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Formal methods for rights management

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Title: Formal methods for rights management

1
Formal methods for rights management

Vicky Weissman

2
The big picture

Digital content providers want to write policies
about their works.
A policy says that under certain conditions an
action is permitted or forbidden.
The ACM has a digital library and policies
members may download articles members may not
republish articles without explicit consent.
They want their policies enforced.

3
The big picture

Digital content providers want to write policies
about their works.
A policy says that under certain conditions an
action is permitted or forbidden.
The ACM has a digital library and policies
members may download articles members may not
republish articles without explicit consent.
They want their policies enforced.

4
The big picture

Digital content providers want to write policies
about their works.
A policy says that under certain conditions an
action is permitted or forbidden.
The ACM has a digital library and policies
members may download articles members may not
republish articles without explicit consent.
They want their policies enforced.

5
Example

Miramax spends 100 million to make the movie
Big Hit.
A warehouse employee borrows a DVD and puts the
movie on the web.
Without appropriate policy writing and
enforcement, people can download the movie for
free, instead of buying it.
If enough people do this, then Miramax is in
trouble.

6
Its not just movies

Music industry voices same concern. (Our IP is
being stolen!)
Digital libraries cant put certain resources
online, because of IP laws.
The Greek Orthodox Archdiocese of America wants
to put resources online, but is wary of
defamation.

7
XrML to the rescue

XrML is an XML-based language for writing
policies.
Semantics is given in 2 ways.
An English interpretation of the syntax.
An English description of an algorithm that says
if a set of XrML policies imply a permission.
Bottom line write policies in XrML, enforce
using the algorithm.

8
Industry likes XrML

XrML endorsed by Adobe, Hewlett-Packard,
Microsoft, Xerox, Barnesandnoble.com, MPEG
International Standards Committee
Microsoft and others plan to make XrML compliant
products.
Will tomorrows DVD player enforce XrML policies?

9
XrML Shortcomings

No formal semantics.
Policies can be ambiguous.
The interpretation of the syntax doesnt quite
match the algorithm.
The algorithms behavior on some (realistic)
input is unintuitive and unintended by language
designers.
E.g. If Alice is a student and any student may
eat lunch, may Alice? Alg. says no.

10
Improving XrML
Joint Work with Joe Halpern CSFW 04

Fix the algorithm to match developers intent.
Translate XrML policies to formulas in
first-order logic.
Prove our translation matches the algorithm.
Algorithm says policies imply permission iff
translated policies imply translated permission.
Why translate?
Lets us compare XrML with languages in CS
literature, borrow complexity results,
extensions,
Gives XrML formal semantics (no ambiguity).

11
Consider complexity

Show that determining if a permission follows
from a set of XrML policies is NP-hard.
Find tractable fragments that are almost as
expressive.

12
First step Present XrML syntax

XrML is an XML-based language.
XrML policies are verbose.
So, we present a syntax that is
more concise and
easy to map to XrML syntax.

13
Basic components

Principals
Agents (e.g., Alice, the University).
Resources
Digital content (e.g., CS431 Syllabus)
Rights
Actions (e.g., download, play, edit)
Properties
Describe a principal (e.g., student, smart).

14
Syntax

Princ p vp Princ ? Princ.
Rsrc s vs
Right r vr
Prop pr
p, s, r, and pr are application-defined,
vp, vs, and vr are variables, ?
is the union operator.

15
Principals revisited

Set of principals is closed under union.
E.g. Principals include, Alice, Bob, and Alice,
Bob
Who is Alice, Bob?
Alice, Bob is Alice and Bob in cahoots.

16
Principals revisited

Set of principals is closed under union.
E.g. Principals include, Alice, Bob, and Alice,
Bob
Who is Alice, Bob?
Alice, Bob is Alice and Bob in cahoots.
Suppose Alice has a key, does Alice, Bob have
it?

17
Principals revisited

Set of principals is closed under union.
E.g. Principals include, Alice, Bob, and Alice,
Bob
Who is Alice, Bob?
Alice, Bob is Alice and Bob in cahoots.
Suppose Alice has a key, does Alice, Bob have
it? Yes.

18
Principals revisited

Set of principals is closed under union.
E.g. Principals include, Alice, Bob, and Alice,
Bob
Who is Alice, Bob?
Alice, Bob is Alice and Bob in cahoots.
Suppose Alice has a key, does Alice, Bob have
it? Yes.
Suppose that Alice is quiet, is Alice, Bob
quiet?

19
Principals revisited

Set of principals is closed under union.
E.g. Principals include, Alice, Bob, and Alice,
Bob
Who is Alice, Bob?
Alice, Bob is Alice and Bob in cahoots.
Suppose Alice has a key, does Alice, Bob have
it? Yes.
Suppose that Alice is quiet, is Alice, Bob
quiet? Not necessarily.

20
Question

Does a set of principals have the properties of
its members?
XrML interpretation of ? doesnt say.
XrML algorithm makes the assumption in one
routine, but not in another.
Since XrML doesnt answer question
We dont make assumption.
But, can easily write policies to force it.

21
Syntax (cont.)

grant cond ? conc.
If cond holds, then conc holds.

22
Syntax (cont.)

grant cond ? conc.
If cond holds, then conc holds.
conc Pr(p) Perm(p, r, s).
Pr(p) means principal p has property pr. Perm(p,
r, s) means p is permitted to exercise right r
over resource s.

23
Syntax (cont.)

grant cond ? conc.
If cond holds, then conc holds.
conc Pr(p) Perm(p, r, s).
Pr(p) means principal p has property pr. Perm(p,
r, s) means p is permitted to exercise right r
over resource s.

24
Syntax (cont.)

grant cond ? conc.
If cond holds, then conc holds.
conc Pr(p) Perm(p, r, s).
Pr(p) means principal p has property pr. Perm(p,
r, s) means p is permitted to exercise right r
over resource s.

25
Syntax (cont.)

grant cond ? conc.
If cond holds, then conc holds.
conc Pr(p) Perm(p, r, s).
Pr(p) means principal p has property pr. Perm(p,
r, s) means p is permitted to exercise right r
over resource s.
cond true conc cond ? cond.

26
Examples

Can write
Alice is nice as true ? Nice(Alice).
Anyone who pays 2 may play Big Hit as
Pay2(x) ? Perm(x, play, Big Hit).
Anyone who is nice and pays 1 may play Big
Hit as Nice(x) ? Pay1(x) ? Perm(x, play,
Big Hit).

27
Examples

Can write
Alice is nice as true ? Nice(Alice).
Anyone who pays 2 may play Big Hit as
Pay2(x) ? Perm(x, play, Big Hit).
Anyone who is nice and pays 1 may play Big
Hit as Nice(x) ? Pay1(x) ? Perm(x, play,
Big Hit).

28
Examples

Can write
Alice is nice as true ? Nice(Alice).
Anyone who pays 2 may play Big Hit as
Pay2(x) ? Perm(x, play, Big Hit).
Anyone who is nice and pays 1 may play Big
Hit as Nice(x) ? Pay1(x) ? Perm(x, play,
Big Hit).

29
Examples

Can write
Alice is nice as true ? Nice(Alice).
Anyone who pays 2 may play Big Hit as
Pay2(x) ? Perm(x, play, Big Hit).
Anyone who is nice and pays 1 may play Big
Hit as Nice(x) ? Pay1(x) ? Perm(x, play,
Big Hit).

30
The syntax given here is a fragment of XrML.
31
XrML Algorithm

Let G be a set of grants.
Auth algorithm
Input G and e, where e is var-free conc.
Output true iff e follows from G.
Auth calls CondMet algorithm
CondMet input d, which is a var-free cond.
Output true iff d holds.

32
Auth algorithm

Auth(G, e)
Find the set D of var-free conds s.t.
? d?D ? g?G, ?. g? d ? e.
(In other words, find D s.t. if any d?D holds,
then a grant in G implies e.)
Return ?d?D CondMet(d).

33
CondMet algorithm

CondMet(d)
If d is true, Return true.
If d is a conc, Return Auth(?, d).
If d is e1 ? ... ? en, where E is the set of conc
in d, Return ?e?E Auth(?, e).

34
Example

Let g true ? Student(Alice), g
Student(x) ? Perm(x, eat, lunch)
May Alice eat lunch?
Auth(g, g, Perm(Alice, eat, lunch))

35
Example

Let g true ? Student(Alice), g
Student(x) ? Perm(x, eat, lunch)
May Alice eat lunch?
Auth(g, g, Perm(Alice, eat, lunch))
Finds D ?

36
Example

Let g true ? Student(Alice), g
Student(x) ? Perm(x, eat, lunch)
May Alice eat lunch?
Auth(g, g, Perm(Alice, eat, lunch))
Finds D ?

37
Example

Let g true ? Student(Alice), g
Student(x) ? Perm(x, eat, lunch)
May Alice eat lunch?
Auth(g, g, Perm(Alice, eat, lunch))
Finds D ?

38
Example

Let g true ? Student(Alice), g
Student(x) ? Perm(x, eat, lunch)
May Alice eat lunch?
Auth(g, g, Perm(Alice, eat, lunch))
Finds D Student(Alice), ?

39
Example

Let g true ? Student(Alice), g
Student(x) ? Perm(x, eat, lunch)
May Alice eat lunch?
Auth(g, g, Perm(Alice, eat, lunch))
Finds D Student(Alice)
Calls CondMet(Student(Alice))

g is lost!
40
Example

Let g true ? Student(Alice), g
Student(x) ? Perm(x, eat, lunch)
May Alice eat lunch?
Auth(g, g, Perm(Alice, eat, lunch))
Finds D Student(Alice)
Calls CondMet(Student(Alice))
Calls Auth(?, Student(Alice))

41
Example

Let g true ? Student(Alice), g
Student(x) ? Perm(x, eat, lunch)
May Alice eat lunch?
Auth(g, g, Perm(Alice, eat, lunch))
Finds D Student(Alice)
Calls CondMet(Student(Alice))
Calls Auth(?, Student(Alice))
Finds D ?
Returns false

42
Algorithm Fix

Let G be a set of grants.
Auth algorithm
Input G and e, where e is var-free conc.
Output true iff e follows from G.
Auth calls CondMet algorithm
CondMet input G and d, where d is a var-free
cond.
Output true iff d holds.

43
Auth algorithm

Auth(G, e)
Find the set D of var-free conds s.t.
? d?D ? g?G, ?. g? d ? e.
(In other words, find D s.t. if any d?D holds,
then a grant in G implies e.)
Return ?d?D CondMet(G, d).

44
CondMet algorithm

CondMet(G, d)
If d is true, Return true.
If d is a conc, Return Auth(G, d).
If d is e1 ? ... ? en, where each ei is a conc,
Return ?i ? n Auth(G, ei).

45
Problem Termination

Auth does not terminate on all inputs.
E.g., g e ? e, where e is var-free.
Auth(g, e)
Finds D e
Calls CondMet(g, e)
CondMet calls Auth(g, e)

46
Termination Fix
Keep track of conc given as input to Auth.

Auth(g, e, ?)
Finds D e
Calls CondMet(g, e, e)
CondMet calls Auth(g, e, e)
Auth(g, e, e) detects loop
Returns false

47
The fixed algorithm is correct.

Auth(G, e) should return true only if there is a
sequence S of grants
d1 ? e1, , dn ? en,
where
each grant is a grant in G under some
substitution
di follows from e1, , ei-1 and
en in e.
Corrected alg finds S, if it exists.

48
Translation

Let sT be the translation of any string s.
Grants are a bit tricky.
(d ? e)T ?x1, , ?xn (dT ? eT),
where x1, , xn are the vars in d and e.
If a grant g is a resource (like a certificate)
gT is a constant.
Everything else translates to itself.
E.g., R(Bob)T R(Bob) (p1?p2)T p1?p2

49
Translation is correct.

Definition A good model satisfies the union
properties (p1?p2 p2?p1, ).
Theorem For every set G of grants and every
var-free conc e in XrML, (fixed) Auth(G, e)
returns true iff ?g?G gT ?
eT is true in every good model.

50
Complexity

Determining if a set of XrML grants imply a conc
is NP-hard.
Given the translation, this is easy to prove.
Given the proof, its easy to see that the result
depends on the ? operator.
Suppose we remove ? from grammar.
XrML translates to Datalog, which is a
well-known tractable fragment of first-order
logic.
Given the translation, finding a tractable,
fairly expressive fragment is easy.

51
But thats not all

We can extend Datalog by adding some negation
without becoming intractable.
We can extend XrML in the same way.
Also, adding functions to Datalog make the
language intractable.
Easy to show same result holds for XrML.

52
Other options

If applications need functions and/or another
type of negation, try another first-order
language.
Lithium HW CSFW 2003 is a fragment of
first-order logic that supports functions and
some negation (different restrictions than
Datalog).
We can restrict XrML to be a fragment of Lithium
and then extend the XrML fragment to include
functions/some neg.

53
Key Points

Digital content providers need to be able to
write their policies, and these policies need to
be correctly enforced.
No matter how carefully you do it, writing a
policy language and/or an enforcement algorithm
without formal semantics WILL BE BUGGY.

54
The End
55
Extensibility

Not hard to add some conclusions
E.g., Pr(s) resource s has property Pr.
Not hard to add some conditions, by extending
CondMet case statement.
But what if we want to add negation?

56
Negation

Suppose that a concl can be ?Pr(p), meaning p
does not have pr.
Shouldnt be a problem, since Auth just does
symbols matching (except on terms) and CondMet
relies on Auth, when called with a concl.

57
Example