------------------------------------------------------------------------------- Mohamed G. Gouda CS 395T Spring 2006 Homework 1 ------------------------------------------------------------------------------- 1. [4 points] Design a protocol for securely transmitting data messages from a process p to a process q. The designed protocol should overcome message forging, message replay, and message loss. In this protocol, the two processes p and q share a key, sk. Process p sends to q data messages encrypted using sk, and q replies by sending ack messages in the clear. Each data message has two nonces, nc and nc', where nc is the nonce of the data message and nc' is the nonce of the next data message. The concatenation of these two nonces, nc and nc', along with the text t of the data message is encrypted using the shared key sk. ------------------------------------------------------------------------------- 2. [3 points] Modify the grid protocol for the distribution of symmetric keys among n processes, where n>2, as follows. Instead of using one sqrt(n)*sqrt(n) grid along with direct keys, the modified protocol uses two sqrt(n)*sqrt(n) grids. In the modified protocol, each process gets 4*sqrt(n) grid keys (taken from the two grids), but has no direct keys. Describe the modified protocol and the two grids that it uses. ------------------------------------------------------------------------------- 3. [3 points] Describe an algorithm for optimal dispersing of a set of (n+1) certificate chains, where one chain is of length 3 or more and all the other chains are of length at most two. (Recall that a dispersal of a chain set is optimal if it minimizes the average number of certificates assigned to each node.) -------------------------------------------------------------------------------