  We construct a system, Sandi, to bring trust in online communication through
accountability. Sandi is based on a unique "somewhat monotone" accountability
score, with strong privacy and security properties. A registered sender can
request from Sandi a cryptographic tag encoding its score. The score measures
the sender's trustworthiness based on its previous communications. The tag is
sent to a receiver with whom the sender wants to initiate a conversation and
signals the sender's "endorsement" for the communication channel. Receivers can
use the sender's score to decide how to proceed with the sender. If a receiver
finds the sender's communication inappropriate, it can use the tag to report
the sender to Sandi, thus decreasing the sender's score.
  Sandi aims to benefit both senders and receivers. Senders benefit, as
receivers are more likely to react to communication on an endorsed channel.
Receivers benefit, as they can make better choices regarding who they interact
with based on indisputable evidence from prior receivers.
  Receivers do not need registered accounts. Neither senders nor receivers are
required to maintain long-term secret keys. Sandi provides a score integrity
guarantee for the senders, a full communication privacy guarantee for the
senders and receivers, a reporter privacy guarantee to protect reporting
receivers, and an unlinkability guarantee to protect senders. The design of
Sandi ensures compatibility with any communication system that allows for small
binary data transfer.
  Finally, we provide a game-theoretic analysis for the sender. We prove that
Sandi drives rational senders towards a strategy that reduces the amount of
inappropriate communication.
