.NH 1
CONCLUSIONS
.XL yw
.PP
Although IX has run for several years as a full-featured
.UX
system, it has not stood the test of abuse outside
the laboratory.
Nevertheless it did enjoy an early success stopping a virus.
Infected code was received over an unclassified data connection,
and from it the virus propagated among user
programs, including some run by the superuser.
It could not, however, propagate into trusted code; and it revealed
itself by attempting to do so.
Thus the system worked exactly as intended.
The same virus infested other
.UX
systems on the local network without being
detected.|reference(duff)
.PP
Dynamic labels are an attractive model for
mandatory security.
They serve well for
preventing unauthorized access and accidental
disclosure, though not for interdicting covert channels.
They are well matched to multilevel applications,
such as multilevel windowed terminals.
The uniformity of the model, with checking at every
transaction, fosters a coherent implementation that
works the same way across the spectrum of devices and system
features.
As a result, much of the work of label checking
is expressible in tables (\*(y7).
.PP
Label policies on the IX model should be useful
in settings where several users work on several
projects, with varying patterns of sharing.
For example, in educational institutions, secrecy labels
could enforce separation among drafts of examinations,
grade books, and students.  In medical labs, secrecy labels
could isolate personal information about experimental
subjects.  Integrity labels could help distinguish maintenance
access from everyday use, even when users and maintainers
are the same people.  IX labels should be adequate
for many classified environments with need-to-know
rules or concerns about inadvertent admixture
of data; the potential for covert channels may be
the least of one's worries about spying.|reference(woodward)
.PP
A potential problem with dynamic labels is label creep.
High data mistakenly written into a low file contaminates
that file irrevocably.  (Bell-LaPadula systems have a 
complementary problem: you can't write
high data into a low file even when you want to.)
The most dramatic problem is that of tar-baby files (\*(y6).
We believe, however, that label creep is largely illusory.
The alternative is overclassification, especially
of compartmented data.|reference(woodward)
.PP
For example, 
an intelligence analyst looking at data from various sources in a
Bell-LaPadula system will naturally choose a label at the
upper bound of all the data.  Outputs then will appear to
belong to all compartments rather than just the compartments
of the contributing inputs.
This ought to be avoidable at a multilevel IX terminal.
Unfortunately, however, we implemented each
.I mux
window as a virtual terminal bearing a single label just as
an ordinary device file does.
Thus the label of the keyboard is tied to the label of
the highest output that the window can receive, and
the host processes will be forced high regardless of
what data is being examined.
A plausible, and not too difficult, extension of
.I mux
would be to provide finer-grained control somewhat
in the spirit of
a ``compartmented mode
workstation'' from IBM.|reference(CMW) 
In that system, which has dynamic labels and per-file
ceilings, input labels are divorced from output labels and
in some cases labels are kept to the granularity of a byte.
.PP
The mechanisms for stream security
and structured privilege are largely concerned with
assuring integrity, by establishing mutual confidence among
actors and by guiding behavior in authorized patterns.
These notions are important for safe and sound operations in
general, whether or not security labeling is in force.
Private paths are broadly applicable.
Any
.UX
system could benefit from them.
Pexing is important for client/server applications
and for multiplexed use of channels.
Stream identifiers, or some equivalent way to
attach characterizing information to channels,
help considerably in safe use of heterogeneous networks.
.PP
The privilege server, though somewhat complicated, should be
useful in classical
.UX 
or almost any system with security features.
In particular, it could be used to realize
security models, such as Clark-Wilson, which
have been recommended for commercial
applications.|reference(clark wilson)
These models are more concerned with who can do what
than with who can see what.
The privilege server's authorization tree
makes clear where rights originate and how they
may be propagated.
It corresponds
well to the way organizations work by delegating
and separating powers.
It is a complement to, not a competitor for,
identification and authentication facilities such as
Kerberos.|reference(kerberos)
.PP
IX integrates mandatory controls
with the
.UX
system in a way that should meet most
needs for confidentiality.
Compact and coherent, IX has several unique features that
engender trust in its behavior:
continuously checked dynamic labels, structured
management of privilege, and private paths.
.SH
Acknowledgments
.PP
In building IX, we were fortunate to start from
the 10th Edition research
.UX
system, the structure of which
had been carefully cultivated by
D. M. Ritchie, D. L. Presotto, and N. Wilson.
D. L. Presotto and A. G. Hume cheerfully modified software
on which IX depended.
K. L. Thompson, F. T. Grampp, and L. A. Wehr influenced
major design decisions.  Through J. D. Weiss and B.
Smith-Thomas we kept in touch with the design of the
B1-level System V/MLS; and through L. A. Wehr, D. Bendet,
and G. B. Green with the B2-level System V release 4ES.
Other technical contacts for those systems included
C. W. Flink, T. F. Houghton, 
W. J. Leighton, and T. L. Vaden.
C. Mumford of the Joel Chandler Harris Foundation
gave enthusiastic help with reference material.
.bp
.SH
References
.LP
|reference_placement
