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

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Why does Merkle's Puzzle requires Eve quadratic complexity of effort to break the system?

The way Applied cryptography 2ED explains the puzzle is as follows (I paraphrase it): Bob generates 2^20 messages of the form x,y where ...
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Creating an SPN cipher from ChaCha20 or Salsa20's double round function

Given that ChaCha20's building blocks have been also used to construct a hash function, can the double round function from either ChaCha20 or Salsa20 be used to construct a secure SP-network cipher? ...
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How would you name and/or describe this algorithm?

First up: this is not a decoding request or anything. This is about terminology. Twice per year, we email another department to ask for a new licence file for their software. They are always slow to ...
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Are collisions in AES-128 theoretically possible when the plaintext is the same? [duplicate]

Is it theoretically possible to find $P$, $K$, and $K^\prime$ such that $E_K(P) = E_{K^\prime}(P)$ where $E$ is AES-128 and $K \ne K^\prime$? I'd imagine this would be infeasible, but is it actually ...
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end-to-end encrypted chat app with symmetric + public key encryption, TLS and cert pinning

A big drawback of end-to-end encrypted messaging services is that, as soon as you lose your device, you lose all your messages. Your private key is gone after all. I was wondering though, wouldn't it ...
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120 views

(Lightweight) Multicast one-way Authentication

Problem: I'm thinking about a lightweight solution to provide source authentication (only one source) to multiple receivers (multicast message). Context: Taking the problem to ground, we can think ...
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34 views

Hiding the identity of a party within the Kerberos authentication scheme

In the Kerberos authentication protocol, as described here: would it be safer to replace step (1) with: $$ A \rightarrow T : A, E_{K_A{_{T}}}(B, N_A) $$ so that a passive adversary does not know ...
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117 views

How to break this PRNG encryption scheme? [closed]

Suppose we have the following symmetric algorithm: key = random bits G = PRNG(seed: key) input = [0,1,1,0,1,0,0,0, 0,1,1,0,1,0,0,1] // "hi" in ascii The key can ...
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Common pitfalls of session key activation?

Assume that initiator $I$ and responder $R$ of a key agreement protocol have agreed on new symmetric session keys $K'_{auth}$ and $K'_{enc}$, e.g. by way of Diffie-Hellman and key derivation, possibly ...
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285 views

Does MAC provide origin authentication? Why not just use symmetric encryption?

Let's assume Bob and Alice has a secret key for MAC. If MAC is constructed by the key and the message, that means that both Alice and Bob can construct the exact same message and MAC. So, Bob can ...
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AES GCM symmetric encryption of data at rest in c#, ciphertext much longer than plaintext

I've written encryption/decryption routines using AES GCM. Code can be found here. I recently realized that the cipher text is longer than I believe it should be. It should add the tag and IV onto ...
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Theoretical security of bit rotation for encyption

This is a theoretical question. I have a idea, encryption by bit rotation in byte. Do not know if something like this already exists. General concept is: First n bytes is rotated to left or right ...
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110 views

How much pre-hashing a password increase security against brute force attack?

I am wondering how much hashing a password used as key for a symmetric key algorithm helps preventing brute force attack on crypted text. Let's assume I want to crypt text A using key B using ...
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1answer
287 views

Number of keys for a monoalphabetic cipher

My teacher stated that… The number of possible keys (E) in a monoalphabetic cipher is $26$! My thinking: Every letter in the alphabet can be $25$ different letters hence it the number of ...
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92 views

How does the key size per data bit influence the security?

The likelihood of breaking, for instance, an AES-128 cipher is 100% after $2^{128}$ tries in brute force, meaning that I've got to try $2^{128}$ keys to certainly break it. What if I (hypothetically) ...
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517 views

How do I calculate the maximum plain text length allowable for a certain cipher text length?

I am encrypting and storing sensitive values in the database. I have a set max length in the database, but in order to provide useful user feedback, I'd like to know the max input I should allow for a ...
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54 views

NIST HMAC Test vectors

I am searching test vectors of the HMAC-SHA256 primitive to verify the correctness of an implementation. Without hesitation I go on the NIST website to see what they offer. While reading the ...
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177 views

Using the same symmetric key in both directions?

A simple symmetric encryption scheme uses the same key, derived from a password, for both directions of the communication. Is this bad practice, and if so what should be done about it? Is it 'good ...
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137 views

Are password salts equivalent to keystream?

I was reading up on keystream when I came upon the definition of it: A keystream is a stream of random or pseudorandom characters that are combined with a plaintext message to produce an encrypted ...
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81 views

Key cracking: Prefer many small texts over one large text?

If an attackers sets out to crack the symmetric key of e.g. AES-CTR, would they prefer to have access to many small cryptotexts or one large cryptotext? I.e. is it more interesting for the attacker to ...
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227 views

The relationship between the key length and encrypt time in Xor algorithm?

I did work on the encryption algorithm and the decryption using the XOR method noticed that when more the key length, the less time spent on encryption and decryption. I have two questions in ...
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Messages exchanged between Bob Alice are encrypted safe?

Can you help me understand the following reasoning? If Alice sends Bob a message and that message is encrypted with two keys simultaneously: a symmetric key (Ks) and Bob's public key. The symmetric ...
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Using IV buffer after altered inside a Rijndael CBC Encryption/Decryption process as IV for next message?

When sending a block to be decrypted or encrypted, with RijndaelCBC, we input the data to decrypt/encrypt and an IV for syncing and to prevent identical outputs for identical inputs. This question is ...
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128 views

How can I protect against the failure of a block or symmetric cipher?

Can I protect against the failure of a block or symmetric cipher by chaining different techniques together? If so what implementation details should I be aware of? Are some combination of ciphers ...
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79 views

Perfect Secrecy in Symmetric Key Systems

For symmetric key systems does perfect secrecy imply that all ciphers in the cipher space are equiprobable? I would think not, but I am finding it hard to come up with a counter example.
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What are the practical limitations of Asymmetric encryption vs Symmetric encryption? [duplicate]

I hear it commonly said that the advantages of Symmetric encryption are speed and no limit to the amount of data that can be symmetrically encrypted. I also hear it commonly said that the advantages ...
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172 views

DH and PKI for KeyExchange

My question is fairly simple. I have more than two nodes which needs to communicate very efficiently from computational point of view. One of the nodes can become a coordinator between the nodes. ...
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233 views

Why have 4th and 5th steps in Needham-Schroeder Protocol?

Why have 4th and 5th steps in Needham-Schroeder Protocol? It is said "These steps assure B that the original message it received (step 3) was not a replay.". But what is a replay here? And I don't ...
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464 views

Is symmetric key encrypted with server's public key secure

I'm trying to implement lightweight yet secure protocol for communicating two trusted parties with each other. Consider following scheme: Alice wants to establish secure two-sided channel with ...
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369 views

brute force attack on KDF vs KEY

if we believe 256bit keys are secure in AES against any brute force attack, is that possible we use a KDF (convert user's weak entered password in truecrypt into a fixed AES key) with too much round ...
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115 views

Question about use of IV in this cipher

Main cipher objective : Generate a keyed CSPRN stream, that does not expose useful information about the state of the CSPRNG, and xor this with the plaintext. To prevent an attacked who could somehow ...
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Encryption strength when using a PBKDF

Suppose: $m$ is a positive integer. $n=2 \times m$. $H: \{0,1\}^{*} \rightarrow \{0,1\}^n$. $p$ is a password. $s$ is a salt. $K=\{0,1\}^n$ is the derived key. $PBKDF(p,s)$ derives a key from $p$ ...
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Is it safe to use the same IV for multiple encryptions in AES CBC mode when you are always encrypting random bytes?

I am generating random bytes (16 or 32 bytes at a time), and I want to encrypt them using a 128 bit AES key in CBC mode. Am I really weakening this horribly if I am reusing the same IV and key each ...
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115 views

What would a backdoor in symmetric key cipher look like? [duplicate]

Pretty much what the title says. DES was suspected of having one, but I couldn't imagine what it could possibly look like if only the NSA knew how to use it. Dual_EC_DRBG's backdoor is asymmetric, but ...
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47 views

Repeating something encrypted and non-encrypted?

If one wants to keep the receiver's name non encrypted, but it also appears in the encrypted message - will it leak information? (other than the receiver's name, of course.) Let's assume a "bad" case ...
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block cipher algorithms with variable block lengths

Rijndael supports block lengths of 128, 192 and 256. AES does not but Rijndael does. What other algorithms support variable block lengths? Or is Rijndael unique in that regard?
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Condition on Vector Boolean Function to be Bijective

Suppose the vector boolean function be $$\begin{align} f:F^n_2 \longrightarrow F_2^n \\ (x_1,\dots ,x_n) \longrightarrow (x_2,\dots x_n,g) \\ \\ g:F^n_2 \longrightarrow F_2 \\ (x_1,\dots ,x_n) ...
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441 views

Are SSL/TLS used for maintaining data confidentiality?

In SSL/TLS, we use symmetric and asymmetric keys to encrypt the data. Does that mean that the keys are used to encrypt the data (preserve the integrity) or provide a secure channel for the data to be ...
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Game with symmetric key

Alice and Bob are playing Rock-paper-scissors. Alice chooses $a \leftarrow\{stone, paper, scissors\}$ and a nonce $R_A$ used as symmetric key for encryption $$A → B : A, R_A(a)$$ Bob chooses $b ...
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What operations are used in symmetric cryptography and why?

In continuation to Why are bitwise rotations used in cryptography?: what other operations are commonly used in symmetric cryptography? Are there particular operations on numbers that are good for ...
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Prepend information about encryption implementation to the stored data

I want to preface this by saying i will not be using any of this code/information in a live project, this is only for learning/fun (so I welcome some speculation) I was looking at how the PHP crypt() ...
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37 views

Adversarial Indistinguishability [closed]

I am trying to devise an encryption scheme such that an adversary can win the following game with probability at most $1/2 + \epsilon$. Adversary gives challenger two plaintext messages $m_1$ and ...
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Cryptography of numbers

Is there any algorithm capable of encrypt securely (symmetric) N numeric digits in a numeric message of M digits (N < M)? I am especially interested in a specific case in which N = 10 and M = N + ...
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141 views

Session key Exchange, some doubt

I have to find a solution to this problem: I have a network composed by 1 server and some client. The server has a couple of keys (public and private) and it shares a secret key with each client. My ...
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1answer
289 views

Encryting data with password for local storage using libsodium

I want to encrypt data that is stored on the hard disk using a user-entered password. For this I look into the sodium docs and all I can find for symmetric encryption is authenticated encryption. ...
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Ephemeral Diffie-Hellman in a symmetric crypto context

I'd like forward secrecy in a case where the two sides have a shared long-term symmetric key. I intend to use AES in GCM. It looks like ephemeral Diffie-Hellman is the best way to generate the session ...
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Are there any quantum-resistant symmetric encryption schemes?

It seems that quite a few currently available encryption schemes will possibly be broken by quantum computing. Are there any symmetric encryption schemes that will remain unbroken (either because of ...
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Symmetric mutual authentication with client using a derived secret

I'm attempting to find a client/server authentication protocol that allows the client and server to authenticate each other when the client doesn't know the server secret but does have a sensitive key ...
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130 views

Secure symmetric encryption algorithm for any-size base62 data

I am searching for a secure algorithm to encrypt base62 (or any other base) data for ids in urls. It should feature: No blocksize etc, limiting the length of the message to a factor of n If you ...
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121 views

Basic question about design of program with data encryption

I need to store encrypted files on a hard disk. The actual code side is not important for me now (I plan to use C# and AES-GCM encryption from Bouncy Castle library, please tell me if you think that's ...