BIP: 112
  Title: CHECKSEQUENCEVERIFY
  Authors: BtcDrak 
           Mark Friedenbach 
           Eric Lombrozo 
  Status: Draft
  Type: Standards Track
  Created: 2015-08-10
==Abstract== This BIP describes a new opcode (CHECKSEQUENCEVERIFY) for the Bitcoin scripting system that in combination with BIP 68 allows execution pathways of a script to be restricted based on the age of the output being spent. ==Summary== CHECKSEQUENCEVERIFY redefines the existing NOP3 opcode. When executed it compares the top item on the stack to the nSequence field of the transaction input containing the scriptSig. If it is greater than or equal to (1 << 31), or if the transaction version is greater than or equal to 2, the transaction input sequence is less than or equal to (1 << 31) and the top stack item is less than the transaction input sequence, script exection continues as if a NOP was executed, otherwise the script fails. BIP 68's redefinition of nSequence prevents a non-final transaction from being selected for inclusion in a block until the corresponding input has reached the specified age, as measured in block height or block time. By comparing the argument to CHECKSEQUENCEVERIFY against the nSequence field, we indirectly verify a desired minimum age of the the output being spent; until that relative age has been reached any script execution pathway including the CHECKSEQUENCEVERIFY will fail to validate, causing the transaction not to be selected for inclusion in a block. ==Motivation== BIP 68 repurposes the transaction nSequence field meaning by giving sequence numbers new consensus-enforced semantics as a relative lock-time. However, there is no way to build Bitcoin scripts to make decisions based on this field. By making the nSequence field accessible to script, it becomes possible to construct code pathways that only become accessible some minimum time after proof-of-publication. This enables a wide variety of applications in phased protocols such as escrow, payment channels, or bidirectional pegs. ===Examples=== ====Contracts With Expiration Deadlines==== =====Escrow with Timeout===== An escrow that times out automatically 30 days after being funded can be established in the following way. Alice, Bob and Escrow create a 2-of-3 address with the following redeemscript. IF 2 3 CHECKMULTISIGVERIFY ELSE CHECKSEQUENCEVERIFY DROP CHECKSIGVERIFY ENDIF At any time funds can be spent using signatures from any two of Alice, Bob or the Escrow. After 30 days Alice can sign alone. The clock does not start ticking until the payment to the escrow address confirms. ====Retroactive Invalidation==== In many instances, we would like to create contracts that can be revoked in case of some future event. However, given the immutable nature of the blockchain, it is practically impossible to retroactively invalidate a previous commitment that has already confirmed. The only mechanism we really have for retroactive invalidation is blockchain reorganization which, for fundamental security reasons, is designed to be very hard and very expensive to deliberately pull off. Despite this limitation, we do have a way to provide something functionally similar using CHECKSEQUENCEVERIFY. By constructing scripts with multiple branches of execution where one or more of the branches are delayed we provide a time window in which someone can supply an invalidation condition that allows the output to be spent, effectively invalidating the would-be delayed branch and potentially discouraging another party from broadcasting the transaction in the first place. If the invalidation condition does not occur before the timeout, the delayed branch becomes spendable, honoring the original contract. Some more specific applications of this idea: =====Payment Channel Revocation===== Scriptable relative locktime provides a predictable amount of time to respond in the event a counterparty broadcasts a revoked transaction: Absolute locktime necessitates closing the channel and reopen it when getting close to the timeout, whereas with relative locktime, the clock starts ticking the moment the transactions confirms in a block. It also provides a means to know exactly how long to wait (in number of blocks) before funds can be pulled out of the channel in the event of a noncooperative counterparty. =====Hash Time-Locked Contracts===== Hashed Timelock Contracts (HTLCs) can be used to create chains of payments which is required for lightning network payment channels. The scheme requires both CHECKSEQUENCEVERIFY and CHECKLOCKTIMEVERIFY to enforce HTLC timeouts and revokation. In lightning commitment transactions, CHECKSEQUENCEVERIFY and CHECKLOCKTIMEVERIFY enforce a delay between publishing the commitment transaction, and spending the output. The delay is needed so that the counterparty has time to prove the commitment was revoked and claim the outputs as a penalty. =====2-Way Pegged Sidechains===== OP_IF lockTxHeight nlocktxOut [] reorgBounty Hash160(<...>) OP_REORGPROOFVERIFY OP_ELSE withdrawLockTime OP_CHECKSEQUENCEVERIFY OP_DROP OP_HASH160 p2shWithdrawDest OP_EQUAL OP_ENDIF ==Specification== Refer to the reference implementation, reproduced below, for the precise semantics and detailed rationale for those semantics. /* Threshold for nSequence: below this value it is interpreted * as a relative lock-time, otherwise ignored. */ static const uint32_t SEQUENCE_LOCKTIME_THRESHOLD = (1 << 31); /* Threshold for nSequence when interpreted as a relative * lock-time: below this value it has units of blocks, otherwise * seconds. */ static const uint32_t SEQUENCE_UNITS_THRESHOLD = (1 << 30); case OP_NOP3: { if (!(flags & SCRIPT_VERIFY_CHECKSEQUENCEVERIFY)) { // not enabled; treat as a NOP3 if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS) { return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS); } break; } if (stack.size() < 1) return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION); // Note that elsewhere numeric opcodes are limited to // operands in the range -2**31+1 to 2**31-1, however it is // legal for opcodes to produce results exceeding that // range. This limitation is implemented by CScriptNum's // default 4-byte limit. // // If we kept to that limit we'd have a year 2038 problem, // even though the nLockTime field in transactions // themselves is uint32 which only becomes meaningless // after the year 2106. // // Thus as a special case we tell CScriptNum to accept up // to 5-byte bignums, which are good until 2**39-1, well // beyond the 2**32-1 limit of the nLockTime field itself. const CScriptNum nSequence(stacktop(-1), fRequireMinimal, 5); // In the rare event that the argument may be < 0 due to // some arithmetic being done first, you can always use // 0 MAX CHECKSEQUENCEVERIFY. if (nSequence < 0) return set_error(serror, SCRIPT_ERR_NEGATIVE_LOCKTIME); // To provide for future soft-fork extensibility, if the // operand is too large to be treated as a relative lock- // time, CHECKSEQUENCEVERIFY behaves as a NOP. if (nSequence >= SEQUENCE_LOCKTIME_THRESHOLD) break; // Actually compare the specified sequence number with the input. if (!CheckSequence(nSequence)) return set_error(serror, SCRIPT_ERR_UNSATISFIED_LOCKTIME); break; } bool CheckSequence(const CScriptNum& nSequence) const { // Relative lock times are supported by comparing the passed // in operand to the sequence number of the input. const int64_t txToSequence = (int64_t)txTo->vin[nIn].nSequence; // Fail if the transaction's version number is not set high // enough to trigger BIP 68 rules. if (static_cast(txTo->nVersion) < 2) return false; // Sequence numbers above SEQUENCE_LOCKTIME_THRESHOLD // are not consensus constrained. Testing that the transaction's // sequence number is not above this threshold prevents // using this property to get around a CHECKSEQUENCEVERIFY // check. if (txToSequence >= SEQUENCE_LOCKTIME_THRESHOLD) return false; // There are two kinds of nSequence: lock-by-blockheight // and lock-by-blocktime, distinguished by whether // nSequence < SEQUENCE_UNITS_THRESHOLD. // // We want to compare apples to apples, so fail the script // unless the type of nSequence being tested is the same as // the nSequence in the transaction. if (!( (txToSequence < SEQUENCE_UNITS_THRESHOLD && nSequence < SEQUENCE_UNITS_THRESHOLD) || (txToSequence >= SEQUENCE_UNITS_THRESHOLD && nSequence >= SEQUENCE_UNITS_THRESHOLD) )) return false; // Now that we know we're comparing apples-to-apples, the // comparison is a simple numeric one. if (nSequence > txToSequence) return false; return true; } ==Reference Implementation== A reference implementation is provided in the following git repository: https://github.com/maaku/bitcoin/tree/checksequenceverify ==Deployment== We reuse the double-threshold switchover mechanism from BIPs 34 and 66, with the same thresholds, but for nVersion = 4. The new rules are in effect for every block (at height H) with nVersion = 4 and at least 750 out of 1000 blocks preceding it (with heights H-1000..H-1) also have nVersion = 4. Furthermore, when 950 out of the 1000 blocks preceding a block do have nVersion = 4, nVersion = 3 blocks become invalid, and all further blocks enforce the new rules. It is recommended that this soft-fork deployment trigger include other related proposals for improving Bitcoin's lock-time capabilities, including: [https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki BIP 65]: OP_CHECKLOCKTIMEVERIFY, [https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki BIP 68]: Consensus-enforced transaction replacement signalled via sequence numbers, and [https://github.com/bitcoin/bips/blob/master/bip-0113.mediawiki BIP 113]: Median-Past-Time-Lock. ==Credits== Mark Friedenbach invented the application of sequence numbers to achieve relative lock-time, and wrote the reference implementation of CHECKSEQUENCEVERIFY. The reference implementation and this BIP was based heavily on work done by Peter Todd for the closely related BIP 65. BtcDrak authored this BIP document. Thanks to Eric Lombrozo help with example usecases. ==References== [https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki BIP 68] Consensus-enforced transaction replacement signalled via sequence numbers [https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki BIP 65] OP_CHECKLOCKTIMEVERIFY [https://github.com/bitcoin/bips/blob/master/bip-0113.mediawiki BIP 113] Median past block time for time-lock constraints [http://lists.linuxfoundation.org/pipermail/lightning-dev/2015-July/000021.html HTLCs using OP_CHECKSEQUENCEVERIFY/OP_LOCKTIMEVERIFY and revocation hashes] [http://lightning.network/lightning-network-paper.pdf Lightning Network] [http://diyhpl.us/diyhpluswiki/transcripts/sf-bitcoin-meetup/2015-02-23-scaling-bitcoin-to-billions-of-transactions-per-day/ Scaling Bitcoin to Billions of Transactions Per Day] [http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-August/010396.html Softfork deployment considerations] [https://gist.github.com/sipa/bf69659f43e763540550 Version bits] [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2013-April/002433.html Jeremy Spilman Micropayment Channels] ==Copyright== This document is placed in the public domain.