Re: Encryption size

From: Ertugrul Soeylemez (never_at_drwxr-xr-x.org)
Date: 10/30/05 

Date: Sun, 30 Oct 2005 16:19:44 +0100

Lassi Hippeläinen <lahippel.at.ieee.org@moon.invalid> (Sun, 30 Oct 2005 11:00:25 +0200):
> Ertugrul Soeylemez wrote:
>
> > Lassi Hippeläinen <lahippel.at.ieee.org@moon.invalid> (Sat, 29 Oct 2005
> > 12:03:41 +0300):
> >> RSA can be seen as a block cipher. The block size is the size of
> >> the modulus, which you can choose.
> >
> > You can't really tell if RSA is a block cipher or not. A block is a
> > fixed number of of units. If you define a unit to be a byte, then
> > it is a block cipher, but you can as well define a unit to be one
> > cleartext unit, which is just a large number. In that case, RSA is
> > not a block cipher.
>
> The only meaningful way to define RSA block size is the size of the
> modulus, as I mentioned above. Usually it is 1024 or 2048 bits, but
> there is no mathematical reason why it has to be a power of two. It's
> only a habit that makes sense when data records have lengths of
> 2^N. Also 1536 bits is in use.

Yes, but you can also see one cleartext number 'm' as a unit, because it
is encrypted as a single whole, not like in other ciphers, where the
individual bytes are meaningful. In other words: RSA does not rely on
the underlying data representation. That's why RSA can't really be seen
as a block cipher.

In fact, this is even better, because you can use anything as the
underlying unit. If you build a computer, where each line has three
states, then you might define a byte to be of size 9 tribits, in which
case it could take 3^9 different numbers. This doesn't hurt RSA, but it
well hurts all symmetric block ciphers.

Also you can as well choose the key size in another unit than bits. My
key is of size 2^4096, but I could as well say that it's 3^2585.

Regards.

-----
Public key "Ertugrul Soeylemez <never@drwxr-xr-x.org>" (id: CE402012)
Fingerprint: 0F12 0912 DFC8 2FC5 E2B8 A23E 6BAC 998E CE40 2012

HKP: hkp://subkeys.pgp.net/
LDAP: ldap://keyserver.pgp.com/
HTTP: http://www.keyserver.de/

Relevant Pages

  • Re: newbie Qs about RSA, OAEP
    ... just say use 2048-bit RSA keys and not worry about it [worry about the rest ... > RSA block, you encrypt the message with a block cipher, and encrypt only ... > the keys (the block cipher key, the MAC key, maybe the IV if there is ...
    (sci.crypt)
  • Re: newbie Qs about RSA, OAEP
    ... > Are there recommended minimum/maximum lengths for RSA keys? ... RSA block, you encrypt the message with a block cipher, and encrypt only ... each protocol has its own way of indicating length. ...
    (sci.crypt)
  • Re: Encryption size
    ... Ertugrul Soeylemez wrote: ... > You can't really tell if RSA is a block cipher or not. ... The only meaningful way to define RSA block size is the size of the modulus, ...
    (comp.security.misc)
  • Re: Encryption size
    ... > modulus, which you can choose. ... You can't really tell if RSA is a block cipher or not. ... HTTP: http://www.keyserver.de/ ...
    (comp.security.misc)