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

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Patent Status & Usability of AE Modes [on hold]

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
332 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
108 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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Brute Force attack on Symmetric Cryptography [migrated]

If I have a specific key length say 4 and a given ciphertext, how do I use str.matches("^[a-zA-Z0-9\-#\.\!\;\,\(\)\/%&\s]*$")) to find a key in order to get the plaintext. So far I have a class ...
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1answer
39 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
97 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
50 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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47 views

How secure are key-derived S-boxes?

I have seen some papers which show that random s-boxes aren't secure. But I have never seen anything on key-derived s-boxes. How does one even begin to attack a cipher for which the non-linearity is ...
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1answer
118 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
56 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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46 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
71 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
71 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
17 views

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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0answers
45 views

Is this a good way of storing a symmetric encryption/decryption key?

I'm creating an Android app that enables users to send the following sensitive info to other users: Name, surname and email Current user location This information is sent by User A, encrypted using ...
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1answer
49 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
111 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
37 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 ...
2
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0answers
52 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
544 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
45 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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1answer
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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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2answers
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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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1answer
43 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
73 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
912 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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1answer
46 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
60 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
73 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
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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
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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
101 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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4answers
951 views

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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2answers
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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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3answers
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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
54 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
112 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
208 views

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
36 views

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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2answers
762 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
170 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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0answers
32 views

Decrypt the encrypted message with similar symmetric key

Machine A generates a fingerprint and uses it as a symmetric key to encrypt the message. After that, the encrypted message is sent to Machine B without sending the symmetric key. In machine B, it can ...
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1answer
93 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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4answers
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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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2answers
137 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
51 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 ...