Enforce first four bits are zero for all spending keys and phi.

src/uint252.h

@@ -0,0 +1,48 @@
+#ifndef UINT252_H
+#define UINT252_H
+
+#include <vector>
+#include "uint256.h"
+#include "serialize.h"
+
+// Wrapper of uint256 with guarantee that first
+// four bits are zero.
+class uint252 {
+private:
+    uint256 contents;
+
+public:
+    ADD_SERIALIZE_METHODS;
+
+    template <typename Stream, typename Operation>
+    inline void SerializationOp(Stream& s, Operation ser_action, int nType, int nVersion) {
+        READWRITE(contents);
+
+        if ((*contents.begin()) & 0xF0) {
+            throw std::ios_base::failure("spending key has invalid leading bits");
+        }
+    }
+
+    const unsigned char* begin() const
+    {
+        return contents.begin();
+    }
+
+    const unsigned char* end() const
+    {
+        return contents.end();
+    }
+
+    uint252() : contents() {};
+    explicit uint252(const uint256& in) : contents(in) {
+        if (*contents.begin() & 0xF0) {
+            throw std::domain_error("leading bits are set in argument given to uint252 constructor");
+        }
+    }
+
+    uint256 inner() const {
+        return contents;
+    }
+};
+
+#endif