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Re: NewBie:cryptography : DES vs Blowfish?
From: shawn () nunleys com
Date: Sat, 2 Nov 2002 13:47:19 -0800
Which is more secure... that's a broad question, of course. If your
intention is to determine the security of the protocol itself, and not the
implementation of it in a particular application, then you must look at the
various cryptanalysis studies done against each protocol. DES has been
proven, time and again, to be very resistant to cryptanalysis attacks, but it
does have a limited key size. Most implementations use 56bit keys, and even
with triple DES you're only using 2 56bit keys. Blowfish and GOST have not
had as much study, but the key sizes can be bigger. So, barring any newly
discovered weaknesses (it can happen) you can get more brute-force attack
protection from Blowfish and GOST.
(plagiarized from aliennetworks.com)
DES (Data Encryption Standard)
DES is, by far, the worlds most popular encryption solution. DES is a block
cipher; it encrypts data in 64-bit blocks. It is also a symmetric algorithm;
this implies that it uses the same algorithm and key for decryption and
encryption. With a key length of 56 bits, DES performs 16 rounds (Loops) which
are comprised of multiple XOR and data substitutions. One of DES strengths is
an avalanche effect that is caused by a portion of the result of round 16
being fed back into round one of the next data block; This causes an increase
in data diffusion that can make DES very hard to crack.
The math behind DES - All possible 64 bit plaintext blocks can be mapped onto
all possible 64 bit cipher text blocks in 264! Possible ways. The DES
algorithm, with a 56 bit key, will produce around 256 of these mappings
(Thats about 100000000000000000 possible mappings). When DES was developed,
there was no known way to crack it in a single lifetime; Now, with
differential and linear cryptanalysis tools at hand, new advances in computer
hardware, and great strides in number theory, a dedicated DES cracking machine
could be built for under $1,000,000 that could break any message in just a few
hours. In 1984 DES chips capable of performing 256,000 encryptions and
decryptions per second were available; in 1987 chips that could do 512,000
were available. In 1993 , Michael Wiener designed a machine that could do a
brute-force crack on any DES message, decoding it in 3.5 hours. There are many
other attacks that can break DES (Eventually), including differential
cryptanalysis, Dif-Lin, meet-in-the-middle, and chosen plaintext. These
attacks all share two common factors; theyre very difficult to execute in
practice, and they can become expensive. Are their other ways? The algorithm
is over 20 years old; the NSA no longer uses it for encrypted transmissions;
and the source code and substitution boxes are available in the public domain.
The combination of readily available data on the algorithm, exponential
increases in computing power, and increasing value of transmitted data is
giving many companies a reason to reevaluate DES as their choice for encrypted
transmissions. Regrettably, they have few options that offer actual security.
Blowfish is a fast 64 bit block cipher with a variable-length key. Key
expansion in Blowfish can create a 448 bit keyspace quite a bit larger than
DES. The major drawback of Blowfish is that it is optimized for applications
where the keyspace does not change very often. In spite of this shortcoming,
Blowfish passes many security tests, and provides a very good level of
security. Blowfish is in the public domain (Anyone can go get the source code
for free), and is easy to implement.
The math behind Blowfish - Blowfish consists of 16 rounds, or loops.
Cryptanalysis of Blowfish by Serge Vaudenay reveals a partial differential
attack that can recover the plaintext array in 28r+1 chosen plaintexts. There
is also a class of known weak keys that can increase the effectiveness of this
attack by a factor of two.
[end of plagiarism]
In my opinion, it's not the protocol that's the issue nearly as much as the
technical implementation and your procedures for protecting your keyspace.
Whenever you must resort to using symmetric-key crypto, you face some tough
issues with regard to key distribution.
Quoting DocValde <DocValde () gmx de>:
Hallo Roberto Ramsis,
am Freitag, 1. November 2002 um 12:37:15 schrieben Sie:
i needed to know,which is more secure : DES , BLOWFISH,GOST? and which is
DES is obsolete since few years, (not only) since its key length is
fixed to 56bit
effective. Even TripleDES has only 112bit effective.
With GOST, i had not to deal yet, but Blowfish seems to be quite good and
fast enough for
almost any use. Be sure to use 128bit keylength upwards.
Malte von dem Hagen
eMail: DocValde () gmx de