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Merge pull request #211 from btcdrak/bip112-amend

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[WIP] Update BIP112
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@ -2,7 +2,8 @@
BIP: 112
Title: CHECKSEQUENCEVERIFY
Authors: BtcDrak <btcdrak@gmail.com>
Mark Friedenbach <mark@friedenbach.org>
Mark Friedenbach <mark@friedenbach.org>
Eric Lombrozo <elombrozo@gmail.com>
Status: Draft
Type: Standards Track
Created: 2015-08-10
@ -18,14 +19,13 @@ 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.
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
@ -51,105 +51,12 @@ 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.
// Threshold for nLockTime: below this value it is interpreted as block number,
// otherwise as UNIX timestamp (already defined in Bitcoin Core).
static const unsigned int LOCKTIME_THRESHOLD = 500000000; // Tue Nov 5 00:53:20 1985 UTC
// Threshold for inverted nSequence: below this value it is interpreted
// as a relative lock-time, otherwise ignored.
static const uint32_t SEQUENCE_THRESHOLD = (1 << 31);
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 > txToInvSequence)
return false;
return true;
}
https://github.com/maaku/bitcoin/commit/33be476a60fcc2afbe6be0ca7b93a84209173eb2
===Examples===
==Example: Escrow with Timeout==
====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
@ -171,6 +78,164 @@ 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 <lockTxHash> nlocktxOut [<workAmount>] reorgBounty Hash160(<...>) <genesisHash> 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<uint32_t>(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:
@ -181,21 +246,13 @@ 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 = 8. The new rules are
in effect for every block (at height H) with nVersion = 8 and at least
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 = 8. Furthermore, when 950 out of the 1000 blocks
preceding a block do have nVersion = 8, nVersion = 3 blocks become
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.
When assessing the block version as mask of ~0x20000007 must be applied
to work around the complications caused by
[http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-August/010396.html BIP101's premature use]
of the [https://gist.github.com/sipa/bf69659f43e763540550 undecided version bits proposal].
By applying ~0x20000007 with nVersion = 8, the thresholds should be tested
comparing block nVersion >= 4 as this will save a bit for future use.
It is recommended that this soft-fork deployment trigger include other
related proposals for improving Bitcoin's lock-time capabilities, including:
@ -219,28 +276,33 @@ done by Peter Todd for the closely related BIP 65.
BtcDrak authored this BIP document.
Thanks to Eric Lombrozo help with example usecases.
==References==
BIP 68: Consensus-enforced transaction replacement signalled via
sequence numbers
https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki
[https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki BIP 68] Consensus-enforced transaction replacement signalled via sequence numbers
BIP 65: OP_CHECKLOCKTIMEVERIFY
https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki
[https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki BIP 65] OP_CHECKLOCKTIMEVERIFY
BIP 113: Median past block time for time-lock constraints
https://github.com/bitcoin/bips/blob/master/bip-0113.mediawiki
[https://github.com/bitcoin/bips/blob/master/bip-0113.mediawiki BIP 113] Median past block time for time-lock constraints
HTLCs using OP_CHECKSEQUENCEVERIFY/OP_LOCKTIMEVERIFY and
revocation hashes
http://lists.linuxfoundation.org/pipermail/lightning-dev/2015-July/000021.html
[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.