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I'm proofreading an article mentioning the cost reduction of P2TR for Lightning channels in comparison to P2WSH. To that end, I'm looking for the precise weight, vsize and serialized length of a 2-of-2 P2WSH input.

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TL;DR:
A 2-of-2 P2WSH input should be conservatively estimated with 96 vbytes. Standard 2-of-2 P2WSH inputs generally weigh 95.5, 95.75 or 96.0 vbytes, depending on whether the r-values in the signatures are low or high. A wallet using signature grinding (which is highly recommended) will always produce 95.5 vbyte inputs. 96.0 vbytes is the conservative estimate allowing for high-r signatures.

Composition of a P2WSH input

Each input commits to spending a specific UTXO by providing its transaction outpoint:

PREVOUT: hash (32 bytes)
         index (4 bytes)

The scriptSig for a P2WSH input is empty, however, the scriptSig length must be provided as 0:

SCRIPTSIG: length (1 byte)
           <no content>

Each transaction input has its own sequence number:

SEQUENCE: nSequence (4 bytes)

A 2-of-2 P2WSH input requires a witness stack in the transaction's witness block. The witness stack has four elements: the witness script, the two signatures, and the dummy element that OP_CHECKMULTISIG additionally pops from the stack but never uses. The dummy element is provided in the form of a length 0 push, just like the scriptSig in the non-witness part of the input.

WITNESS: item count (1 byte)
         dummy element length (1 byte)
         <no content>
         1st signature length (1 byte)
         1st signature (71 or 72 bytes)¹
         2nd signature length (1 byte)
         2nd signature (71 or 72 bytes)¹
         witness script length (1 byte)
           signature count (OP_2, 1 byte)
           1st pubkey length (1 byte)
           1st pubkey (33 bytes)
           2nd pubkey length (1 byte)
           1nd pubkey (33 bytes)
           pubkey count (OP_2, 1 byte)
           OP_CHECKMULTISIG (1 byte)

Conservative weight, vsize, and size estimate

A P2WPKH transaction input adds to a transactions…

weight:

non-witness data: 4 × (32 + 4 + 1 + 4) = 164 WU
witness script: 1 + 1 + 33 + 1 + 33 + 1 + 1 = 71 WU
entire witness: 1 + (1 + 0) + (1 + 72) + (1 + 72) + (1 + witness script) = 220 WU
non-witness + witness = 164 + 220 = 384 WU

vsize:

32 + 4 + 1 + 4 + [1 + (1 + 0) + (1 + 72) + (1 + 72) + (1 + 71)] / 4 = 96 vbytes

serialized byte length:

32 + 4 + 1 + 4 + [1 + (1 + 0) + (1 + 72) + (1 + 72) + (1 + 71)] = 261 bytes

If the signing wallet uses signature grinding, the r-value is always 32 bytes, reducing a signature to 71 bytes and the above maxima to 382 WU, 95.5 vbytes, and 259 bytes respectively.


¹ also see What is the maximum size of a DER encoded ECDSA signature?.

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