# All Questions

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### Truncating ciphertexts on ring-LWE schemes

On the section 5.4 of the paper Improved Security for a Ring-Based Fully Homomorphic Encryption Scheme, the authors explain how to discard some bits of the ciphertexts to get smaller ciphertexts and ...
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### Why do $\alpha$ and the private key value in diffie-hellman should be from $2$ to $p-2$

I just started learning Diffie-Hellman Key Exchange. I couldn't get the reason of making $\alpha$ and private key for Alice and Bob constrained between $2$ and the prime number generated minus $2$ ...
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### Why do stream ciphers use a nonce?

My question maybe will be stupid, but my problem is that I do not understand why stream ciphers need a key and also a nonce. As far as I understand, the keystream is generated with the nonce. The same ...
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### how to calculate maximum data size of an RSA signature

I am reading a content, and am not sure if I follow how they come up with the maximum data size which can be signed by an RSA key. The author states: "...if you are trying to sign a message using a ...
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### How were semagrams encrypted in the pre-digital era?

Historically messages in languages that use alphabets have been encrypted manually according to some kind of algorithm (e.g. mono- and poly-alphabetic ciphers). But how wew messages encrypted in a ...
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### Is there any reason not to use EdDSA with Weierstrass curves?

I'm volunterely working for a crypto library and we're planning on adding Curve25519 support (finally). At the same point I had the idea of adding support for EdDSA in the same run. Our library is ...
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### Confused about how chosen ciphertext attacks work?

When performing a CCA, the adversary gets the public key. At some point, he will send a message m to the challenger, and the challenger will send back either $E_{pk}(m)$ or $E_{pk}(r)$, where r is ...
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### Can Pseudorandom values be used as secure keys?

I want to derive $n$ keys from a master key. I pick a master key $mk$ and use pseudo-random function (PRF) to generate $n$ pseudo-random values, $v_i=PRF(mk,i)$ where $1\leq i \leq n$. Next, I use ...
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### Why are the first 4-6 characters from ssh-keygen (RSA) the same everytime? [duplicate]

New to StackExchange, so please bear with the newbie. At my current job we are often creating many ssh keys with ssh-keygen (RSA). One thing I have noticed is that the first few characters (say the ...
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### Will entropy be lost by using a DRBG?

Lets assume we have a DRGB (deterministic random bit generator) which is seeded by a good true RBG (random bit generator). Before any bit has been read from the DRBG, the entropy is clearly the number ...
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### Message recovery using CBC

Take for example that alice has a message m that is 412 bits long, which she encrypts using AES-128 (which has a 128-bit blocksize) in CBC mode with a symmetric key k to produce ciphertext: c = Enc k ...
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### How about a homomorphic integer sorting in a MPC context?

I want to implement the ATV-FHE scheme as described by Adriana López-Alt, Eran Tromer, Vinod Vaikuntanathan: On-the-Fly Multiparty Computation on the Cloud via Multikey Fully Homomorphic Encryption ...
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### What are the roles of the simulator in simulation based proofs

I'm trying to better understand simulation based proofs in the UC model - but the guidelines to construct a simulator confuse me. To my understanding, the simulator is activated in two ways: ...
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### Brute force attack multiple stream cipher

Assume that a scheme used to secure a specific system $Π=(Gen,Enc,Dec)$ which is thought have indistinguishable multiple encryption in the presence of an eavesdropper. For any adversary $A$ that can ...
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### AES 256 Encryption - Is it really easy to decrypt if you have the key?

So this might sound like a crazy question but bear with me for a minute. I can't find any info on the internet and so am here, although this might have been a good place to start. I've recently ...
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### How to verify a signature with openssl methods from the ECDSA class?

I'm trying to verify my signature like I explained in my question before. But I have some problems to program my own verification without the verify methods of openssl. Therefore I'm going on to use ...
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### How does non-linearity (S-box) add security to AES?

I'm trying to understand how AES guarentees security. One of the named points on the web is that AES uses a non-linear step (the SubBytes step). But how exactly does the Rijndael key schedule add ...
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### Key Derivation Function (KDF): Can a key derived from KDF be considered as a secure key?

Consider a case where we have a master key MS that is used in pseudo-random function to generate a set of pseudo-random values. Then we use key derivation function to derive a key from each of ...
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### how do I implement identity-based signature scheme using PBC library [migrated]

i want to implement cha cheon id base signature scheme using pbc library....i'm very new to pbc library can any one help me.. and i'm very new to stack exchange please forgive me if I post anything ...
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### Trying to determine what encryption method was used [closed]

I have a database I'm trying to decode. The database is an 1997 Microsoft Access database, and the program itself was written around 2001 in Visual C++. The database is used for decoding VIN numbers. ...
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### Implementing RSA in C# when Public Key is Known [migrated]

I'm trying to implement an RSA encryption function, but I'm new to cryptography. I know the public key and have data to encrypt, but I can't get the implementation correct in C#. I've tried using: ...
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### What followed findings of A. Lenstra et al. concerning shared factors of practical RSA moduli?

A. Lenstra et al. had a paper in 2012 "Ron was wrong, Whit is right", in which one reads: "What surprised us most is that many thousands of 1024-bit RSA moduli, including thousands that are contained ...
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### Taking half of a X bits hash key as secure as using a X/2 bit hash function?

This question comes from a (nitpicking) comment by me in this SO question. I said that using the half of the output from a 256 bit hash function is worse than using a 128 bit hash function. My ...
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### How to use a stream cipher to sign a message?

Are there known constructions which allow to sign a message M (let's say it will be sent as clear text) using a stream cipher C and a symmetric secret key K? So, we have M, C, K and we want to send ...
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### Help defining a shift cipher mathematically

For an assignment question, I am trying to prove formally that double encryption with two keys by the shift-cipher encryption function results in a shift cipher as well. If the Shift-cipher encryption ...
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### How to calculate $v_k(s)$ in Quadratic Arithmetic Program?

In the paper Pinocchio: Nearly Practical Verifiable Computation, there are some polynomials $v_k(x)$, $w_k(x)$ and $y_k(x)$ representing the structure of there circuit. However, as in the "concrete ...
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### David Chaum's anonymous credential

I am a newbie in cryptography (I know the basic RSA, AES stuffs) and am interested in anonymous authentication systems. Looks like David Chaum is the godly figure in the field. I see lots of people ...
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### What's the perfect chain length : number of chains ratio for rainbow tables?

I'm building a SHA-1 rainbow table to crack basic passwords (i.e. up to 10 character, 0-9,a-z). But I can't seem to calculate that golden ratio that would give me the most coverage. Also is there ...
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### Degree of M and Max Sequence Length for LFSR

For the LFSR mod 2, determine the degree of m and maximum sequence length s_(i+3)≡ s_i+s_(i-2)+s_(i-4) mod 2 determine the degree of m and the maximum ...
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### Miller Rabin - Error probability of .5 a possibility?

I'm testing the property of Miller Rabin that the error probability is at most 1/4 when only a single base a is chosen and we iterate only one time. We are testing odd integers 90,000 to 100,000. ...
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### argon2 versus bcrypt/PBKDF2 password hashing security gain

I wonder if it can be approximated how much of a security margin the new argon2 hash, winner of the password hashing competition, can give over bcrypt or PBKDF2, for an attacker using large GPU ...
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### Does CP-ABE have any disincentive to share secret keys

I want to use a CP-ABE scheme to encrypt data to be accessed by multiple unknown (at time of encryption) users. Reading the papers related to CP-ABE, there does not appear to be any disincentive for ...
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### Multiple levels of encryption

Is it feasible to have data encrypted at different levels? I'd like to encrypt an xml database (such as used in www.keepass.info) whereby two keys can exist: Key #1 decrypts the entire database ...
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### Why can you reverse a modulo function when knowing its primes

We are dealing with cryptography in school right now and superficially went over the Rabin cryptosystem with the (apparently usual) example of p=7 and q=11 etc (we didn't do RSA). I understand that ...
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### Exactly how slow is asymmetric encryption?

I'm currently doing a project on RSA, and have just come across this website. https://www.namecheap.com/support/knowledgebase/article.aspx/798/67/what-is-an-rsa-key-used-for On here, it says that ...
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### Which is better? Serialize and then encrypt or Encrypt and then Serialize

Let say in a simple Java program, Considering all the facts, like security and performance, Which is the best way to follow. I tried searching for a answer but proper answer is not out there.
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### security definition

I am new in cryptography and I saw this question in a book. I really don't know what this question means and what is the answer of it(it's not my homework etc. but I'd love to know the answer of ...
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### secure PRF or not

I am new in Cryptography and I saw this question in a note I solved it but I'm not sure about my answers. Let $F : \{0, 1\}^n × \{0, 1\}^n→ \{0, 1\}^n$ be a secure PRF (i.e. a PRF where the key ...
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### RSA time function

I am doing a paper on textbook RSA (highschool-level). I am explaining and proving the significance of the public key exponent in time and security. So far, I have encrypted a constant message with a ...
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### attack on modified DES

any idea for attack on this modified DES? Each round sub-key is 32 bits long, and the round function simply $XOR$s the round sub-key with the input to the round (i.e. $f(k, R) = k_i ⊕ R)$ . For ...
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### Which gives better deterministic encryption SIV or Plain ECB mode?

Lets say , if we encrypt a plain text message $msg$ with key $key$ in below two ways. Which is the below would give better deterministic encryption and why ? AES-ECB($key$ , $msg$) SIV($key$, NIL , ...
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### Encryption and HMAC key derivation

I'm deriving keys from a user-supplied passphrase for symmetric encryption using AES-256 in CBC mode with a HMAC for authenticity/integrity. My key derivation function currently looks like this: ...
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### Execution time RSA [closed]

I am doing a paper focusing on the public key exponent in textbook RSA. However, I need to know the time complexity for encryption time. I have heard both $O(\hspace{.02 in}\log_2(x))$ and $O(x^2)$ as ...
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### Reference AES-NI assembler code for AES-128-GCM? [closed]

I want to implement AES-128-GCM in assembler using Intel AES-NI instructions. I have found reference code in the Intel AES-NI Whitepaper but I don't see code for the GCM mode there (or indeed any use ...