K = HMAC_K(V || 0x00 || int2octets(x) || bits2octets(h1))

   The bits2octets transform takes as input a sequence of blen bits and
   outputs a sequence of rlen bits.  It consists of the following steps:

   1.  The input sequence b is converted into an integer value z1
       through the bits2int transform:

          z1 = bits2int(b)
   2.  z1 is reduced modulo q, yielding z2 (an integer between 0 and
       q-1, inclusive):

          z2 = z1 mod q

       Note that since z1 is less than 2^qlen, that modular reduction
       can be implemented with a simple conditional subtraction:
       z2 = z1-q if that value is non-negative; otherwise, z2 = z1.

   3.  z2 is transformed into a sequence of octets (a sequence of rlen
       bits) by applying int2octets.