Re: asymmetric vs symmetric ciphers - block size?
From: Adam O'Brien (adamobrien54321_at_yahoo.co.uk)
Date: 05/19/03
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Date: 19 May 2003 07:56:27 -0700
digitalhome@ananzi.co.za (Clive) wrote in message news:<3e90be2d.0305181107.49e1cd79@posting.google.com>...
> Can anyone briefly explain why in general asymmetric ciphers need
> larger block sizes than symmetric ones?
>
> Your help is much appreciated.
> Clive Barker
For asymmetric ciphers the keys need to be bigger than for symmetric
ciphers. This is because the 'brute force' method of cryptanalysis are
different in the two cases. For symmetric ciphers one has to check
every key in the key space (minor tweaks aside). For an asymmetric
cipher like (standard) RSA you don't have to ckeck every possible key
- e.g. you know that p and q will be odd. For RSA the 'brute force'
attacks are actually quite sophisticated (GNFS), so the keys must be
longer.
Now, given that you are using a larger key you may as well use a
larger block size. Again to use RSA, large values of p and q imply
large ciphertext values. So it would just be wilfully inefficient to
restrict the size of input blocks to lower values.
Adam
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