Encryption is the process of transforming plaintext using a cipher into ciphertext to make it unreadable to anyone except those possessing the key. Decryption is the process of transforming that ciphertext back into plaintext, using the key.

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El Gamal special construction

The algorithm: I have to determine if the following algorithm is secure: $p,q \in \mathbb{P}$ and $N = p \cdot q$ $e \in [2, N - 1]$ such that $\gcd(e, N) = 1$ $r_1, r_2$ are random numbers ...
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Single public key for NaCL signing and encryption?

Bob wants to use NaCL (specifically the Python NaCL library) to both a) sign some publically available plaintext for anybody to authenticate as being written by Bob and b) encrypt some other secret ...
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Combining public key signature schemes and private key encryption

I'm working on my bachelor thesis and I have some questions. I want to combine a CPA-secure private key encryption scheme and some unforgeable public key signature scheme (in my case some sanitizable ...
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Is an android encrypted SDcard readable/decryptable easily by a desktop machine?

If I encrypt my SDCard on my android phone using the built-in encryption software, and my phone dies for whatever reason, would I be able to easily by just using the pin/password to access that data ...
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60 views

Does a “commutative” asymmetric system exist?

Completely crypto noob here. I'm looking for a cryptosystem made of a func f + key1 + key2 such that they satisfy: g(f(msg, key1),key2)=msg g(f(msg, key2),key1)=msg key1 != key2 Does such a ...
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Encrypt-then-authenticate variant used in IPsec

I am trying to understand the variant of Encrypt-then-Authenticate (EtA) that is used in IPsec. For this I have looked at this chapter. What I got so far is that the integrity and confidentiality ...
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190 views

Why multiple homomorphic operations on a ciphertext leaks no information about the plaintext?

Scenario: Assume I encrypt message $m$ using Paillier encryption, so I would get $c=E(m)$. I give the identical $c$ to two different parties, parties $D$ and $E$. Party $D$ computes: $c_{D}= E(m)^{...
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Why does GPG ciphertext length differ with fixed plaintext length?

Thank you for your reply. Based on your inputs. I was investigated further, find details below Step 1) Input string for encryption is UUID format Input data = c0c6f311-a793-4d65-9374-1c0e45f5382c ...
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106 views

Are all encryption tools made equal?

I'd like to encrypt work-related data backups stored on an external drive. The backups contain proprietary company information and trade secrets. My current backup program, Comodo Backup, allows me ...
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208 views

Is simple XOR with secret key using DHE secure?

Client and server have given generator 3 and modulus which is 256 bit prime same all the time for all clients and server (simply hardcoded). They both generate a private exponent and send result to ...
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Whats the point of an IV if public? [duplicate]

What is the point of the initialization vector in cryptographic modes like AES-CBC when it is sent in the public? Cant any attacker just listen and reverse the IV? and as such defeating its purpose? ...
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Why don't use random padding in RSA?

Consider the following scenario: Sender S sends a message to receiver R. He uses a hybrid encryption scheme with RSA as key-encapsulation algorithm and AES-256-CTR (4byte-counter) and SHA256-HMAC. ...
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102 views

Device intercommunication with commands

I want to create a smart house project, and in that I bought some simple RF modules. Those modules don't support crypto out of the box but I want to connect them to a device that I would program to ...
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164 views

Modes of encryption for hard drives?

What are the currently recommended modes of encryption for large disks? Basic requirements would seem to be that you can update an encrypted sector independently of others, which rules out most ...
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377 views

Is appending the hash of the plaintext to the end of an encrypted message sufficient to ensure integrity?

Is appending the hash of the plaintext to the end of an encrypted message sufficient to ensure integrity? My reasoning is that the attacker doesn't know all the plaintext, so by having a hash, the ...
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930 views

Feistel cipher vs Substitution-permutation network - number of rounds

So I have been doing for the last couple of days comparison between Feistel cipher and Substitution-permutation network. Now I can say I know a lot about them and their similarities and differences ...
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190 views

Is it a good idea to use Lagrange/Newtonian interpolation for encryption?

I see that Lagrange interpolation is commonly used for secret sharing, but could it be used for encryption? The goal is to reduce database I/O and compute new values on the fly. Suppose the use case ...
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What are good combinations of public key algorithms or primitives for long term security?

There has been talk in literature about doing multiple encryption with different block ciphers or stream ciphers several times before, and the benefits and risks of such efforts. There may be hidden ...
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does re-encrypting the same value with multiple keys reduce security

I found myself wondering today, how much security is lost if you take a plaintext - assume that its content, including any metadata is unknown to an attacker, for example it may be random data - and ...
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Compression step of PGP

We do compression in PGP. Does this step increase/decrease or either have any effect regarding the authentication of a message?
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269 views

Derive both MAC and AES keys from same PBKDF2?

When doing password-based encryption, is it OK to create one instance of PBKDF2 from the password, and then use it to create both the AES key and the MAC key? (Or should a new instance of PBKDF2 be ...
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1k views

Determine the Key given ciphertexts and plaintexts?

Given two sets of plaintexts and their corresponding ciphertexts, how does one determine the key $K$? The ciphertexts are: $$C_1= (P_1 \oplus K_0) + K_1$$ and $$C_2= (P_2 \oplus K_0) + K_1$$ $K = ...
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Why is $-1$ an illegal message in the Goldwasser-Micali Encryption Scheme

I'm not sure why in the Goldwasser-Micali encryption scheme with a Blum integer $N$, the message $-1$ is always a illegal message. Can you give me some direction for starting? What is illegal ...
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2k views

What's the difference between the long term key and the session keys?

I'm currently studying secret key cryptography, and I've come across the terms "long-term key" and "session key". What's the difference between these two kinds of keys?
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How to compute the attacker's probability?

Given a random bit string $R=r_1r_2\cdots r_n$, let us encrypt each bit $c_i=E(r_i,k_i)$ where $k_i$ is taken from the stream of encryption keys. Suppose that an attacker can guess the correct $r_i$ ...
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648 views

Can you identify the public key used to encrypt something?

If I encrypt a string with a public key, does the encrypted ciphertext reveal the public key I used to encrypt it? Basically, I don't want anyone to know who the ciphertext is addressed to. I'm ...
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How to compare two datasets „anonymously”?

Ok, I hope this question makes some sense because I am not so sure how to word it any differently… Imagine the following situation: There are 10 defined colors (blue, orange, yellow etc.) There are ...
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Secure encrypt-then-sign with RSA

I understand that when you want to encrypt and sign data with RSA the generally recommended approach is sign-then-encrypt. However, I have encrypted data that I need to sign, to prove the author of ...
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GPG and PAR2 error correction data from the plain archive, will it compromise security?

I have the following scenario: Archives compressed with 7z, hundreds of MiB in size GPG to encrypt the archives (binary, without ASCII armor) PAR2 to create error correction data Question 1. ...
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283 views

Zero-Knowledge Challenge-Responce Protocol

Good day to everyone. I am trying to implement an e voting system (just for reference -it is not important though-it is described at the Internet Voting Protocol Based on Improved Implicit Security ...
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208 views

Does hash_df use binary or an ASCII hexadecimal representation for numbers to be passed to the hash function?

I'm implementing the HASH_DRBG algorithm. As per NIST-SP800 90 document for HASH DRBG Generation, section 10.4.1 "Derivation Function Using a Hash Function (Hash_df)", the Hash_df algorithm contains ...
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Salting Hash stored on RFID tag?

In my application, I am trying to prevent counterfeit widgets from being used on our gadgets; think HP ink-cartridges. I will be doing such with UHF RFID tags embedded in the widgets. Unfortunately, ...
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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 ...
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How do substution-permutation networks work?

I understand what's going on in higher level. But i don't understand what's really going on in page 3 and 4 of this slide. Specifically, How S0 and S1 is used? What goes in what goes out? How ...
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Can a book cipher be cracked without key?

Book cipher works this way : Choose a reference book. For each word find its position in the book. replace each word by its coordinates (ie: 2,3,4 = page 2, 3rd line, 4th word). How would ...
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Limit the length of AES encoded output to certain value

First let me be clear upfront that am not Crypto expert. I'm having a problem where I will have to use NFC card. Now given the limitation of NFC cards memory sizes, I will need to constraint my system ...
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253 views

RSA enc/decryption with multiple prime modulus using CRT

Every information I found on internet about RSA-CRT encryption/decryption uses only two primes. I'm interested in my project in doing that using multiple (up to 8) primes. The general idea is to ...
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55 views

How to compromise the privacy of NaCl encrypted messages, when nonce is reused?

I want to know to how break NaCl encryption when a nonce used more than once for a given key private key. According to PyNaCl docs: It is VITALLY important that the nonce is a nonce, i.e. it is a ...
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343 views

Deciphering “easy” ciphers without hints

I've been keen on IT Security for a long time now and I've learned a lot about networking & security. However trying a "decryption challenge" I'm lacking what I think is basic encryption/...
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174 views

How to clock a linear feedback shift register?

I have some trouble understanding what happens when a LFSR is clocked. I have tried many examples but I do not get the correct result. Can anyone point out my errors? I have the following polynomial: ...
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Attacks on WinRAR volumes reusing same password?

I have quite a few WinRAR volumes that are all encrypted with the same password. The password is long and unbruteforceable, but I recently read that WinRAR uses PBDKF2 as its key-derivation function ...
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How can recovered 5-letters plain text help me to recover reused OTP key

I have 10 cipher texts ciphered with One Time Pad (OTP) using the same key. I need to recover the key (or in other words, to recover the 11th cipher text which I assumed would require me to recover ...
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Using ECB as RSA encryption mode, but encrypted messages are unique

As I understand, ECB should not be used as encryption mode unless you are encrypting single blocks of data which are always unique and only are encrypted once. I have a collection of ids represented ...
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117 views

How to make PKCS conformation oracle for trying Bleichenbacher’s RSA attack?

I was trying to understand Bleichenbacher’s CCA attack and thought of work it out in python. Can some one throw some light on the the logic behind the oracle used to check PKCS conformation? Will a ...
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213 views

How can a good pseudo-random number generator be made?

I am trying to make a pseudo-random number generator so that I can use it in a synchronous stream cipher for encrypting plain text. I want it to generate numbers which are as random as possible. What ...
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Use of ElGamal encryption for signature generation

If RSA (textbook RSA that is) generates a digital signature by using the sender's private key, couldn't any cryptosystem (the only two that come to mind are RSA and ElGamal) capable of asymmetric ...
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Last Round in DES

I am studying DES for my Cryptoexam .As far as I understood , the initial plaintext is permuted and then the left half is called L0 and Right half is called R0. After that for each round the process ...
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How can I calculate the Rijndael SBox?

I would like to implement the Rijndael subBytes() operation using calculation instead of tables, because I like to play with this on different wordsizes, as an ...
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Indistinguishable encryptions in the presence of an eavesdropper equivalence

I'm trying to prove that definition 5 and definition 6 in this document are equivalent. This is what I've done at the moment: Asume that the scheme has Indistinguishable encryptions in the presence of ...