# All Questions

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### Determine safety of exercise hash functions

There have been several attacks against hash functions such as $h_{sha1}$ and $h_{md5}$. Professor A suggests a new function $h_{666}$ with an output length of 666 bits. Professor B suggests ...
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### Determine if encryption is safe against CPA

The following encryption scheme encrypts each block of length $n$ of the plaintext separately: $c_i = k_1 \oplus F(k_2 \oplus p_i)$ Where $F$ is a strong pseudo-random permutation (i.e. it is easy ...
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### How broken is a xor of two multiply with carry generators?

How broken is a xor of two LCGs? How Brittle Are LCG-Cracking Techniques? Based on the above two questions, I have simply one additional question: how broken is a XOR of two 64-bit multiply with ...
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### Collision or second preimage for the ChaCha core?

Daniel J. Bernstein's ChaCha core is an evolution of the Salsa20 core. Both are functions over the set of 512-bit bitstrings, partitioned as sixteen 32-bit words. Can we exhibit collisions, or ...
191 views

### Understanding Meet-in-the-Middle attack on block ciphers

I understand how to implement a Meet-in-the-Middle attack on 2DES/3DES, by computing for one half all possible keys and storing the outcome and then subsequently do the same for the other half but ...
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### Worst case one way function

the worst-case one way function is defined as follows $$\forall A \exists x : pr(A(f(x))\in f^{-1}(f(x)))\neq 1$$ can you give any example of such function?
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### What is Polymorphic encryption? [closed]

Well i have downloaded some cryptographic algorithm. Do there are those I didn't understand so this is my question: What is polymorphic RC4 or Polymorphic xor encryption? I heard this in malware ...
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### Eavesdropping experiment with private key

I am extremely new to cryptography, still trying to learn basics. I got stuck in the following example of an eavesdropping experiment with a private key. \begin{array} {|r|r r r|r|} \hline C & ...
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### SQL-Like queries in CRYPTDB doesn't work [closed]

I have downloaded and built Cryptdb and it works well. Most of queries on encrypted database run without any issue but, the query with LIKE key word receives an ...
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### How are subkeys, in Feistel block ciphers, derived for each round?

I'm bit confused about the subkeys that are used in Feistel ciphers. I know that all the subkeys ki are derived from the main key K, but how? Say I have a simplified Feistel block cipher of i rounds, ...
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### Is secure communication without public-key crytography feasible?

We all know about public-key cryptosystems, and we know some of these are computationally secure (for the time being, of course). Are there any algorithms out there which allow safe communication ...
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### Cardinality of the key space $\{0,1\}^\ell$

Imagine we have a scheme whose key is chosen from the set $\{0,1\}^\ell$. My question is: What is the size of key space for this scheme?
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### 2DES vs. two-key triple encryption, with no decryption step

So studying double encryption (double DES) and understanding why it is subject to a Meet-in-the-middle attack, I tried to see if the same attack could be applied to a triple encryption with only two ...
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### What is search space of this problem? [duplicate]

I have an exam and i have no time to solve this problem(I think about it a lot but i do not solve it).please help me and solve it for me: $A$ is a random number and has $512$ bits.We make $B$ ...
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### What is “witness encryption”?

I recently skimmed over tho papers on time-lock encryption: “Time-release Protocol from Bitcoin and Witness Encryption for SAT” by Liu, Garcia, and Ryan “How to Build Time-Lock Encryption” by Jager ...
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### Relationship between an elements order and the DLP [closed]

How can I use these properties to attack the DHKE? I know that the order of $a$ is always $2$ for $a = P - 1$ in $Z_p$. The subgroups generated by a will be $\{1,a\}$.
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### Turning a 64 bit block cipher into a 128 bit block cipher

There are quite a few block cipher modes of operation that require 128 bits. There are also modes of operation where a higher block size than 128, e.g. a block size of 256 bit would even be practical. ...
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### 3DES over Galois Counter Mode (GCM) for authenticated encryption

Is it possible to use DES or 3DES over Galois Counter Mode (GCM) to provide authenticated encryption? I haven't seen any implementation yet.
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### Encrypt a file for multiple persons with different keys?

I thought a problem and want to ask if there's a more elegant solution. The scenario: A company with a fileserver, which hosts encrypted documents. The files need to be accessible by about a dozen ...
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### 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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### Concerns about use of MAC in my specification

Here is the pseudocode for a routine I am designing: ...
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### What are the differences between collision attack and birthday attack?

From my understanding both types of attack, collision and birthday, are based on the principle of two randomly/pseudo-randomly chosen plaintext to hash to the same value. I don't want to launch any ...
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### what are the advatanges and disavantages of the digital signature algorithms for JWT?

I am using a Java JWT library jose4j. There is a list of digital signatures algorithms which can be used. HMAC using SHA-2 RSASSA-PKCS1-V1_5 Digital Signatures with with SHA-2 Elliptic Curve Digital ...
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### Encryption(AES) and verification per message

I want to send/receive messages with A (an peer) using TCP socket. I want to encrypt messages using AES CBC. Only I and A know a symmetric key K1 used in AES CBC, and K2 used in HMAC. The size of each ...
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### Compromised Issuing CA [closed]

I'm configuring a PKI infrastructure with an offline root CA and several issuing CAs. Among other topics, I'm struggling to decide how the revokation of an Issuing CA certificate works. My Root CA ...
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### Why develop Edward curve formulas that deviate from unification?

Edward curves were considered initially because they provide a unified formula for both doubling and addition, thus having inherent side-channel resistance. But a lot of work has been done recently ...
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### is this possible to use asymmetric encryption schemes as order preserving encryption

Are we able to use order preserving encryption in asymmetric schemes, is this work efficiently. is this secure at any one of the situations
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### How can complexity be increased or decreased in AES?

I have been studying data compression for a while. For educational purposes, after a lot of reading, I managed to create a software that performs encryption and authentication using AES256-GCM. ...
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### Voting protocol - How many dishonest tallying centres can the protocol cope with?

We have the voting protocol that is described here. I am looking at the following exercise: Determine how many dishonest tallying centres the protocol can cope with before the following properties ...
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### Establish Trust By Signing Random Seed

I'm trying to get my server to establish trust with a client. Neither is on the Internet, so there are no certificate authorities. The typical way to establish trust in my domain is for a client to ...
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### What does the acronym “CRT” stand for in the PKCS #1 specification?

What does the acronym "CRT" stand for in the PKCS #1 specification? Does it stand for "Chinese Remainder Theorem"? Thanks, Pete
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### What's the structure of server-key-exchange message during TLS handshake

Here is a relevant question: With OpenSSL and ECDHE, how to show the actual curve being used? Using ...
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### Proof that two parties can communicate

Suppose we have three parties, Alice, Bob, and Carol. Alice can receive messages from Bob and Carol individually, but can only send messages to both simultaneously. Additionally, Alice cannot tell if ...
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### Can I use an ECDH Shared Secret from the same Private / Public Key Pair?

I'd like to know if using the ECDH shared secret of a static EC Private Key with it's own corresponding static EC Public Key causes a problem / weakness. (edit) not asking if it's ok to re-use the ...
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### How HMAC.Update function works?

I know how standard HMAC(key,msg) function works, and I want to write a Delphi port of HMAC class that implements Update function. The problem is that I don't understand how this Update function works ...
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### SHA512 faster than SHA256?

I'm getting this strange result that SHA512 is around 50% faster than SHA256. I'm using .net's SHA512Managed and SHA256Managed ...
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### What is difference between Cryptographic Transformation, Encryption and Hash? [duplicate]

I read in a book that: let $f(K,R)$ is cryptographic transformation of $R$ using $K$, $K\{R\}$ means $R$ is encrypted using $K$ and $h\{R\}$ is the hash of $R$ and then the example follows. I want to ...
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### How to generate own secure elliptic curves?

I know that the algorithm used to generate the Brainpool curves and the NIST curves is published. The algorithm should be this one (RFC5639 Appendix A). From what it looks like it's rather slow to ...
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### EC: Why does $h>200$ need to hold?

From BSI-TR-03111 (PDF), on page 15: The class number of the principal order belonging to the endomorphism ring of E SHOULD be at least 200. This value commonly is referred to as $h$ in that ...
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### What are the differences between the elliptic curve equations?

I think we're all aware of the "classical" Weierstrass (short?) elliptic curve equation: $y^2\equiv x^3 + ax +b \pmod p$. Well known examples of these curves include the NIST's and Brainpool ones. ...
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### Period of pseudo random sequence generated from (5, 2, 0) LFSR

I was reading about the Linear Feedback Shift Registers and I am confused about the technique to find the period of a primitive polynomial. Consider the polynomial $x^5 + x^2 + 1$. As this is a ...
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### Brute-force attack given multiple hash prefixes

(Context: I'm auditing some code which I suspect to be insecure, but I'd like to be able to quantify this.) Suppose you have a 56-bit secret key (\$secret), and ...
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### Securely start a client server connection without ssl/tls [closed]

NOTE: Pardon my ignorance of encryption, I learned about these concepts today Going to rephrase this question, but I think I already have fine answers provided. I need to have a block of data ...
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### Compression in Symmetric-Encryption?

When using symmetric encryption is it important to compress the data first? I think that compression will make data look "more random" and this might help the ciphertext be harder to crack but I am ...
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### Digital signature in a group chat scenario?

I understand that RSA is great for 1-to-1 digital signature. In the group chat of N people, other than storing and maintaining N public keys, is there a simpler way of authenticating each and everyone ...
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### Get AES key from client request

I get a request from a client who encrypts the message with the wrong AES key. Is there a way for me to know what AES key the client actually used? I understand I may get the decrypted msg which is ...
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### How to quickly detect incorrect password in encrypted file without compromising security?

I am developing a software to encrypt/decrypt files/streams using symmetrical encryption algorithm. During encryption phase data wil be compressed and encrypted. When decrypting data, I want to be ...