# All Questions

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### Is it possible to construct a hash-based public key encryption scheme?

I know hashes can be used to construct symmetric encryption schemes (HMAC running in counter mode), signature schemes (SPHINCS), pseudorandom generators, etc. One thing that's missing is public key ...
28 views

### Algorithm to find primes q and p with q|p-1?

I understand that if $p$ is prime then $p-1$ must be composite (at least divisible by $2$ as it is even). But how does an algorithm find a prime $q$ such that $q \cdot r = p - 1$. I thought prime ...
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### why is index “a non-negative integer less than 2^64” in the KDF for UMACs?

https://tools.ietf.org/html/rfc4418#section-3.2.1 describes the parameters for KDF as follows: ...
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### Hey, anyone know if this is a color cryptography? If yes what? [on hold]

Me and a group of friends are solving an ARG, we found some videos where every frame was an image like this one.
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### Are there any symmetric Partial Homomorphic Encryption Schemes?

Note that I am asking specifically for Partial Homomorphic Encryption (PHE) schemes and not Somewhat Homomorphic Encryption (SME) schemes. I believe there are several SHE schemes that are symmetric ...
49 views

### RSA, factoring with middle known bits of one prime?

It was shown by Coppersmith, that we can factor the RSA modulus with half of the most significant bits of one of the primes. And for the lsb case, assume that we known $p_0,k$, and $p=p_12^k+p_0$. ...
22 views

### Decrypting a weak PRNG Mersenne Twister Stream Cipher

I am currently working on the crypto-challenges presented here. I created a simple stream cipher using a Mersenne Twister RNG. It basically works as follows: Seed a RNG with the key (basically up to ...
32 views

### Can some one help and explain the approach for decryption? [on hold]

E-crime department had to decrypt the message, upon inspecting the USB Stick police found the following message: DJOCIT WWDN TYWTNAPRP BBMV SIWRVHCUTO. It is believed that before the incident, the ...
27 views

### Generating anomalous elliptic curves with given order [on hold]

First of all please see this paper, I want to fully understand the section 3 and 4 of this paper and write a script that generates an anomalous elliptic curve with given prime order. Furthermore how I ...
23 views

### General factoring and one-way functions

Let a function $f$ be one-way, if there exists a probabilistic polynomial time algorithm to find the preimage of $y = f(x)$ for uniformly chosen $x$ with non-negligible probability. Define the ...
317 views

### What security risks does exposing the size of the plaintext entail?

For this question my focus is exclusively on symmetric block ciphers (with a particular interest in AES256 in CBC mode). I have read much discussion on various ways to pad the plaintext so that the ...
3k views

### How to detect a failed AES256 decryption programmatically?

I have implemented a simple encryption/decryption program based on AES256 in CBC mode1. Actually, it is more precise to describe it as a compression+encryption / decryption+decompression program. If ...
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### How to compute the number of instructions in python [on hold]

Could you please tell me how I can compute the number of instructions of a program written in python. thanks
61 views

### How do discrete logarithm with modulo a prime and a non-prime compare?

Let $c_N = g^i \mod N$ and $N=p \cdot q$ and $c_P = f^j \mod P$ and $P$ a prime We assume $N,P$ has the same bit-length. $P$ is the best type of prime you can choose (e.g. safe prime). $N$ is a ...
37 views

### What is / How would I calculate the number of bits(?) I need to make the probability of a collision very small?

If I wanted to create a TOTP-esque algorithm that generated a string n characters long, with each character being a base64 character, generated from a user secret <...
63 views

### Limitations of boolean and arithmetic circuits

Much of modern cryptography is based around working with boolean or arithmetic circuits. For example in Multi-Party Computation the 'famous' results allow for the secure computation of any function ...
48 views

### Cipher recommendation for known-plaintext attack

I need to encrypt a 4-digit number. The cipher-text can be up to 11 digits long and there are no limits for the key size. Considering that the attacker has access to both the cipher-text and the ...
48 views

Elliptic curves are usually defined over prime rings (fields), but what if we chose a ring of composite order? Let $n = pq$ for $p,q$ large primes. Say I have elliptic curve $y^2 = x^3 + ax + b$ over ...
23 views

### Bullteproofs with ranges not in power of two

I am new to bulletproofs,From what I understand, if i was to do a range proof using bulletproofs , both the start and end number have to be to the power of 2. is there any algorithm/approach which ...
68 views

### Learning elliptic curve cryptography for specific application

I would like to develop a protocol for specific purpose. This protocol will utilize asymmetric cryptography in which one private key can be paired with numerous public keys: messages encrypted with ...
56 views

### NaCl allows decryption with the same public key as for encryption

I followed the description here: Public-key authenticated encryption: crypto_box and created an example that encrypts a message with Bob's public key and signs it with Alice's secret key : ...
34 views

### specific scenario question relating to using NACL crypto_box in a protocol

Scenario 1: SenderA sends message1 to ReceiverB message1 is encrypted using NACL crypto_box using SenderA secret key and ReceiverB public key. we understand that the NACL crypto_box authenticator ...
24 views

### any one can give me answer this question…it is very helpful [on hold]

The Caesar cipher shifts all the letters in a piece of text by a certain number of places. The key for this cipher is a letter which represents the number of place for the shift. So, for example, a ...
54 views

### Second preimage attack complexity when the digest is malleable

An attacker has a hash $h$ and a message $m$. He wants to find $m^\prime$ with the same length as $m$ which hashes to $h^\prime$. However, $m^\prime$ can only differ from $m$ in the 1 bits (i.e. ...
35 views

### Which archive file format with encryption is most secure? [on hold]

If I had to bundle a bunch of files into a compressed archive file with encryption, which file format would be most secure? RAR? Zip? Other? What is recommended?
55 views

### Is there FHE (fully homomorphic encrpytion) for string/text processing?

I had a quick look at FHE libraries like SEAL, HeLib, and Palisade and they deal with addition or multiplication of encrypted data. Is there a FHE that deals with string? If no, is that because it is ...
86 views

### Is it feasible to find n hashes that sum up to a given hash?

Consider two sets $A=\{a_1,a_2,\cdots,a_n\}, B=\{b_1,b_2,\cdots,b_m\}$; We can calculate the hash sum of those sets: $$HASHSUM(𝐴)=(ℎ𝑎𝑠ℎ(a_1)+ ℎ𝑎𝑠ℎ(𝑎_2)+\cdots +ℎ𝑎𝑠ℎ(𝑎_𝑛))$$ and HASHSUM(𝐵...
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### Generate Private Key with AES 256 CTR [migrated]

How to generate a private key with AES 256-CTR ? I need a private key with CTR as CBC is no longer supported by my client. Please advise Details: We need to connect to an SFTP instance of client ...