How does relative timelock work? I mean, I know the theoretical description, but how to use nSequence concretely ? Can you do me a practical example with 2 sample transactions one of which locked with nSequence from another?


In order to use a relative time lock, you need to provide the requirements in the scriptPubKey to which the Bitcoin is sent.


scriptPubKey for escrow with 30 day timeout:

    2 <Alice's pubkey> <Bob's pubkey> <Escrow's pubkey> 3 CHECKMULTISIG
    <Alice's pubkey> CHECKSIG

Then, in order to spend it before 30 days, the scriptSig that satisfies the first conditional statement (multisig script) must be provided, i.e.:

scriptSig: 0 <signature1> <signature2>

Or after 30 days, alice can provide:

scriptSig: <signature>

See BIP112


Note that in order to set a relative locktime: the tx must have the following properties:

  • version must be 2 or greater
  • nSequence must not have 32nd bit set
  • nSequence must have the 23rd bit set (0x400000) if it is a lock-time type, unset for block height type
  • for relative lock-time type the granularity of each bit is 512 seconds
  • for relative block height type each bit represents 1 block

For 30 days, I believe it would be as follows:

30 * 24 * 60 * 60 = 2592000 seconds
2592000 / 512 = 5062.5 ~= 5063 or 0x13C7
sequence = 0x13C7 | 0x400000 = 0x4013C7 or 4199367
nSequence = 0xC7134000 (little endian)

The sequence is the very last 4 bytes of the transaction, see https://en.bitcoin.it/wiki/Transaction

  • Thank you, very clear. Do u know where to add nSequence parameter inside a transaction? – Bruce Wayne Jan 15 '19 at 8:50
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    The sequence or locktime is the very last 4 bytes of the transaction, see en.bitcoin.it/wiki/Transaction – JBaczuk Jan 15 '19 at 12:54
  • @JBaczuk - Has the impact of little endian encoding been taken into consideration after performing BIP 68 bit encoding? – skaht Jan 15 '19 at 19:24
  • @skaht right, you'd need to convert to little endian before putting it in the transaction – JBaczuk Jan 15 '19 at 21:19
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    The nSequence field is not the same as the nLockTime field. The nSequence field is per-input, and inputs can have different values set. See General format (inside a block) of each input of a transaction - Txin in the bitcoin wiki link. – arubi Jan 16 '19 at 8:54

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