6
votes
2answers
792 views

Computationaly hard detokenization algorithm for credit card numbers

I am designing a vault that tokenizes credit card numbers (a plaintext that consists of 16 decimal digits), with the following requirements: Given some plaintext, the vault returns an index. If ...
1
vote
1answer
147 views

Is it ok to send part of digital signature if we have bandwidth constraints?

When creating digital signatures is it ok to send part of it when we have bandwidth constraints?
1
vote
1answer
107 views

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 ...
1
vote
1answer
104 views

Addition-only PHE in F#

Using homomorphic encryption, I would like to be able to take an encrypted integer and either add 1 or -1 for a new encrypted value. I do not want the encrypted value to be recoverable - just the ...
6
votes
4answers
1k views

How do institutions like banks do RSA with big primes?

When encrypting with RSA it is often infeasible to decrypt by just doing c^d mod n, because for example when using the primes $(p,q)=(12553,1233)$, which are small ...
1
vote
1answer
83 views

Using cryptographic hash to match AES key size

I am making a tool for myself where I intend to store cipher text. I wish to use AES together with arbitrary sized keys to create these cipher text strings. The idea is to use a cryptographic hash ...
2
votes
1answer
778 views

How is HKDF-Expand better than a simple hash?

Why do we need HKDF-Expand if we can simply hash the pseudorandom key to make it longer? The docs (in the link) say: The second stage "expands" the pseudorandom key to the desired length... ...
3
votes
2answers
425 views

Best way to get 32 bytes from PBKDF2

Use the PBKDF2 output as an HMAC key, hashing two different inputs - once for 20 bytes and once for 12 : ...
2
votes
1answer
233 views

ASCII: weakness

Suppose I have an encryption function $e$ of a symmetric cipher with plaintext space $$\mathcal{M}=\{1,2,\ldots,2^{24} \}.$$ Now, consider a list $L$ of integers: $$67, 114, 121, 112, 116, 111.$$...
1
vote
1answer
47 views

Logarithm in Private Set Intersection

Is the logarithm in the private set intersection protocol mentioned in Section 3.4, Step 5 of http://www.ece.umd.edu/~danadach/MyPapers/set-int.pdf, a base 10 or 2? How do we find this out?
1
vote
0answers
46 views

ElGamal Homomorphic Encryption Formula Question [duplicate]

With Pubic Key $(G, q, g, h)$ where $G$ is a group, $q$ prime, $g$ a generator of $G$, Am I right in thinking that: $$\mathrm{Enc}(m;r) := (g^r, h^r \cdot g^m)$$
0
votes
1answer
149 views

Homomorphic Encryption Notation Question

What does the following notation mean in a homomorphic encryption scheme? ENC(x;r) What does x and ...
8
votes
1answer
178 views

Is the Couvreur et al. polynomial time attack on McEliece practical?

There's a new e-print out on arXiv titled "A Polynomial Time Attack against Algebraic Geometry Code Based Public Key Cryptosystems" by Alain Couvreur, Irene Márquez-Corbella and Ruud Pellikaan: "...
3
votes
1answer
274 views

Derive both MAC and AES keys from same PBKDF2?

When doing password-based encryption, is it OK to create one instance of PBKDF2 from the password, and then use it to create both the AES key and the MAC key? (Or should a new instance of PBKDF2 be ...
1
vote
1answer
122 views

Efficient Robust Private Set Intersection Questions

I am trying to implement Efficient Robust Private Set Intersection using additive ElGamal. I am trying to run the full protocol mentioned in Section 3.4 on the following inputs: $p = 17$ (prime) $g ...
2
votes
1answer
139 views

Can you explain the counter in the Key derivation function of PACE?

I try to understand the PACE-Protocol, especially the Key Derivation Function in ECDH on page 33. Can someone explain me, what the Counter is good for? Often there are iteration counters in KDF - but ...
1
vote
2answers
542 views

Shamir's ID based signatures

I am currently implementing Shamir's ID based signature algorithm as proposed in Adi Shamir, Identity-Based Cryptosystems and Signature Schemes. Advances in Cryptology: Proceedings of CRYPTO 84, ...
1
vote
1answer
273 views

Usage of Fermat primes in RSA

Why are Fermat primes ($2^{2^n}+1$) useful as $e$ (the public key) in RSA encryption? I understand why $2^n+1$ primes are useful, because they would simply be ...
3
votes
1answer
122 views

Does re-iterate AES128 with different keys gives any benefict?

I'm not cryptography expert (well, I'm here after all!) but I have this doubt… Suppose I have 2 different keys: secretkey1 secretkey2 And a plain text: myReallySecretPlainText If I use AES128 ...
1
vote
0answers
64 views

How to specify last $t$ bits are only sent when a signature is sent?

I have read that in cases where there is a limit on the amount of data sent over the network, one idea is to sent only $t$ bits of data rather than sending all the bits. For example, in the Schnorr ...
1
vote
1answer
91 views

Authentication and deniability

I have the following problem: Alice wants to send Bob a message such that Bob knows it is from Alice but at the same time he cannot prove that Alice send him this message. The solution I came up with,...
1
vote
1answer
125 views

What is the padding scheme for the original Tiger hash?

The specification doesn't seem to indicate how to pad messages to the requisite 512-bit boundary. What is the padding scheme for the original Tiger hash?
1
vote
0answers
68 views

Is it feasible to break an encrypted and later encoded message?

A message is sent from a person to another. The plain message is first encrypted, even with a weak algorithm - say, DES. Then, the encrypted message is encoded with a simple substitution, which is ...
3
votes
0answers
206 views

Secret sharing - no dealer, modifiable, verifiable

I need to find a secret sharing scheme with these properties: The scheme is set up by all participants, ie no single entity called the dealer The parameters of secret sharing need to be modifiable,...
18
votes
0answers
713 views

Hardness of finding mutual discrete logarithms of small generators in $\mathbb{Z}_p$

Suppose you want to select a prime $p$ such that finding e.g. $\log_2(3)$ in $\mathbb{Z}_p$ is expected to be either at least as hard as the general Discrete Logarithm Problem in $\mathbb{Z}_p$, or at ...
2
votes
1answer
243 views

One-way function and uninvertible function [closed]

How can I prove the following: If $f$ is a one-way function, then it is an uninvertible function?
1
vote
1answer
753 views

Z*p Generator (Cyclic Group) in java

I am trying to implement ElGamal in java without using the in-built libraries. The problem occurs when I am trying to find the generators of the number(in this case 11). Everything works perfectly ...
1
vote
1answer
237 views

ElGamal encryption with private key [closed]

Does the encryption with a private key and decryption with a public key works in ElGamal likewise what is done when we use RSA for sender authentication?
3
votes
2answers
417 views

Avoiding overflow when encrypting with RSA

When encrypting with RSA one calculates $ m^e \pmod n $ by doing the following: m^e % n Where $m$ is what we encrypt. Often $e$ is a very big number to make it ...
2
votes
3answers
306 views

Generating bilinear pairing parameters - running time of finding member of p-torsion group

Update: Question completely rephrased. I want to create the parameters for a bilinear pairing (the Tate pairing in this case). In case you're interested I'm following this thesis, specifically the ...
1
vote
1answer
158 views

Method to calculating e in RSA

For simplicity I choose two small primes for p and q. p=3 q=11 n=33 Φ(n)=20 Now we need ...
1
vote
1answer
476 views

Why do we use 1024 / 160 bit primes in DSA?

I am looking at DSA's parameter generation and don't understand why for $p$ a 1024 bit prime is needed if $q$ is chosen as a $160$ bit prime. I thought that the security of DSA relates on the discrete ...
3
votes
1answer
407 views

Predicting Java's PRNG using partial output

I have a program (a game) that uses Java's nextInt function with parameter 0x10000 to get a random 16-bit number. Java's ...
1
vote
1answer
55 views

Would requiring reciprocal signatures create a stronger Web of Trust?

In this post on Cryptome it's noted that a person called Michael Vario is willy-nilly signing people's public key's and uploading those signatures. He is effectively forging a web of trust. I'm sure ...
4
votes
2answers
225 views

Choice of reduction polynomial in Whirlpool's internal cipher

Whirlpool is an interesting little hash function in the Miyaguchi-Preneel family. In my mind, it's most interesting feature is the design of internal cipher W, where the distinction between key and ...
1
vote
2answers
166 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?
21
votes
3answers
14k views

How cryptographically secure was the original WW2 Enigma machine, from a modern viewpoint?

If cryptanalysts today were to crack the original Enigma machine, “how fast” or “how easily” could they do it? What methods would they use? The original cracking was significantly helped by operator ...
2
votes
0answers
62 views

Efficient Robust Private Set Intersection Additive ElGamal [duplicate]

I am trying to implement Efficient Robust Private Set Intersection using additive ElGamal. I am trying to run the full protocol mentioned in Section 3.4 on the following inputs: $p = 17$ (prime) $g ...
2
votes
2answers
462 views

Random numbers for Rabin-Miller primality tests

I've implemented a Rabin-Miller primality test fuction following Wikipedia and the book Applied Cryptography. Now I'm using it for generating primes with a string seed. The book suggests the following ...
12
votes
3answers
1k views

How long will my encryption remain private?

This is a basic question in cryptography but I have not found a good, comprehensive answer. It is explained that our keys should expire and we should get new, stronger ones with time, reflecting more ...
1
vote
2answers
324 views

What does $\Pi$ represent in cryptography?

I am doing a project in cryptography and while reading a tag generation algorithm for files, I got stuck with the meaning of $\Pi$. The screenshot of the algorithm is inserted below. Here the ...
1
vote
1answer
74 views

when to do elgamal exponentiation computation

Recetly while studying about cryptographic primtives, I have come across below line in wiki : http://en.wikipedia.org/wiki/ElGamal_encryption Encryption under ElGamal requires two exponentiations; ...
-1
votes
1answer
276 views

Are the protocols DH-EKE, B-SPEKE, A-EKE, and SRP are considered challenge-response authenticaion mechanisms? [closed]

I got confused which one is considered challenge-response mechanism which one is not.
4
votes
1answer
246 views

Does changing the random number selected for each message increase security in Schnorr's scheme?

I was reading about Schnorr's scheme and in that for signing the signer generates a random number and continues with his operation. I am trying to implement this in a scenario where the signature ...
1
vote
0answers
97 views

Factorization or discrete logarithm is difficult for an attacker?

I have read that difficulty in breaking many algorithms are based either on Factorization or discrete logarithm. I am reading about schemes that are similar to RSA which make use of integer ...
3
votes
2answers
196 views

ElGamal in private set intersection: how to handle negative numbers?

I am trying to implement the Efficient Robust Private Set Intersection using Additive ElGamal. For this, from the client side, my inputs are $C = \{1, 2\}$. So, the polynomial is $(x-1)(x-2)$ which ...
3
votes
1answer
326 views

A fast non-cryptographic hash function that is “strong enough”?

I'm designing a simple one-time-password mechanism for authentication against a possibly-insecure server - i.e. I don't want to use symmetric shared secrets. The first idea that came into mind was ...
1
vote
1answer
87 views

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 ...
1
vote
1answer
131 views

Cryptography: Oblivious Transfer with at most one transfer?

I am trying find a transfer method for this problem. Suppose Alice and Bob both has access to a function $\text{F}$ with two parameters. $\text{F}(x,y)$ randomly returns $p$ to alice and $q$ to Bob ...
-1
votes
3answers
3k views

How can I find the prime numbers used in RSA?

I got this question in a local hacking event, but I couldn't solve it. Problem Statement ---- Continuing their snooping habit, NSA kept bugging Alice's communication. Resorting to the age old ...

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