This document proposes SIEVE, a decentralized technique to identify malicious nodes in mobile ad hoc networks (MANETs). SIEVE uses rateless coding and the LT decoding process to detect corrupted data packets. It constructs a factor graph based on "checks" that nodes generate when decoding data. These checks contain information about which nodes provided packet data and whether the data was corrupted. SIEVE runs belief propagation on the factor graph to compute the probability that each node is malicious based on the checks. Simulation results show that SIEVE can accurately identify malicious nodes and is robust against various attacks, while having low computational and communication costs suitable for MANETs.