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Key Transparency Architecture

Architecture

Clients

Clients make requests to Key Transparency servers over HTTPS / JSON or gRPC.

Key Transparency Server

The front end servers reveal the mappings between user identifiers (e.g. email address) and their anonymized index in the Trillan Map with the Verifiable Random Function.

The front ends also provide account data for particular users and the mappings between that account data and the public commitments stored in the Trillian Map.

Commitment Table

The commitment table stores account values and the associated commitment key nessesary to verify the commitment stored in the Trillian Map.

Mutation Table

When a user wishes to make a change to their account, they create a signed change request (also known as a mutation) and send it to a Key Transparency frontend.

The frontend then saves the mutation in the mutation table, allowing the database to assign the mutation a monotonically increasing sequence number or timestamp, establishing an authoritative ordering for new mutations.

This strict ordering requirement could be relaxed in the future for performance reasons. Strictly speaking only given sets (batches) of mutations that need to be ordered relative to other sets.

Trillian Map

The Trillian Map stores the sparse merkle tree and is designed to scale to extremely large trees. The Trillian Map is updated in batches.

Trillian Log

The Trillian Log stores a dense merkle tree in the style of Ceritificate Transparency. The Key Transparency Sequencer adds SignedMapRoots from the Trillian Map to the Trillian Log as they are created.

Key Transparency Sequencer

The Key Transparency Sequencer runs periodically. It creates a batch of new mutations that have occurred since the last sequencing run. It then verifies the mutations and applies them to the currently stored values in the map.

After each new map revision, the sequencer sends the new SignedMapRoot (SMR) to the Trillian Log which must sequence the new SMR before the front ends will start using the new map revision.

After each new map revision, the sequencer will also send the new SignedMapRoot, SignedLogRoot, Mutations, and associated proofs to the front ends over a streaming gRPC channel. The frontends will then forward those same notification to active monitors over a streaming gRPC channel.

Mutation

Mutations in Key Transparency are defined as a signed key-value object.

  • The Key must be the valid index associated with the user's identifier.
  • The Value is an object that contains
    • A cryptographic commitment to the user's data.
    • The set of public keys that are allowed to update this account.
  • The mutation also contains the hash of the previous mutation. This helps break race conditions and it forms a hash chain in each account.

Monitors

Monitors process and verify the mutations that make each new epoch. Monitors verify various policy properties of the signed key-values in the Trillian Map. In particular, monitors verify that

  • Back pointers in each leaf do not skip over any values - an operation that would be bandwidth intensive for mobile clients.
  • Mutations are properly signed and verified by public keys declared in the prior epoch.

Monitors also observe the Trillian Log proofs provided by the Key Transparency front end to detect any log forks.

Monitors participate in a primitive form of gossip by signing the Trillian Log roots that they see and make them available over an API.