# All Questions

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### Weak Boneh-Boyen Signature in Composite order group

In the paper Ring Signatures of Sub-linear Size Without Random Oracles, authors have remarked that the scheme is instantiable in composite order setting, too. I am attaching the following reference ...
49 views

### Using Permutation polynomial to compute a MAC

Is the following MAC secure? For a block $y_i$ in a file, we defined a MAC as follows: $Mac_i:PRF(k,i) \cdot g^{y_i \cdot r_i} \bmod p$. Where $p$ is a prime number, $g \in \mathbb{G}$,$PRF(k,i)$ is ...
191 views

### PRF based on the GGM construction

What's the differences between the concepts “pseudorandom generator” and “pseudorandom number generator”? In fact, I want to implement a pseudorandom function based on GGM's construction at ...
247 views

### Python implementation of a blind signature scheme which doesn't involve RSA

RSA seems a bit creepy after the Snowden revelations and i'm looking for a simple python based blind signature library to fiddle around with. So far i've been unable to find anything. What am i ...
134 views

### How do you derive the lambda and beta values for endomorphism on the secp256k1 curve?

Note: This question was reposted from Bitcoin Stack Exchange, where it received alike answers. You can see a little background about this on this bitcointalk post by the late Hal Finney. $\beta$ ...
545 views

### SHA1 Partial Collision

I'm working on a little python script that reads in two fairly similar English sentences (they're in text files - reading them in) and I'm trying to force a SHA1 partial collision (e.g. first 32 bits ...
883 views

### What is the difference between a hash function and a pseudorandom function? [duplicate]

Read the title. I've seen in RFCs that some MAC functions are called "pseudorandom functions". What are those? How are they different than hash functions? Why can't a hash function be used instead?
321 views

### CPA security of a stateless and deterministic encryption system

Why can no stateless and deterministic encryption system be IND-CPA secure? Is there a formal proof for it?
112 views

### RSA generation of private key using public key

In RSA private key generation e*d ≡ 1 mod φ e is public, also n is public. How to prove mathematically, generation of ...
130 views

### Multiple encryption of part of block

Is there any security risk in encrypting parts of blocks twice? I'd like to get around block sizes and make encrypted messages just as long as the unencrypted ones: Block 1: Byte 0 - 7 Block 2: Byte ...
123 views

### Definition of ciphertext security

If I got it right, chosen ciphertext security implies also CPA security. In other words, attacker can submit plaintexts to the challenger (along with ciphertexts). I do not understand why ...
55 views

### Decrypt a message which is encrypted using XOR? [duplicate]

This is a puzzle asked in a contest. Given that encryption , decryption happens as per following rule/code: ...
92 views

### Existence of encryption schemes where $F_{k}^{-1} \big( F_{k}(A) + R \big) = A + O(R)$

Is there a encryption scheme in literature, where I can do something like $$F_{k_1}^{-1} \big( F_{k_2}(A) + R \big) = A + O(R)$$ where, $A$ is a $m \times n$ real matrix, $R$ is a $m \times n$ real ...
500 views

### Why does the DES crypto algorithm NOT use 2 rounds?

Now, if we were to go round by round, you could give a distinct reason for not using a single round since after just one round, the right half of the text comes directly, as-is, to form the left half ...
666 views

### Generation of a cyclic group of prime order

I am trying to implement a cyclic group generator in Java, but I am running into some issues. In many cryptosystems, the following phrase is expressed during the key generation stage. Let G be ...
70 views

### Non-repudiation and digital signature of a dishonest participant

Let's assume a dishonest Alice who sends, encrypts & digital signs a message to Bob. Bob stores the decrypted message and the digital signature in a database. However Alice is a bad girl and ...
256 views

### XOR cipher with three different ciphertexts and repeated key, key length known. How do I find the plaintexts?

Let us say we have three different plaintexts (all alphabets, A-Z): $x$, $y$ and $z$, each of length $21$. Let the key, $a$, be also of length $21$. Now, what we have is $x \oplus a$, $y \oplus a$ ...
63 views

### Why is it a quadratic equation?

In Groth-Sahai NIZK proof system, they have defined something called Quadratic Equation in $\mathbb{Z}_n$ as shown below. But, my idea of quadratic equation was a second order polynomial equation in a ...
231 views

### Parallel Pollard's Rho: Number of distinguished points

When using the parallel version of Pollard's Rho algorithm for discrete logs, each processor performs its own random walk to find distinguished points, and reports the starting point and the ...
81 views

### One-way function definition

I cannot understand why a one-way function $f$ is defined in this way $\text{Pr}(f(A(f(x))) = f(x)) < \frac{1}{p(n)}$ and not $\text{Pr}(A(f(x)) = x) < \frac{1}{p(n)}$ where $A$ is a ...
120 views

### Proof that $gcd(e, \lambda(N)) = 1 \hspace{1mm} \Longleftrightarrow \hspace{1mm} gcd(e, \varphi(N)) = 1$

What is the proof for the fact that $gcd(e, \lambda(N)) = 1 \hspace{1mm} \Longleftrightarrow \hspace{1mm} gcd(e, \varphi(N)) = 1$ Where: $N = P * Q$ where $P$ and $Q$ are both primes. $\varphi(N)$ ...
59 views

### Do any stream ciphers with aperiodic keystreams exist?

Exactly what it says on the tin. I can imagine constructing such keystreams from: The binary expansions of irrational numbers Chaotic systems like the logistic map or the Lorenz attractor. The ...
182 views

I'm trying to solve a question about one time pads but I'm not sure if my assumptions are correct. I am to assume that I have a one time pad with perfect secrecy. This one time pad is used to ...
35 views

### Reducing key shares in Damgård-Dupont threshold RSA

I'm working on understanding and implementing Damgård, I., & Dupont, K. (2005). Efficient Threshold RSA Signatures with General Moduli and No Extra Assumptions. Public Key Cryptography-PKC ...
76 views

### Is there a reference that prove that the AES Key Schedule generate random looking round keys?

Starting from uniformly random generated AES master key, is there a reference that prove that an specific roundkey can be considered as uniformly random generated as well ?
239 views

### How hard is it to find plaintexts whose hashes satisfy $h(a)\oplus h(b)=h(c)$?

Given a cryptographic hash function $h$, for example SHA256, how hard is it to find plaintexts $a,b,c$ such that $$h(a)\oplus h(b)=h(c) \text?$$
729 views

### How secure is it to use password as AES key?

I am developing a steganography app where user encrypt any file into an audio file. The user can enter a password to protect the hidden data. The same password is converted as a 256 bit key and the ...
743 views

It seems that merkle hash tree (MHT) traversals have been discussed somewhat in the literature, but there does not appear to be much written on inserting, deleting, and updating leaves. Is this lack ...
127 views

### Usage of GF(p^m) fields, where p != 2

$GF(2^m)$ Galois fields are widely used in different cryptographic algorithms, for example, in Rijndael. However, $GF(p^m)$ fields are possible with any prime $p$, not only 2, but $GF(2^m)$ fields ...
335 views

### Definition of the Decryption oracle

In the context of public-key encryption, what would be a formal definition of the decryption oracle? I know the informal definition (i.e., a function that is available to the adversary and that ...
30 views

### Any ideas on login using Digital Signature PKCS

I am curious to know about possibility to login into web application just by identifying the user based on digital signature in usb token. any reference in this direction?
531 views

### 1 Billion Bit Encryption?

So, browsing through YouTube, I stumbled on this video interview of John Draper (Captain Crunch), one of the first "hackers". He talks for about 3 minutes (until 27:48) about his home rolled ...
120 views

### How do I communicate the value of the initialization vector to the end user? Should it be part of the encrypted message?

I am taking a cryptography class and our first homework is to implement a 32-bit block cipher. I implemented a simple block-cipher that uses CBC. Currently, my implementation reads both the 32-bit IV ...
111 views

### RSA: How effective is this keypair-trash attack [duplicate]

A question that could very well be part of xkcd's "what if?": Let's say Monica made a piece of software that sends all RSA keypairs to a central database after they're not used anymore. Something ...
68 views

### Applications of GF(p) polynomials

A Galois field of the type $GF(p)$, where $p$ is prime, is normally expressed as the ring of integers modulo $p$. If my understanding is correct, it is also possible to represent its elements as a ...
353 views

Given is a system, which does provide only implementations of fast hash algorithms (MD5, SHA1, SHA-256, SHA-512). There is no implementation of PBKDF2, bcrypt or scrypt available. The system does ...
246 views

### x509 Certificate Signature

I've got a question about the signature of a CA. As I understand, the CA takes the public key of the client and signs it with his own private key by using "md5WithRSAEncryption" (like explained here: ...
141 views

### Is this a valid “fix” for deterministic encryption in encrypted databases?

It appears (after doing some light research) that for encrypted databases to be practical enough to be usable, deterministic encryption is required, specifically with regard to the type of encryption ...
108 views

### Negative exponents in Shoup's threshold RSA?

I'm trying to implement threshold RSA operations, starting with decryption based on Peeters, R., Nikova, S., & Preneel, B. (2008). Practical RSA Threshold Decryption for Things That Think. ...
52 views

### With wrong IV at receiver side, the CFB in better than OFB?

Assume the receiver have a wrong IV (initialization vector), in the CFB mode only the first block of plaintext is wrong but in OFB mode the second and all blocks will be affected. Is that correct? ...
396 views

### Why isn't CTR mode (counter mode) used more often?

For the CTR mode, the design is good for parallelization, yes, it seems the benchmark of the program downloaded from crypto++ proves that on an Intel I7 CPU. My question is that as most of CPU on ...
120 views

### Problem in understanding Blakley's Secret Sharing Scheme

I need to implement Blakley's Secret Sharing Scheme. I have read below mentioned two research papers but still unable to understand how to implement it. Safeguarding cryptographic keys Two Matrices ...
850 views

### How are the AES inverse S-Boxes calculated?

I would like to know, how to calculate the inverse S-box. I followed this link (with affine transformation first, then multiplicative inverse), but the result is wrong. For example, if I use the value ...
111 views

### Hardness assumptions on composite order bilinear groups [closed]

I am not at all knowledgeable in elliptic curve cryptography. So, here lies a couple of questions that I failed to find answers for to my satisfaction. Is there any known Type-III bilinear pairing ...
106 views

### Is there a strong cryptographic reason for GCM's 2^39 - 256 bit limit?

In reading through the original GCM specification (McGrew & Viega '05), the composition of the 128 bit Initialization Vector as a concatenation of a 96b nonce and a 32b unsigned wrapping counter ...
78 views

### Is there a strong cryptographic reason for GCM's 2^39 - 256 bit limit? [duplicate]

In reading through the original GCM specification (McGrew & Viega '05), the composition of the 128 bit Initialization Vector as a concatenation of a 96b nonce and a 32b unsigned wrapping counter ...
51 views

### Is signing a message just encrypting it with private key? [duplicate]

I have a simple question, When I sign a message (in RSA) does the program encrypt the text with my private key? Because I can decrypt it with my public key. Yes or no. Is signing a message just ...
226 views

### Why can't you just clone encrypted data and use it?

Let's take a contactless card for example (one for public transport lets say (i.e. MiCard)), the data is encrypted on the card and encrypted on the reader as far as I know. So how come somebody who ...
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 ...