Re: DES and 3DES
From: Mxsmanic (mxsmanic_at_hotmail.com)
Date: 10/31/04
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- In reply to: Subba Rao: "DES and 3DES"
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Date: Sun, 31 Oct 2004 12:24:33 +0100
Subba Rao writes:
> ? 3DES (same as DES) performs 48 rounds (as opposed to 16 rounds of DES)
> of encryption.
No, 3DES is DES repeated three times with (usually) three different
keys.
> What is the default DES or 3DES key length? Is it 56 bit or 128 bit?
Single DES uses a 56-bit key. Triple DES uses a 168-bit key.
> I am under the impression that 3DES always uses 128 bit encryption.
No, normally 3DES is 168 bits. Of course, you can force 40 bits to zero
to make it effectively 128 bits, if needed.
> It is for that reason alone it is more secure. Am I right on that?
Yes. Triple DES is more secure because the effective key length is
longer than it is with single DES.
> Can 3DES encryption be done with smaller key sizes?
Sure, but there would be no point in doing so.
> If yes, is it part of the best practices?
It's not a good idea.
> For encryption/decryption, does 3DES perform 48 rounds on the
> data OR use 3 separate 56-bit keys on the data?
It uses three separate keys. It's DES done three times, with three
keys.
Not that this is more secure only because DES is not a group. In
cryptography, a "group" is a cipher in which consecutive encryption with
any number of keys is no more secure than encryption with a single key.
A simple XOR of the type used for one-time pads is in this category.
DES is not, though, and so multiple encryptions with different keys in
DES are more secure than just one encryption.
-- Transpose hotmail and mxsmanic in my e-mail address to reach me directly.
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