Introduce concept Key.

Chaos/Cipher/Block/Key.hpp

@@ -0,0 +1,18 @@
+#ifndef CHAOS_CIPHER_BLOCK_KEY_HPP
+#define CHAOS_CIPHER_BLOCK_KEY_HPP
+
+#include <concepts>
+#include <cstdint>
+
+namespace Chaos::Cipher::Block
+{
+
+template<typename T>
+concept Key = requires
+{
+    requires std::constructible_from<T, uint8_t *, uint8_t *>;
+};
+
+} // namespace Chaos::Cipher::Block
+
+#endif // CHAOS_CIPHER_BLOCK_KEY_HPP