From 460163ee0b1268834f707154f6d4056da001ece9 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Sun, 15 Dec 2019 14:44:39 -0800
Subject: Add rationale on security assumptions

---
 bip-taproot.mediawiki | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

(limited to 'bip-taproot.mediawiki')

diff --git a/bip-taproot.mediawiki b/bip-taproot.mediawiki
index f436497..2abb185 100644
--- a/bip-taproot.mediawiki
+++ b/bip-taproot.mediawiki
@@ -24,7 +24,7 @@ This document is licensed under the 3-clause BSD license.
 
 ===Motivation===
 
-This proposal aims to improve privacy, efficiency, and flexibility of Bitcoin's scripting capabilities without adding new security assumptions. Specifically, it seeks to minimize how much information about the spendability conditions of a transaction output is revealed on chain at creation or spending time and to add a number of upgrade mechanisms, while fixing a few minor but long-standing issues.
+This proposal aims to improve privacy, efficiency, and flexibility of Bitcoin's scripting capabilities without adding new security assumptions<ref>'''What does not adding security assumptions mean?''' Unforgeability of signatures is a necessary requirement to prevent theft. At least when treating script execution as a digital signature scheme itself, unforgeability can be [https://github.com/apoelstra/taproot proven] in the Random Oracle Model assuming the Discrete Logarithm problem is hard. A [https://nbn-resolving.de/urn:nbn:de:hbz:294-60803 proof] for unforgeability of ECDSA in the current script system needs non-standard assumptions on top of that. Note that it is hard in general to model exactly what security for script means, as it depends on the policies and protocols used by wallet software.</ref>. Specifically, it seeks to minimize how much information about the spendability conditions of a transaction output is revealed on chain at creation or spending time and to add a number of upgrade mechanisms, while fixing a few minor but long-standing issues.
 
 ==Design==
 
@@ -41,7 +41,7 @@ As a result we choose this combination of technologies:
 * 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''' 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, the public key 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.</ref>
 
-Informally, the resulting design is as follows. A new witness version is added (1), whose programs consist of 32-byte encodings of points ''Q''. ''Q'' is computed as ''P + hash(P||m)G'' for a public key ''P'', and the root ''m'' of a Merkle tree whose leaves consist of a version number and a script. These outputs can be spent directly by providing a signature for ''Q'', or indirectly by revealing ''P'', the script and leaf version, inputs that satisfy the script, and a Merkle path that proves ''Q'' committed to that leaf. All hashes in this construction (the hash for computing ''Q'' from ''P'', the hashes inside the Merkle tree's inner nodes, and the signature hashes used) are tagged to guarantee domain separation.
+Informally, the resulting design is as follows: a new witness version is added (version 1), whose programs consist of 32-byte encodings of points ''Q''. ''Q'' is computed as ''P + hash(P||m)G'' for a public key ''P'', and the root ''m'' of a Merkle tree whose leaves consist of a version number and a script. These outputs can be spent directly by providing a signature for ''Q'', or indirectly by revealing ''P'', the script and leaf version, inputs that satisfy the script, and a Merkle path that proves ''Q'' committed to that leaf. All hashes in this construction (the hash for computing ''Q'' from ''P'', the hashes inside the Merkle tree's inner nodes, and the signature hashes used) are tagged to guarantee domain separation.
 
 == Specification ==
 
-- 
cgit v1.2.3

