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BIP-141 defines a witness reserved value as part of a flexible commitment structure:

Double-SHA256(witness root hash|witness reserved value)

The purpose is to support commitments (i.e., data associated with a block) beyond the witness root hash. That part makes sense, but it seems like the value of witness reserved value should be defined in BIP-141. For example, should it be omitted altogether when hashing the witness root hash? Should it be set to 0x00? Some other value?

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It's a reserved value.

That means it is reserved for future extensions, but for now there are no rules about it. It's encouraged to not use it for anything, as that may conflict with later use.

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  • When creating a coinbase Null Data output, what value does the miner use for "witness reserved value" given that a hash for (witness root hash|witness reserved value) is required?
    – Rich Apodaca
    Jun 11, 2017 at 18:49
  • Any value at all is allowed.
    – Pieter Wuille
    Jun 11, 2017 at 18:53
  • It seems that a node must know the witness reserved value used by a miner in order to properly validate the commitment in the coinbase output. The commitment is defined in terms of the double SHA-256 of (witness root hash|witness reserved value). In other words, the commitment in the coinbase output depends on the value of witness reserved value chosen by the miner. Yet this value doesn't appear anywhere explicitly. I'm missing something, but what is it?
    – Rich Apodaca
    Jun 11, 2017 at 21:41
  • The value is included in the coinbase witness, so every validating node knows it. It can be any 32-byte value, though.
    – Pieter Wuille
    Jun 11, 2017 at 22:43
  • Thanks - I think I found it: "the coinbase's input's witness must consist of a single 32-byte array for the witness reserved value." In other words, the value of "witness reserved value" is found in the coinbase transaction's witness. A coinbase transaction has only one input, so its only witness field contains the "witness reserved value" as its only item. Right?
    – Rich Apodaca
    Jun 12, 2017 at 0:02

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