Distributed content-addressable storage systems use self-verifying data to protect data integrity and to enable graceful scaling. One feature commonly missing from these systems, however, is the ability to identify the owner of a piece of data in a non-repudiable manner. While a solution that associates a certificate with each block of data is conceptually simple, researchers have traditionally claimed that the cost of creating and maintaining certificates is too great. In this paper, we demonstrate that systems can, in fact, efficiently map data to its owner in a secure and nonrepudiable fashion. To reduce the cost of creating and maintaining certificates, we extend the traditional contentaddressable interface to allow the aggregation of many small data blocks into larger containers. The aggregation is performed in a way that also supports self-verifying data at the granularity of the block and container, fine-granularity access, and incremental updates. We describe two prototype implementations and present preliminary performance results from deployments on PlanetLab and a local cluster.