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Make consensus checking of tweaks in pubkey.* Taproot-specific

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That results in a much safer interface (making the tweak commit
to the key implicitly using a fixed tag means it can't be used for
unrelated tweaking).
This commit is contained in:
Pieter Wuille 2021-02-01 16:21:59 -08:00
parent a4bf84039c
commit 2fbfb1becb
3 changed files with 33 additions and 9 deletions
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22
src/pubkey.cpp
View file

@ -188,11 +188,25 @@ bool XOnlyPubKey::VerifySchnorr(const uint256& msg, Span<const unsigned char> si
return secp256k1_schnorrsig_verify(secp256k1_context_verify, sigbytes.data(), msg.begin(), &pubkey);
}
bool XOnlyPubKey::CheckPayToContract(const XOnlyPubKey& base, const uint256& hash, bool parity) const
static const CHashWriter HASHER_TAPTWEAK = TaggedHash("TapTweak");
uint256 XOnlyPubKey::ComputeTapTweakHash(const uint256* merkle_root) const
{
secp256k1_xonly_pubkey base_point;
if (!secp256k1_xonly_pubkey_parse(secp256k1_context_verify, &base_point, base.data())) return false;
return secp256k1_xonly_pubkey_tweak_add_check(secp256k1_context_verify, m_keydata.begin(), parity, &base_point, hash.begin());
if (merkle_root == nullptr) {
// We have no scripts. The actual tweak does not matter, but follow BIP341 here to
// allow for reproducible tweaking.
return (CHashWriter(HASHER_TAPTWEAK) << m_keydata).GetSHA256();
} else {
return (CHashWriter(HASHER_TAPTWEAK) << m_keydata << *merkle_root).GetSHA256();
}
}
bool XOnlyPubKey::CheckTapTweak(const XOnlyPubKey& internal, const uint256& merkle_root, bool parity) const
{
secp256k1_xonly_pubkey internal_key;
if (!secp256k1_xonly_pubkey_parse(secp256k1_context_verify, &internal_key, internal.data())) return false;
uint256 tweak = internal.ComputeTapTweakHash(&merkle_root);
return secp256k1_xonly_pubkey_tweak_add_check(secp256k1_context_verify, m_keydata.begin(), parity, &internal_key, tweak.begin());
}
bool CPubKey::Verify(const uint256 &hash, const std::vector<unsigned char>& vchSig) const {

15
src/pubkey.h
View file

@ -236,7 +236,20 @@ public:
* sigbytes must be exactly 64 bytes.
*/
bool VerifySchnorr(const uint256& msg, Span<const unsigned char> sigbytes) const;
bool CheckPayToContract(const XOnlyPubKey& base, const uint256& hash, bool parity) const;
/** Compute the Taproot tweak as specified in BIP341, with *this as internal
* key:
* - if merkle_root == nullptr: H_TapTweak(xonly_pubkey)
* - otherwise: H_TapTweak(xonly_pubkey || *merkle_root)
*
* Note that the behavior of this function with merkle_root != nullptr is
* consensus critical.
*/
uint256 ComputeTapTweakHash(const uint256* merkle_root) const;
/** Verify that this is a Taproot tweaked output point, against a specified internal key,
* Merkle root, and parity. */
bool CheckTapTweak(const XOnlyPubKey& internal, const uint256& merkle_root, bool parity) const;
const unsigned char& operator[](int pos) const { return *(m_keydata.begin() + pos); }
const unsigned char* data() const { return m_keydata.begin(); }

5
src/script/interpreter.cpp
View file

@ -1486,7 +1486,6 @@ template PrecomputedTransactionData::PrecomputedTransactionData(const CMutableTr
static const CHashWriter HASHER_TAPSIGHASH = TaggedHash("TapSighash");
static const CHashWriter HASHER_TAPLEAF = TaggedHash("TapLeaf");
static const CHashWriter HASHER_TAPBRANCH = TaggedHash("TapBranch");
static const CHashWriter HASHER_TAPTWEAK = TaggedHash("TapTweak");
static bool HandleMissingData(MissingDataBehavior mdb)
{
@ -1869,10 +1868,8 @@ static bool VerifyTaprootCommitment(const std::vector<unsigned char>& control, c
}
k = ss_branch.GetSHA256();
}
// Compute the tweak from the Merkle root and the internal pubkey.
k = (CHashWriter(HASHER_TAPTWEAK) << MakeSpan(p) << k).GetSHA256();
// Verify that the output pubkey matches the tweaked internal pubkey, after correcting for parity.
return q.CheckPayToContract(p, k, control[0] & 1);
return q.CheckTapTweak(p, k, control[0] & 1);
}
static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion, const std::vector<unsigned char>& program, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror, bool is_p2sh)