# Certificate

## What is a certificate?​

Cross-subnet communication in Topos lies in the submission of certificates—data structures that certify subnet state transitions.

It is the role of every subnet to:

Once broadcast and delivered throughout the TCE, the certificate can be verified and the cross-subnet message executed in the receiving subnet.

## Composition​

A certificate is defined as:

• subnet_id is the unique identifier of the subnet;
• prev_state_hash is the previous subnet state commitment (from the previous certificate);
• state_hash is the current subnet state commitment;
• proof is the zkSTARK proof of validity;
• XS_list represents the list of included cross-subnet messages;
• proof_XS_list is the list of inclusion proofs of cross-subnet messages in the proven state transition;