Comment 6 for bug 509562

I found the what is at fault here, the following program demonstrates the error:

#!/usr/bin/python
from M2Crypto import BN, EVP, RSA, util, Rand, m2, X509
from binascii import hexlify, unhexlify
i=0
while True:
   i=i+1
   try:
      iv = (hex(BN.rand(16 * 8))[2:34]).replace('L', 'c')
      uh = unhexlify(iv)
   except:
      print "found failure, i=%s, iv=%s len=(%s)\n" % (i,iv,len(iv))
      raise

basically,
  iv = (hex(BN.rand(16 * 8))[2:34]).replace('L', 'c')
is not guaranteed to create something that is consumable by unhexlify.
Run it, and you'll very quickly see something like:

$ /tmp/go.py
found failure, i=36, iv=3f99a2dfad8a6d936772bd181f8653c len=(31)

Traceback (most recent call last):
  File "/tmp/go.py", line 11, in <module>
    uh = unhexlify(iv)
TypeError: Odd-length string