[NTLUG:Discuss] ssh key generation

[Burton Strauss]

Also, make sure you have enough sources of entropy.  

Strangely: If you are just leaving the system alone, you are actually
defeating yourself as you aren't introducing much entropy.  Get on the web,
move the mouse, delete files, etc.

You can see the state of the pool via 

# cat /proc/sys/kernel/random/entropy_avail   
3648

-----Burton 

# man 4 random
RANDOM(4)                                 Linux Programmer's Manual
RANDOM(4)

NAME
       random, urandom - kernel random number source devices

DESCRIPTION
       The  character  special  files  /dev/random  and /dev/urandom
(present since Linux 1.3.30) provide an
       interface to the kernel's random number generator.  File /dev/random
has major device  number  1  and
       minor device number 8.  File /dev/urandom has major device number 1
and minor device number 9.

       The random number generator gathers environmental noise from device
drivers and other sources into an
       entropy pool.  The generator also keeps an estimate of the number of
bits of  noise  in  the  entropy
       pool.  From this entropy pool random numbers are created.

       When  read,  the /dev/random device will only return random bytes
within the estimated number of bits
       of noise in the entropy pool.  /dev/random should be suitable for
uses that need  very  high  quality
       randomness  such  as  one-time  pad  or  key  generation.  When the
entropy pool is empty, reads from
       /dev/random will block until additional environmental noise is
gathered.

       When read, /dev/urandom device will return as many bytes as are
requested.  As a result, if there  is
       not  sufficient  entropy  in  the entropy pool, the returned values
are theoretically vulnerable to a
       cryptographic attack on the algorithms used by the driver.  Knowledge
of how to do this is not avail-
       able  in  the current non-classified literature, but it is
theoretically possible that such an attack
       may exist.  If this is a concern in your application, use /dev/random
instead.

CONFIGURING
       If your system does not have /dev/random and /dev/urandom created
already, they can be  created  with
       the following commands:

               mknod -m 644 /dev/random c 1 8
               mknod -m 644 /dev/urandom c 1 9
               chown root:root /dev/random /dev/urandom

       When  a Linux system starts up without much operator interaction, the
entropy pool may be in a fairly
       predictable state.  This reduces the actual amount of noise in the
entropy pool below  the  estimate.
       In  order to counteract this effect, it helps to carry entropy pool
information across shut-downs and
       start-ups.  To do this, add the following lines to an appropriate
script  which  is  run  during  the
       Linux system start-up sequence:

            echo "Initializing kernel random number generator..."
            # Initialize kernel random number generator with random seed
            # from last shut-down (or start-up) to this start-up.  Load and
            # then save 512 bytes, which is the size of the entropy pool.
            if [ -f /var/random-seed ]; then
                 cat /var/random-seed >/dev/urandom
            fi
            dd if=/dev/urandom of=/var/random-seed count=1

       Also, add the following lines in an appropriate script which is run
during the Linux system shutdown:

            # Carry a random seed from shut-down to start-up for the random
            # number generator.  Save 512 bytes, which is the size of the
            # random number generator's entropy pool.
            echo "Saving random seed..."
            dd if=/dev/urandom of=/var/random-seed count=1

PROC INTERFACE
       The files in the directory /proc/sys/kernel/random  (present  since
2.3.16)  provide  an  additional
       interface to the /dev/random device.

       The  read-only  file entropy_avail gives the available entropy.
Normally, this will be 4096 (bits), a
       full entropy pool.

       The file poolsize gives the size of the entropy pool. Normally, this
will be 512 (bytes).  It can  be
       changed  to  any  value  for which an algorithm is available.
Currently the choices are 32, 64, 128,
       256, 512, 1024, 2048.

       The file read_wakeup_threshold contains the number of bits of entropy
required  for  waking  up  pro-
       cesses   that   sleep   waiting  for  entropy  from  /dev/random.
The  default  is  64.   The  file
       write_wakeup_threshold contains the number of bits of entropy below
which we wake up  processes  that
       do  a  select() or poll() for write access to /dev/random.  These
values can be changed by writing to
       the files.

       The     read-only     files     uuid     and     boot_id     contain
random     strings      like
       6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.   The  former is generated
afresh for each read, the latter was
       generated once.

FILES
       /dev/random
       /dev/urandom

AUTHOR
       The kernel's random number generator was written by Theodore Ts'o
(tytso at athena.mit.edu).

SEE ALSO
       mknod (1)
       RFC 1750, "Randomness Recommendations for Security"

Linux                                            2003-10-25
RANDOM(4)



-----Original Message-----
From: discuss-bounces at ntlug.org [mailto:discuss-bounces at ntlug.org] On Behalf
Of brian at pongonova.net
Sent: Thursday, June 23, 2005 5:29 PM
To: discuss at ntlug.org
Subject: Re: [NTLUG:Discuss] ssh key generation

Unless you're trying to protect national security secrets, 8192 bits is
overkill.  2048 bits is more than sufficient, and probably won't be broken
in your lifetime...

But yes, expect several hours of processing on your 8K-bit key.

  --Brian

On Thu, Jun 23, 2005 at 06:00:54PM -0500, Ralph Green, Jr. wrote:
> Howdy,
>   I am generating some new public/private key pairs to use for 
> communicating to a new server.
> I have not done this in a while and I am a little concerned about how 
> long it is taking.
> The machine is a P2/366 running SuSE 9.3.  I issued the command:
>   ssh-keygen -b 8192 -t dsa
>  It has been running for two and a half hours so far.  Is it 
> conceivable that it is running ok?  I don't seem to be swapping even 
> though the system only has 192 meg of ram.

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