From 61c6d1a8440db09c44d7fd367a6f2c641ea93d40 Mon Sep 17 00:00:00 2001
From: Antoine Poinsot <darosior@protonmail.com>
Date: Wed, 1 Dec 2021 23:45:33 +0100
Subject: [PATCH] script/sign: basic signing support for Miniscript descriptors

Try to solve a script using the Miniscript satisfier if the legacy
solver fails under P2WSH context. Only solve public key and public key
hash challenges for now.

We don't entirely replace the raw solver and especially rule out trying to
solve CHECKMULTISIG-based multisigs with the Miniscript satisfier since
some features, such as the transaction input combiner, rely on the
specific behaviour of the former.
---
 src/script/descriptor.cpp            |  2 +-
 src/script/sign.cpp                  | 87 +++++++++++++++++++++++++++-
 src/test/descriptor_tests.cpp        |  9 ++-
 test/functional/wallet_miniscript.py |  2 +-
 4 files changed, 92 insertions(+), 8 deletions(-)

diff --git a/src/script/descriptor.cpp b/src/script/descriptor.cpp
index 864eb8864fd..110006b6b6d 100644
--- a/src/script/descriptor.cpp
+++ b/src/script/descriptor.cpp
@@ -1012,7 +1012,7 @@ public:
         return false;
     }
 
-    bool IsSolvable() const override { return false; } // For now, mark these descriptors as non-solvable (as we don't have signing logic for them).
+    bool IsSolvable() const override { return true; }
     bool IsSingleType() const final { return true; }
 };
 
diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index 0d74a661a59..15da4667e0c 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -10,6 +10,7 @@
 #include <policy/policy.h>
 #include <primitives/transaction.h>
 #include <script/keyorigin.h>
+#include <script/miniscript.h>
 #include <script/signingprovider.h>
 #include <script/standard.h>
 #include <uint256.h>
@@ -381,6 +382,74 @@ static CScript PushAll(const std::vector<valtype>& values)
     return result;
 }
 
+/**
+ * Context for solving a Miniscript.
+ * If enough material (access to keys, hash preimages, ..) is given, produces a valid satisfaction.
+ */
+struct Satisfier {
+    typedef CPubKey Key;
+
+    const SigningProvider& m_provider;
+    SignatureData& m_sig_data;
+    const BaseSignatureCreator& m_creator;
+    const CScript& m_witness_script;
+
+    explicit Satisfier(const SigningProvider& provider LIFETIMEBOUND, SignatureData& sig_data LIFETIMEBOUND,
+                       const BaseSignatureCreator& creator LIFETIMEBOUND,
+                       const CScript& witscript LIFETIMEBOUND) : m_provider(provider),
+                                                                 m_sig_data(sig_data),
+                                                                 m_creator(creator),
+                                                                 m_witness_script(witscript) {}
+
+    static bool KeyCompare(const Key& a, const Key& b) {
+        return a < b;
+    }
+
+    //! Conversion from a raw public key.
+    template <typename I>
+    std::optional<Key> FromPKBytes(I first, I last) const
+    {
+        Key pubkey{first, last};
+        if (pubkey.IsValid()) return pubkey;
+        return {};
+    }
+
+    //! Conversion from a raw public key hash.
+    template<typename I>
+    std::optional<Key> FromPKHBytes(I first, I last) const {
+        assert(last - first == 20);
+        Key pubkey;
+        CKeyID key_id;
+        std::copy(first, last, key_id.begin());
+        if (GetPubKey(m_provider, m_sig_data, key_id, pubkey)) return pubkey;
+        m_sig_data.missing_pubkeys.push_back(key_id);
+        return {};
+    }
+
+    //! Conversion to raw public key.
+    std::vector<unsigned char> ToPKBytes(const CPubKey& key) const { return {key.begin(), key.end()}; }
+
+    //! Satisfy a signature check.
+    miniscript::Availability Sign(const CPubKey& key, std::vector<unsigned char>& sig) const {
+        if (CreateSig(m_creator, m_sig_data, m_provider, sig, key, m_witness_script, SigVersion::WITNESS_V0)) {
+            return miniscript::Availability::YES;
+        }
+        return miniscript::Availability::NO;
+    }
+
+    //! Time lock satisfactions.
+    // TODO
+    bool CheckAfter(uint32_t value) const { return false; }
+    bool CheckOlder(uint32_t value) const { return false; }
+
+    //! Hash preimage satisfactions.
+    // TODO
+    miniscript::Availability SatSHA256(const std::vector<unsigned char>& hash, std::vector<unsigned char>& preimage) const { return miniscript::Availability::NO; }
+    miniscript::Availability SatRIPEMD160(const std::vector<unsigned char>& hash, std::vector<unsigned char>& preimage) const { return miniscript::Availability::NO; }
+    miniscript::Availability SatHASH256(const std::vector<unsigned char>& hash, std::vector<unsigned char>& preimage) const { return miniscript::Availability::NO; }
+    miniscript::Availability SatHASH160(const std::vector<unsigned char>& hash, std::vector<unsigned char>& preimage) const { return miniscript::Availability::NO; }
+};
+
 bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreator& creator, const CScript& fromPubKey, SignatureData& sigdata)
 {
     if (sigdata.complete) return true;
@@ -416,9 +485,21 @@ bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreato
     {
         CScript witnessscript(result[0].begin(), result[0].end());
         sigdata.witness_script = witnessscript;
-        TxoutType subType;
+
+        TxoutType subType{TxoutType::NONSTANDARD};
         solved = solved && SignStep(provider, creator, witnessscript, result, subType, SigVersion::WITNESS_V0, sigdata) && subType != TxoutType::SCRIPTHASH && subType != TxoutType::WITNESS_V0_SCRIPTHASH && subType != TxoutType::WITNESS_V0_KEYHASH;
+
+        // If we couldn't find a solution with the legacy satisfier, try satisfying the script using Miniscript.
+        // Note we need to check if the result stack is empty before, because it might be used even if the Script
+        // isn't fully solved. For instance the CHECKMULTISIG satisfaction in SignStep() pushes partial signatures
+        // and the extractor relies on this behaviour to combine witnesses.
+        if (!solved && result.empty()) {
+            Satisfier ms_satisfier{provider, sigdata, creator, witnessscript};
+            const auto ms = miniscript::FromScript(witnessscript, ms_satisfier);
+            solved = ms && ms->Satisfy(ms_satisfier, result) == miniscript::Availability::YES;
+        }
         result.push_back(std::vector<unsigned char>(witnessscript.begin(), witnessscript.end()));
+
         sigdata.scriptWitness.stack = result;
         sigdata.witness = true;
         result.clear();
@@ -592,8 +673,8 @@ class DummySignatureChecker final : public BaseSignatureChecker
 {
 public:
     DummySignatureChecker() = default;
-    bool CheckECDSASignature(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const override { return true; }
-    bool CheckSchnorrSignature(Span<const unsigned char> sig, Span<const unsigned char> pubkey, SigVersion sigversion, ScriptExecutionData& execdata, ScriptError* serror) const override { return true; }
+    bool CheckECDSASignature(const std::vector<unsigned char>& sig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const override { return sig.size() != 0; }
+    bool CheckSchnorrSignature(Span<const unsigned char> sig, Span<const unsigned char> pubkey, SigVersion sigversion, ScriptExecutionData& execdata, ScriptError* serror) const override { return sig.size() != 0; }
 };
 }
 
diff --git a/src/test/descriptor_tests.cpp b/src/test/descriptor_tests.cpp
index 6b2ef74e192..7e11e362bee 100644
--- a/src/test/descriptor_tests.cpp
+++ b/src/test/descriptor_tests.cpp
@@ -528,9 +528,12 @@ BOOST_AUTO_TEST_CASE(descriptor_test)
     CheckUnparsable("wsh(and_b(and_b(older(1),a:older(100000000)),s:pk(L4gM1FBdyHNpkzsFh9ipnofLhpZRp2mwobpeULy1a6dBTvw8Ywtd)))", "wsh(and_b(and_b(older(1),a:older(100000000)),s:pk(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204)))", "and_b(older(1),a:older(100000000)) is not sane: contains mixes of timelocks expressed in blocks and seconds");
     CheckUnparsable("wsh(and_b(or_b(pkh(L4gM1FBdyHNpkzsFh9ipnofLhpZRp2mwobpeULy1a6dBTvw8Ywtd),s:pk(Kx9HCDjGiwFcgVNhTrS5z5NeZdD6veeam61eDxLDCkGWujvL4Gnn)),s:pk(L4gM1FBdyHNpkzsFh9ipnofLhpZRp2mwobpeULy1a6dBTvw8Ywtd)))", "wsh(and_b(or_b(pkh(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204),s:pk(032707170c71d8f75e4ca4e3fce870b9409dcaf12b051d3bcadff74747fa7619c0)),s:pk(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204)))", "and_b(or_b(pkh(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204),s:pk(032707170c71d8f75e4ca4e3fce870b9409dcaf12b051d3bcadff74747fa7619c0)),s:pk(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204)) is not sane: contains duplicate public keys");
     // Valid with extended keys.
-    Check("wsh(and_v(v:ripemd160(095ff41131e5946f3c85f79e44adbcf8e27e080e),multi(1,xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0)))", "wsh(and_v(v:ripemd160(095ff41131e5946f3c85f79e44adbcf8e27e080e),multi(1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0)))", "wsh(and_v(v:ripemd160(095ff41131e5946f3c85f79e44adbcf8e27e080e),multi(1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0)))", UNSOLVABLE, {{"0020acf425291b98a1d7e0d4690139442abc289175be32ef1f75945e339924246d73"}}, OutputType::BECH32, {{},{0}});
-    // Valid under sh(wsh()) and with a mix of xpubs and raw keys
-    Check("sh(wsh(thresh(1,pkh(L4gM1FBdyHNpkzsFh9ipnofLhpZRp2mwobpeULy1a6dBTvw8Ywtd),a:and_n(multi(1,xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0),n:older(2)))))", "sh(wsh(thresh(1,pkh(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204),a:and_n(multi(1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0),n:older(2)))))", "sh(wsh(thresh(1,pkh(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204),a:and_n(multi(1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0),n:older(2)))))", UNSOLVABLE | MIXED_PUBKEYS, {{"a914767e9119ff3b3ac0cb6dcfe21de1842ccf85f1c487"}}, OutputType::P2SH_SEGWIT, {{},{0}});
+    Check("wsh(and_v(v:ripemd160(095ff41131e5946f3c85f79e44adbcf8e27e080e),multi(1,xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0)))", "wsh(and_v(v:ripemd160(095ff41131e5946f3c85f79e44adbcf8e27e080e),multi(1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0)))", "wsh(and_v(v:ripemd160(095ff41131e5946f3c85f79e44adbcf8e27e080e),multi(1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0)))", DEFAULT, {{"0020acf425291b98a1d7e0d4690139442abc289175be32ef1f75945e339924246d73"}}, OutputType::BECH32, {{},{0}});
+    // Valid under sh(wsh()) and with a mix of xpubs and raw keys. Note how we can only satisfy a single
+    // branch (timelock sat isn't implemented yet) but we can solve and sign the script nonetheless.
+    Check("sh(wsh(thresh(1,pkh(L4gM1FBdyHNpkzsFh9ipnofLhpZRp2mwobpeULy1a6dBTvw8Ywtd),a:and_n(multi(1,xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0),n:older(2)))))", "sh(wsh(thresh(1,pkh(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204),a:and_n(multi(1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0),n:older(2)))))", "sh(wsh(thresh(1,pkh(03cdabb7f2dce7bfbd8a0b9570c6fd1e712e5d64045e9d6b517b3d5072251dc204),a:and_n(multi(1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0),n:older(2)))))", SIGNABLE | MIXED_PUBKEYS, {{"a914767e9119ff3b3ac0cb6dcfe21de1842ccf85f1c487"}}, OutputType::P2SH_SEGWIT, {{},{0}});
+    // An exotic multisig, we can sign for both branches
+    Check("wsh(thresh(1,pk(xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc),a:pkh(xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0)))", "wsh(thresh(1,pk(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL),a:pkh(xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0)))", "wsh(thresh(1,pk(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL),a:pkh(xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0)))", SIGNABLE, {{"00204a4528fbc0947e02e921b54bd476fc8cc2ebb5c6ae2ccf10ed29fe2937fb6892"}}, OutputType::BECH32, {{},{0}});
 }
 
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/test/functional/wallet_miniscript.py b/test/functional/wallet_miniscript.py
index 2252f1e4240..283136e1686 100755
--- a/test/functional/wallet_miniscript.py
+++ b/test/functional/wallet_miniscript.py
@@ -59,7 +59,7 @@ class WalletMiniscriptTest(BitcoinTestFramework):
             lambda: len(self.ms_wo_wallet.listunspent(minconf=0, addresses=[addr])) == 1
         )
         utxo = self.ms_wo_wallet.listunspent(minconf=0, addresses=[addr])[0]
-        assert utxo["txid"] == txid and not utxo["solvable"]  # No satisfaction logic (yet)
+        assert utxo["txid"] == txid and utxo["solvable"]
 
     def run_test(self):
         self.log.info("Making a descriptor wallet")