Move plain public key in output rationale to design section

Rebased by Pieter Wuille

1 files changed, 2 insertions, 1 deletions

diff --git a/bip-taproot.mediawiki b/bip-taproot.mediawiki
index bd9de86..d92f955 100644
--- a/bip-taproot.mediawiki
+++ b/bip-taproot.mediawiki
@@ -42,6 +42,7 @@ As a result we choose this combination of technologies:
 * As Schnorr signatures also permit '''batch validation''', allowing multiple signatures to be validated together more efficiently than validating each one independently, we make sure all parts of the design are compatible with this.
 * Where unused bits appear as a result of the above changes, they are reserved for mechanisms for '''future extensions'''. As a result, every script in the Merkle tree has an associated version such that new script versions can be introduced with a soft fork while remaining compatible with bip-taproot. Additionally, future soft forks can make use of the currently unused <code>annex</code> in the witness (see [[#Rationale]]).
 * While the core semantics of the '''signature hashing algorithm''' are not changed, a number of improvements are included in this proposal. The new signature hashing algorithm fixes the verification capabilities of offline signing devices by including amount and scriptPubKey in the digest, avoids unnecessary hashing, uses '''tagged hashes'''<ref>'''Why use tagged hashes?''' So far, nowhere in the Bitcoin protocol are hashes used where the input of SHA256 starts with two (non-double) SHA256 hashes, making collisions with existing uses of hash functions infeasible.</ref> (according to bip-schnorr) and defines a default sighash byte.
+* The '''public key is directly included in the output''' in contrast to typical earlier constructions which store a hash of the public key or script in the output. This has the same cost for senders and is more space efficient overall if the key-based spending path is taken. <ref>'''Why is the public key directly included in the output?''' While typical earlier constructions store a hash of a script or a public key in the output, this is rather wasteful when a public key is always involved. To guarantee batch verifiability, ''q'' must be known to every verifier, and thus only revealing its hash as an output would imply adding an additional 32 bytes to the witness. Furthermore, to maintain [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-January/012198.html 128-bit collision security] for outputs, a 256-bit hash would be required anyway, which is comparable in size (and thus in cost for senders) to revealing the public key directly. While the usage of public key hashes is often said to protect against ECDLP breaks or quantum computers, this protection is very weak at best: transactions are not protected while being confirmed, and a very [https://twitter.com/pwuille/status/1108097835365339136 large portion] of the currency's supply is not under such protection regardless. Actual resistance to such systems can be introduced by relying on different cryptographic assumptions, but this proposal focuses on improvements that do not change the security model. Note that using P2SH-wrapped outputs only have 80-bit collision security. This is considered low, and is relevant whenever the output includes data from more than a single party (public keys, hashes, ...). </ref>.
 
 Not included in this proposal are additional features like new sighash modes or opcodes that can be included with no loss in effectiveness as a future extension. Also not included is cross-input aggregation, as it [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-March/015838.html interacts] in complex ways with upgrade mechanisms and solutions to that are still [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-October/016461.html in flux].
 
@@ -56,7 +57,7 @@ The notation below follows that of bip-schnorr.
 A Taproot output is a SegWit output (native or P2SH-nested, see [https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki BIP141]) with version number 1, and a 32-byte witness program.
 The following rules only apply when such an output is being spent. Any other outputs, including version 1 outputs with lengths other than 32 bytes, remain unencumbered.
 
-* Let ''q'' be the 32-byte array containing the witness program (second push in scriptPubKey or P2SH redeemScript) which represents a public key according to bip-schnorr <ref>'''Why is the public key directly included in the output?''' While typical earlier constructions store a hash of a script or a public key in the output, this is rather wasteful when a public key is always involved. To guarantee batch verifiability, ''q'' must be known to every verifier, and thus only revealing its hash as an output would imply adding an additional 32 bytes to the witness. Furthermore, to maintain [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-January/012198.html 128-bit collision security] for outputs, a 256-bit hash would be required anyway, which is comparable in size (and thus in cost for senders) to revealing the public key directly. While the usage of public key hashes is often said to protect against ECDLP breaks or quantum computers, this protection is very weak at best: transactions are not protected while being confirmed, and a very [https://twitter.com/pwuille/status/1108097835365339136 large portion] of the currency's supply is not under such protection regardless. Actual resistance to such systems can be introduced by relying on different cryptographic assumptions, but this proposal focuses on improvements that do not change the security model. Note that using P2SH-wrapped outputs only have 80-bit collision security. This is considered low, and is relevant whenever the output includes data from more than a single party (public keys, hashes, ...). </ref>.
+* Let ''q'' be the 32-byte array containing the witness program (second push in scriptPubKey or P2SH redeemScript) which represents a public key according to bip-schnorr.
 * Fail if the witness stack has 0 elements.
 * If there are at least two witness elements, and the first byte of the last element is 0x50<ref>'''Why is the first byte of the annex <code>0x50</code>?''' Like the <code>0xc0</code>-<code>0xc1</code> constants, <code>0x50</code> is chosen as it could not be confused with a valid P2WPKH or P2WSH spending. As the control block's initial byte's lowest bit is used to indicate the public key's Y quadratic residuosity, each script version needs two subsequence byte values that are both not yet used in P2WPKH or P2WSH spending. To indicate the annex, only an "unpaired" available byte is necessary like <code>0x50</code>. This choice maximizes the available options for future script versions.</ref>, this last element is called ''annex'' ''a''<ref>'''What is the purpose of the annex?''' The annex is a reserved space for future extensions, such as indicating the validation costs of computationally expensive new opcodes in a way that is recognizable without knowing the outputs being spent. Until the meaning of this field is defined by another softfork, users SHOULD NOT include <code>annex</code> in transactions, or it may lead to PERMANENT FUND LOSS.</ref> and is removed from the witness stack. The annex (or the lack of thereof) is always covered by the transaction digest and contributes to transaction weight, but is otherwise ignored during taproot validation.
 * If there is exactly one element left in the witness stack, key path spending is used: