Re: newbie Q's about RSA, OAEP

From: Michael Amling (nospam_at_nospam.com)
Date: 11/28/03 

Date: Fri, 28 Nov 2003 17:27:53 GMT

Dominic wrote:
> I've found articles covering the maths behind these algorithms, but is
> there anything which gives a good introduction to their practical use?
>
> Are there recommended minimum/maximum lengths for RSA keys?

   1024-bit modulus is about the minimum these days. You could use 768
for low-value messages. 16K-bit modulus is the largest I've heard of
anyone using.

>
> Is it safe (not necessarily efficient) to code long messages in RSA by
> splitting it into blocks and coding each separately (as you would with
> Rijndael). Are ECB, CBC modes applicable in that case?

   Standard procedure is that if the message doesn't fit into a single
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
one) with RSA.
   Maybe someone else can actually answer your question.

> Should you generally use OAEP rather than coding the message uncooked?

   Yes. Always.

> I know this is a good idea when encoding short messages to avoid
> dictionary attacks, but is it useful with longer messages?

   Yes. It protects against more than just dictionary attacks.

>
> Is there an accepted scheme for indicating the length of the message?

   AFAIK, each protocol has its own way of indicating length.

--Mike Amling

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