# Tagged Questions

An asymmetric cipher is an encryption scheme using a pair of keys, one to encrypt and a second to decrypt a message. This way the encrypting key need not be kept secret to ensure a private communication. Similarly in public key authentication, the verification key can be public and the signing key ...

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### Can you tell me how to create a cipher please? just a little simple one [closed]

I don't how to make a cipher text at all, can you help me?
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### Factoring an RSA modulus given high bits of a factor

I have {e,N,C} and part of p; can I get q from this example : ...
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### Why does choosing the first coprime e greater than half of φ(n) result in the same d (private exponent)

While reading on RSA's algorithm, I attempted a simplified implementation and noticed the following: When choosing the public exponent $e$, if the value chosen is the first coprime after $φ(n)/2$ ...
766 views

### How to build an electro-mechanical public key cipher machine?

It is generally assumed that asymmetric encryption schemes were invented in 1973 at GCHQ in Britain and, independently, in 1976 at the MIT. Imagine, if the abstract idea of having a public key and a ...
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### How to use the hardcoded server's public key to register a client application?

I'm designing the security for a smartphone app that can be used to make car reservations. Clients have to register them with their driver's licence through the application and can then book a car. ...
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### How does this solution guarantee authentication of B and privacy?

The exercise is: What could be changed on the second message " Kya(Kxb(Ra,Ks)) " to guarantee B's authenthicity and also privacy? My teacher's solution was "add a destination" , so it would be ...
142 views

### ECDSA non random k - Retrieve private key [closed]

At the moment I'm playing around with ECDSA. I know that I can retrieve the private key if the same K is used to sign two messages. I tried to use this script: Ruby Script to crack the private key. ...
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### Need help understanding this exercise's solution

We were given this exercise, which asks us to identify the types of keys used on this diagram: The solution was Kxa -> A's private key Kya -> A's public key KxB -> B's private key My question ...
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### ECDSA key recovery - floating point values

I'm currently attempting to recover an ECDSA key. I have $m$, $m'$ and signatures $(r, s)$, $(r', s')$, and I know that $k$ is constant, the curve is NIST P-192 and the hash function of the. As such,...
258 views

### Is there a administrator key that can decrypt all ciphertexts encrypted with different keys by different persons in a group?

here is the scenario: $A, B, C, \ldots, Z$ are in a group with a administrator $Admin$, each of the users in the group has his own key $Key_A, Key_B$, and so on... everyone encrypt his own ...
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### What are the most appropriate PKI based cryptographic protocols for peer to peer communicaton?

I've been trying to find out which is the most appropriate PKI based cryptographic protocol for peer to peer communication. I do understand that they vary with respect to the peer to peer architecture ...
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### Key Distribution Center Key Protocol Analysis

As shown in the image, why is Circle 1 to be B and Circle 2 to be A? Why is not the opposite, that is, Circle 1 to be A and Circle 2 to be B? like this picture From my understanding, Notation B in ...
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### Different kind of PGP keys or IDs, how to interpret them?

When people say "Here's my PGP key", they tend to come up with all sorts of different stuff. Sometimes they provide an 8-digit hexadecimal number, which seems to be a key ID (although I'm not sure). ...
1k views

### Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC)?

Related to "Is it possible to derive the encryption method from encrypted text?". Given ciphertexts generated by any of the major asymmetric ciphers (RSA, ElGamal, ECC, etc..) can these ciphertexts ...
32k views

### How does asymmetric encryption work?

I've always been interested in encryption but I have never found a good explanation (beginners explanation) of how encryption with public key and decryption with private key works. How does it ...
484 views

### Is there an algorithm to find the number of intersections of two sets?

Suppose both I and my friend have a set of integer numbers. We want to know the number of common elements in our two sets but without knowing elements of the sets of each other. So I don't want my ...
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### Use MDC to provide origin authentication for public key encryption?

Why can't we use an MDC (“Manipulation Detection Code”, as also handled in this Q&A) to provide origin authentication when using public-key encryption?
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### Why is a-symmetric so much harder to compute and why does it need such big keys? [duplicate]

I read that a symmetric encryption requires a way bigger key size to have the same security as symmetric encryption with smaller keys. But why is this, and why is it so much harder to compute?
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### Can I change an encryption key used without decrypting first

I'm looking to solve for a little challenge I have in regards to changing the encryption on a message with asymmetric keys. I'd love to be able to swap the encryption to a different key in memory and ...
749 views

### Are there any signature schemes for underpowered devices (8-bit microcontroller)?

I am currently researching into a small scale home automation system, aiming for cost. The system architecture is basically one master and several slaves which are connected in parallel. Recently i'...
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### Padding schemes for asymmetric encryption with provable security in the standard model

Most padding schemes for asymmetric encryption (OAEP, OAEP+) are only proven secure in the random oracle model. Although no attacks are known, it would be nice to find a padding scheme with provable ...
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### How to convert roots of Weber polynomial to Hilbert class polynomial over modulo prime?

Using any non square root discriminant $D$, we should be able to find the Weber class polynomial. How can I convert the roots of a Weber polynomial to a Hilbert class polynomial over modulo prime?
5k views

### RSA with small exponents?

Just to establish notation with respect to the RSA protocol, let $n = pq$ be the product of two large primes and let $e$ and $d$ be the public and private exponents, respectively ($e$ is the inverse ...
1k views

### Are there asymmetric cryptographic algorithms that are not based on integer factorization and discrete logarithm?

In the computer security class (in which cryptography is a big chapter) that I took, I remembered the professor said about current asymmetric cryptography algorithms are based on integer factorization ...
4k views

### Why does RSA need p and q to be prime numbers?

This is a clarifying thread after finding: What makes RSA secure by using prime numbers?. Despite the fact that this question was answered already, I am still struggling to really grasp the ...
1k views

### NIST Diffie-Hellman prime: how was it picked? Where did it come from?

According to this Matasano Crypto challenge, the NIST "likes" the following prime modulus, which appears to be expressed in hexadecimal: ...
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### Creating ECDH using OpenSSL

For academic reasons, I'm playing around with OpennSSL 1.0.2g. I tested RSA encryption/decryption. I created key exchange with DHKE. But I'm struggling to find a way, to create ECDH, using only ...
117 views

### Can an adversary distinguish a private key from a pseudo-random string of the same length?

Apologies if this is a dumb question but allow me to describe the dilemma I have: Suppose that I am protecting a private key on a device using a password & PBKDF2. The obvious attack would be an ...
208 views

### Finding the subgroup in isogeny-based cryptography

Isogeny-based cryptography is one of the newest post-quantum cryptography. Hardness of this system is based on finding isogeny between two elliptic curves. Also this is theorem: Elliptic curves ...
11k views

### What makes RSA secure by using prime numbers?

I am just learning about the RSA algorithm. Looking at the first two steps: Choose two distinct prime numbers $p$ and $q$. Compute $n = pq$. I have some probably stupid questions: Why do $p$ ...
332 views

### Security of the shared secret key for a smart meter (Linky)

Reading the specs of a new electricity smart meter (called Linky in France), I was surprised about the chosen encryption method (128 bits symmetric key AES), based on a single secret key (called CCC ...
441 views

### What is the random tape?

I am reading the paper "Toward plaintext-aware public-key encryption without random oracles" by Bellare and Palacio. So I can't understand what is the tape and random tape?
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### How to calculate the exponent in modular exponentiation?

I have a problem when calculating power in modular, $a^b \bmod c = d$. where we can know values of $a$, $c$ and d, but we don't know values of $b$. example : $29^b \bmod 1024 = 365$. So, how can I ...
31 views

### How to add passphrase to an RSA key?

If I want to protect a private RSA key, do I just XOR the password into the key, or do I do something else? Is it secure? Edit: I got a good answer from a comment - Hash the password/passphrase with ...
22 views

### private-key can't be traced (GPG)?

I am very new to cryptography. I was reading about GPG/PGP Basics, and I have a doubt: As per my understanding, Everyone knows the algorithm (GPG) with which I generated my key pair. And If I give my ...
31 views

### Is a good idea to generate a public key from a secret key using a functional encryption scheme?

FE - functional encryption PK - public key I want to design a PK cryptosystem that has a unique PK for each encryptor. Namely, the encryptor submits a unique value (or an attribute) to the private ...
2k views

### Why is PuTTYgen-created RSA public exponent(e) not in {3,17,65537}?

As far as I know, RSA public exponent(e) should be one of {3,17,65537}. However, I found PuTTYgen-created RSA public exponent(e) is 0x25(37) by default,as follows, (PuTTYgen version: 0.66) I am ...
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### How do you work out the private key in a RSA cryptosystem knowing a certain public key?

The public key: n = 85 and e = 19 I know that n is the product of two prime numbers, now is it right of me to figure them out by process of elimination which I got p or q equals 17 and 5. Or is ...
133 views

### Simulation-Based Proof: When a Secret Key is Involved

Assume we have a protocol in which a party receives an encrypted random polynomial. The polynomial is encrypted using his public key. We want to construct a simulator for this party (so this party ...
12k views

### The modulus of RSA public key

I am studying the RSA cryptosystem. The public key consists of $(n, e)$, the modulus (product of two large primes), and the encryption exponent. I want to separate the modulus $n$ and exponent $e$. A ...
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### Why ssh-gen makes difference between PEM and PKCS8?

Correct me if I am wrong, but PKCS8 is format to store private key info. It could be binary-enoded (DER) or Base64 encoded (<...
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### Securely register a client in server using AES an RSA

I've designed the following protocol to send the client's public key to the server. Initially, the client knows server's public key, but the server does not know the client's public key. client ...
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### Polynomials and efficient computability

In public key crypto, the popular definitions of security (CPA, CCA1,2) depend on PPT adversaries. I'm trying to understand why adversaries should be PPT. It's clear that adversaries should be at ...
5k views

### Why can’t DSA be used for encryption?

This question at StackOverflow mentions that DSA cannot be used for encrypt. But Both RSA and DSA can be used to generate public and private keys, right? (Or am I wrong?). Then why can't I use the ...
870 views

### Additive ElGamal cryptosystem using a finite field

I'm trying to implement a modified version of the ElGamal cryptosystem as specified by Cramer et al. in "A secure and optimally efficient multi-authority election scheme", which possesses additive ...
143 views

### The difficulty of computing discrete logs

I understand that in Diffie-Hellman it should be hard to compute $a$ given $g$ and $g^a$. In computational Diffie-Hellman, it appears to be hard to compute $(g^{ab})$ from $g^a$ and $g^b$. As for ...
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### What is the private key in RSA?

I'm new to cryptography and I have a doubt: I read some pages a bit different definition for the RSA private key: In 1 - (n, d) In 2 - ...
245 views

### Help with RSA-2048 crypto ransom virus [closed]

The RSA-2048 Crypto Ransom Virus has devastated me, I tried the backup method, the previous version, the Shadow Explorer, it deleted all my restore points, Its took out 5 HDs and my USB pen that ...