  In an ideal network, every packet would be attributable to its sender, while
host identities and transmitted content would remain private. Designing such a
network is challenging because source accountability and communication privacy
are typically viewed as conflicting properties. In this paper, we propose an
architecture that guarantees source accountability and privacy-preserving
communication by enlisting ISPs as accountability agents and privacy brokers.
While ISPs can link every packet in their network to their customers, customer
identity remains unknown to the rest of the Internet. In our architecture,
network communication is based on Ephemeral Identifiers
(EphIDs)---cryptographic tokens that can be linked to a source only by the
source's ISP. We demonstrate that EphIDs can be generated and processed
efficiently, and we analyze the practical considerations for deployment.
