key: store CKey key material as std::byte

`CKey` already exposes its private key data as `std::byte` through `data()`/`begin()`/`end()`, but internally stored it as `unsigned char`.

Store the key material as `std::byte` end-to-end, and only cast to `unsigned char*` at libsecp256k1 call boundaries. This avoids repeated `reinterpret_cast` and makes the code consistently use C++20’s byte type for opaque key data.

Author: l0rinc

src/key.cpp

@@ -162,7 +162,7 @@ bool CKey::Check(const std::byte* vch) { return Check(UCharCast(vch)); }
 void CKey::MakeNewKey(bool fCompressedIn) {
     MakeKeyData();
     do {
-        GetStrongRandBytes(MakeWritableByteSpan(*keydata));
+        GetStrongRandBytes(*keydata);
     } while (!Check(begin()));
     fCompressed = fCompressedIn;
 }
@@ -317,7 +317,7 @@ EllSwiftPubKey CKey::EllSwiftCreate(std::span<const std::byte> ent32) const
 
     auto success = secp256k1_ellswift_create(secp256k1_context_sign,
                                              UCharCast(encoded_pubkey.data()),
-                                             keydata->data(),
+                                             UCharCast(begin()),
                                              UCharCast(ent32.data()));
 
     // Should always succeed for valid keys (asserted above).
@@ -336,7 +336,7 @@ ECDHSecret CKey::ComputeBIP324ECDHSecret(const EllSwiftPubKey& their_ellswift, c
                                           UCharCast(output.data()),
                                           UCharCast(initiating ? our_ellswift.data() : their_ellswift.data()),
                                           UCharCast(initiating ? their_ellswift.data() : our_ellswift.data()),
-                                          keydata->data(),
+                                          UCharCast(begin()),
                                           initiating ? 0 : 1,
                                           secp256k1_ellswift_xdh_hash_function_bip324,
                                           nullptr);

src/key.h

@@ -50,7 +50,7 @@ class CKey
 
 private:
     /** Internal data container for private key material. */
-    using KeyType = std::array<unsigned char, 32>;
+    using KeyType = std::array<std::byte, 32>;
 
     //! Whether the public key corresponding to this private key is (to be) compressed.
     bool fCompressed{false};
@@ -117,7 +117,7 @@ class CKey
 
     //! Simple read-only vector-like interface.
     unsigned int size() const { return keydata ? keydata->size() : 0; }
-    const std::byte* data() const { return keydata ? reinterpret_cast<const std::byte*>(keydata->data()) : nullptr; }
+    const std::byte* data() const { return keydata ? keydata->data() : nullptr; }
     const std::byte* begin() const { return data(); }
     const std::byte* end() const { return data() + size(); }