Merge pull request bitcoin#171 from jonasnick/footnote16

Clarify bip-taproot digest difference to bip143 regarding sub-hashes

bip-taproot.mediawiki

@@ -145,7 +145,7 @@ As the message for signature verification, transaction digest is ''hash<sub>TapS
 The total number of bytes hashed is at most ''211'' (excluding sub-hashes such as `sha_prevouts`)<ref>'''What is the number of bytes hashed for the signature hash?''' The total size of the input to ''hash<sub>TapSighash</sub>'' (excluding the initial 64-byte hash tag) can be computed using the following formula: ''178 - is_anyonecanpay * 52 - is_none * 32 + has_annex * 32''.</ref>.  Sub-hashes may be cached across signatures of the same transaction.
 
 In summary, the semantics of the [https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki BIP143] sighash types remain unchanged, except the following:
-# The way and order of serialization is changed.<ref>'''Why is the serialization in the transaction digest changed?''' Hashes that go into the digest and the digest itself are now computed with a single SHA256 invocation instead of double SHA256. There is no expected security improvement by doubling SHA256 because this only protects against length-extension attacks against SHA256 which are not a concern for transaction digests because there is no secret data. Therefore doubling SHA256 is a waste of resources. The digest computation now follows a logical order with transaction level data first, then input data and output data. This allows to efficiently cache the transaction part of the digest across different inputs using the SHA256 midstate. Additionally, digest computation avoids unnecessary hashing as opposed to BIP143 digests in which parts may be set zero and before hashing them. Despite that, collisions are made impossible by committing to the length of the data (implicit in <code>hash_type</code> and <code>spend_type</code>) before the variable length data.</ref>
+# The way and order of serialization is changed.<ref>'''Why is the serialization in the transaction digest changed?''' Hashes that go into the digest and the digest itself are now computed with a single SHA256 invocation instead of double SHA256. There is no expected security improvement by doubling SHA256 because this only protects against length-extension attacks against SHA256 which are not a concern for transaction digests because there is no secret data. Therefore doubling SHA256 is a waste of resources. The digest computation now follows a logical order with transaction level data first, then input data and output data. This allows to efficiently cache the transaction part of the digest across different inputs using the SHA256 midstate. Additionally, sub-hashes can be skipped when calculating the digest (for example `sha_prevouts` if <code>SIGHASH_ANYONECANPAY</code> is set) instead of setting them to zero and then hashing them as in BIP143. Despite that, collisions are made impossible by committing to the length of the data (implicit in <code>hash_type</code> and <code>spend_type</code>) before the variable length data.</ref>
 # The digest commits to the <code>scriptPubKey</code><ref>'''Why does the transaction digest commit to the <code>scriptPubKey</code>?''' This prevents lying to offline signing devices about output being spent, even when the actually executed script (<code>scriptCode</code> in BIP143) is correct. This means it's possible to compactly prove to a hardware wallet what (unused) execution paths existed.</ref>.
 # If the <code>SIGHASH_ANYONECANPAY</code> flag is not set, the digest commits to the amounts of ''all'' transaction inputs.<ref>'''Why does the transaction digest commit to the amounts of all transaction inputs?''' This eliminates the possibility to lie to offline signing devices about the fee of a transaction.</ref>
 # The digest commits to all input <code>nSequence</code> if <code>SIGHASH_NONE</code> or <code>SIGHASH_SINGLE</code> are set (unless <code>SIGHASH_ANYONECANPAY</code> is set as well).<ref>'''Why does the transaction digest commit to all input <code>nSequence</code> if <code>SIGHASH_SINGLE</code> or <code>SIGHASH_NONE</code> are set?''' Because setting them already makes the digest commit to the <code>prevouts</code> part of all transaction inputs, it is not useful to treat the <code>nSequence</code> any different. Moreover, this change makes <code>nSequence</code> consistent with the view that <code>SIGHASH_SINGLE</code> and <code>SIGHASH_NONE</code> only modify the digest with respect to transaction outputs and not inputs.</ref>