+ /**
+ * \brief Get the offset and length for the X coordinate.
+ *
+ * This function returns the offset and length (in bytes) of
+ * the X coordinate in an encoded non-zero point.
+ *
+ * \param curve curve identifier.
+ * \param len receiver for the X coordinate length (in bytes).
+ * \return the offset for the X coordinate (in bytes).
+ */
+ size_t (*xoff)(int curve, size_t *len);
+