What Is Transactions as Proof of Stake (TaPoS)?
Also known as: TaPoS, TAPOS, Transaction as Proof of Stake, Transactions as Proof of Stake
In short
TaPoS (Transactions as Proof of Stake) is a mechanism that requires every transaction to include a reference to a recent block, thereby showing which chain history the transactions endorse; it has been used in BitShares, Steem and EOSIO-based networks.
The TaPoS concept emerged in the BitShares ecosystem pioneered by Daniel Larimer. In this structure, when signing a transaction, the user adds to it reference information derived from the number and hash of a recent block. The transaction is valid only on the chain that contains this block. Thus, token holders, with every transaction they send, in effect endorse a particular chain history; the name comes from transactions acting as a kind of proof of stake.
The mechanism has practical benefits. A transaction cannot be replayed on a different fork or on another network derived from the same root; this provides protection against replay attacks. In addition, an alternative chain secretly produced and later published by a malicious party would not contain honest users' transactions, so it would be harder for it to gain acceptance. The ref_block_num and ref_block_prefix fields in EOSIO transactions are the implementation of this idea.
TaPoS is not a consensus algorithm on its own; it is an additional security layer that complements mechanisms such as DPoS. In practice, transactions also usually carry an expiration time, and a transaction that does not get into a block within this time is considered invalid.
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