Header chain

The witness appends Zcash block headers to the program. A header is accepted only if its previous hash equals the hash of the last accepted header and its proof of work meets the target encoded in its bits field. The program keeps the most recent four thousand and ninety six headers in a ring and discards older ones. A memo is acted on only once the header of its block has been accepted and at least twelve headers have been accepted after it.

The witness is anonymous and keyless. Anyone can submit a header. A wrong header is rejected and costs the submitter the transaction fee. There is no reward for being the witness beyond the program continuing to work.