# Data Availability Certificates​

Data Availability Certificates (DACerts) allow the Sanko Sequencer to circumvent the costs associated with posting transaction data to the settlement layer, resulting in significantly more effective cost-effective transaction processing compared to other rollups.

 A DACert contains:

* the hash of a data block
* an expiration time
* proof that N-1 Committee members have signed the (hash, expiration time) pair, consisting of
  1. the hash of the Keyset used in signing
  2. a bitmap saying which Committee members signed
  3. a BLS aggregated signature (over the BLS12-381 curve) proving that those parties signed.

Because of the 2-of-N trust assumption, a DACert constitutes proof that the block's data (i.e., the preimage of the hash in the DACert) will be available from at least one honest Committee member, at least until the expiration time.

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