Questions tagged [symmetric]

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

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One-time pad question (Old ciphers) [closed]

The plaintext SECRETIVE is encrypted using One Time Pad with key DDDDDDDDD. Which of the following is TRUE? The ciphertext will be the same as using Caesar cipher with key of 4 The ciphertext is the ...
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336 views

Security of AES after key expansion 4-MB with ECB and CBC

My question is, how does the security depend or change if I encrypt with AES-128 in total 4-MB of Data? With Electronic Code Book Mode and Cipher Block Chaining Mode ? Does an adversary still ...
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1answer
116 views

Detect MITM with hashed session-key — Is my protocol secure?

I have the following setup: An app ("client") wants to communicate securely with a device I created ("server"). The client has the public key of the server (and a hashed password) preinstalled. I use ...
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1answer
76 views

Can a message be encrypted with BOTH mono-alpha and poly-alpha ciphers?

For my class assignment, I was given this text; Qpuvu vo vv stazpuykzh Hhycmlmv asb. Ad dhyaqva wh uyo uzb jpwww Avlw. Tkpjllfdmbp zoatz vzjavfvna ZLHqh. Amva luy yyvlz yu bhk zua zlhvvalaz yow ...
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1answer
79 views

Change a secret key in an embedded device

There is a device that uses a secret key Sk to encrypt data. Assume this Sk is compromised. Could the following scheme be considered safe, in case we want to change the compromised Sk: We store some ...
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Combining classical attacks and quantum cryptanalysis

I'm regularly reading that AES-256 is secure against quantum computers because Grover's algorithm will only halve the key strength (i.e. "only" require $\sqrt{2^l} = 2^{l/2}$ operations instead of the ...
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In what cases might PRESENT be implemented as encryption-only?

In the paper, under section 4.1 Goals and environment of use we read: In applications that demand the most efficient use of space, the block cipher will often only be implemented as encryption-only. ...
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Does repeated xoring of the (same) key K lower the entropy of K?

If a symmetric encryption key $K$ has been generated by some recommended (NIST for example) RBG then we should have a key with high entropy. But if we xor other bit-strings (e.g. IVs, other keys, etc....
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Why are matrices so common in symmetric encryption?

Matrices have been used in symmetric ciphers since the Hill Cipher (before?) all the way up to modern ciphers such as Twofish and AES. I understand matrices can be invertible, therefore making them ...
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Patent Status & Usability of AE Modes [closed]

Post Structure: Sources Cited Question & Details Before You Answer Useful Info From Comments Sources Cited: https://blog.cryptographyengineering.com/2012/05/19/how-to-choose-authenticated-...
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1answer
251 views

Unforgeable CPA-secure private-key encryption scheme that is not CCA-secure

This is exercise with 4.26 in Katz and Lindell's introduction to cryptography 2nd ed. The question is: Show a CPA-secure private-key encryption scheme that is unforgeable but is not CCA-secure. ...
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345 views

Is there a symmetric cipher that is explicitly designed to be slow?

I am trying to find a symmetric cipher that is explicitly designed to be slow. I realize that this is generally not desirable, but I'd like many cycles and lots of electronic action. I could modify ...
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3answers
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Could ANY cipher serve as the foundation for a public key cryptosystem?

My reasoning goes like this: In principle, any trapdoor function can serve as the foundation for an asymmetric encryption scheme. All ciphers are trapdoor functions, since it's easier to encrypt ...
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1answer
63 views

Hash ratchets with truncating hash function

I'm working on a forward secret messaging system that relies on hash ratchet. I'm using BLAKE2b as the one-way function, which by default produces 512-bit digests. These digests are truncated by my ...
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2answers
362 views

Decrypt Base64 Encoded Monoalphabetic Cipher

I'm attempting to decrypt a body of ciphertext which has been encrypted using a monoalphabetic cipher. The trouble I'm having is that the plaintext was base64 encoded before being encrypted. In this ...
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130 views

Oblivious Encryption: Oblivious Transfer Based Encryption

I’m aware of a couple of OT protocols, most of which use Public Key Cryptography, Key Establishment (or similar constructs) as the base. I don’t really have specific knowledge on other OT protocols, ...
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Why do block ciphers use key schedules instead of round constants? (Even-Mansour)

Let's take AES as an example. What would be wrong with just having a 256 bit key that you XOR into your input and then XOR into your output? No key expansion at all. I believe it's even known as the ...
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What was the BassOmatic cipher, and what made it so weak?

According to Wikipedia, this homebrew cipher was originally used in PGP, before Phil Zimmermann replaced it with IDEA. Supposedly, insecurities in the algorithm were pointed out to him, leading to ...
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159 views

Multi-Party Offline Key Exchange Using Public and Private Keys

I hope the title already explains the problem I have in mind; but let me lay out some more details. Given public-private key pairs $P_A, P_B, P_C, ... P_Z$, (first $3$ of which are owned by our usual ...
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1answer
78 views

Non-derivable public key asymmetrical encryption? [duplicate]

The need we have is to pass encrypted messages but such that the encryption key can't be generated from the decryption key. This rhymes with vanilla asymmetrical crypto, but in every schema I've seen,...
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420 views

How is a ciphertext encrypted using random numbers decrypted using a symmetric key algorithm

We use true random number generators as a source of keys for encryption. What I can not wrap my head around is: How is this cipher text decrypted in case of symmetric key algorithm as we need the same ...
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Symmetric encryption with partial knowledge of key

While reading Laurent Bussard's thesis, Section 4.5.1, p91 (p119 of the pdf) I wondered about the following paragraph (emphasis mine): the prover first picks a random symmetric key $k \in_R \{0, 1\}...
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1answer
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Trivial Broadcast Encryption Scheme for video conferencing how well does it handle users joining or leaving a call?

Consider the trivial Broadcast Encryption Scheme for video conferencing, where each user shares a long-lived key with the broadcaster, and the session key used to encrypt a multicast message (or ...
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2answers
329 views

Encryption using static AES256 symmetric key vs ephemeral session key

For a hackathon I am designing an information exchange system for healthcare. Healthcare data is stored on unsecured systems. I want to assure confidentiality, integrity and authenticity of this data ...
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1answer
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Using AWS KMS to manage secret keys

For my use case, I need to maintain a secret key with the client to generate and verify HMAC for requests. We can have multiple clients and we can not make assumptions such as a client's server will ...
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Sharing salt for deterministic symmetric key derivation

I have a requirement to generate and share unique symmetric keys for a large numbers of small data objects. In many cases the storage and retrieval of the key for each object will be a significant ...
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3answers
590 views

Understanding MACs and why cleartext is sent over

Assume both Alice and Bob have a symmetrical key, let's call it $k$. They agree on a cryptographic hash function, let's call it $\mathsf{HASH}$. Now Alice wants to send an authenticated message. She ...
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55 views

question on encryption of multiple distinct messages

When I study symmetric encryption, it is quite straightforward to understand the following: in the presence of eavesdropper, simply using a PRG $G(k)$ and XORing multiple messages won't provide ...
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Why does Telegram roll its own crypto?

Telegram recently updated its symmetric-key encryption scheme MTProto, which seems to fix some known vulnerabilities. Nevertheless, I still do not understand why they roll their own crypto. What ...
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1answer
379 views

Can non-repudiation be ensured without public-key cryptography?

Message authentication is ensured by using HMAC, which can be thought of as a symmetric-key construction. Source authentication in multicast communications can be guaranteed using TESLA, which is ...
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219 views

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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1answer
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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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1answer
88 views

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
1k views

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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2answers
93 views

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
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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
122 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 ...
3
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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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2answers
139 views

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}, ...
2
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1answer
37 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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1answer
142 views

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
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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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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
169 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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2answers
122 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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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 $...