3

I read the bip143, but didn't understand how to calculate the scriptCode of p2wsh.

bip143 told us:

A new transaction digest algorithm is defined, but only applicable to sigops in version 0 witness program
Double SHA256 of the serialization of:
  1. nVersion of the transaction (4-byte little endian)
  2. hashPrevouts (32-byte hash)
  3. hashSequence (32-byte hash)
  4. outpoint (32-byte hash + 4-byte little endian) 
  5. scriptCode of the input (serialized as scripts inside CTxOuts)
  6. value of the output spent by this input (8-byte little endian)
  7. nSequence of the input (4-byte little endian)
  8. hashOutputs (32-byte hash)
  9. nLocktime of the transaction (4-byte little endian)
  10. sighash type of the signature (4-byte little endian)


The item 5:

For P2WPKH witness program, the scriptCode is 0x1976a914{20-byte-pubkey-hash}88ac.
For P2WSH witness program,
if the witnessScript does not contain any OP_CODESEPARATOR, the scriptCode is the witnessScript serialized as scripts inside CTxOut.
if the witnessScript contains any OP_CODESEPARATOR, the scriptCode is the witnessScript but removing everything up to and including the last executed OP_CODESEPARATOR before the signature checking opcode being executed, serialized as scripts inside CTxOut. (The exact semantics is demonstrated in the examples below)

I don't kown how to find out if "the signature checking opcode being executed". Then I read the example "Native P2WSH" in bip143 and I am even more confused.

Here is the first example of "Native P2WSH":

The following is an unsigned transaction: 0100000002fe3dc9208094f3ffd12645477b3dc56f60ec4fa8e6f5d67c565d1c6b9216b36e0000000000ffffffff0815cf020f013ed6cf91d29f4202e8a58726b1ac6c79da47c23d1bee0a6925f80000000000ffffffff0100f2052a010000001976a914a30741f8145e5acadf23f751864167f32e0963f788ac00000000
  
    nVersion:  01000000
    txin:      02 fe3dc9208094f3ffd12645477b3dc56f60ec4fa8e6f5d67c565d1c6b9216b36e 00000000 00 ffffffff
                  0815cf020f013ed6cf91d29f4202e8a58726b1ac6c79da47c23d1bee0a6925f8 00000000 00 ffffffff
    txout:     01 00f2052a01000000 1976a914a30741f8145e5acadf23f751864167f32e0963f788ac
    nLockTime: 00000000
  
  The first input comes from an ordinary P2PK:
    scriptPubKey: 21036d5c20fa14fb2f635474c1dc4ef5909d4568e5569b79fc94d3448486e14685f8ac value: 1.5625
    private key:  b8f28a772fccbf9b4f58a4f027e07dc2e35e7cd80529975e292ea34f84c4580c
    signature:    304402200af4e47c9b9629dbecc21f73af989bdaa911f7e6f6c2e9394588a3aa68f81e9902204f3fcf6ade7e5abb1295b6774c8e0abd94ae62217367096bc02ee5e435b67da201 (SIGHASH_ALL)
  
  The second input comes from a native P2WSH witness program:
    scriptPubKey : 00205d1b56b63d714eebe542309525f484b7e9d6f686b3781b6f61ef925d66d6f6a0, value: 49
    witnessScript: 21026dccc749adc2a9d0d89497ac511f760f45c47dc5ed9cf352a58ac706453880aeadab210255a9626aebf5e29c0e6538428ba0d1dcf6ca98ffdf086aa8ced5e0d0215ea465ac
                   <026dccc749adc2a9d0d89497ac511f760f45c47dc5ed9cf352a58ac706453880ae> CHECKSIGVERIFY CODESEPARATOR <0255a9626aebf5e29c0e6538428ba0d1dcf6ca98ffdf086aa8ced5e0d0215ea465> CHECKSIG
  
  To sign it with a nHashType of 3 (SIGHASH_SINGLE):
  
  hashPrevouts:
    dSHA256(fe3dc9208094f3ffd12645477b3dc56f60ec4fa8e6f5d67c565d1c6b9216b36e000000000815cf020f013ed6cf91d29f4202e8a58726b1ac6c79da47c23d1bee0a6925f800000000)
  = ef546acf4a020de3898d1b8956176bb507e6211b5ed3619cd08b6ea7e2a09d41
  
    nVersion:     01000000
    hashPrevouts: ef546acf4a020de3898d1b8956176bb507e6211b5ed3619cd08b6ea7e2a09d41
    hashSequence: 0000000000000000000000000000000000000000000000000000000000000000
    outpoint:     0815cf020f013ed6cf91d29f4202e8a58726b1ac6c79da47c23d1bee0a6925f800000000
    scriptCode:   (see below)
    amount:       0011102401000000
    nSequence:    ffffffff
    hashOutputs:  0000000000000000000000000000000000000000000000000000000000000000 (this is the second input but there is only one output)
    nLockTime:    00000000
    nHashType:    03000000
  
  scriptCode:  4721026dccc749adc2a9d0d89497ac511f760f45c47dc5ed9cf352a58ac706453880aeadab210255a9626aebf5e29c0e6538428ba0d1dcf6ca98ffdf086aa8ced5e0d0215ea465ac
                                                                                       ^^
               (please note that the not-yet-executed OP_CODESEPARATOR is not removed from the scriptCode)
  preimage:    01000000ef546acf4a020de3898d1b8956176bb507e6211b5ed3619cd08b6ea7e2a09d4100000000000000000000000000000000000000000000000000000000000000000815cf020f013ed6cf91d29f4202e8a58726b1ac6c79da47c23d1bee0a6925f8000000004721026dccc749adc2a9d0d89497ac511f760f45c47dc5ed9cf352a58ac706453880aeadab210255a9626aebf5e29c0e6538428ba0d1dcf6ca98ffdf086aa8ced5e0d0215ea465ac0011102401000000ffffffff00000000000000000000000000000000000000000000000000000000000000000000000003000000
  sigHash:     82dde6e4f1e94d02c2b7ad03d2115d691f48d064e9d52f58194a6637e4194391
  public key:  026dccc749adc2a9d0d89497ac511f760f45c47dc5ed9cf352a58ac706453880ae
  private key: 8e02b539b1500aa7c81cf3fed177448a546f19d2be416c0c61ff28e577d8d0cd
  signature:   3044022027dc95ad6b740fe5129e7e62a75dd00f291a2aeb1200b84b09d9e3789406b6c002201a9ecd315dd6a0e632ab20bbb98948bc0c6fb204f2c286963bb48517a7058e2703
  
  scriptCode:  23210255a9626aebf5e29c0e6538428ba0d1dcf6ca98ffdf086aa8ced5e0d0215ea465ac
               (everything up to the last executed OP_CODESEPARATOR, including that OP_CODESEPARATOR, are removed)
  preimage:    01000000ef546acf4a020de3898d1b8956176bb507e6211b5ed3619cd08b6ea7e2a09d4100000000000000000000000000000000000000000000000000000000000000000815cf020f013ed6cf91d29f4202e8a58726b1ac6c79da47c23d1bee0a6925f80000000023210255a9626aebf5e29c0e6538428ba0d1dcf6ca98ffdf086aa8ced5e0d0215ea465ac0011102401000000ffffffff00000000000000000000000000000000000000000000000000000000000000000000000003000000
  sigHash:     fef7bd749cce710c5c052bd796df1af0d935e59cea63736268bcbe2d2134fc47
  public key:  0255a9626aebf5e29c0e6538428ba0d1dcf6ca98ffdf086aa8ced5e0d0215ea465
  private key: 86bf2ed75935a0cbef03b89d72034bb4c189d381037a5ac121a70016db8896ec
  signature:   304402200de66acf4527789bfda55fc5459e214fa6083f936b430a762c629656216805ac0220396f550692cd347171cbc1ef1f51e15282e837bb2b30860dc77c8f78bc8501e503
  
  The serialized signed transaction is: 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

01000000
0001
02
fe3dc9208094f3ffd12645477b3dc56f60ec4fa8e6f5d67c565d1c6b9216b36e 00000000
48
47304402200af4e47c9b9629dbecc21f73af989bdaa911f7e6f6c2e9394588a3aa68f81e9902204f3fcf6ade7e5abb1295b6774c8e0abd94ae62217367096bc02ee5e435b67da201
ffffffff
0815cf020f013ed6cf91d29f4202e8a58726b1ac6c79da47c23d1bee0a6925f8 0000000000
ffffffff

01
00f2052a01000000
1976a914a30741f8145e5acadf23f751864167f32e0963f788ac

0003

47304402200de66acf4527789bfda55fc5459e214fa6083f936b430a762c629656216805ac0220396f550692cd347171cbc1ef1f51e15282e837bb2b30860dc77c8f78bc8501e503
473044022027dc95ad6b740fe5129e7e62a75dd00f291a2aeb1200b84b09d9e3789406b6c002201a9ecd315dd6a0e632ab20bbb98948bc0c6fb204f2c286963bb48517a7058e2703
4721026dccc749adc2a9d0d89497ac511f760f45c47dc5ed9cf352a58ac706453880aeadab210255a9626aebf5e29c0e6538428ba0d1dcf6ca98ffdf086aa8ced5e0d0215ea465ac

So here are my questions:

  1. Here "witnessScript" is RedeemScript, right?
  2. What is meaning of 'CODESEPARATOR' here?
  3. Why calculate both scriptCode(OP_CODESEPARATOR is not removed && OP_CODESEPARATOR is removed) and both put them into witness data?
0

Yes, witnessScript is just the RedeemScript, which would also be the last element of the witness data. I am not sure about the OP_CODESEPARATOR, or why a script would contain that in the first place.

0

Yes, here witnessScript refers for P2WSH to what was previously called the redeemScript for P2SH.

CODESEPARATOR is an (interesting [1]) OP code which allows to customize the behaviour of the CHECKSIG OP code. It allows to section the script code for a signature to commit to only part of it.

BIP143 slightly changed its behaviour for SegWit scripts.

why need calculate both scriptCode(OP_CODESEPARATOR is not removed && OP_CODESEPARATOR is removed) and both put them into witness data?

Hmm I guess you are refering to these lines:

please note that the not-yet-executed OP_CODESEPARATOR is not removed from the scriptCode

It's to illustrate the new behaviour : the CODESEPARATOR OP is not removed along with the code section it bounds (previously it was).


[1] CODESEPARATOR was initially used to execute the scriptPubkey and scriptSig as a single stack (inserted in-between). It was then removed by Satoshi when patching the 1 RETURN bug.

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