## ICS 247 - Security Algorithms Homework 4, 50 Points Due: Friday, February 27, 2004

Consider an SDR solution where users are associated with the leaves of a complete binary tree T. Every node v in T has a secret key K(v), which is known to each of v's descendents. The root node of T also has a secret key X, and each non-root node has a key that is derived from X, such that, for each node v, if X(v) is the key for v, then L(X(v)) is the key for v's left child and R(X(v)) is the key for v's right child, where L and R are (different) one-way hash functions. For each leaf node v, we store at v the X(u) key values for each sibling u of the path from v to the root. We then define set differences the same as in the SDR method, where S[v,w] is the set that is rooted at node v but excludes the subtree rooted at node w. The difference between this scheme and SDR, however, is that the way we assign a key for S[v,w] is to use f(K(v),X(v)), where f is a one-way function.

1. 10 points. Rigorously show that for any set of r revoked leaves, there are O(r) S[v,w] sets that cover all the non-revoked users.
2. 10 points. Show that each user need store only O(log n) keys, where n is the number of users, using the above scheme (as opposed to the regular SDR method that requires O(log^2 n) keys per user).
3. 10 points. Show that, for any revoked leaf x in the above scheme, x is unable to decrypt any message from the leader using only the keys stored at x.
4. 10 points. Show that two revoked users in the above scheme can collude so that one of them can read one of the messages sent by the leader.
5. 10 points. Show that in the SDR approach defined in class (using the paper by Naor et al.), no two revoked users can collude to decrypt a message sent by the leader.