Transactions between accounts of a subnet or across multiple subnets mutate and modify the internal states of the involved subnets. These actions are called state transitions.
In the Topos ecosystem, the state of subnets is not known to other subnets and, as such, the validity of state transitions cannot be trivially guaranteed. Subnets must rely on a proof of computational integrity whose verification is fast enough for the ecosystem to scale as the number of subnets increases, and to ensure global consistency across subnets without depending on known identities to assess validity of the so-called state transitions.
For this purpose, the Topos protocol leverages zero-knowledge STARKs by batching all subnet transactions in order to generate a proof attesting to the validity of these transactions. These generated proofs are included into certificates to be propagated to the rest of the ecosystem. Upon reception, the fast verification of zkSTARK proofs allows the ecosystem to verify a very large number of state transitions and hence scale exponentially.
zkSTARKs (in the sense of FRI-based zkSTARKs as the Topos protocol is relying on) are a relatively recent cryptographic technology to enforce computational integrity while preserving privacy, and whose verification scales exponentially faster than the underlying computation size. In addition, the construction is specifically tailored for repeated executions of a given computation, making it perfectly suited for blockchain applications. The zero-knowledge property of the STARK proof is a fundamental property leveraged by Topos to maintain subnet privacy while enforcing correct state transitions.
The certificate STARKs can prove the correct execution of any arbitrary subnet state transition and can be verified by anyone. As such, developers are given the freedom to build any type of applications which can be composable with all existing applications in the ecosystem.