PuTTY compatibility: Does gen_rsa_priv_key still need to regenerate primes until modulus is a multiple-of-8-bits long ?

[Sajin.Valoth at Emerson.com]

Matt,
   Thanks, that was really quick!

   Regards,
   -- Sajin


-----Original Message-----
From: Matt Johnston [mailto:matt at ucc.asn.au]
Sent: Fri 9/12/2008 12:49 PM
To: Valoth, Sajin [NETPWR/EMBED/MA]
Cc: dropbear at ucc.asn.au
Subject: Re: PuTTY compatibility: Does gen_rsa_priv_key still need to regenerate primes until modulus is a multiple-of-8-bits long ?
 
On Fri, Sep 12, 2008 at 10:32:01AM -0500, Sajin.Valoth at Emerson.com wrote:
> Hi all,
>  
>      With dropbear 0.48.1 running on a MIPS Au1550 platform, 
>      RSA key generation occasionally takes a very long time (multiple
> minutes);
>      the problem was traced to the loop shown below in
> gen_rsa_priv_key()
>      which executes many times, until a value of the modulus, n,
>      is finally computed that is a multiple-of-8-bits long.
...
>  
>           However, using more recent versions of PuTTY  (0.58 and 0.60),
> I don't see any problems
>           with keys generated by dropbear where the modulus is not a
> multiple-of-8-bits long.
>  
>           So, my question is: should the loop above be deprecated since
> it doesn't appear to be necessary any more?

Yes, it looks like this was fixed in PuTTY 0.54 in 2004 [1],
so can be removed. Thanks for letting me know.

Cheers,
Matt

[1] http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/rsa-non8mult-verify-fail.html


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