  Topos is an open interoperability protocol designed to reduce as much as
possible trust assumptions by replacing them with cryptographic constructions
and decentralization while exhibiting massive scalability. The protocol does
not make use of a central blockchain, nor uses consensus to ensure consistent
delivery of messages across a heterogeneous ecosystem of public and private
blockchains, named subnets, but instead relies on a weak causal reliable
broadcast implemented by a distributed network which we call
$\textit{Transmission Control Engine}$ (TCE). The validity of cross-subnet
messages is ensured by the $\textit{Universal Certificate Interface}$ (UCI) and
stems from zkSTARK proofs asserting the validity of subnets' state transitions
executed by the Topos zkVM. Such proofs of computational integrity are publicly
verifiable by any other participants in and out the protocol such as other
subnets or audit companies. The interface between the TCE and subnets leverages
the ICE-FROST protocol, an innovative threshold signature scheme, whose static
public key allows for uniquely identifying subnets after they register in the
protocol. The Topos protocol is designed to provide $\textit{uniform security}$
to the ecosystem and to handle any type of subnets (e.g., permissioned,
permissionless) in order to fit any business use cases and pave the way for
global adoption and a new standard for the Internet base layer.
