-to-key
version of a password in the case of a user, or a "random"
number in the case of a server).
---
Reasons for having the client control the quality of
the password are: 1) The client knows what restrictions
are appropriate. For example, Moera knows the information
about a user that shouldn't be used as a passwd: the user's
middle name, phone number or whatever. Kerberos shouldn't
have to keep track of that stuff just to check passwords.
2) The client can also determine how restrictive to be: for
example, "kadmin" giving the administrator a hint that the
password is too short or easy, but letting it be overridden;
"register" being more picky about a new user's password.
3) Kerberos shouldn't handle cleartext passwords (see below).
Note: Kerberos would still need to keep track of information
such as previous keys for each principal and/or the time the
password was last modified. But the client should decide
what to do with that information.
---
A reason for having Kerberos control the quality of the DES
key is: Kerberos needs to know what keys are "good" anyway,
since it's responsible for assigning good random (session and
server) keys.
---
Reasons for sending keys to Kerberos in DES form (not ascii)
have been mentioned already: preventing disclosure of keys
in other realms, having to trust administrators not to
snatch copies of cleartext passwords, etc.
Jennifer
