'Initialise a HKDF_SHA256 object.'

def __init__(self, prk, info='', length=32):

    self.hashLen = const.SHA256_LENGTH
    if (length > (self.hashLen * 255)):
        raise ValueError(("The    OKM's    length    cannot    be    larger    than    %d." % (self.hashLen * 255)))
    if (len(prk) < self.hashLen):
        raise ValueError(('The    PRK    must    be    at    least    %d    bytes    in    length    (%d    given).' % (self.hashLen, len(prk))))
    self.N = math.ceil((float(length) / self.hashLen))
    self.prk = prk
    self.info = info
    self.length = length
    self.ctr = 1
    self.T = ''
