The proposed digest algorithm is adapted from BIP143<sup>[1][1]</sup> as it minimizes redundant data hashing in verification, covers the input value by the signature and is already implemented in a wide variety of applications<sup>[2][2]</sup>.
* If the `ANYONECANPAY` flag is not set, `hashPrevouts` is the double SHA256 of the serialization of all input outpoints;
* Otherwise, `hashPrevouts` is a `uint256` of `0x0000......0000`.
#### hashSequence
* If none of the `ANYONECANPAY`, `SINGLE`, `NONE` sighash type is set, `hashSequence` is the double SHA256 of the serialization of `nSequence` of all inputs;
* Otherwise, `hashSequence` is a `uint256` of `0x0000......0000`.
#### scriptCode
In this section, we call `script` the script being currently executed.
This means `redeemScript` in case of P2SH, or the `scriptPubKey` in the general case.
* If the `script` does not contain any `OP_CODESEPARATOR`, the `scriptCode` is the `script` serialized as scripts inside `CTxOut`.
* If the `script` contains any `OP_CODESEPARATOR`, the `scriptCode` is the `script` but removing everything up to and including the last executed `OP_CODESEPARATOR` before the signature checking opcode being executed, serialized as scripts inside `CTxOut`.
Notes:
1. Contrary to the original algorithm, this one does not use `FindAndDelete` to remove the signature from the script.
2. Because of 1, it is not possible to create a valid signature within `redeemScript` or `scriptPubkey` as the signature would be part of the digest.
This enforces that the signature is in `sigScript` .
3. In case an opcode that requires signature checking is present in `sigScript`, `script` is effectively `sigScript`.
However, for reason similar to 2, it is not possible to provide a valid signature in that case.
#### value
The 8-byte value of the amount of Bitcoin this input contains.
#### hashOutputs
* If the sighash type is neither `SINGLE` nor `NONE`, `hashOutputs` is the double SHA256 of the serialization of all output amounts (8-byte little endian) paired up with their `scriptPubKey` (serialized as scripts inside CTxOuts);
* If sighash type is `SINGLE` and the input index is smaller than the number of outputs, `hashOutputs` is the double SHA256 of the output amount with `scriptPubKey` of the same index as the input;
* Otherwise, `hashOutputs` is a `uint256` of `0x0000......0000`.
1. In the original algorithm<sup>[3][3]</sup>, a `uint256` of `0x0000......0001` is committed if the input index for a `SINGLE` signature is greater than or equal to the number of outputs.
