> Josh Berezin wrote:
>
> It's like this: I'm trying to figure out exactly what we're using
> here, so we can report it to Marketing-folk. I didn't write any of the
> SSL code, I'm just asking it a few questions.
>
> SSL_CIPHER_description reports we're using 3DES(168). Makes sense.
> SSL_CIPHER_get_bits() reports we're using 192 bits. (both in bits_used
> and bits_possible)
>
> So, which is right? 168 seems to make more sense, since DES is usually
> 56 bits, and 168 is 56 * 3. But I guess it could be 192 == 64 * 3.
> What is the explanation for the discrepancy?
There's actually another alternative figure...
DES keys are usually generated and distributed as 8 bytes which is 64
bits. However each byte has a parity bit meaning that only 7 bits are
actually usable. 3DES involves the DES operations on distinct keys means
the key is distributed as 192 bits but only 168 bits are usable.
Thats not the whole story though. Something called the "meet in the
middle attack" can reduce the actual effective number of bits to 112.
This attack requires huge resources to implement though.
So the key gets distributed as 192 bits of these 168 bits are actually
used but the effective strength is 112 bits.
Steve.
--
Dr Stephen N. Henson. http://www.drh-consultancy.demon.co.uk/
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Senior crypto engineer, Celo Communications: http://www.celocom.com/
Core developer of the OpenSSL project: http://www.openssl.org/
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