3
votes
1answer
260 views

Construct IND-CPA secure encryption scheme by combining two given schemes

I have two encryption schemes $\Pi_0, \Pi_1$, at least one of them is IND-CPA secure but I don't know which one. The task is to construct a scheme $\Pi$ that is guaranteed to be CPA secure and to ...
3
votes
1answer
106 views

Decisional Diffie-Hellman: compute Legendre symbol of $g^{ab}$ from $g^a$ and $g^b$?

From https://en.wikipedia.org/wiki/Decisional_Diffie%E2%80%93Hellman_assumption: Importantly, the DDH assumption does not hold in the multiplicative group $\mathbb {Z} _{p}^{*}$, where $p$ is ...
3
votes
2answers
121 views

Implementation of ECC over binary field

I am supposed to implement ECC over binary field (in C++) for equations of the type - $y^2 + xy = x^3 + ax + b$, as my project. I wish to include the following features : The user will enter a prime ...
3
votes
2answers
171 views

Best modern cipher for morse code?

I am looking for a modern cipher to encode morse code, ie. input is [A-Z 0-9] (base36 as I learned in the comments below) and output is the same, one character at a ...
3
votes
2answers
113 views

Are parts of a pseudorandom string also a pseudorandom string?

I am working on an exercise that encrypts a message using only the first $n$ bits of a pseudorandom generator's output $G(k)$, i.e. $F_k(m) := G(k)_{0...n−1} \oplus m$. $k$ is chosen uniformly at ...
3
votes
2answers
177 views

secure PRF or not

I am new in Cryptography and I saw this question in a note I solved it but I'm not sure about my answers. Let $F : \{0, 1\}^n × \{0, 1\}^n→ \{0, 1\}^n$ be a secure PRF (i.e. a PRF where the key ...
3
votes
2answers
81 views

Is there a way to do single key-pair asymmetric encryption?

All of the information on asymmetric encryption I can find uses key-exchanging to get a mutual symmetric key. What I am trying to do is encrypt a message with a public key and have it only readable ...
3
votes
2answers
272 views

What is the use case for XOF functions (i.e. SHAKE128/256)?

FIPS 202 defines 2 functions: SHAKE128 SHAKE256 As extendable-output functions (XOFs) that can have variable output length. But in Appendix A.2 marks: it is possible to use an XOF as a hash ...
3
votes
2answers
175 views

Is there any relation between two strings with the same MD5 hash?

Is there any relation between two strings with the same MD5 hash? For example these two strings: ...
3
votes
1answer
64 views

Making Pascal Paillier' output absolute

Can we make subtraction result of cipher texts encrypted by Pascal Paillier absolute. Just like we use method Math.abs() in Java ? For example, if we subtract 0 from 1: 1-0 = 1, it is positive but 0-1 ...
3
votes
1answer
335 views

Do Weak Elliptic Curves Exist?

I'm running into very contradictory opinions when try to understand if weak elliptic curves exist. I'm not interested in the case when a curve's weakness is attributed to properties of an EC's prime, ...
3
votes
1answer
134 views

Can the round function $F$ in a Feistel Network be practically any non-invertible function?

This question might seem silly, but how true is the fact that the round function $F$ does not have to be invertible? I am curious to know this, because non-invertible functions can be very lossy, i.e....
3
votes
2answers
236 views

Memory-hard proof-of-work: are they ASIC-resistant?

Is a memory-hard proof-of-work scheme necessarily resistant to speedups from custom ASICS? Background: Bitcoin uses a proof-of-work scheme based on SHA256 hashing. The scheme is compute-bound. ...
3
votes
2answers
162 views

How to find the value of a vector modulo a basis in lattice-based cryptography

In Gentry's paper on fully homomorphic encryption using ideal lattices, he finds the values of vectors modulo a certain basis. For instance: $\psi \leftarrow \psi' \mod B$ Taken from page 69 of ...
3
votes
2answers
116 views

Is reverse NTRU still secure?

I'm currently prototyping something with the NTRU encryption scheme but I wish to use it in "reverse" -- distribute private keys so anyone can decrypt, but keep the public keys secret and thus only ...
3
votes
2answers
115 views

Homomorphic Encryption vs. Garbled circuits

I'm currently trying to understand all the differences between homomorphic encryption and garbeled circuits. As I understood the use of homomorphic encryption hides either the data or the computation ...
3
votes
1answer
224 views

How can AES + cycle walking be used as an FPE?

I'm a bit confused after reading several articles that seem to provide conflicting information. Basically I was wondering if AES by itself can be used as an FPE? And if so is this something ...
3
votes
3answers
102 views

Authenticate ephemeral key exchange

At the moment I'm designing a crytosystem for a VPN server. One of the requirements is Perfect Forward Secrecy, but I'n not sure how to implement authentication on the ephemeral key exchange. If both ...
3
votes
1answer
554 views

Encryption algorithm used in WPA/WPA2

I want to know how EXACTLY the WPA password gets encrypted I have been searching through and I was able to catch: The actual password is converted to some hash Salting is applied to the hash The ...
3
votes
1answer
120 views

How can you produce visual cryptography?

I would like to be able to "code" images such as the one seen here on Wikipedia. Is this generally produced with software, or does this require complex programming? Ideally, I would love to be able ...
3
votes
1answer
568 views

How to prove that a function is not pseudorandom?

I am currently enrolled in a cryptography course, which uses the book by Katz and Lindell. I'm struggling with the exercies which ask for proofs, like the following one: Let G(k) be a PRG with |k|...
3
votes
1answer
145 views

Is ElGamal IND-CCA1?

We all know that textbook ElGamal falls due to chosen ciphertext attacks, because of its multiplicative homomorphic property ($E(A)*E(B)=E(AB)$). However these attacks require the ciphertext ($E(A)$ ...
3
votes
1answer
75 views

RSA: Range of public modulus

I know the public modulus is the product of two primes. For key with length L, the public modulus N should be such ...
3
votes
1answer
140 views

RSA with $\lambda(n)$ or $\varphi(n)$

The PKCS #1 v2.0 specifications suggest using $\lambda(n) = \mathrm{lcm}(p-1,q-1)$. What is the benefit of choosing $\lambda(n)$ over $\varphi(n) = (p-1)(q-1)$ in RSA key generation?
3
votes
1answer
178 views

What are the differences between the elliptic curve equations?

I think we're all aware of the "classical" Weierstrass (short?) elliptic curve equation: $y^2\equiv x^3 + ax +b \pmod p$. Well known examples of these curves include the NIST's and Brainpool ones. ...
3
votes
1answer
141 views

Can we reduce Diffie-Hellman problem to “Discrete-log inversion” problem?

Let $G$ be a cyclic multiplicative group of order $n$. Let $g$ be a (public) generator of $G$. The Diffie-Hellman (DH) problem asks: Given $g^x, g^y\in G$ for $x, y\in \mathbb{Z}^*_n$, to compute $g^{...
3
votes
2answers
308 views

What makes SHA-2 and SHA-3 have security levels half their output hash length?

The tag wiki for sha-256 says, in part: SHA-256 ... processes the input in 512-bit block and has a 256-bit output, for a 128-bit security level. I'm not a cryptographer, just an interested ...
3
votes
1answer
101 views

what is the public information in Elliptic curve cryptosystems [closed]

Currently my knowledge about Elliptic curve is quite limited to the textbook and I don't know how a practical Elliptic curve cryptosystem works. I read an example about key exchange using Elliptic ...
3
votes
2answers
192 views

How to calculate if probability is negligible or not

If i have probability $Q = 2C(A\times B)$ where $A$ and $B$ are unknown probabilities and $C$ is a non-negligible probability, what can i speculate about probability $Q$ and how can i calculate bounds ...
3
votes
1answer
361 views

Where is the authentication tag stored in file encrypted using AES-GCM?

So I am using cryptopp to encrypt a file in a project and I am using AES-GCM. I have a 256bit key and a 128bit ivec and a 128bit tag that I use to encrypt files. However, I want to know where the ...
3
votes
2answers
229 views

In ECC, how do I prove that point addition is commutative?

I am studying elliptic curve cryptography and this question is related to the commutative property of point addition operation. Point addition $P_3(x_3,y_3)$ of two points $P_1(x_1, y_1)$ and $P_2(...
3
votes
1answer
288 views

Why are there no MACs inspired by block cipher modes other than CBC and CFB?

I've been studying message authentication codes and I was wondering why a MAC can only be produced with AES in CBC and CFB mode and why not the other modes such as ECB, OFB and counter. Why are CBC ...
3
votes
2answers
1k views

PRNG determinism (and java.security.SecureRandom)

Looking at getDefaultPRNG() in java.security.SecureRandom makes me very unsure about how to use it. On my desktop, it uses sun.security.provider.NativePRNG, which reads from ...
3
votes
1answer
159 views

Is a strong block cipher usable as a strong sponge function?

From the literature, it looks like the security proofs of sponge functions depend on how well they approximate a random permutation, Since a block cipher also ideally behaves like a random permutation ...
3
votes
2answers
270 views

Block size vs key size confusion

I understand that if a block cipher has $k$-bit keys and $n$-bit input/output blocks, then if $k>n$, we can expect one message-ciphertext pair to narrow us down (I think?) to $2^{k-n}$ possible ...
3
votes
1answer
163 views

Is the salt used with PBKDF2 secret?

I have two network endpoints that need to use the same derived key. Can the server send the salt to the client 'in the clear'?
3
votes
1answer
90 views

Why develop new stream ciphers when block ciphers can be trivally made into streams?

I've just been reading about a new drop-in replacement for the RC4 stream cipher, but why develop new stream ciphers when AES can be made into a stream with a little XORing?
3
votes
2answers
209 views

Attacking unuauthenticated RSA

In symmetric cryptography, a scheme is not considered secure if the data transmitted is only encrypted. The adversary will be able to modify messages to generate encrypted messages of his choice ...
3
votes
2answers
197 views

CBC-MAC key can be added to ciphertext?

I use AES both CBC and CBC-MAC to encrypt some stuff. I generate one key for CBC and one different key for CBC-MAC. Does the second key (for CBC-MAC) need to be secret? How to join such key with the ...
3
votes
2answers
371 views

Diffie–Hellman Ephemerality Nomenclature

Below is a short excerpt of available DH cipher suites available on a machine. I understand EDH is ephemeral DH, and that ECDH is for Elliptic-Curve DH which is computationally faster. ECDHE is both ...
3
votes
2answers
534 views

Whitening and randomness extraction

The Von Neumann randomness extractor is explicitly mentioned in the randomness extractor page of Wikipedia, but strangely enough that page does not contain any reference to whitening the results of an ...
3
votes
2answers
183 views

What exactly does a key do?

I am getting to grips with cryptography as a total newbie, and am struggling with encryption "keys" and how to visualize them. From http://computer.howstuffworks.com/encryption.htm/printable: ...
3
votes
1answer
1k views

Montgomery Ladder vs Double-and-Add

I would like to know what (if any) are the advantages of using Montgomery Power ladder over the Double-and-Add-Always algorithm. I think that firstly, Monty would be slightly faster than DoubleAndAdd. ...
3
votes
2answers
180 views

Reductionist proofs of decisional problems to computational

Are they any reductionist proofs where an attacker $\mathcal{I}$ for a well established computationally "hard" problem $\mathsf{Π}$ is employing an attacker $\mathcal{A}$ who we assume is able to ...
3
votes
1answer
112 views

Does $i^n=j^n$ for $i, j \in GF(2^q)$ and $i \neq j$ for some $n<2^q-1$

Let $i, j \in GF(2^q)$ and $i \neq j$ and $i,j\neq0$. Is that possible that $i^n=j^n$ for some $n$ such that $0 < n < 2^q-1$? I am looking for a proof if the answer is no, or for a method to ...
3
votes
2answers
551 views

Is that possible to use encryption higher than 256-bit?

When browsing websites with Chrome sometimes I can see a green lock and encryption info inside. It says the connection with this website is 128-bit or 256-bit encrypted with TLS1.2. So I'm wondering ...
3
votes
1answer
287 views

Security proof of FO(Fujisaki-Okamoto) hybrid encryption

The proof of FO hybrid encryption is hard to understand. $\:$ Especially, how does the challenger respond to the decryption queries when the challenger can only have some encryption queries? Can ...
3
votes
1answer
1k views

LFSR get output from characteristic polynomial?

Say you have a characteristic polynomial of an LFSR: $$f(X) = X^4 + X^3 + 1$$ How can I use this function f to get the output of the LFSR, given some initial state? Obviously I can create the LFSR ...
3
votes
2answers
287 views

Is a random oracle controled by the challenger?

When proving a Crypto scheme security under random oracle model, is the random oracle always controlled by the challenger? What if the Hash is only used by the adversary?
3
votes
3answers
202 views

Is it meaningful to consider the leakage of master key of KGC?

In the Identity-based cryptography, the user's private key in the systems is delivered by the key generation center(KGC). My question is : Is it meaningful to consider the leakage of master key of KGC?...

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