analysing cryptographic algorithms, potentially uncovering weaknesses in them (e.g. "breaking" them or casting doubts on their actual security)

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268 views

Solid summary of what encryption remains strong after recent events [closed]

Following the recent government hacking (and subsequent news and fallout), I'm wondering what crypto-systems remain strong (and why) and what crypto-systems were exploited too heavily to be reliable. ...
4
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3answers
210 views

Why to try get key out of white box crypto? How can one protect WBC itself?

Help me understand the following. With WBC the question is mainly being stated as can one try to break WBC to identify the key. My question is: Why would one ever want to do that anyways? If one can ...
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1answer
155 views

Derive a public EC key from two public EC keys

Alice has two EC key pairs: $a_1$, $a_2$ are private keys (integers), $A_1$, $A_2$ are the corresponding public keys (points). Alice and Bob want to create a new public key $C$. Alice must prove that ...
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5answers
576 views

How to prove the security of the PRNG?

Are there any realties tests or criterias that prove the security of the PRNG? What kind of tests or criteria?
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1answer
334 views

WIN LM Password Hashes (using John the ripper) [closed]

In a problem set, we've been asked to use John The Ripper to crack a list of Win LANMAN passwords. They are of the following form: 953A55E36AB85C0B9DA92C18555E42E1:0169295E0A6A6B35B2DD9932D10C1978 ...
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2answers
327 views

How do we prove that AES, DES etc. are secure?

How do we prove that AES, DES etc. are secure? I've read about them and know the basics on how they work, but I still don't quite understand how we can be sure they are secure? What are the proofs?
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1answer
268 views

What is adversary's advantage in cryptography and why we use it?

The definition of adversary's advantage seems a bit odd for me and I am wondering why do we use it to measure the power of an adversary rather than just use the probability of a PPT adversary ...
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3answers
243 views

How to attack a fixed LCG with partial output?

To show some colleague programmers exactly how broken C's rand() is (at least on Windows) I decided to break it. So everyone knows the exact parameters, MSVC's ...
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0answers
108 views

Career advice for our readers [closed]

I am a first year math PhD student with work experience and an MS in math. Do you have any advice regarding internships involved with cryptography and other mathematics other than the NSA? Thanks.
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1answer
227 views

How much does a successful Distinguishing Attack “break” the attacked stream cipher?

I started reading about the most recent cryptanalysis of RC4 and it's variants. One of the papers I just finished reading ("Two Linear Distinguishing Attacks on VMPC and RC4A and Weakness of RC4 ...
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2answers
202 views

Is this a structural weakness of Feistel networks?

I'm doing a lot of reading about Feistel networks. Something occurred to me a bit ago that I hadn't realized previously, namely that in any Feistel construction there are bits of the plaintext that ...
5
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1answer
103 views

Is there any existing analysis for this construction to turn a tweakable blockcipher into a PRF?

I'm basically looking at this construction to turn a tweakable blockcipher $E_c(x)$ taking a key $k$, nonce $n$, counter $c$ (forming tweak $t = c||n$) and an input $x$ into a PRF on an ...
4
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1answer
147 views

Can a PRNG with a fixed amount of entropy always be detected?

Given a fixed seed (i.e. no ongoing source on entropy), is there (practically or theoretically) any PRNG algorithm that can generate an infinite sequence of random numbers such that an observer cannot ...
15
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1answer
855 views

What NIST protocol was allegedly backdoored by NSA in 2006?

From a recent NY Times article: Cryptographers have long suspected that the agency planted vulnerabilities in a standard adopted in 2006 by the National Institute of Standards and Technology ...
3
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2answers
139 views

Feedback requested on a method of posting a message without revealing the author

So I was thinking about variations on the Dining Cryptographers problem - In some cases, it's useful to be able to post a message without revealing the source, but with the additional constraint of ...
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6answers
340 views

Do ciphertexts leak information about their algorithmic creators?

If an adversary holds thousands of encrypted files that were encrypted with the same cipher, can he determine which cipher method had been used on the files? Assume that the adversary has no prior ...
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2answers
332 views

Quadratic residue problem on composite integers

Its believed that the quadratic residue modulo $n=p·q$ for large primes $p$ and $q$ is intractable, which forms the basis of some cryptosystems. However, it is solvable if the factors of $n$ are ...
6
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3answers
698 views

Any practical uses of machine learning for cryptography?

I am about to go study for my masters in machine learning, data mining and high performance computing, but have recently become very interested in cryptography after taking Dan Boneh's Cryptography ...
4
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2answers
269 views

Is there any research about cryptography on nondeterministic Turing machines?

I know it's a highly theoretical topic, but I was wondering if there was any research out there about what cryptography would be like assuming that we had access to nondeterministic Turing machines. ...
8
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2answers
684 views

Why does DES use exactly 16 rounds?

Why does DES use exactly 16 rounds and not more or less than 16? What is the significance of using 16 rounds?
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2answers
241 views

How secure would this code be against cryptanalysis?

Simple version: Create software that takes a database of the dictionary, alphabet, and phrases. Randomly generate a database of random strings of letters/numbers/symbols of varying length. Randomly ...
4
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1answer
657 views

Recent attacks on RSA

At Blackhat 2013 this week, there was a talk saying RSA is (essentially) doomed in the near future. networkworld.com ~ “Black Hat: Elliptic curve cryptography coming as smarter algorithms threaten ...
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3answers
3k views

Specification of the Megamos crypto algorithm

It has recently emerged that a paper that was scheduled to appear at Usenix Security 2013, titled "Dismantling Megamos Crypto: Wirelessly Lockpicking a Vehicle Immobiliser", has been censored ...
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2answers
325 views

Can you break a multi language code using Frequency analysis?

Let say that I wrote a 26 letter alphabet, each letter of my alphabet represent a letter from the latin alphabet. I'm writing in 3 languages, only I know which languages. Grammar is the one from my ...
2
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0answers
106 views

Jumblar: Using map locations as passwords

I had the idea of converting map locations into passwords. Basically Jumblar takes a hash of the user's location and stores this as a comment on the PGP network. The 'stored-hash'(Vague-Hash) ...
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1answer
121 views

Can you explain what the AES paper means by “sharing active S-boxes”?

I am reading the "Biclique cryptanalysis of the full AES" paper. What do they mean by "sharing active S-boxes"? How can this concept can be advantageous to make a bicycle? If there is someone who ...
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2answers
169 views

Why is it said that if we have a duplicate ciphertext block it can leak our information?

I have been recently going through a question: http://security.stackexchange.com/questions/15767/what-do-key-size-and-block-size-mean-in-cryptography and it created in me a series of doubts. Referring ...
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1answer
60 views

Applications of 3-collisions

I recently read Improved Generic Algorithms for 3-Collisions by Joux and Lucks (Asiacrypt 2009), available as http://eprint.iacr.org/2009/305.pdf. I was wondering about applications of this technique ...
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388 views

why public key cryptography (RSA) is use in sites like Google, facebook (what is the need to ecode data in these sites)?

The RSA algorithm is used for encryption (plain text to cipher text) and decryption (cipher text to plain text) of data. What is the need for using public keys in these sites; why is data is encrypted ...
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1answer
234 views

How to prove a cipher resistant to differential cryptanalysis?

How do you prove that a cipher is resistant to differential cryptanalysis? It's said that Rijndael has been proven resistance to differential cryptanalysis. How do cryptographers do that?
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4answers
540 views

Decrypting files with an unknown method but a known result

I am assuming there is a very simple way of working this out by brute force, but I am not sure if there is a better way. I have a file of data that I wish to get (my data, generated by a machine). ...
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1answer
236 views

Seemingly simple decryption question

Forgive the very novice question, but... Given an algorithm which, upon inputting an unchanging 4-character string, generates a variable 6-character string, but having no idea what the algorithm is ...
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2answers
399 views

Given a message and signature, find a public key that makes the signature valid

Given a message $M$ and a signature $S$, is it feasible to find a RSA public key $(n,e)$ such that $S$ verifies as a valid signature on $M$ (using this public key)? What if we're given one public key ...
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2answers
274 views

What are the standard procedures in cryptanalysis to analyze unknown ciphertext?

What are the "standard procedures in cryptanalysis" to analyze unknown ciphertext? In other words: Are there any protocols, officially acknowledged checklists or something like that which represent a ...
7
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0answers
590 views

Blowfish in CBC mode with multi-use keys. Is the 64-bit blocksize a fatal issue if one were to encrypt TBs of data?

Crashplan uses 448-bit Blowfish to encrypt the data you send them. The mode used is CBC, keys are multiple use, and IVs are generated with SecureRandom from JCE. Keys are not derived from the ...
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1answer
441 views

Can two cipher letters per plaintext letter easily defeat character frequency analysis?

For a class 5 years ago I wrote a paper about "defeating character frequency analysis by using two cipher letters per plaintext letter" ...
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1answer
2k views

Security strength of RSA in relation with the modulus size

NIST SP 800-57 §5.6.1 p.62–64 specifies a correspondence between RSA modulus size $n$ and expected security strength $s$ in bits: ...
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2answers
177 views

differential and linear cryptanalysis

I have been reading about differential and linear cryptanalysis. They were mainly introduced by Adi Shamir and Biham to show weakness of DES. However, many of articles state that they have been ...
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1answer
247 views

How insecure in practice?

I am in attempt to understand relative insecurity of certain encryption schemes. Particularly of interest is DES and RC2. I know AES is better and should be used to encrypt. But practically, if ...
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2answers
752 views

Why are bitwise rotations used in cryptography?

Any understanding I have of cryptography stops right around the cipher level. As such, I'm just curious as to why bit shifts and moreover circular bit shift are so prevalent in cryptography.
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1answer
297 views

What do recent announcements about solving the DLP in $GF(2^{6120})$ mean for RSA

After just reading the post Do recent announcements about solving the DLP in $GF(2^{6120})$ apply to schemes proposed for cryptographic use? I was a bit confused. DSA, ElGamal and others are based on ...
12
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1answer
440 views

Do recent announcements about solving the DLP in $GF(2^{6120})$ apply to schemes proposed for cryptographic use?

A recent paper by Göloğlu, Granger, McGuire, and Zumbrägel: Solving a 6120-bit DLP on a Desktop Computer seems to "demonstrate a practical DLP break in the finite field of $2^{6120}$ elements, using ...
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4answers
6k views

Recommended skills for a job in cryptology [on hold]

First let me apologize if this is an ill posed question. Let me also note that I do not in any way seek a comprehensive answer, simply your thoughts on what makes for a valuable asset to a company ...
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2answers
300 views

Are there any hand ciphers not obsoleted by computer cryptanalysis? [duplicate]

Computerized cryptanalysis has obviously made formerly "secure" hand ciphers like Playfair, Four Square, and the Hill Cipher obsolete because they can be defeated in seconds. But is there a hand ...
4
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1answer
458 views

Hill-cipher, disordered alphabet

I am going to apply a simple substitution cipher to my input, then encrypt the result with a Hill cipher. How can this be broken, in a chosen-plaintext threat model? In other words, instead of the ...
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2answers
280 views

Help me describe/identify this challenge-response protocol/algorithm?

My area of expertise is reverse engineering, specifically embedded systems. I do attack cryptographic systems, but this largely involves key recovery or exploiting the implementation. I was asked to ...
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1answer
787 views

Using an MD5 hash as a password

Suppose Alice is using a password prompt that only accepts up to 32 characters for any particular password. Memorization of long strings of random characters is not one of Alice's strengths, so she ...
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1answer
243 views

Security of Salsa20 with some known plaintext?

Basic question- if I'm encoding a bunch of known filetypes with salsa20, will it still be secure if the plaintext header is known? Assume that a different IV and Key are generated before each file is ...
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1answer
2k views

repeating-key xor and hamming distance

I read that to break repeating-key xor you can do the following: try a keysize $n$ and compute the hamming distance between the first $n$ bits of the encrypted string and the bits $n+1$ to $2n$ of the ...
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3answers
271 views

Is it possible to work out the hash algorithm from a list of known message-hash pairs?

For example, in my situation I know hash(20) = 486e9638177faf1f34e49910491b77af. I also know the hashes for all values from 0 to 20. Is it possible to work out the ...