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Questions tagged [symmetric]

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

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How does an S-box introduce confusion

I'm trying to find the logic for introducing the S-box, but having trouble understanding why. In a simple cipher, e.g. c = pt xor key I understand that I can ...
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0answers
19 views

What are dual codes and the codewords denoted by these dual codes in terms of trace?

I am currently reading a research paper (linked below) that mentions "Consider only maps that vanish at 0, their short codes $C_f$ and their duals $C_f^\perp$. The duals can be written as $C_f^\perp = ...
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1answer
24 views

What is the link between the parity check matrix, double-error-correcting codes and APN permutations?

I am currently reading a research paper (linked below) which mentions that a map $f:V:=GF(2^{m}) \rightarrow V$ which vanishes at 0 is APN if it satisfies the condition that it is a binary code $C_{f}$...
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1answer
42 views

What does 2-flat mean when discussing APN permutations?

I am currently reading a research paper that uses the term 2-flat. However, I don't understand what this term means. For example, the paper mentions in its definition of APN that for all distinct $a$,...
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0answers
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Prove that if a SES is statistically secure then it is computationally secure as well

Statistical security definition has a condition that the adversary is not computationally bounded which makes it stricter than Computational security but how would one mathematically prove it using ...
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2answers
62 views

Are all block and stream ciphers symmetric?

I'm trying to categorize my knowledge. Can I place block ciphers and stream ciphers under symmetric with RSA/DH under asymmetric?
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2answers
98 views

Generate one-time-pad with pre-shared key and public randomness

I know very little about cryptography, so I apologize if the question does not make sense. Let's say we design a function $F(P, X) \rightarrow K$ that takes in a pre-shared private key $P$ and a ...
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1answer
62 views

How to compare one time pad security and efficiency with AES?

It seems that the minimal size of Pseudorandom permutation (PRP) output is $128$ bits. In general, I'd like to see if there's any minimal size of the output of pseudorandom function (PRF) as well? for ...
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1answer
73 views

Can POS devices really generate random numbers?

I am doing an implementation of a system designed by a payment company and I am a little skeptical about the way they have designed the end to end cryptography. The system is as follows: There are ...
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2answers
61 views

How many keys in a Hybrid Cryptosystem?

If symmetric cryptosystems have $C^2_n$ keys, and asymmetric systems have $2n$ keys, how many does a hybrid system have? This was in an online quiz from school Company A is considering to adopt ...
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1answer
104 views

How Secure is gpg Command Using Symmetric AES-256 From Being Cracked

How secure is any data encrypted using the following command: gpg -o *encrypted_output_file_name* --symmetric --cipher-algo AES256 *file_to_be_encrypted* ...
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71 views

Computing a probability

Let $(w_1,\cdots,w_l) \in (\{0,1\}^\lambda)^l$, and $r_1,\cdots,r_l $ are chosed as uniformly random from $Z^*_q$ such that $r_1+\cdots+r_l=0$ and $q$ is a large prime number. Also, Let $F$is a ...
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1answer
119 views

Explaining Chaotic Cryptography

I will venture myself into studying chaotic cryptography. However, I find it hard to explain what chaotic cryptography entails to those that have some knowledge of cryptography. Neither can I explain ...
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1answer
71 views

Format of hybrid crypto envelope structure?

From posts including the accepted answer to How to encrypt file with RSA and AES that anyone with RSA key can decrypt them? hybrid crypto (AKA envelope encryption) a GPG-encrypted file has an ...
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1answer
46 views

Send content to owner of gpg public key, nested content symmetric encryption

If I want to encrypt a text content file using a symmetric algorithm like AES, is there a standard format gpg will expect, where it can detect a symmetric key (encrypted with recipient gpg public key ...
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1answer
52 views

PRGs exist if computationally secret encryption schemes exist

I have understood the "equivalences" between PRP, PRF and PRG. Now, I just read an statement in my lecture notes that made me think that there has to be a conversion between PRG and computationally ...
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1answer
54 views

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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1answer
309 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 ...
2
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1answer
77 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
63 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
69 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 ...
5
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1answer
114 views

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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3answers
401 views

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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1answer
52 views

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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0answers
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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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0answers
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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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CPA-secure private-key encryption scheme that is unforgeable but 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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2answers
344 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
120 views

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
50 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 ...
2
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2answers
222 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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1answer
93 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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1answer
138 views

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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0answers
169 views

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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0answers
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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
73 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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2answers
217 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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0answers
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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
56 views

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
239 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
60 views

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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0answers
57 views

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
563 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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2answers
50 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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421 views

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
196 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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158 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
47 views

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
79 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 ...