57
votes
Is Triple DES still considered safe to use?
Well, yes and no.
Triple DES using 3 different keys is still considered secure because there are no known attack which completely break its security to a point where it is feasible nowadays to crack ...
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47
votes
Accepted
How was the special DES key 0E329232EA6D0D73 found?
That value 0E329232EA6D0D73 was found by brute force. I would be surprised if there was a significantly better method: that would be tantamount to a cryptanalytic break of DES, very different from the ...
- 138k
33
votes
Accepted
Why is double encryption that's equivalent to single encryption no better than single encryption?
This is simply saying that if a cryptosystem has a functional composition that is
$$ h_{k}(x) = f_{k_1}(g_{k_2}(x)) $$
then you can find a key for single encryption that works as the double encryption....
- 47.5k
29
votes
Accepted
Can neural cryptanalysis be applied to AES?
No. Neuro-Cryptanalysis fails on serious ciphers, including DES and AES.
Sebastien Dourlens's Neuro-differential cryptanalysis of DES (in sections 5.4.2 and 5.4.3 of his 1996 mémoire) learns an S-box. ...
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21
votes
Purpose of DES parity bits
Parity of DES key bytes was introduced on request of US authorities during the design of DES in the late 1970s:
it mitigates the risk of accidental key alteration; in particular, any all-zeros or all-...
- 138k
20
votes
Accepted
Purpose of DES parity bits
They are there to check if the key was indeed correctly retrieved. It could for instance be that the key is a result of key decryption or key agreement. In that case, or simply during transmission, ...
- 91.7k
17
votes
Accepted
Why is DES not an ideal cipher?
DES uses a 56-bit key (formally, a 64-bit key, out of which 8 bits are simply ignored), so it represents a family of exactly $2^{56}$ permutations(*).
If you were to select a permutation at random ...
- 86.6k
15
votes
Converting normal alphabets to 64 bit plain text
What's asked requires choices not explicit in the question:
1. Character encoding into bytes
The most common and recommendable convention nowadays is UTF-8, which encodes any character as a sequence ...
- 138k
14
votes
For a typical n-bit symmetric key, how many keys would be considered too weak to use?
I understand that all zeros or all ones would be weak for any cipher.
This isn't actually true. For good cipher there are no weak keys.
And certain ciphers, e.g. DES, have a list of weak keys. But ...
- 2,930
13
votes
Is DES secure under CBC?
No, it will be insecure. There are two reasons;
Due to the smaller key size 56-bit; DES was tested for brute-force attack since published.
DES_CHALL, 96 days to find the CES challenge key in 1997.
...
- 47.5k
13
votes
What is DES-EDE3-CBC?
At least in the context of PKCS#5 (which is commonly seen through the encryption of PEM files), DES-EDE3-CBC is Triple DES with three keys, used in CBC mode, with unspecified padding.
Yes, “EDE” means ...
12
votes
S-box basic question
Good question and yes, it would appear that data is lost. And technically, something clearly must disappear as 6 don't go into 4. But consider the overall architecture of DES:
Trace the flow of the ...
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12
votes
Is Triple DES still considered safe to use?
NIST just recently (11/27/2017) put out a bulletin that Triple-DES will be deprecated in the future, and will be disallowed in protocols like TLS and IPsec, with a future deprecation timeline to be ...
- 121
12
votes
Cryptographic methods that do not use prime numbers?
There is none. All cryptography involves the number 2, which is prime, whenever dealing with information in strings of bits—or in esoteric cases like ROT13, well, there's a prime number right there, ...
- 47.9k
11
votes
Feistel-Network - why wire crossing?
Let's look at a picture of a generic feistel cipher
Notice that no keying material is used during or after that final swap. So, we can conclude that the final swap does not impact security at all.
...
- 38.5k
11
votes
Accepted
Why is the DES s-box non-linear? Why does it make the cracking of the cipher more difficult?
It is of course possible to write DES or any block cipher as a system of non-linear equations involving the plaintext bits, the ciphertext bits, and the key bits, which hold with probability 1. In ...
- 4,405
11
votes
Accepted
Can I find the encryption key if I know the plain text and the encrypted text (DES and AES)?
The most cost-effective way to do this is to try each key and see if it works. The expected number of trials is $2^{55}$ with DES, $2^{127}$ with AES-128, and $2^{255}$ with AES-256.
You can speed ...
- 47.9k
11
votes
Accepted
In the RSA DES challenges, how did the contestants know they had found the right key considering they weren't given any plaintext?
One can still access the challenge rules from the archive.org
Each contest is based on a specified cipher. A brief piece of printable ASCII text (containing byte values in hexadecimal notation from ...
- 47.5k
11
votes
Why use complex encryption algorithms?
We use more complex encryption algorithms than XOR with a random or pseudo-random keystream for a number of reasons:
In order to get a short secret key in symmetric encryption. XOR with a true ...
- 138k
11
votes
Converting normal alphabets to 64 bit plain text
First of all you should not use DES, it is not secure and, arguably, never has been secure as the key size of 56 bits is well below 128 bits. The strength of the key after analysis is even less than ...
- 91.7k
10
votes
AES vs DES S-boxes
A good source for this kind of questions is the book The Design of Rijndael by Joan Daemen and Vincent Rijmen. On page 35 they write about their choice for the used S-box $S_{RD}$:
Design criteria ...
- 101
9
votes
Feistel-Network - why wire crossing?
It's there to facilitate a simple implementation. As there is no key addition applied afterwards, the final swapping of the halves does not contribute towards security.
The Feistel cipher entry on ...
- 19.5k
9
votes
DES strength and weakness
The wikipedia article @SEJPM links to is about as high level of an overview as you can really get. We can elaborate on some of the points.
DES is weak against Brute force in this day and age.
...
- 19.5k
9
votes
S-box basic question
DES uses a Feistel network. The S-box results gets XOR-ed with the other half so no information is lost. It doesn't need to be invertible.
- 11.7k
9
votes
Accepted
How to find fixed points for DES weak keys
I answer in hopefully didactic order.
What does the author mean by the intermediate texts exactly?
The intermediate texts after $n$ rounds are the 64-bit quantities $L_n\mathbin\|R_n$, numbering ...
- 138k
8
votes
Accepted
What is the main limitation of DES?
According to Introduction to Modern Cryptography by Katz and Lindell, in the section titled Security of DES on page 218 in the second edition.
After almost 30 years of intensive study, the best ...
- 38.5k
8
votes
Accepted
Would DES be secure with 128 bit keys?
No, DES* (which I'll call your "DES modified to use 128 bit keys") would not be as secure as AES; two reasons spring immediately to mind:
Block size; DES* would still have 64 bit blocks; most block ...
- 145k
7
votes
Is a 4DES or 5DES system possible?
There is a very interesting paper that relates to this exact question (but you wouldn't guess it from the title). The paper is titled Efficient Dissection of Composite Problems, with Applications to ...
- 27.6k
7
votes
Accepted
In differential cryptanalysis for DES, why is F(0...0) assumed to be always 0...0?
The question arises from a misunderstanding: The attack described in the paper does not work with actual inputs and outputs, but with differences between them. Hence Differential Cryptanalysis.
...
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7
votes
Accepted
How can I calculate the avalanche effect in symmetric algorithms?
You appear to be under the impression that AES should score better than DES, because AES is stronger than DES.
The issue is that your test is extremely crude; any decent block cipher will pass the ...
- 145k
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