sign: Add CreateMuSig2Nonce

diff --git a/src/key.cpp b/src/key.cpp
index 01fa3d27..02398332 100644
--- a/src/key.cpp
+++ b/src/key.cpp
@@ -13,6 +13,7 @@
 #include <secp256k1.h>
 #include <secp256k1_ellswift.h>
 #include <secp256k1_extrakeys.h>
+#include <secp256k1_musig.h>
 #include <secp256k1_recovery.h>
 #include <secp256k1_schnorrsig.h>
 
@@ -349,6 +350,39 @@ KeyPair CKey::ComputeKeyPair(const uint256* merkle_root) const
     return KeyPair(*this, merkle_root);
 }
 
+std::vector<uint8_t> CKey::CreateMuSig2Nonce(MuSig2SecNonce& secnonce, const uint256& sighash, const CPubKey& aggregate_pubkey, const std::vector<CPubKey>& pubkeys)
+{
+    // Get the keyagg cache and aggregate pubkey
+    secp256k1_musig_keyagg_cache keyagg_cache;
+    if (!MuSig2AggregatePubkeys(pubkeys, keyagg_cache, aggregate_pubkey)) return {};
+
+    // Parse participant pubkey
+    CPubKey our_pubkey = GetPubKey();
+    secp256k1_pubkey pubkey;
+    if (!secp256k1_ec_pubkey_parse(secp256k1_context_static, &pubkey, our_pubkey.data(), our_pubkey.size())) {
+        return {};
+    }
+
+    // Generate randomness for nonce
+    uint256 rand;
+    GetStrongRandBytes(rand);
+
+    // Generate nonce
+    secp256k1_musig_pubnonce pubnonce;
+    if (!secp256k1_musig_nonce_gen(secp256k1_context_sign, secnonce.Get(), &pubnonce, rand.data(), UCharCast(begin()), &pubkey, sighash.data(), &keyagg_cache, nullptr)) {
+        return {};
+    }
+
+    // Serialize pubnonce
+    std::vector<uint8_t> out;
+    out.resize(MUSIG2_PUBNONCE_SIZE);
+    if (!secp256k1_musig_pubnonce_serialize(secp256k1_context_static, out.data(), &pubnonce)) {
+        return {};
+    }
+
+    return out;
+}
+
 CKey GenerateRandomKey(bool compressed) noexcept
 {
     CKey key;
diff --git a/src/key.h b/src/key.h
index 22f96880..97ed27cc 100644
--- a/src/key.h
+++ b/src/key.h
@@ -7,6 +7,7 @@
 #ifndef BITCOIN_KEY_H
 #define BITCOIN_KEY_H
 
+#include <musig.h>
 #include <pubkey.h>
 #include <serialize.h>
 #include <support/allocators/secure.h>
@@ -220,6 +221,8 @@ public:
      *                               Merkle root of the script tree).
      */
     KeyPair ComputeKeyPair(const uint256* merkle_root) const;
+
+    std::vector<uint8_t> CreateMuSig2Nonce(MuSig2SecNonce& secnonce, const uint256& sighash, const CPubKey& aggregate_pubkey, const std::vector<CPubKey>& pubkeys);
 };
 
 CKey GenerateRandomKey(bool compressed = true) noexcept;
diff --git a/src/musig.cpp b/src/musig.cpp
index 7ebb8e55..5cf638d5 100644
--- a/src/musig.cpp
+++ b/src/musig.cpp
@@ -111,3 +111,10 @@ bool MuSig2SecNonce::IsValid()
 {
     return m_impl->IsValid();
 }
+
+uint256 MuSig2SessionID(const CPubKey& script_pubkey, const CPubKey& part_pubkey, const uint256& sighash)
+{
+    HashWriter hasher;
+    hasher << script_pubkey << part_pubkey << sighash;
+    return hasher.GetSHA256();
+}
diff --git a/src/musig.h b/src/musig.h
index bb469df4..03324fc4 100644
--- a/src/musig.h
+++ b/src/musig.h
@@ -18,6 +18,8 @@ struct secp256k1_musig_secnonce;
 using namespace util::hex_literals;
 constexpr uint256 MUSIG_CHAINCODE{"868087ca02a6f974c4598924c36b57762d32cb45717167e300622c7167e38965"_hex_u8};
 
+constexpr size_t MUSIG2_PUBNONCE_SIZE{66};
+
 //! Compute the full aggregate pubkey from the given participant pubkeys in their current order.
 //! Outputs the secp256k1_musig_keyagg_cache and validates that the computed aggregate pubkey matches an expected aggregate pubkey.
 //! This is necessary for most MuSig2 operations.
@@ -58,4 +60,6 @@ public:
     bool IsValid();
 };
 
+uint256 MuSig2SessionID(const CPubKey& script_pubkey, const CPubKey& part_pubkey, const uint256& sighash);
+
 #endif // BITCOIN_MUSIG_H
diff --git a/src/psbt.h b/src/psbt.h
index f8098b04..f0de079f 100644
--- a/src/psbt.h
+++ b/src/psbt.h
@@ -794,7 +794,7 @@ struct PSBTInput
 
                     std::vector<uint8_t> pubnonce;
                     s >> pubnonce;
-                    if (pubnonce.size() != 66) {
+                    if (pubnonce.size() != MUSIG2_PUBNONCE_SIZE) {
                         throw std::ios_base::failure("Input musig2 pubnonce value is not 66 bytes");
                     }
 
diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index 8575c0b9..5ca3f988 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -7,8 +7,10 @@
 
 #include <consensus/amount.h>
 #include <key.h>
+#include <musig.h>
 #include <policy/policy.h>
 #include <primitives/transaction.h>
+#include <random.h>
 #include <script/keyorigin.h>
 #include <script/miniscript.h>
 #include <script/script.h>
@@ -100,6 +102,34 @@ bool MutableTransactionSignatureCreator::CreateSchnorrSig(const SigningProvider&
     return true;
 }
 
+std::vector<uint8_t> MutableTransactionSignatureCreator::CreateMuSig2Nonce(const SigningProvider& provider, const CPubKey& aggregate_pubkey, const CPubKey& script_pubkey, const CPubKey& part_pubkey, const uint256* leaf_hash, const uint256* merkle_root, SigVersion sigversion, const SignatureData& sigdata) const
+{
+    assert(sigversion == SigVersion::TAPROOT || sigversion == SigVersion::TAPSCRIPT);
+
+    // Retrieve the private key
+    CKey key;
+    if (!provider.GetKey(part_pubkey.GetID(), key)) return {};
+
+    // Retrieve participant pubkeys
+    auto it = sigdata.musig2_pubkeys.find(aggregate_pubkey);
+    if (it == sigdata.musig2_pubkeys.end()) return {};
+    const std::vector<CPubKey>& pubkeys = it->second;
+    if (std::find(pubkeys.begin(), pubkeys.end(), part_pubkey) == pubkeys.end()) return {};
+
+    // Compute sighash
+    std::optional<uint256> sighash = ComputeSchnorrSignatureHash(leaf_hash, sigversion);
+    if (!sighash.has_value()) return {};
+
+    MuSig2SecNonce secnonce;
+    std::vector<uint8_t> out = key.CreateMuSig2Nonce(secnonce, *sighash, aggregate_pubkey, pubkeys);
+    if (out.empty()) return {};
+
+    // Store the secnonce in the SigningProvider
+    provider.SetMuSig2SecNonce(MuSig2SessionID(script_pubkey, part_pubkey, *sighash), std::move(secnonce));
+
+    return out;
+}
+
 static bool GetCScript(const SigningProvider& provider, const SignatureData& sigdata, const CScriptID& scriptid, CScript& script)
 {
     if (provider.GetCScript(scriptid, script)) {
@@ -755,6 +785,12 @@ public:
         sig.assign(64, '\000');
         return true;
     }
+    std::vector<uint8_t> CreateMuSig2Nonce(const SigningProvider& provider, const CPubKey& aggregate_pubkey, const CPubKey& script_pubkey, const CPubKey& part_pubkey, const uint256* leaf_hash, const uint256* merkle_root, SigVersion sigversion, const SignatureData& sigdata) const override
+    {
+        std::vector<uint8_t> out;
+        out.assign(MUSIG2_PUBNONCE_SIZE, '\000');
+        return out;
+    }
 };
 
 }
diff --git a/src/script/sign.h b/src/script/sign.h
index 2b4db200..4a0782da 100644
--- a/src/script/sign.h
+++ b/src/script/sign.h
@@ -23,6 +23,7 @@ class SigningProvider;
 
 struct bilingual_str;
 struct CMutableTransaction;
+struct SignatureData;
 
 /** Interface for signature creators. */
 class BaseSignatureCreator {
@@ -33,6 +34,7 @@ public:
     /** Create a singular (non-script) signature. */
     virtual bool CreateSig(const SigningProvider& provider, std::vector<unsigned char>& vchSig, const CKeyID& keyid, const CScript& scriptCode, SigVersion sigversion) const =0;
     virtual bool CreateSchnorrSig(const SigningProvider& provider, std::vector<unsigned char>& sig, const XOnlyPubKey& pubkey, const uint256* leaf_hash, const uint256* merkle_root, SigVersion sigversion) const =0;
+    virtual std::vector<uint8_t> CreateMuSig2Nonce(const SigningProvider& provider, const CPubKey& aggregate_pubkey, const CPubKey& script_pubkey, const CPubKey& part_pubkey, const uint256* leaf_hash, const uint256* merkle_root, SigVersion sigversion, const SignatureData& sigdata) const =0;
 };
 
 /** A signature creator for transactions. */
@@ -53,6 +55,7 @@ public:
     const BaseSignatureChecker& Checker() const override { return checker; }
     bool CreateSig(const SigningProvider& provider, std::vector<unsigned char>& vchSig, const CKeyID& keyid, const CScript& scriptCode, SigVersion sigversion) const override;
     bool CreateSchnorrSig(const SigningProvider& provider, std::vector<unsigned char>& sig, const XOnlyPubKey& pubkey, const uint256* leaf_hash, const uint256* merkle_root, SigVersion sigversion) const override;
+    std::vector<uint8_t> CreateMuSig2Nonce(const SigningProvider& provider, const CPubKey& aggregate_pubkey, const CPubKey& script_pubkey, const CPubKey& part_pubkey, const uint256* leaf_hash, const uint256* merkle_root, SigVersion sigversion, const SignatureData& sigdata) const override;
 };
 
 /** A signature checker that accepts all signatures */