diff --git a/kex_gex.py b/kex_gex.py
index cf4b6d3..2b6e11c 100644
--- a/kex_gex.py
+++ b/kex_gex.py
@@ -4,7 +4,8 @@
 # are provided by the server.  a bit more work is required on our side (and a
 # LOT more on the server side).
 
-from message import Message, inflate_long, deflate_long
+from message import Message
+from util import inflate_long, deflate_long, generate_prime
 from secsh import SSHException
 from transport import MSG_NEWKEYS
 from Crypto.Hash import SHA
@@ -95,10 +96,11 @@ class KexGex(object):
         # generate prime
         while 1:
             # does not work FIXME
+            # the problem is that it's too fscking SLOW
             self.transport.log(DEBUG, 'stir...')
             self.transport.randpool.stir()
             self.transport.log(DEBUG, 'get-prime %d...' % preferred)
-            self.p = number.getPrime(preferred, self.transport.randpool.get_bytes)
+            self.p = generate_prime(preferred, self.transport.randpool)
             self.transport.log(DEBUG, 'got ' + repr(self.p))
             if number.isPrime((self.p - 1) // 2):
                 break
diff --git a/util.py b/util.py
index 2834972..fd78af3 100644
--- a/util.py
+++ b/util.py
@@ -1,6 +1,7 @@
 #!/usr/bin/python
 
-import struct
+import sys, struct, traceback
+from Crypto.Util import number
 
 def inflate_long(s, always_positive=0):
     "turns a normalized byte string into a long-int (adapted from Crypto.Util.number)"
@@ -87,3 +88,30 @@ def safe_string(s):
     return out
 
 # ''.join([['%%%02X' % ord(c), c][(ord(c) >= 32) and (ord(c) <= 127)] for c in s])
+
+def bit_length(n):
+    norm = deflate_long(n, 0)
+    hbyte = ord(norm[0])
+    bitlen = len(norm) * 8
+    while not (hbyte & 0x80):
+        hbyte <<= 1
+        bitlen -= 1
+    return bitlen
+
+def generate_prime(bits, randpool):
+    hbyte_mask = pow(2, bits % 8) - 1
+    x = randpool.get_bytes((bits+7) // 8)
+    if hbyte_mask > 0:
+        x = chr(ord(x[0]) & hbyte_mask) + x[1:]
+    n = inflate_long(x, 1)
+    n |= 1
+    n |= (1 << (bits - 1))
+    while 1:
+        # loop catches the case where we increment n into a higher bit-range
+        while not number.isPrime(n):
+            n += 2
+        if bit_length(n) == bits:
+            return n
+
+def tb_strings():
+    return ''.join(traceback.format_exception(*sys.exc_info())).split('\n')