Questions tagged [symmetric]

Symmetric cryptosystems assume two communicating entities share a pre-established secret key.

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What is the use of Byte-Substitution,Shiftrows,MixColumn steps in AES, if 128 bit key is known for decryption?

I have just started learning AES algorithm. S-Box is using Galois Fields $\text{GF}(2^8)$ math behind its computation and Shiftrows & MixColumn steps also doing some math at both encryption and ...
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Which symmetric algorithm should I use for storing files in untrusted servers?

I have gigabites of files, which I have to backup. I have some servers for storing this files. But these servers are untrusted. So I need to encrypt files, then backup files. When my PC breaks down, ...
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Is this method of speeding up a symmetrical algorithm secure? (AES output joining)

I was trying to find out a way of speeding up a given symmetrical algorithm the reason and programing launguage in this case are unimportant. I have came up with the following thing: Divide data into ...
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1answer
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After ECDH with Curve25519, is it pointless to use anything stronger than AES128?

Is the following reasoning correct: After ECDH with Curve25519, the resulting shared secret will be an EC public key with a bit strength of 128 bits. This public key would then be hashed (let's say ...
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How to be sure to retrieve the good key when brute-forcing a cipher?

Imagine the case of the one-time pad. It is proved to be secure because the key could be everything. We have no guarantee that this key or that key was the one used while encrypting our data. ...
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1answer
518 views

What is the advantage of a symmetric encryption system over an asymmetric encryption system?

What are the reasons why somebody would choose a symmetric system over an asymmetric system considering its disadvantage such as a key management and a key transmission? Some people say a symmetric ...
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3answers
132 views

Can someone steal the key between 2 clients which are using Symmetric Encryption?

Can someone which is not authorized be able to listen between the conversation of 2 clients and catch the key? Because if I understood correctly one client needs to send the key through a channel so ...
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1answer
108 views

Simple Electromagnetic Analysis vs symmetric cryptography

I've asked this question here and was advised to post this question on this forum in hopes of an answer. I'm currently taking a security class and i came into this issue when writing a report about ...
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Can the key-generation algorithm always be uniform?

Assume that the encryption uses $n$-bit keys to encrypt $l(n)$-length messages. If a symmetric key encryption scheme is defined as $\Pi_{1} = ( \mathrm{Enc}, \mathrm{Dec} )$, then for every $x_{0}, ...
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1answer
46 views

Secure scheme to share symmetric key using asymmetric keys and certificates with N parties

I need to distribute symmetric AES key into number of end points which will use it to decrypt data. The AES key will be the same on all end points, it is not possible to generate unique on every end ...
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Why is this authentication protocol with a nonce and tree parties not secure?

Why is the following authentication service not secure? C⟶S: $I_C$ S⟶C: $N$ C⟶S: $E(K_{C,A}, N)$ S⟶A: $E(K_{S,A}, \{I_C, E(K_{C,A}, N)\})$ A⟶S: $E(K_{S,A}, N)$ where: C = client S = server A = ...
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Can I find the encryption key if I know the plain text and the encrypted text (DES and AES)?

If I have the plain text and its output after encryption with a key K1, is it algorithmically feasible to find K1? I am specifically interested in the cases of DES and AES encryption algorithms.
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Should text be hashed before being used as an encryption key?

Let's say I have an API that provides an encryption wrapper around another API. I give the consuming programmer the option of either passing a string or byte array as an encryption key. The byte ...
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1answer
57 views

Is the private key of a private/public key pair also a symmetric key?

I think I've got a fairly good (practical, not mathematical) understanding of symmetric versus asymmetric cryptorgraphy, so the difference between those is not the question. In a local discussion ...
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Is HMAC a suitable substitute for HKDF?

My PHP version does not include the hash_hkdf() function, and upgrading versions is not worth the trouble for my project (which is for personal pleasure, not for ...
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814 views

Symmetric cipher key size vs number of rounds. Longer key = more secure cipher?

I would have questions related to discussion under this article. Bruce Schneier answered questions regarding key size vs number or rounds: Why do you need more rounds with longer keys? And how did ...
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1answer
188 views

Is there any symmetric encryption algorithm cannot guess the key when encryption and decryption value are known?

I want symmetric encryption algorithms that cannot have their keys recovered when given plaintext and ciphertext. I tried XOR cipher algorithm, but when I've obtained ciphertext and plaintext, the key ...
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127 views

How can we encrypt a file to be just readable (and not writable) after decryption?

Is it possible to encrypt a file (e.g., a text document) such that when a user decrypts it, he/she be able to just read the file and not be able to change the content of the file? I mean, the result ...
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1answer
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What is the advantage of CFB over OFB?

I was wondering if CFB can be better than OFB in any ways. I would like to know in what cases one would use CFB over OFB.
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1answer
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Looking for a specific cryptosystem (encryption two times with two different keys and then decryption with another different key )

I am looking for a specific cryptosystem with the following properties: Suppose a User (Say $User_1$) encrypts a message $M$ with the key $K_1$ (the resulted ciphertext is $C_1$). Another user, say $...
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772 views

How can I share a number with an opponent?

Let's say I have a message (eg: Hello, world!) and I want to encrypt this message using symmetric encryption (AES-256). I am using a list of keys, where I randomly ...
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0answers
241 views

The right way to encrypt sensitive data with an user password

I am developing something like 1Password and I want to be sure that method that I use is secure. Main idea of the app is storing user passwords, and is protected with a global master password. Main ...
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1answer
240 views

Deterministic symmetric indistinguishable encryption

Are there any block-cipher modes (or symmetric encryption schemes in general) that are fully-deterministic, meaning same inputs lead to the same outputs (like for all modes with constant IV) and ...
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Is using the same IV in AES similar to not using an IV in the first place?

So if I understand how an IV works with AES, I'm supposed to generate a different IV for every message because using only a key, I will get the same encryption if the message was encrypted twice (...
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159 views

Broadcast encryption not using symmetric keys

Is there a common scheme for a broadcast style encryption that doesn't involve and exchange of a common key? I'm aware that traditionally what would be done is there would be a common symmetrical key ...
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1answer
85 views

Is symmetric encryption with a key containing the userId passes as authentication?

The scenario: There is a web application where ownership check is time expensive so during the development all users were able to access all other user's objects (let's say apples) by changing the ...
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Safely using ciphers that take small key sizes

Is it feasible to use triple encryption for block ciphers with the maximum keysize being the only significant security issue? I mean $C = E_{k_1}(E_{k_2}(E_{k_3}(P)))$ and $P = D_{k_1}(D_{k_2}(D_{k_3}(...
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1answer
125 views

What does GHASH really do?

I try to understand the GCM-MODE used in TLS. The problem I have had is described below. 1:textbook describes how GHASH works: 2: I get some debuginfo from openssl code that contain enc_key(32 bytes)...
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818 views

What is the main limitation of DES?

I know this question seems too general, not specific. Our professor asked this question today, and I have no idea how to answer it. Maybe it's the key size, maybe another thing? Any idea?
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Are sufficiently large key sizes enough to deter quantum attacks for symmetric key ciphers? [duplicate]

Are sufficiently large key sizes enough to deter quantum attacks for symmetric key ciphers such as AES?
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63 views

Establish more secure symmetric crypto system given A and B have small initial secret

Alice and Bob have a secret number X that is known only to them. However, the number is only 14 bits, and Trudy knows they share such a secret and can intercept messages between them. Instead of using ...
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1answer
944 views

If a key is used to encrypt that same key, is the computational cost to brute force it decreased?

If a key $d$ is used to encrypt a message $m$ that is identical to the key $d$, resulting in a cipher $c$ $c = \text{Encrypt}(m, d) = \text{Encrypt}(d, d)$ is the computational cost to brute force $...
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What is the computational cost of guessing symmetric keys for two ciphers that decrypt to the same input?

A message m is encrypted with two symmetric keys, resulting in two ciphers c, c1 = encrypt(m, key1) c2 = encrypt(m, key2) What is the computational cost of guessing symmetric keys for both ciphers ...
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2answers
256 views

Role of session identifier in Otway-Rees protocol

According to Wikipedia, Otway-Ress protocol is as follow: The protocol can be specified as follows in security protocol notation, where Alice is authenticating herself to Bob using a server $S$ ($M$...
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580 views

XOR and cipher stream

I am new to cryptography and I am very confused. In fact, I am so confused that I have trouble providing a suitable title for my thread... I know that a Substitution Cipher is designed to substitute ...
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1answer
117 views

How to solve rubix-cube like symmetric encryption

So for a project at University I wrote a symmetric encryption system that was very similar to how a rubix cube would work. I had each byte of my data mapped to a element in a grid, with 6 grids in ...
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249 views

Apply visual cryptography to a video stream?

I read that there are different ways to apply visual cryptography. One is to generate two images that when overlapped provide the original image. The main issue of using this approach with a video is ...
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1answer
428 views

Best method to achieve authenticated encryption with openssl

Problem I want to send a plaintext document between from myself to someone else over an untrusted link. I want to firstly stop third parties from reading the document, I want the receiver to be able ...
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1answer
546 views

Which cipher to use with openssl for shortest output?

I'd like to encrypt my 12-24 words seed for my bitcoin wallet with openssl command line tool to be able to have it with me on paper without the risk that somebody could use it Which cipher should I ...
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1answer
721 views

In BBC Sherlock's The Blind Banker, what type of cryptographic function is used by the Black Lotus?

In BBC’s Sherlock, Season 1, Episode 2 “The Blind Banker”, the Black Lotus crime syndicate uses ancient Chinese number characters to refer to pages and words in a commonly known book (which was ...
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1answer
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How to find the type of symmetric cipher? [closed]

How to find the type of symmetric cipher that was encrypted. I have ciphertext, plaintext, key. there is a command in openssl?
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1answer
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What is the general purpose of concealment algorithms introduced in…? [closed]

What is the general purpose of concealment algorithms introduced in https://eprint.iacr.org/2003/050.pdf?
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Do I need to use HMAC in this crypto wallet backup application?

I'm building an application for securely backing up cryptocurrency private keys or 12-word backup phrases that crypto wallets use as a seed to generate private keys. For the purpose of this question, ...
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1answer
866 views

How can an all-zero encryption key result in plaintext ?

In the Krack-attack paper, the section 6.3 says that a vulnerability on Android devices involves installing an all-zero encryption key when the message 3 is re-sent. In his demonstration, he shows ...
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Shortest key in symmetric algorithm used only once is unsecured?

Considering a message of n bits, one ridiculously short key of 1 bit and using this cryptographic function which performs the <...
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1answer
542 views

How does Permutation and Substitution work?

I'm a little confused on how permutation and substitution works, I've been reading about S-DES and how it goes through P-Boxes and S-Boxes but what goes on inside P or S in the algorithm? Are these p ...
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Why do we use multiple rounds of encryption?

I understand that DES and AES use multiple rounds of encryption to secure the cipher text output, what is the real reason this is carried out? Is there a certain point performing multiple rounds ...
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Is it enough to encrypt then sign when using symmetric-key encryption?

I'm doing some tests in Java, encrypting messages exchanged between two entities. I'm using a symmetric key to encrypt the messages. Right now I've stumbled upon a question. Is it enough to encrypt ...
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271 views

Encrypt password for oneself on paper

Assume the following: You have to encrypt a file (ZIP-file) containing import information. This file should be password-protected and you want to leave yourself the password encrypted (on paper, ...
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1answer
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Why does the Gameboy Advance serial encryption include the destination byte address?

The Gameboy Advance encrypts data sent over its serial port. It assembles a random number to seed a linear congruential generator. Then, it xors this PRN, the data, a symmetric key, and the byte ...