core: move away from db towards hash-based auth

src/core/Challenge.hpp

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <string>
+
+class CChallenge {
+  public:
+    CChallenge(const std::string& fingerprint, const std::string& challenge, int difficulty);
+    CChallenge(const std::string& jsonResponse);
+
+    std::string fingerprint() const;
+    std::string challenge() const;
+    std::string signature() const;
+    bool        valid() const;
+
+  private:
+    std::string getSigString();
+
+    std::string m_sig, m_fingerprint, m_challenge;
+    bool        m_valid      = false;
+    int         m_difficulty = 4;
+
+    struct SChallengeJSON {
+        std::string fingerprint, challenge, sig;
+        int         difficulty = 4, solution = 0;
+    };
+};