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 ...
- 7,449
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 ...
- 132k
31
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....
- 45.7k
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. ...
- 132k
19
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, ...
- 89.2k
19
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-...
- 132k
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 ...
- 85.9k
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 ...
- 132k
14
votes
Accepted
Why does applying 56-bit DES twice only give 57 bits of security?
Decrypt the ciphertext with every possible key and store the result: $2^{56}$ decryptions. Now encrypt the (known) plaintext of the ciphertext with every possible key: $2^{56}$ encryptions. Now you ...
- 3,860
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 ...
- 2,920
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.
...
- 45.7k
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 ...
- 14.5k
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, ...
11
votes
Is there any open-source white-box implementation of AES or DES?
I add my whitebox AES implementation on GitHub in:
C++
Java
C++ version implements both Chow's (mixing bijections, input/output encodings, external encodings) and Karroumi's (dual AES in each column)...
- 231
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.
...
- 38.2k
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 ...
- 4,375
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 ...
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 ...
- 45.7k
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 ...
- 132k
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 ...
- 89.2k
11
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 ...
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
Accepted
Is DES slow in hardware or only in software?
DES is slow in software because it was designed back in the early 70's even before the 8086 processor existed, and uses several bit oriented operations that are just not implemented efficiently in a ...
- 12.9k
9
votes
what is the current actual budget - as of 2015 - needed to build a DES breaker machine?
Re-using their design might be no good idea - there are cheaper designs for sure. This new DES cracker would just need to try every possible key - like the one of the EFF already did.
DES was a big ...
- 3,860
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 ...
- 19.4k
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.
...
- 19.4k
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.
- 11.5k
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 ...
- 132k
8
votes
Accepted
Why does the DES crypto algorithm NOT use 2 rounds?
DES with 2 rounds is broken. It is trivial to find a way to get the key with much less work than for the full DES (and even that is broken).
DES is a Feistel cipher, so we have two halves, the left ...
- 3,860
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