3
votes
2answers
930 views

If we can find prime numbers larger than 17 milion digits, why can't we find all 1024bit primes? [duplicate]

"Largest Known Prime Number Discovered; Has 17,425,170 Digits" http://www.sciencedaily.com/releases/2013/02/130213225424.htm If we can find prime numbers larger than 17 milion digits, why can't we ...
2
votes
3answers
722 views

decrypt numbers multiplied by a certain floating point number

A friend has about 10K numbers in a an excel sheet. He wanted to encrypt these numbers in order to share the file with a 3rd party that will help him reorganize the tables. What he did, to hide the ...
0
votes
1answer
114 views

GCM implementation

I want to write GCM implementation myself (only for research purposes). So far, I have successfully written CTR implementation (for AES), but now I ran into few problems with GCM. If I'm not wrong, ...
-1
votes
2answers
484 views

How to choose an algorithm

I have some understanding of Cryptography and I would like to know if there is a tool or website to help me choose an algorithm based on my own needs (I'm being vague in purpose). I'm looking for a ...
3
votes
1answer
105 views

Is this JS library using openssl genrsa -rand improperly?

pem, a JS wrapper around openssl, has the following code in it: var params = ['genrsa', '-rand', '/var/log/mail:/var/log/messages', keyBitsize ]; Am ...
3
votes
0answers
79 views

Exponentiation with fully homomorphic encryption [duplicate]

I have often heard that because a fully homomorphic encryption scheme allows for both additions and multiplications on encrypted data, most other operations can be simulated. I don't understand how ...
-2
votes
1answer
97 views

Hashes and Ciphers [closed]

So I'm brand new to cryptography, I'm confused on a few fundamental issues, and my normal avenues of research aren't bearing any fruit. I'm hoping someone can describe the relationship between ...
3
votes
3answers
364 views

How does an oblivious test of plaintext equality work?

Assume an ElGamal Cryptosystem. Assume a set of three players, $P_1$, $P_2$ and $P_3$. The private key $x$ is shared among the players. The player $P_1$ has a piece of the private key $x_1$, $P_2$ has ...
1
vote
1answer
106 views

Send an encrypted question and return answer without decryption

Oke bear with me for a minute. Once upon a time someone mentioned a hypothetical way where you would send an encrypted question to Google and Google would return an answer without decrypting it. As ...
2
votes
1answer
1k views

Cryptool - DES (ECB and CBC ) - double encyption

So, I uploaded a text file in Cryptool and encrypted it with DES-ECB with the key FF FF FF FF FF FF FF FF. I encrypted the cipher text again, with DES-CBC using ...
2
votes
1answer
290 views

PBKDF2 for login key, scrypt for encryption key

I need to derive two keys from a single password client side. One for in-browser login, the other for encryption. Is the following secure ? ...
-2
votes
3answers
58 views

Reduction of a modular exponentiation's base does not change the result

Can anyone help me to prove this property used in the RSA correctness theorem? $a^b\bmod{n}\equiv (a\bmod{n})^b\bmod{n}$ Specifically, here is what I have done: this is what i mean. i can't ...
16
votes
1answer
3k views

How to communicate by email with forward secrecy and deniability?

My correspondents and I would like to communicate privately by email. We used to employ PGP but that might have been shooting ourselves in the foot. PGP doesn't have forward secrecy. If any of our ...
0
votes
2answers
117 views

Solve the congruence using Pohlig-Hellman algorithm

Use the Pohlig-Hellman algorithm to compute a solution to: $3^x\equiv 2 \pmod {65537}$ My attempt: $p-1 = 65537-1 = 65536= 2^{16}$ $x= 2^0x_0+2^1x_1+2^2x_2+...+2^{15}x_{15}$ For $x_0$: $2^{65536/...
1
vote
1answer
150 views

Hardness of problem related to bilinear pairings

Let $e: \mathbb G_1 \times \mathbb G_1 \rightarrow \mathbb G_T$ be an efficient bilinear pairing. Note that the pairing is symmetric (i.e., Type 1). The problem is, given $g \in \mathbb G_1$ and $e(g,...
2
votes
1answer
173 views

Gap problem for Learning With Errors

Informally, a "Gap problem" arises when solving the computational (or search) version using an oracle for the decisional version. This definition of Gap Problem was introduced by Okamoto and ...
7
votes
1answer
777 views

understanding forking lemma

Every time when I read a paper that has digital signature, when it comes to prove the security of a digital signature scheme, many chances that the author will use the forking lemma. The forking ...
10
votes
1answer
836 views

Why are the outputs of the md5sum tool and Crypto++'s MD5 different?

Could you please tell me what is the difference of coreutils' md5sum and sha*sum tools (...
0
votes
2answers
231 views

AES ECB experiment

Generally speaking, ECB mode shouldn't be used. ECB seems to be more of a basic building block than anything else. Because I thought it would be interesting to experiment with encryption (this method ...
4
votes
2answers
169 views

Randomness re-use in LWE encryption scheme

Let me describe the scheme first, it is the scheme proposed by O. Regev when he introduced the LWE assumption. $sk = \textbf{s} \in \mathbf{Z}_q^n$ $pk = \textbf{A}\textbf{s}+\textbf{e}$ where $\...
0
votes
2answers
118 views

Schnorr's identication protocol

Schnorr's identification protocol is said to be a honest verifier zero knowledge protocol. Does it imply it will not work if the verifier is dishonest? I'm wondering if this is the case, it would ...
1
vote
2answers
149 views

ECDSA Public key randomness

If I have a 256bit ECDSA public key how likely is it that the same public key will be generated by another person. Will it take on average 2^256 tries to generate same public key? If I xor the first ...
1
vote
1answer
181 views

RSA Key composition

I'm trying to figure out how it is composed the RSA key. Follow what I know : For a RSA 2048, 2048 is the bit length of the module. The public key is composed by $(e,M)$ when $e$ is the public ...
2
votes
2answers
439 views

Using hash for one time pad key

Recently I've been delving into security algorithms, I already knew some of the (easy) math behind AES and RSA and how to and not to implement it. But well, i got a bit bored so i thought I'd just do ...
-4
votes
1answer
61 views

encryption of message “DATACONFANTIALITY” with key playfair using playfair cipher

actually i have solved the above mentioned problem but i want to check weather it is correct or not. my question is that how to encrypt the message DATACONFANTIALITY using the playfair cipher where ...
4
votes
1answer
280 views

“Practical” operations supported by functional encryption?

I'm curious about what operations have been developed into functional encryption schemes. What I mean by that is: what operations can be performed over encrypted ciphertexts? Obviously homomorphic ...
0
votes
1answer
113 views

If you know the messages, and the results of HMAC(key, message) can you find the key?

If the attacker has 1 billion 4KB (random-noise) messages and sha256-HMACs for those messages (all using the same key), can they calculate the key that was used, before the universe dies?
2
votes
1answer
61 views

In a additive group is it hard to calculate $bg$ given $ag, g, abg$

The ECDH problem defined that given $g,ag,bg$ it is difficult to calculate $abg$. But it is also difficult to calculate $bg$ given $ag,g,abg$. where $g$ is generator and a,b are elements of group.
0
votes
1answer
213 views

Reducing size of hash function

If I have some data I hash with SHA256 as hash=SHA256(data) and then copy only the first 8 bytes of the hash instead of the whole 32 bytes. How easy is it to find a ...
0
votes
0answers
143 views

Affine transformation in finite field SubBytes

Can anyone please explain how this computation was done? I picked it up from How are the AES S-Boxes calculated? The affine transformation is as follows. The input bits are multiplied against the ...
1
vote
2answers
182 views

How to show that a function is not a PRP?

So I have to answer these questions, but I honestly have no idea what I am doing or where to begin. Can I get some advice on how to tackle these? Consider a PRP candidate function $f$ on $K = X = \{0,...
4
votes
0answers
97 views

Anonymisation resilient to key leak

Problem statement Elbonian citizens have been issued a card holding a random $B_j\in\{0,1\}^b$, $0\le j<2^n$, $b\ge n+20$ (representative example $b=64$, $n=28$). The habit has grown to use this ...
1
vote
3answers
326 views

Brute force against computational security

One-time pad is secure under brute force with respect to either of the definitions \begin{align} \Pr[M = m] &= \Pr[M = m | C = c]\\ \Pr[C = c | M = m_0] &= \Pr[C = c | M = m_1] \end{align} ...
2
votes
1answer
867 views

What is a fair exchange scheme?

The Wikipedia entry Fair exchange has only two lines, and the only paper I can find describing such a protocol is extremely complicated. Is this a new field in cryptology or can someone give an ...
1
vote
1answer
77 views

Hill Cipher the point of a known plaintext attack question

Is the point of a known plaintext attack on a Hill Cipher to get the matrix key?
0
votes
0answers
393 views

Substitution-permutation network

Can anyone in 6th grade English explain the steps in the Substitution-permutation network algorithm or just point a link that does so? (which is included in many of the top modern algorithms) ...
7
votes
1answer
3k views

Is CAST5 still a secure algorithm to use?

I just installed GnuPG for Windows. The documentation says the default symmetric cipher is CAST5. In PGP also, default cipher was CAST5. Is CAST5 secure to use? Any known attacks reported on this ...
1
vote
1answer
267 views

Simple message authentication code

I used to figure that message authentication codes function like digital signatures: a hash of a message is calculated and then encrypted with a secret key. To verify the authenticity of a message, ...
1
vote
1answer
571 views

Why don't PyCrypto and M2Crypto produce the same result as OpenSSL here (AES-128 ECB)

If I run the following, for openSSL: echo -n "0123456789abcdef0123456789abcdef" | openssl aes-128-ecb -nosalt -nopad | xxd and enter the key "YELLOW SUBMARINE", ...
0
votes
1answer
150 views

Figure out the encryption being used [closed]

There is a web API (for a game) that sends and receives data encrypted in hex format. I'm interested in accessing the api outside of the game client (mobile app). When sending a request with an ...
0
votes
1answer
378 views

Convert numeric message from RSA ciphertext to alphabetic

I have a set of numeric values that are supposed to represent a message encoded via textbook RSA. For example: 60889 37946 13359 61629 35960 13747 I've decrypted ...
0
votes
0answers
67 views

Are Barreto-Naehrig Curves suitable for pairing-based cryptography?

If Barreto-Naehrig Curves are suitable for pairing-based cryptography, can I use the library available at Optimal ATE Pairing?
2
votes
1answer
701 views

How to accurately calculate Unicity Distance for English?

The Unicity Distance for the DES cipher is around $8.6$ characters, and can be calculated using the $U=H(k)/D$ formula, where $D = R - r$, and where $R = 8$ is the number of bits in a byte (ASCII is 7 ...
0
votes
0answers
71 views

Find a polynomial time algorithm for the following problem

Let $p$ be a prime number and let $c \in \mathbb{Z}_p$ and $e \in (\mathbb{Z}/(p-1))^{\ast}$. Put $c \equiv_p m^e$. Find a polynomial time algorithm that given $p$, $e$, and $c$ will compute $m$. I ...
0
votes
1answer
102 views

Implications of reversing CBC encryption

What would be the security implications if we reverse CBC encyption? Here is a picture of CBC encryption: Here is a picture of CBC decryption: My guess is that the encryption would work if we ran ...
-1
votes
1answer
145 views

Complexity of attacks on affine cipher

I am interested in the complexity of attacking affine ciphers under the following two scenarios during a Ciphertext only attack during a Chosen Plaintext attack For an alphabet of size $m$, how ...
1
vote
1answer
256 views

How to implement the Salsa20 hash function?

I am currently implementing Salsa20 from the specification as an exercise in learning and self-flagellation. I have Sections 1-7 passing the test examples provided. I am now stuck on Section 8, The ...
1
vote
2answers
318 views

Given infinite unencrypted and encrypted texts, can I find the algorithm?

Given an infinite amount of both unencrypted and the corresponding encrypted text, would I be able to calculate the algorithm? I am relative new to cryptography and don't know if I would be able to ...
0
votes
1answer
505 views

Is this encryption scheme perfectly secure?

Let $m = 6$, and let $\mathbb{Z}_m$ denote the set $\{0,…,m-1\}$. Let $X \mod m$ denote the remainder obtained when dividing $X$ by $m$. (a) Consider the symmetric encryption scheme in which the ...
2
votes
1answer
368 views

What is the period of a cipher?

For the following explanation of RC4: It is a variable key-size stream cipher with byte-oriented operations. The algorithm is based on the use of a random permutation. Analysis shows that the ...

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