analysing cryptographic algorithms, potentially uncovering weaknesses in them (e.g. "breaking" them or casting doubts on their actual security)
2
votes
2answers
121 views
Can you help with that definition for a CCA?
The following is a definition taken from Introduction to Modern Cryptography by Katz and Lindell.
I'm having a hard time understanding some basic concepts! Can you please help me?
...
4
votes
1answer
125 views
What are the consequences of a MAC tag collision?
I've seen some proofs of MAC security that are based on the extremely-unlikely event that two MAC tags collide (ie, they are equal for distinct messages). Suppose that this extremely unlikely event ...
6
votes
2answers
2k views
How does a chosen ciphertext attack work, with a simple example?
Can someone please explain - using a simple example - how a chosen ciphertext attack works?
6
votes
2answers
297 views
For public-key encryption, why does COA resistance imply CPA resistance?
My professor told me:
If a public-key crypto-system is secure against ciphertext-only attacks,
then it is also secure against a chosen-plaintext attack.
Why is this true? Is there a proof that ...
9
votes
3answers
398 views
Are these emerging threats against AES affecting your designs?
Recentally, an attack on AES was discovered which reduces its computationally complexity, by a very slight amount.
The first key recovery attack on the full AES-128 with computational
complexity ...
2
votes
2answers
153 views
Getting the encryption method and key from the encrypted data and the raw data
I have some pairs of plaintext and ciphertext data, from which I need to be able to decrypt the other passwords stored in the database.
The password field typically contains something like
...
4
votes
1answer
404 views
Linear Cryptanalysis
What is the principle of linear cryptanalysis, as applied to a block cipher ? For instance, this page gives the rough outline of differential cryptanalysis.
6
votes
1answer
366 views
How to deduce enigma settings given a partial plaintext?
Assuming some large block of text is encrypted with an enigma machine and I only know a small subset of letters before and after encryption, how do I go about figuring out the enigma settings from ...
3
votes
2answers
633 views
Decrypting DES with decrypted and encrypted data
I got two 8 bytestrings
One which is decrypted is:
FF FF FF FF FF FF FF FF
and one which is encrypted is:
16 05 78 B0 0A C2 78 7F
Encryption mode is CipherMode.ECB
I need the key used in DES ...
5
votes
2answers
845 views
How to attack a classical cipher using known partial plaintext?
I have a ciphertext generated by a classical cipher. I do not know what was cipher used to generate it. I do however have the beginning of the plaintext.
What are the cryptanalysis approaches for ...
5
votes
1answer
255 views
If the PSK is known, is it possible to decrypt traffic from other clients in a WPA2 wlan network?
If in a public WLAN WPA2-PSK is used, but the PSK is more or less publicly available, does this mean that an attacker with that PSK can easily decrypt wlan traffic from/to other clients of that WLAN?
...
12
votes
2answers
329 views
What is the general justification for the hardness of finding preimages for cryptographic hash functions?
Since most cryptographic hash functions are simple, compact constructions does this simplicity impose a limit on the complexity and the size of a function that can generate preimages? That is, given a ...
3
votes
1answer
161 views
Is the last step of an iterated cryptographic hash still as resistant to preimage attacks as the original hash?
Considering a cryptographic hash, such as MD5 or SHA2, denoted by the function $H(m)$ where $m$ is an arbitrary binary string, there is a lot of material available that deals with potential weakness ...
12
votes
2answers
949 views
How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?
I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world.
How can we reason about the symmetric ciphers cryptanalysis ...
2
votes
2answers
254 views
How can one break a monoalphbetic substitution chipher at pseudorandom text?
Does anybody know how to break monoalphbetic substitution cipher, if it is applied to some pseudorandom text (for example to some surrogate key filed in a database)?
Let us assume that we have only ...
5
votes
4answers
172 views
Changing algorithms during encryption
Inspired by "Guarding against cryptanalytic breakthroughs: combining multiple hash functions", I am curious if there is a cryptographic reason to use only one algorithm during encryption.
For ...
5
votes
3answers
1k views
Cracking Pre-Paid Cards
Is it possible to deduce the original function that used to generate those pre-paid cards number that are used for charging your mobile phone credits?
For Example, If I've collected about 1000 of ...
3
votes
2answers
230 views
If Bob steals Alice's private key, how exactly would he read her encrypted documents?
So Bob grabs Alice's secret key when she isn't looking and her encrypted files, doesn't he need to know her passphrase to read her files?
What I am reading is that no he does not need it but as far ...
6
votes
1answer
248 views
How exactly would someone attempt to analyse ciphertext produced by popular encryption products such as Truecrypt/PGP?
I am interested in understanding what the process would be if an attacker wished to attempt to decrypt data secured by common tools such as OpenPGP, Truecrypt or the like. Are there any documented ...
10
votes
4answers
473 views
About Cryptography in a Character Language
Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it.
Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem ...
6
votes
1answer
436 views
Implementation of Tao Xie and Denguo Feng's MD5 attack
It is well known that MD5 is completely broken today - however, to understand the theory behind the attacks I am looking for an implementation of the collision attacks described in the 2009 paper A ...
-4
votes
1answer
213 views
3
votes
1answer
228 views
What is the best method to determine the language used in a monoalphabetic substitution cipher?
Working on a cipher (which I assume to be a mono-alphabetic substitution cipher due to the letter frequency) I struggle with the fact that I don't know which language the plain text is written in. ...
5
votes
4answers
239 views
How to forge Schnorr signatures if you can guess the challenge
Underlying the Schnorr signature is an identification protocol: let $G$ be a cyclic group where discrete log is "hard" and choose $g$ as a generator of $G$. Now have Alice pick a random (secret) ...
20
votes
2answers
953 views
How do I apply differential cryptanalysis to a block cipher?
I've read a lot of summaries of block ciphers particularly with regards to the NIST competitions stating that reduced-round block ciphers are, for example, vulnerable to differential cryptanalysis.
I ...
5
votes
2answers
260 views
Analysis of Repeatedly Enciphered Plaintext using Same Algorithm / Key
Please forgive the impracticality of this question, but I'm curious about the behaviors of encryption algorithms applied to their own output.
Suppose I have an encryption algorithm E and (using the ...
4
votes
1answer
226 views
If a cryptanalytic breakthrough is made, what process should be followed?
If a researcher manages to make a cryptanalytic breakthrough on a cryptographic algorithm or protocol that is in use, what should they do?
Has this ever happened before? What are the implications for ...
2
votes
2answers
148 views
How are constructs with data-dependent swaps and rotations cryptanalyzed?
Linear and differential cryptanalysis seem well suited for constructs with a (relatively) simple fixed structure of boolean expressions. But some ciphers incorporate swaps of array elements where the ...
10
votes
2answers
391 views
How long would the 100 Year Cryptography Project have secured its data had it been started 100 years ago?
The goal of the Tahoe-LAFS 100 Year Cryptography project is to "enhance Tahoe-LAFS's cryptographic system so that Tahoe shipped today/next year might remain safe from cryptographic attacks for a 100 ...
3
votes
1answer
122 views
Proof that Alternating Step Generator and modifed ASG' have equivalent security?
The Alternating Step Generator (ASG) is a PRNG combining 3 LFSRs. Output of the ASG is the XOR of the output of two clock-gated LFSRs. At each step, a single one of these LFSRs is clocked, according ...
6
votes
3answers
159 views
Number of intersections of two sets
Suppose both I and my friend have a set of integer numbers. We want to know the number of common elements in our two sets but without knowing elements of the sets of each other. So I don't want my ...
3
votes
3answers
252 views
Where can I find useful data for cryptography/coding theory?
When implementing cryptographic/coding theory algorithms one need to use data like big prime numbers, numbers in Z_n and their inverses, irreducible polynomials in Z_n[x] and so on... While sometimes ...
2
votes
1answer
289 views
The use of cribs
I'm looking to incorporate more crib usage in breaking ciphers in unknown enciphering schemes, or at least to gleam what information I may. This seems to be a big hurdle to me, and I'm looking for ...
13
votes
3answers
1k views
Reverse engineering a hash?
I understand this may not be the best place to ask a question like this, but I believe that this community may be the best/only place I can ask such a question.
I have inputs and outputs from an ...
8
votes
4answers
189 views
Tactics available to help prove security of a new system?
I believe that the accepted tactic to "prove" a system as secure is to allow the crypto-community to review it and if no vulnerabilities are found over a long period of time (5 or 6 years), then a new ...
10
votes
1answer
337 views
Security of N bit HMAC
Lets say that I am using 128 bit HMAC. How many operations are needed to find "non secure" message. Is birthday attack possible?
6
votes
1answer
119 views
Are there any tools for expressing the cipher operations as a system of equations?
The first step In Algebraic-attack is expressing the cipher operations as a system of equations. Is there an automated tool to do this?
