refactor: move util::Xor to Obfuscation().Xor

This is meant to focus the usages to narrow the scope of the obfuscation optimization.

`Obfuscation::Xor` is mostly a move.

Co-authored-by: maflcko <6399679+maflcko@users.noreply.github.com>

src/streams.h

@@ -11,6 +11,7 @@
 #include <span.h>
 #include <support/allocators/zeroafterfree.h>
 #include <util/check.h>
+#include <util/obfuscation.h>
 #include <util/overflow.h>
 #include <util/syserror.h>
 
@@ -27,27 +28,6 @@
 #include <utility>
 #include <vector>
 
-namespace util {
-inline void Xor(std::span<std::byte> write, std::span<const std::byte> key, size_t key_offset = 0)
-{
-    if (key.size() == 0) {
-        return;
-    }
-    key_offset %= key.size();
-
-    for (size_t i = 0, j = key_offset; i != write.size(); i++) {
-        write[i] ^= key[j++];
-
-        // This potentially acts on very many bytes of data, so it's
-        // important that we calculate `j`, i.e. the `key` index in this
-        // way instead of doing a %, which would effectively be a division
-        // for each byte Xor'd -- much slower than need be.
-        if (j == key.size())
-            j = 0;
-    }
-}
-} // namespace util
-
 /* Minimal stream for overwriting and/or appending to an existing byte vector
  *
  * The referenced vector will grow as necessary
@@ -279,7 +259,7 @@ public:
      */
     void Xor(const std::vector<unsigned char>& key)
     {
-        util::Xor(MakeWritableByteSpan(*this), MakeByteSpan(key));
+        Obfuscation().Xor(MakeWritableByteSpan(*this), MakeByteSpan(key));
     }
 
     /** Compute total memory usage of this object (own memory + any dynamic memory). */