Dash Core

Dash Core Developer Documentation

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Locktime and Sequence Number

One thing all signature hash types sign is the transaction's locktimelocktime - Part of a transaction which indicates the earliest time or earliest block when that transaction may be added to the block chain.. (Called nLockTime in the Dash Core source code.) The locktime indicates the earliest time a transactiontransaction - A transaction spending satoshis. can be added to the block chainblock chain - A chain of blocks with each block referencing the block that preceded it. The most-difficult-to-recreate chain is the best block chain..

Locktime allows signers to create time-locked transactions which will only become valid in the future, giving the signers a chance to change their minds.

If any of the signers change their mind, they can create a new non-locktime transaction. The new transaction will use, as one of its inputsinputs - An input in a transaction which contains three fields: an outpoint, a signature script, and a sequence number. The outpoint references a previous output and the signature script allows spending it., one of the same outputsoutputs - An output in a transaction which contains two fields: a value field for transferring zero or more duffs and a pubkey script for indicating what conditions must be fulfilled for those duffs to be further spent. which was used as an input to the locktime transaction. This makes the locktime transaction invalid if the new transaction is added to the block chain before the time lock expires.

Care must be taken near the expiry time of a time lock. The peer-to-peer networknetwork - The Dash P2P network which broadcasts transactions and blocks. allows block time to be up to two hours ahead of real time, so a locktime transaction can be added to the block chain up to two hours before its time lock officially expires. Also, blocks are not created at guaranteed intervals, so any attempt to cancel a valuable transaction should be made a few hours before the time lock expires.

Previous versions of Dash Core provided a feature which prevented transaction signers from using the method described above to cancel a time-locked transaction, but a necessary part of this feature was disabled to prevent denial of service attacks. A legacy of this system is a four-byte sequence numbersequence number - Part of all transactions. A number intended to allow unconfirmed time-locked transactions to be updated before being finalized; not currently used except to disable locktime in a transaction in every input. Sequence numbers were meant to allow multiple signers to agree to update a transaction; when they finished updating the transaction, they could agree to set every input's sequence number to the four-byte unsigned maximum (0xffffffff), allowing the transaction to be added to a block even if its time lock had not expired.

Even today, setting all sequence numbers to 0xffffffff (the default in Dash Core) can still disable the time lock, so if you want to use locktime, at least one input must have a sequence number below the maximum. Since sequence numbers are not used by the network for any other purpose, setting any sequence number to zero is sufficient to enable locktime.

Locktime itself is an unsigned 4-byte integer which can be parsed two ways:

  • If less than 500 million, locktime is parsed as a block heightblock height - The number of blocks preceding a particular block on a block chain. For example, the genesis block has a height of zero because zero block preceded it.. The transaction can be added to any block which has this height or higher.

  • If greater than or equal to 500 million, locktime is parsed using the Unix epoch time format (the number of seconds elapsed since 1970-01-01T00:00 UTC---currently over 1.395 billion). The transaction can be added to any block whose block time is greater than the locktime.

Updated about a year ago



Locktime and Sequence Number


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