Private State

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Private state semantics and the UTXO model

Private state must be treated differently from public state and this must be expressed in the semantics of the Noir language.

Private state is encrypted and therefore is "owned" by a user or a set of users (via shared secrets) that are able to decrypt the state.

Private state is represented in an append-only database since updating a record would leak information about the transaction graph.

The act of "deleting" a private state variable can be represented by adding an associated nullifier to a nullifier set. The nullifier is generated such that, without knowing the decryption key of the owner, an observer cannot link a state record with a nullifier.

Modification of state variables can be emulated by nullifying the a state record and creating a new record to represent the variable. Private state has an intrinsic UTXO structure and this must be represented in the language semantics of manipulating private state.

Abstracting UTXO's from App's / Users

The goal of Noir's contract syntax is abstract the UTXO model away from an app user / developer, contract developers are the only actor who should have to think about UTXO's.

This is achieved with two main features:

1. Users sign over transactions, not over specific UTXO's
2. Noir contracts support developer defined unconstrained getter functions to help dApp's make sense of UTXO's. e.g getBalance(). These functions can be called outside of a transaction context to read private state.