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

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Is knowing the distribution of input domain only way to do cipher text only attacks?

Imagine there is a $n$ bit block cipher. The attacker has harvested some how all possible $2^n$ cipher texts for some given key. Without the knowledge of the distribution of input domain as in ...
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374 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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decryption many time pad [duplicate]

I have eleven ciphertexts that were encrypted with the same key (which I don't know). I want to decrypt the last ciphertext. I read similar question like Many time pad attack but I can't solve my ...
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182 views

Breaking RSA, given a special kind of oracle that decrypts related ciphertexts for us

Let $c=E^{RSA}_{e}(w)$ be the ciphertext belonging to the plaintext $w$ if an $RSA$ system is used. Assume that the public exponent $e$ satisfies $e \le 10$. Furthermore, assume there is an oracle ...
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280 views

Brute force a ciphered message? [closed]

I wrote my own cipher to encrypt messages. I would like to test a sample ciphered message to see how strong it is. Are there any tools for such task either in Windows or Linux ?
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108 views

How does compression before encryption leak info about the input?

Apparently current best practices recommend that you do not compress before you encrypt. For example in this blog entry (*): http://sockpuppet.org/blog/2013/07/22/applied-practical-cryptography/ It ...
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81 views

How do we arrive to the equation for solving D in RSA?

We have been shown that DE is congruent to 1 mod phi(n). How do we arrive to the real equation for figuring out d that is d = e^-1 mod phi(n). I want to know the entire process. I am just curious, ...
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123 views

Weak Stream Cipher cryptanalysis

Hi I have a stream cipher which uses a keystream of: F(r) || F(r+1) || F(r+2) || ... Suppose F(r) is the RSA encryption algorithm: ...
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Applying differential cryptanalysis to ciphers with addition mod $2^{32}$

I am studying differential analysis and have a question. Consider the following simple cipher: $$c_1 = S(m_1 \oplus k_1) \oplus k_2$$ (Plaintext $m_1$ xor with key $K_1$, then result goes into an ...
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68 views

Is the following aggregation scheme private?

Is the following scheme private? By private i mean an untrusted aggregator (UA) cannot reveal anything other then an aggregate function output on plaintext data Each party holds a secret key $k_i$ ...
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161 views

Would this program be useful in cryptography?

I know nothing of encrypting. I'm not even sure how to tag this. I wrote a program that can calculate this pretty quickly on my macbook pro 2.3GHz IntelCore i7. The two exponents are Mersenne primes, ...
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249 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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309 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 ...
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231 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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144 views

How to solve the reverse of an equation that uses MOD?

I've been tasked with reverse engineering an unknown crypto function. The function uses the following constants: $a=380951$: I noticed that this is a prime number $b=3182$: I noted that this is a ...
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114 views

How does this Cesar Cipher work?

Suppose I have this cryptogram: http://pastebin.com/QpfRMBg1 Clearly it's a Ceasar Cipher which decrypts to: http://pastebin.com/r50heZuU with a simple shift of 10... OKay, no big deal. Let's take a ...
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608 views

The use of cribs [closed]

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 ...
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Importance of salt when deriving an encryption key

I'm very curious to know this and a bit confuses too: Suppose, I have two files encrypted using AES-128bit with keys PBKDF2-derived from the same password and the same salt. If an attacker does ...
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LFSR and Markov chain question

This may seem an elementary question about LFSRs, and their link to Markov chains. LFSRs show Markov chain behaviour in that there can be a transition matrix defined over the LFSR, this follows from ...
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How do you test the security of your cipher? [closed]

I got asked this question and I didn't know what to answer. How do you test the security of your cipher? What comes to my mind now would be to test it with famous attacks: padding attacks, ...
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55 views

Why does knowing the number of points on a curve help solve ECCDLP?

Perhaps, this is a really obvious question, but I am still having trouble understanding how this all fits together. Why is knowing the number of points on an Elliptic Curve helpful in cracking it? ...
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115 views

How to solve custom made ciphers? [closed]

How would you go about solving basic custom made ciphers - e.g. one generated from this website http://www.wikihow.com/Write-a-Custom-Cipher-Carefully? This is solving it without knowing the method ...
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187 views

Breaking Double Encryption

I am trying to understand how an attacker knows when he has successfully decrypted a ciphertext for an assignment. As such, some pointers/hints for the following questions would be greatly ...
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118 views

Cryptanalysis of an certain algorithm

Is an attacker of an cryptographic algorithm allowed to change order of sub-keys but not changing their number in attack? Would it be called an attack on broken algorithm of cipher?
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Secret key model for a compression function cryptanalysis?

How do we analyze the security of a compression function in the secret key model? More formally, let $H$ be a hash function and $F(IV,M)$ its compression function. Suppose we generate a MAC as ...
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312 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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286 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 ...
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Deciphering a key from XOR encrypted cypher using boolean logic

Assume there's an unencrypted message A, and an encrypted message B. You know that message B was encrypted using a simple XOR method of A with a private key K, resulting in message B. Thus, B = A ⊕ K ...
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335 views

Security analysis of a “one-time pad” type hill cipher

Suppose the Hill cipher were modified to something like a one-time pad cipher, where Alice wants to send a message to Bob, and she chooses a key matrix randomly everytime a new message is sent (and ...
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61 views

Perl DES PCBC as protection against decryption/crypt analysis

Is error propagation in DES PCBC a good method to prevent decryption/crypt analysis by third parties? If I had a very large file encrypted with Perl's DES with the PCBC option, and then removed the ...
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Cracking Stream Cipher

I need to crack a stream cipher. I have a set of plain text and encrypted text pairs. The hint to the question asks me to use entropy analysis and look at the hex dump of the encrypted text. However, ...
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365 views

How can i predict the next number of a long sequence of seemingly random numbers? [closed]

How can I accurately predict the next random number between 1 and 999, given past results. I have no knowledge of the PRNG or algorithm used. How can I predict the next number of a long sequence of ...
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Why is the security of block cipher not a function of key and tweak?

The proofs and security of block ciphers constructed especially of Luby Rackoff Constructions and Feistel Networks is based on the number of queries and round functions. The security measure is always ...
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Can I use the cipher text ouput of only one DES rotation as plaintext to check validity?

I have made a python code for one DES rotation. Can I use the resulting cipher text as a plain text for the same rotation with the same key? I think the answer is no, I can't do so. But I need to be ...
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259 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. ...
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260 views

Measuring Shannon's diffusion

Shannon's idea of diffusion is fundamental to cryptography. Besides being a descriptive idea, is there any work on measuring or expressing it? Saying something like "System A has more diffusion than ...
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Cryptanalysing Affine cipher

I am trying to cryptanalyse a cipher–text encrypted by Affine cipher. The encryption formula is: $c = f(x) = (ax+b)\bmod m$, where $a$ and $b$ are unknown constants; $x$ is a plain-text symbol, and ...
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317 views

Other than brute force, are there any attacks on Threefish-512 using only a single known plaintext block?

As per title, other than brute force, are there any attacks on Threefish-512 using only a single plaintext block? Are there any attacks like this in any other cipher?
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Autocorrelation in Autokey

I know that Vigenère can broken using auto-correlation (as explained at cryptool-online.org). Now my question is: Why does this also work with autokey (tested it using crypttool)? To prove that I ...
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(n,n) Shamir secret sharing [duplicate]

In (n,n) Shamir secret sharing if n shareholders do not have the public values (X values) can they still obtain the secret with only Y values?
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Cryptography — with a semi-priveleged user in the middle — to prevent request-tampering with another server

I'm working on a chat server for a mobile app I am writing. I would like to use a different application server for non-chat related operations and another application for chat operations. I would ...
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Is Chaocipher a secure cipher under ciphertext-only attack?

Chaocipher was invented by John F. Byrne in 1919. The algorithm was recently revealed -- see Moshe Rubin's Chaocipher Revealed, the Algorithm While a known plaintext attack successfully finds the ...
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Example of second preimage attack [closed]

Please, I'm trying to understand second-preimage attacks on cryptographic hash functions. I understand the definition well: given a message and it's hash, an attacker finds another message which ...
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135 views

Is there any analysis of freebsd's “geli” encrypted geometry provider?

I've done a few obvious searches on google and google scholar and not turned up any security analysis of FreeBSD's geli. Is there some out there, or has it gone unanalyzed since its introduction ...
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194 views

How good is the RandomPool generator in Crypto++ library?

How good is the RandomPool generator in Crypto++ library? I never heard of it before… I wonder if there is a Fortuna C implementation that I could use in an Objective-C project. Also: How secure is ...
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AES and Known-plaintext attack [duplicate]

Can you help me to finding some papers that shows AES is secure against known-plaintext attack?
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Computing A, B given A+K and B+K [duplicate]

In his writeup of the Solitaire keystream generator algorithm, Bruce Schneier warns not to use the same key for two different messages: The first rule of an output-feedback mode stream cipher, any ...
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Partial encryption of a file?

I want to partially encrypt a large block of data using AES. Meaning: given plaintext[0:1100], I would like to encrypt ...
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Cryptanalysis not based on method used to encrypt? [closed]

When doing cryptanalysis on RSA or something number theory based, there are various attacks that all seem to involve number theory (Wikipedia: RSA_(algorithm) ~ Attacks_against_plain_RSA). Similarly, ...
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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.