# Certificate

## What is a certificate?​

Cross-subnet communication in Topos lies in the submission of certificates—data structures that certify cross-subnet transactions. Any subnet participants can send an outgoing transaction towards another subnet.

It is the role of every subnet to:

• identify key participants;
• batch such transactions in a certificate;
• include a cryptographic proof of the validity of the state transition since the previously committed state;
• sign the certificate.
##### tip

The previously committed state is the state committed in the previous certificate, or the initial state committed during registration.

Once broadcast and delivered by the TCE, the certificate can be verified and the cross-subnet transaction included in the receiving subnet.

## Composition​

A certificate is defined as:

• subnet_id is the unique identifier of the subnet;
• cert_prev is a reference to the subnet's previous certificate;
• sig is the Schnorr-based signature that authenticates the certificate;
• XS_list represents the list of included cross-subnet transactions;
• proofs is the list of STARK proofs;
• proofs_pub_inputs is the set of STARK public inputs against which the proofs are verified;

• sig_pub_key is the subnet's signature public key;
• init_state_comm is the commitment of the subnet's initial state.