[PYTHON-CRYPTO] randpool.getBytes(self,N) bug if N < 20 bytes in amkCrypto 0.1.3 .

[Adam Fleming]

I was just experimenting with the library when this issue came up, but if
you call GetBytes with N < 20, the result increases the value of entropy,
which is clearly daft. The problem is that dsize is larger than num when
subtracted from it, so when num is subtracted from self.entropy it increases.

The following is an alteration to the routine to allow less than 20 byte
reads, I expect your flames shortly over this issue. :

def getBytes(self,N) :
      s=''
      i, pool = self._getPos, self._randpool
      h=self.hash.new()
      dsize=self.hash.digestsize
      num = N
      while num>0
           h.update(self._randpool[i:i+dsize])
           s=s+h.digest()
           num=num-dsize
           i=(i+dsize) % self.bytes
           if i<dsize:
                self.stir()
                i=self._getPos

      self._getPos =i
      self.entropy=-8*N
      if self.entropy<0 :
                self.entropy=0
      return s[:N]

Enjoy,
Adam.