Add MuSig2SecNonce class for secure allocation of musig nonces

Author: achow101

src/musig.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <musig.h>
+#include <support/allocators/secure.h>
 
 #include <secp256k1_musig.h>
 
@@ -62,3 +63,43 @@ CExtPubKey CreateMuSig2SyntheticXpub(const CPubKey& pubkey)
     extpub.pubkey = pubkey;
     return extpub;
 }
+
+class MuSig2SecNonceImpl
+{
+private:
+    //! The actual secnonce itself
+    secure_unique_ptr<secp256k1_musig_secnonce> m_nonce;
+
+public:
+    MuSig2SecNonceImpl() : m_nonce{make_secure_unique<secp256k1_musig_secnonce>()} {}
+
+    // Delete copy constructors
+    MuSig2SecNonceImpl(const MuSig2SecNonceImpl&) = delete;
+    MuSig2SecNonceImpl& operator=(const MuSig2SecNonceImpl&) = delete;
+
+    secp256k1_musig_secnonce* Get() const { return m_nonce.get(); }
+    void Invalidate() { m_nonce.reset(); }
+    bool IsValid() { return m_nonce != nullptr; }
+};
+
+MuSig2SecNonce::MuSig2SecNonce() : m_impl{std::make_unique<MuSig2SecNonceImpl>()} {}
+
+MuSig2SecNonce::MuSig2SecNonce(MuSig2SecNonce&&) noexcept = default;
+MuSig2SecNonce& MuSig2SecNonce::operator=(MuSig2SecNonce&&) noexcept = default;
+
+MuSig2SecNonce::~MuSig2SecNonce() = default;
+
+secp256k1_musig_secnonce* MuSig2SecNonce::Get() const
+{
+    return m_impl->Get();
+}
+
+void MuSig2SecNonce::Invalidate()
+{
+    return m_impl->Invalidate();
+}
+
+bool MuSig2SecNonce::IsValid()
+{
+    return m_impl->IsValid();
+}

src/musig.h

@@ -11,6 +11,8 @@
 #include <vector>
 
 struct secp256k1_musig_keyagg_cache;
+class MuSig2SecNonceImpl;
+struct secp256k1_musig_secnonce;
 
 //! MuSig2 chaincode as defined by BIP 328
 using namespace util::hex_literals;
@@ -26,4 +28,35 @@ std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkey
 //! Construct the BIP 328 synthetic xpub for a pubkey
 CExtPubKey CreateMuSig2SyntheticXpub(const CPubKey& pubkey);
 
+/**
+ * MuSig2SecNonce encapsulates a secret nonce in use in a MuSig2 signing session.
+ * Since this nonce persists outside of libsecp256k1 signing code, we must handle
+ * its construction and destruction ourselves.
+ * The secret nonce must be kept a secret, otherwise the private key may be leaked.
+ * As such, it needs to be treated in the same way that CKeys are treated.
+ * So this class handles the secure allocation of the secp256k1_musig_secnonce object
+ * that libsecp256k1 uses, and only gives out references to this object to avoid
+ * any possibility of copies being made. Furthermore, objects of this class are not
+ * copyable to avoid nonce reuse.
+*/
+class MuSig2SecNonce
+{
+private:
+    std::unique_ptr<MuSig2SecNonceImpl> m_impl;
+
+public:
+    MuSig2SecNonce();
+    MuSig2SecNonce(MuSig2SecNonce&&) noexcept;
+    MuSig2SecNonce& operator=(MuSig2SecNonce&&) noexcept;
+    ~MuSig2SecNonce();
+
+    // Delete copy constructors
+    MuSig2SecNonce(const MuSig2SecNonce&) = delete;
+    MuSig2SecNonce& operator=(const MuSig2SecNonce&) = delete;
+
+    secp256k1_musig_secnonce* Get() const;
+    void Invalidate();
+    bool IsValid();
+};
+
 #endif // BITCOIN_MUSIG_H