3
votes
0answers
232 views

Evaluating Algebraic Complexity of a S-box

While studying the design and the desirable properties of an AES S-box , I came to know that Algebraic Complexity is also an important property of an S-box which is usually considered while evaluating ...
2
votes
1answer
32 views

Does pseudorandom secret sharing require a central dealer?

I'm trying to figure out whether pseudorandom secret sharing (as described in this paper) requires a central dealer, i.e. one party who doles out keys to the other players and thus who would be able ...
1
vote
0answers
26 views

Decrypt ARC4 messages that are using the same keystream [duplicate]

I have 38 16-byte messages and 1 15-byte message encrypted using ARC4. https://www.dlitz.net/software/pycrypto/api/current/Crypto.Cipher.ARC4-module.html The same keystream was used in all messages. ...
2
votes
2answers
100 views

Question about the key in RC4

I am planning to develop a more secure version of the RC4 algorithm. Since I´m not an encryption expert, I´ld like you to help me find answers to the following questions: Does the value of the key ...
0
votes
0answers
8 views

What is the difference between openssl pkeyutl -sign and openssl rsautl -sign [migrated]

What is the difference between openssl pkeyutl -sign -pkeyopt digest:sha256 and openssl rsautl -sign ? Do they do the same ...
0
votes
0answers
61 views

Analyzing complexity of HMAC-SHA-256? [on hold]

I'm new to cryptography, please don't mind me if I'm asking anything wrong. How can I analyse the complexity of HMAC-SHA-256 if the size of my key is 128 bits and my message contains 128 bits ...
6
votes
2answers
311 views

Determine LFSR phase quickly?

I know it's possible with work backwards from the output bits of an LFSR to determine its feedback polynomial in a O(n) fashion. I'm also curious if, given an LFSR state and polynomial, is it ...
1
vote
1answer
40 views

Determine if encryption is safe against CPA

The following encryption scheme encrypts each block of length $n$ of the plaintext separately: $c_i = k_1 \oplus F(k_2 \oplus p_i)$ Where $F$ is a strong pseudo-random permutation (i.e. it is easy ...
-2
votes
1answer
42 views

Shuffleless PRNG function with non-repeating values?

I need a simple PRNG function of type: Integer = PRNG(n, maxval) as I would like to count from 0 to maxval, not in a linear manner, but in a pseudorandom manner where I still use every value, but ...
1
vote
0answers
22 views

Chance to find HMAC key/salt when having knowledge of the hashed data?

I'm not sure how to word this... I'm working with a HMAC (I think of it being a "salted hash"). I know the entire string being hashed, I do NOT know the salt. I also know the first 8 characters of the ...
1
vote
0answers
225 views

Montgomery Reduction

I'm taking a hardware cryptography class and working on a problem that focuses on Montgomery Reduction. So by definition: Computing a x b (mod N) Pick R, s.t. R > N, gcd (R,N) = 1 Compute N^-1 ...
1
vote
1answer
72 views

Understanding Meet-in-the-Middle attack on block ciphers

I understand how to implement a Meet-in-the-Middle attack on 2DES/3DES, by computing for one half all possible keys and storing the outcome and then subsequently do the same for the other half but ...
3
votes
4answers
331 views

Are specially designed fonts sometimes used in cryptography?

As a font designer, I was thinking that it would be very easy to design a special font that could either jumble characters of a language (or of a code) or simply (with the features offered by the OTF ...
0
votes
0answers
13 views

Adapt existing GOST code in C to hash a file [migrated]

I am trying to hand-code the GOST hash function using C. I came across the following code by Markku-Juhani Saarinen (from link). ...
5
votes
1answer
259 views

Compact digital signature for noisy data

The question asks for a signature scheme, with a public verification procedure, that is tolerant to minor alterations of the signed data during its transmission [possibly in analog form over some ...
0
votes
0answers
15 views

Understanding JCE Cipher Algorithm Strings [migrated]

I am new to cryptographic ciphers and I am trying to understand the strings that JCE uses to specify cryptographic algorithms. For example, using the documentation at: ...
5
votes
1answer
56 views

Keys in HMAC and NMAC

I'm quite new to this topic and have several questions concerning HMAC and NMAC: Why does NMAC need two keys? How can it be attacked if we just used some sort of initialization vector instead of the ...
2
votes
2answers
211 views

Is Encryption without knowing the input directly possible at all?

I know the question is rather unusual, but let me clarify what I'm searching for: There is one person, lets call her $Alice$. $Alice$ has $n$ plaintexts $t_1..t_n$ and $n$ public keys $pk_1..pk_n$ of ...
0
votes
0answers
22 views

Why doesn't COMODO use their root cert for the SSL certs they sell? [migrated]

I noticed that OS X ships with a COMODO Root Certificate, but that the certs they sell to websites stem from the AddTrust External CA Root. Why is this?
5
votes
1answer
88 views

Hard-core predicates: should the adversary be given $1^n$?

In most (all?) classical sources such as the book of Goldreich (2001), hard-core predicated are defined thus: A polynomial-time computable predicate $b : \{0,1\}^* \to \{0,1\}$ is a hard-core of a ...
1
vote
1answer
466 views

Static DH/Static ECDH certificate using OpenSSL

I've been searching for some time and have found no way to create a working certificate and private key pair with a DH or ECDH public key using OpenSSL. Once I do this, I'd like to sign it with an RSA ...
4
votes
1answer
2k views

Who uses the RSA BSAFE library? [duplicate]

Who uses RSA's BSAFE library? Does anyone know what products use it, or have any statistics on how many end users use something that is built on BSAFE? Background: BSAFE is one of the oldest ...
16
votes
2answers
2k views

Why should I use an Initialization Vector (IV) when I have unique keys?

I took a look at “Why, or when, to use an Initialization Vector?” but my question is not the same. I have unique keys encrypting each plaintext (in CBC mode, AES-256) and I do not use a key to ...
0
votes
0answers
8 views

Does we need IV if we're generating a new Symmetric key for every message? [duplicate]

We need to encrypt individual messages, for every message we: Generate a new AES key Encrypt the message with the key (AES/CBC/PKCS5Padding) Encrypt the symmetric key with with a public key ...
1
vote
0answers
24 views

Improve security of transposition ciphers

As we know, breaking a Transposition Cipher is quite trivial and can be easily done by either exploiting single letter frequencies (not $d$-gram frequencies though, given that $2 < d < L$), or ...
9
votes
2answers
661 views

Why Elliptic Curves?

What is the benefit of using elliptic curves over the standard finite field, when the cyclic subgroup we consider of the EC's solution group is just isomorphic to some integer residue class of prime ...
0
votes
1answer
21 views

How to prove the hardness of Rabin's function?

I am unable to prove the following theorem: If for a $1/(\log(n))$ fraction of the quadratic residues $q\pmod n$ one could find a square root of $q$, then one could factor $n$ in random polynomial ...
0
votes
1answer
20 views

Stern identification scheme to signature scheme

I'm trying to understand the comment in the page 103 of the book Postquantum Cryptography. about convert the identification Stern procedure for signing. Can you write an example of signature of this ...
3
votes
1answer
46 views

Implementing forward secrecy in OpenPGP with public key authentication

I am trying to consider the feasibility of implementing an encrypted file sharing system, which both provides forward secrecy but also allows public key verification. I am stuck on the marriage ...
0
votes
0answers
5 views

Length of output of AES 128 encryption in python [migrated]

When I am trying to convert to hex the output of my AES 128 encryption code using pycrypto. I am using hexlify .the output is 64 bits for a 32 bits input. The output is always double the size for any ...
3
votes
1answer
90 views

Why don't use random padding in RSA?

Consider the following scenario: Sender S sends a message to receiver R. He uses a hybrid encryption scheme with RSA as key-encapsulation algorithm and AES-256-CTR (4byte-counter) and SHA256-HMAC. ...
1
vote
2answers
92 views

RSA private key d knowing e,n

I need to find RSA private key $(d, N)$ knowing $(e, N)$. It's "own" RSA implementation. As i know $p$ is random 70 bit number, then $q$ is $p-2^{10} < q < p+2^{10}$ $d$ is max 16 bit long with ...
5
votes
1answer
472 views

Can hash functions improve a lousy PRNG?

If I have a poor PRNG, could a cryptographically secure hash function make a better output? Say the PRNG can produce as much numbers as you wish, reasonably random, but not good enough to be used in ...
0
votes
1answer
31 views

What values constitute failing for ENT tests?

The dieharder tests give a clear PASSED, WEAK, etc and I'm wondering what those thresholds ...
5
votes
1answer
1k views

Reusing keys with AES-CBC

I heard that key/IV pairs must not be reused in AES-CTR, or when using any stream cipher for that matter. Yet the attacks described do not seem to apply to AES-CBC. Is reusing the same key several ...
5
votes
1answer
105 views

Can Alice send Bob a secure message with only DSA and no key exchange?

Suppose Bob has a DSA key pair, and Alice has Bob's public key. Is there any secure way that Alice can send Bob a message only he can read if Bob can not send any reply? I think the answer is no, ...
4
votes
0answers
56 views

Better lower bound on min-entropy

In “Randomness Condensers for Efficiently Samplable, Seed-Dependent Sources” by Dodis, Ristenpart, and Vadhan (PDF), I have seen the statement that: any tuple of distributions $(X,Z)$ is $ε$-close ...
2
votes
1answer
61 views

Hash functions to generate one time pad

Newbie question... may be... Stream cypher, will XOR message bits with one time pad bits Suppose I have a pseudo random number generator, like a Mersenne twister or sort of. I could use the ...
0
votes
1answer
68 views

Does the following linear equation hold in bilinear pairings?

Does the following hold in bilinear pairings? $$e(g^{a_1x_1}g^{a_2x_2},g^{c_1}g^{c_2})=e(g^{x_1+x_2},g^{a_1a_2(c_1+c_2)})$$
2
votes
1answer
81 views

Decryption of message in IBE without random oracle using bilinear pairing registration?

I find the following IBE scheme from the videos posted and i don't understand the decryption algorithm, will any one please elaborate the 6th step scheme Setup($\lambda$) : $(\mathbb{G}, ...
2
votes
1answer
428 views

AES Affine Transformation in $GF(2^4)$

in $GF(2^8)$ the AES affine transformation is defined as \begin{equation*} b( x ) = ( x^7 + x^6 + x^2 + x ) + a ( x )( x^7 + x^6 + x^5 + x^4 + 1) mod (x^8 + 1) \end{equation*} where the polynom $( x^7 ...
1
vote
0answers
65 views

Does the encryption algorithm DES perform random permutations? If so how is information not lost?

Before round 1 begins, all 64 bits of an incoming message and all 56 bits of the secret key are separately permuted (shuffled) My question is this : Is the shuffling of the bits performed ...
7
votes
1answer
107 views

Can Grover's algorithm be parallelized?

Using a quantum computer, Grover's algorithm can search an unordered list of length $N$ in time $\sqrt{N}$. Applied to cryptography this means that it can recover $n$ bit keys and find preimages for ...
0
votes
0answers
58 views

Multiplicative inverse ($17^{-1} \mod 31$)?

So. Sorry for bothering you with such a simple question, but I can't really get this done. It's just an exam question in which I need to use CRT in order to calculate the RSA signature of a msg ...
1
vote
1answer
60 views

How can a lattice attack be applied to ECDSA signatures?

The aim is to check if it is possible to break the ECDSA cryptosystem under the following criteria. Suppose that each ECDSA signature is generated by using the GLV method for point multiplication ...
2
votes
1answer
34 views

Finding keys of modified DES encryption

This is a question I had in my exam today, and I'll be glad if someone can help me to find the answer. A student built an encryption algorithm (something between DES and 3DES), in which the ...
7
votes
1answer
2k views

Zero knowledge proof protocol example?

Alice is color blind. She never knows if her gloves are matched. Her brother Bob always teases her saying her gloves are mismatched and she should go change them. Alice wants to know if ...
1
vote
1answer
55 views

Session key Exchange, some doubt

I have to find a solution to this problem: I have a network composed by 1 server and some client. The server has a couple of keys (public and private) and it shares a secret key with each client. My ...
0
votes
1answer
33 views

What is the run-time on finding collisions with MACs,…?

I've recently read the claim that finding collisions on block cipher may be not a difficult task, if an attacker can control the key and the message. So, given a pseudo-random function $f:\{0,1\}^m ...
1
vote
1answer
37 views

ElGamal and Paillier key sizes for short messages

I am using ElGamal and Paillier schemes to encrypt a large number of short messages: typical 4-byte integers. I do this for the homomorphic properties of these schemes. However, the way the ...

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