https://bitkey.build/building-in-the-open/

# Signing privacy

Predicate blind signing combines blind Schnorr signatures with zero-knowledge proofs that make assertions about transaction attributes. This allows the server to enforce signing policies without learning any identifying information about the transaction itself.

# Vault privacy

For transactions involving funds stored in the vault, the server only signs for the funds outside the vault, adding an extra layer of protection. To preserve privacy while proving funds exist, we use a zero-knowledge proof system similar to the proof-of-solvency approach utilized by exchanges. This method allows the app to prove to the server that sufficient funds exist for a transaction without revealing any specific details about wallet balances or transaction history.

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