  Reputation systems guide our decision making both in life and work: which
restaurant to eat at, which vendor to buy from, which software dependencies to
use, and who or what to trust. These systems are often based on old ideas and
are failing in the face of modern threats. Fraudsters have found ways to
manipulate them, undermining their integrity and utility. Generative AI adds to
the problem by enabling the creation of real-looking fake narratives at scale,
creating a false sense of consensus. Meanwhile, the need for reliable
reputation concepts is more important than ever, as wrong decisions lead to
increasingly severe outcomes: wasted time, poor service, and a feeling of
injustice at best, fraud, identity theft, and ransomware at worst.
  In this extended abstract we introduce Sandi, a new kind of reputation system
with a single well-defined purpose: to create trust through accountability in
one-to-one transactions. Examples of such transactions include sending an email
or making a purchase online. Sandi has strong security and privacy properties
that make it suitable for use also in sensitive contexts. Furthermore, Sandi
can guarantee reputation integrity and transparency for its registered users.
  As a primary application, we envision how Sandi could counter fraud and abuse
in direct communication. Concretely, message senders request a cryptographic
tag from Sandi that they send along with their message. If the receiver finds
the message inappropriate, they can report the sender using this tag. Notably,
only senders need registered accounts and do not need to manage long-term keys.
The design of Sandi ensures compatibility with any communication system that
allows for small binary data transmission.
