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Re: [FW-1] Antwort: Re: [FW-1] Des / 3des
Marcus Brosda wrote:
>
> Hi,
>
> the difference between the 112-bit and the 168-bit version is, that the 168-bit
> version uses three keys with 56 bit length and encrypts the message with key A,
> then key B and then key C.
>
> The 112-bit version has only two keys of 56-bit length and encrypts the message
> first with key A, then with key B and then again with key A.
>
> In both cases the message is encrypted three times (3des) with a summary of 168
> bit key length. But the effective key length of the both algorithms differ.
No, this is not correct. In many 3DES implementations you do use three keys. One
key, k1, is used to encrypt the block, k2 is used to "decrypt" (note that
"decrypting" just means the reverse of DES, reverse-DES encrypts just as well as
DES), and then k3 is used to encrypt again.
Yes, 168-bits of keying material is used. But it only equivalent to 112-bits of
protection. There is a known attack on DES that reduces the keyspace on successive
encryptions (it's not all that practical, but it exists). This is why a "2DES,"
algorthim is essentially useless and cannot be used to give 112-bits of protection.
You can actually do 3DES with 112-bits of key, encrypt with k1, decrypt with k2,
and encrypt with k1, and it is just as secure as using three separate keys.
3DES also has the property that if you use a single key for all three steps,
the algorithm reduces to DES with the same key which may be useful for
interoperability.
--
Crist J. Clark [email protected]
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