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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Why is ElGamalEngine in bouncy castle restricting input data to be less than the length of the group parameter p?

I am currently using the ElGamalEngine of bouncy castle to implement exponential ElGamal for the purpose of making ElGamal additively homomorphic. To do this i raise the message to be encrypted to the ...
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2answers
181 views

Asymmetric encryption that is secure for (almost) any foreseeable future

Recently, I've been reading a couple of papers about building cryptosystems that are secure for up to 30 years. But for some applications, this seems a bit low to me. So I am wondering if considering ...
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2answers
385 views

Encryption of a number with RSA bigger than $n$

I'm having trouble understanding why encrypting a number $m$, which is larger than $n=p\cdot q$. I've looked into the following threads: Why RSA can't handle numbers above 76? Why does plain RSA ...
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1answer
63 views

Ensuring that an operation takes a relatively specific amount of time, but easily verify the result

I want an algorithm of some sort that can ensure that an operation takes a fairly specific amount of time, but proof that this operation was done can be completed relatively inexpensively. For ...
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3answers
247 views

Question on RSA trapdoor permutation

You plan to use a public-key cryptosystem based on the RSA trapdoor permutation in three different real-life applications, in which the attacker has, respectively, only one of the following resources: ...
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1answer
29 views

In NTRU digital envelope, do everyone know the hash function $H$ used?

I have a simple question about the NTRU original cryptosystem using a digital envelope. The article can be found here: http://grouper.ieee.org/groups/1363/lattPK/submissions/ntru.pdf It is said that ...
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2answers
61 views

Is this RSA signing and verification process secure?

I need to verify that messages in a communications program are from valid senders. Below is a highlevel outline of how I think the process would work. The connections themselves are done over TLS. The ...
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2answers
74 views

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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1answer
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 ...
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1answer
106 views

Is this a good communication scheme?

I'm writing a wrapper for a chat program to allow for end-to-end encryption. I'm using Python and PyCrypto to accomplish this. I'd like to use a well known library to do this, but to my knowledge, ...
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3answers
63 views

Defining Symmetric vs Asymmetric cryptosystems

I'm writing a project on the advantages and disadvantages of symmetric vs asymmetric cryptosystems. At the moment I am trying to define both systems using mathematical notation. Under the heading "...
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1answer
246 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 ...
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1answer
86 views

In what cases will RSA not work? [duplicate]

I know that there are cases when RSA will not work like when the number to feed into the system is greater than the modulus. I was wondering if there were any other cases when RSA won't work I looked ...
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1answer
379 views

where should private and public keys be stored? [closed]

I am kind of confused whether should a newly generated key pair be stored in a keystore or in just a normal plain text file? From what I read from online source, there are serveral explainations ...
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1answer
81 views

Altering the message space of Paillier

Is there any obstacle if we restrict in Paillier the plaintext values to be drawn from $\mathbb{Z}_N^*$ instead of $\mathbb{Z}_N$ as the original scheme indicates.
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2answers
413 views

Practical Attack on RSA

Currently I am designing an RSA based application, and I am thinking of how long should the key be in order to be secure against attacks. I know that RSA 4096 bit key can be recovered using Sound ...
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2answers
80 views

public key certificate on TLS/SSL

Currently I am working on a public key certificate. I got a question: Why is a public key certificate needed for TLS/SSL? Wouldn't TLS/SSL itself is enough? What is the role of the certificate?
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1answer
59 views

Is the product of two primes only factorisable by those two primes? [closed]

The question is as per the subject line. Does the fundamental theorem of arithmetic imply (prove?) that if I multiply two primes, then those two primes are the only factors of the product? I.e. 17 x ...
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1answer
369 views

What does the cipher-text overhead means?

I have two questions here. What does the cipher-text overhead means? Why we were suggested not to encrypts the plain-text itself instead just encrypts only the key (i.e random number)
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1answer
58 views

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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1answer
70 views

Are any public key algos commutative?

Are there any public key encryption algorithms that allows for arbitrary ordering of crypto operations (commutative). That is, given a plaintext "message1", the following operations work to doubly ...
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2answers
106 views

How to encrypt incomplete block with RSA as a block cipher?

I have a question about an RSA academic example. If we are using block-RSA with a block-size of 3 and we have this message: "android". It is broken into 'and', 'roi', 'd'. How to encrypt the last ...
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1answer
135 views

How long can a RSA public key be?

If I create an RSA of 4096 bits, what would be the range of the length in bytes of the public key in DER format? Also, will the private key always be exactly 4096 bits (512 bytes)? Does the public ...
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1answer
56 views

How can I prove I have a certain Facebook account by signature?

I would like to know how to prove I have certain Facebook account (or that I can login to that account).
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2answers
48 views

Certificate Path Verification of X509 Certificate

I have the trusted root certificate which I am validating using the self signed key. I also have the intermediate certificate, which i am validating using the root certificate. Now when i am ...
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1answer
51 views

How to deliver an encrypted message to multiple recipients without revealing keys

Let's say I have generated a private and a public key. I create a message and encrypt it with the public key. I deliver this encrypted message and a private key to multiple recipients. Why would I do ...
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2answers
423 views

Client-Server application how can data be encrypted so only clients can read it

Requirements We are building a system that connects multiple clients through a restful API. It allows the creation of groups of trusted clients. We need a way to store group shared data encrypted ...
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1answer
175 views

RSA: how does it work and how is it more secure than symmetric systems

Before starting the question, I know that RSA is naturally a key exchange cryptosystem rather than an antilogarithm fully implemented for encrypting and decrypting data since the length of the secret ...
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1answer
107 views

What are the potential (major) flaws in this security scheme?

It’s been a few years since I read anything involving crypto. I’ve come across this scheme and I wonder how secure it is: Alice generates an RSA keypair (we assume Alice is using proper random ...
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2answers
369 views

Homomorphic Encryption: how does the equality test on ciphertexts work?

Let's suppose we have a asymmetric crypto-system $H$ which is homomorphic with respect to some function $F$. Alice encrypts a message $m$ with her private key $e$ in the crypto-system $H$ and ...
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1answer
771 views

Is it possible to find plaintext from ciphertext if (n) and (a) are known for RSA?

I have a couple of questions pertaining to a RSA problem. I need to decipher some ciphertext and find out what the original plaintext was. Modulus: $n = 2537$ and exponent: $a = 11$ Encrypting ...
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1answer
83 views

What cryptographic paradigm is appropriate for this use case?

This is a very newbie question... I have to quickly implement a solution for our application that works like this... We have a binary executable that is given a configuration file. I need to encrypt ...
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1answer
94 views

How to choose the appropriate public (i, m) and private (j, m) keys?

I studied some encryption and decryption and I have found some very interesting problem to solve on the internet. I hope I am writing to right site - there are so many in StackExchange otherwise I ...
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1answer
381 views

Solving congruences using PARI

I'm having trouble finding info in the docs about how to solve a system of congruences. The closest I can find is 'matsolvemod' in here: http://pari.math.u-bordeaux.fr/dochtml/html.stable/Vectors,...
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30 views

What are the benefits of being able to choose (most of) the public key in a PKC?

We're developing a public key cryptosystem whose ciphertexts are actually much more long than the plaintexts (which, according to this question, is more a benefit than an issue). However, we have a ...
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1answer
26 views

In Paillier homomorphism, how do you deal with bit length? [closed]

I'm trying to figure out homomorphic encryption and would like to multiply two paillier encrypted numbers. Like so: [a].[b]=[a+b] To try this I got this Paillier.java and tried the following: <...
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1answer
37 views

Do RSA keys contain a checksum mechanism? [duplicate]

I'm new to cryptography, and I was making an experiment a few minutes ago, but I want to have some deeper knowledge about RSA public and private keys. I have a set of keys One begins with ...
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1answer
71 views

What's is the simplest possible example of using cryptography to sign a message?

I am interested in explaining to a total layman the basics of how a network can know that you are authorized without having to compromise your private key. Using an example consisting of simple ...
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1answer
149 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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1answer
46 views

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 ...
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1answer
25 views

How to run a (provably probably) secure global referendum?

The following question is extremely broad and speculative, for which I apologize, but it strikes me as exceedingly important, so I will ask it nonetheless. Suppose an event of global significance ...
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1answer
22 views

CCA on PKCS#5 using error feedback

My question is about attacking PKCS#5 with CCA using the error feedback. What we learnt in class about PKCS#5: CBC requires padding to whole block boundary PKCS#5: standard padding If ...
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4answers
200 views

How is a message digest decrypted?

Trying to get my head around digital signatures and hit a slight block. The book I am using gives the following information Device A hashes a message to create a message digest Device A encrypts ...
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1answer
117 views

Multiparty encryption and decryption

Is it possible to send a single encrypted data message to multiple users so that the users receiving the message can only decrypt certain parts of the message, depending on their rights. For example, ...
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2answers
156 views

Random Coin Flip

Quoting an old question: Consider the following protocol for two parties A and B to flip a fair coin (more complicated versions of this might be used for Internet gambling): A trusted ...
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1answer
129 views

how to get the file used to brute force any encryption method [closed]

Bitlocker as example uses AES 128 Bit encryption as default - if it was brute forced how it can be done , I mean where's the file that will be brute forced to generate the password (If I'm right that ...
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1answer
1k views

generate RSA public key having public modulus and exponent [closed]

i want to generate an RSA public public key file using openssl (or other tools) having public modulus and exponent, so i can use it later to encrypt files i have this: ...
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1answer
53 views

Key exchange resilience to DoS attack

I am designing a user registration protocol for a system where I control both clients and servers. The aim is to establish a symmetric key for subsequent authentication, while minimizing the work ...
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1answer
50 views

Can a Cramer-Shoup public key have two different private keys?

Since I am new to the Cramer-Shoup algorithm, I would like to know: Is it possible to have two different private keys for a public key due the key pair generation algorithm that Cramer-Shoup uses? I ...
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1answer
293 views

Why does rsyncrypto require a public key during decryption?

I recently came across a project called rsyncrypto that sacrifices some security so that you can have rsync-friendly files. My understanding is that it works by generating a symmetric key for each ...