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54 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
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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 ...
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31 votes
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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....
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29 votes
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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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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-...
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17 votes
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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, ...
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17 votes
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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 ...
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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 ...
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14 votes
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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 ...
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  • 3,820
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 ...
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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. ...
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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

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, ...
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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)...
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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. ...
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11 votes
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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 ...
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11 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 ...
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11 votes
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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 ...
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11 votes
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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 ...
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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 ...
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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 ...
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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 ...
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  • 101
9 votes
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Complement property of DES

I think I got it: I'm going to show that after round i the result will be $\overline{L_i}$ and $\overline{R_i}$ when the input was $\overline{L_{i-1}}$ and $\overline{R_{i-1}}$ and using $\overline k$ ...
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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 ...
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9 votes
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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 ...
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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 ...
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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. ...
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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.
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9 votes
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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 ...
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  • 124k
8 votes

What is an active S-Box?

You run the algorithm with two different plaintexts (whose difference is usually small – just a few bits, everything else being equal). Wherever these plaintexts lead to different inputs to an S-box ...
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