Merge pull request #1037 from nuttycom/clarify_zip246_sighash_version_motivation

ZIP 246: Add motivation for sighash algorithm versioning.

Author: SamHSmith

zips/zip-0246.rst

@@ -32,41 +32,58 @@ field, as specified in section 7.1 of the Zcash protocol specification
 Abstract
 ========
 
-This ZIP defines the digest algorithms associated with the v6 transaction format.
-
-This proposal also defines the new concept of "sighash versioning": where previously
-each transaction version had a single associated digest algorithm, going forward it
-will be possible for signers to use any digest algorithm within the closed set
-specified for a given transaction version (and made available in consensus via network
-upgrades).
+This ZIP defines the sighash algorithms associated with the v6 transaction
+format.
 
+This proposal also defines the new concept of "sighash algorithm versioning":
+where previously each transaction version had a single associated sighash
+algorithm, going forward it will be possible for signers to use any sighash
+algorithm within the closed set specified for a given transaction version (and
+made available in consensus via network upgrades).
 
 ==========
 Motivation
 ==========
 
-TBD
-
-TODO: Motivate sighash versioning
+The motivation for sighash algorithm versioning is that we want to be able to
+specify the V6 transaction format and signatures without knowing what the
+signatures for atomic swaps in action bundles will need to commit to. This
+allows for future signature hash algorithms to be specified for V6 transactions
+that support signing parts of a transaction while leaving the remainder of the
+transaction malleable, or that commit to additional state that nodes may start
+tracking in a future upgrade and that is not part of an individual transaction
+being signed.
 
+Being able to remove support for sighash algorithm versions makes it possible
+to respond effectively in the case that a bug is found in a previously deployed
+sighash algorithm.
 
 ============
 Requirements
 ============
 
-- Continue to support existing functionality of the protocol (multisig,
+- Continue to support existing functionality of the protocol (e.g. multisig and
   signing modes for transparent inputs).
 
-- It should be possible to update this ZIP with additional digest versions after the first
-  version has been deployed in a network upgrade.
+- It should be possible to update this ZIP with additional sighash algorithm
+  versions that might have divergent behavior with respect to previous
+  versions, after previous versions have been deployed in network upgrades.
+
+- It should be possible to disable support for sighash algorithm versions in
+  network upgrades.
 
+- At the time that one is making a signature, the set of valid sighash
+  algorithm versions is known to consensus.
+
+- Sighash version information is present alongside the signature. 
 
 ================
 Non-requirements
 ================
 
-TBD
-
+- Sighash algorithm versioning as defined in this ZIP does not need to
+  constrain how signature hashes are constructed for future transaction
+  versions.
 
 =============
 Specification
@@ -79,21 +96,27 @@ Sighash versioning
 Rough summary:
 
 - Sighash versions are numbered starting from 0 for each tx version.
-- v0 is by convention the "commit to everything" sighash digest. Other versions can commit to whatever makes sense for desired functionality within that tx version.
-- Have a single byte encoded alongside the signature (not appended the way transparent does) that permits the signer to specify which sighash version they are using.
-- Consensus rules choose the digest algorithm for each signer based on that byte.
+- v0 is by convention the "commit to all effecting data" sighash algorithm. Other
+  versions can commit to whatever makes sense for desired functionality within
+  that tx version.
+- Have a single byte encoded alongside the signature (not appended in the manner
+  of transparent sighash types) that permits the signer to specify which sighash
+  algorithm version they are using.
+- Consensus rules choose the digest algorithm for each signer based on that
+  byte.
 
 ----------
 v0 Digests
 ----------
 
-The v0 digests are based on the v5 transaction digest algorithm defined in ZIP 244 [#zip-0244]_.
+The v0 digests are based on the v5 transaction digest algorithm defined in
+ZIP 244 [#zip-0244]_.
 
 TxId Digest
 ===========
 
-The overall structure of the TxID digest is as follows; each name referenced here will either be
-referenced or described in detail below::
+The overall structure of the TxID digest is as follows; each name referenced
+here will either be referenced or described in detail below::
 
     txid_digest
     ├── header_digest *