+ *
+ * If `nbits' is greater than zero, it is used as an upper limit
+ * for the number of bits in x. This is safe provided that (a) you
+ * use twice as many bits in x as the number of bits you expect to
+ * use in your session key, and (b) the DH group is a safe prime
+ * (which SSH demands that it must be).
+ *
+ * P. C. van Oorschot, M. J. Wiener
+ * "On Diffie-Hellman Key Agreement with Short Exponents".
+ * Advances in Cryptology: Proceedings of Eurocrypt '96
+ * Springer-Verlag, May 1996.