[bitcoin-dev] [BIP-draft] CHECKSEQUENCEVERIFY - An opcode for relative locktime

I have written the following draft BIP for a new opcode CHECKSEQUENCEVERIFY by Mark Friedenbach, which introduces a form of relative-locktime to Bitcoin's scripting language. https://github.com/btcdrak/bips/blob/bip-checksequenceverify/bip-csv.mediawiki <pre> BIP: XX Title: CHECKSEQUENCEVERIFY Authors: BtcDrak <btcdrak at gmail.com> Mark Friedenbach <mark at friedenbach.org> Status: Draft Type: Standards Track Created: 2015-08-10 </pre> ==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 inverse of the nSequence field of the transaction input containing the scriptSig. If the inverse of nSequence is less than the sequence threshold (1 << 31), the transaction version is greater than or equal to 2, and the top item on the stack is less than or equal to the inverted nSequence, script evaluation continues as though a NOP was executed. Otherwise the script fails immediately. 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 heiht 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. ==Specification== Refer to the reference implementation, reproduced below, for the precise semantics and detailed rationale for those semantics. 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 unlike CHECKLOCKTIMEVERIFY we do not need to // accept 5-byte bignums since any value greater than or // equal to SEQUENCE_THRESHOLD (= 1 << 31) will be rejected // anyway. This limitation just happens to coincide with // CScriptNum's default 4-byte limit with an explicit sign // bit. // // This means there is a maximum relative lock time of 52 // years, even though the nSequence field in transactions // themselves is uint32_t and could allow a relative lock // time of up to 120 years. const CScriptNum nInvSequence(stacktop(-1), fRequireMinimal); // 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 (nInvSequence < 0) return set_error(serror, SCRIPT_ERR_NEGATIVE_LOCKTIME); // Actually compare the specified inverse sequence number // with the input. if (!CheckSequence(nInvSequence)) return set_error(serror, SCRIPT_ERR_UNSATISFIED_LOCKTIME); break; } bool CheckSequence(const CScriptNum& nInvSequence) const { int64_t txToInvSequence; // Fail under all circumstances if the transaction's version // number is not set high enough to enable enforced sequence // number rules. if (txTo->nVersion < 2) return false; // Sequence number must be inverted to convert it into a // relative lock-time. txToInvSequence = (int64_t)~txTo->vin[nIn].nSequence; // Sequence numbers under SEQUENCE_THRESHOLD are not consensus // constrained. if (txToInvSequence >= SEQUENCE_THRESHOLD) return false; // There are two types of relative lock-time: lock-by- // blockheight and lock-by-blocktime, distinguished by // whether txToInvSequence < LOCKTIME_THRESHOLD. // // We want to compare apples to apples, so fail the script // unless the type of lock-time being tested is the same as // the lock-time in the transaction input. if (!( (txToInvSequence < LOCKTIME_THRESHOLD && nInvSequence < LOCKTIME_THRESHOLD) || (txToInvSequence >= LOCKTIME_THRESHOLD && nInvSequence >= LOCKTIME_THRESHOLD) )) return false; // Now that we know we're comparing apples-to-apples, the // comparison is a simple numeric one. if (nInvSequence > txInvToSequence) return false; return true; } https://github.com/maaku/bitcoin/commit/33be476a60fcc2afbe6be0ca7b93a84209173eb2 ==Example: 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 <Alice's pubkey> <Bob's pubkey> <Escrow's pubkey> 3 CHECKMULTISIGVERIFY ELSE <LOCKTIME_THRESHOLD + 30*24*60*60> CHECKSEQUENCEVERIFY DROP <Alice's pubkey> 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. ==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-00XX.mediawiki BIP XX]: 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. ==References== BIP 68: Consensus-enforced transaction replacement signalled via sequence numbers https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki BIP 65: OP_CHECKLOCKTIMEVERIFY https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki BIP XX: Median past block time for time-lock constraints https://github.com/bitcoin/bips/blob/master/bip-00XX.mediawiki HTLCs using OP_CHECKSEQUENCEVERIFY/OP_LOCKTIMEVERIFY and revocation hashes http://lists.linuxfoundation.org/pipermail/lightning-dev/2015-July/000021.html ==Copyright== This document is placed in the public domain.