# All Questions

1answer
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### RSA algorithm private key extraction

Decrypting messages can be done by using the following formula: $$M = C^d \mod n$$ Where $M$ is the decrypted message, $C$ is the encrypted message and $d$ is the private key Theoretically ...
1answer
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### Is this a good communication scheme?

I'm writing a wrapper for a chat program to allow for end-to-end encryption. I'm using Python and PyCrypto to accomplish this. I'd like to use a well known library to do this, but to my knowledge, ...
2answers
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### RSA numerical questions

Let $n$ be the modular arithmetic, $p$ and $q$ the two large primes such that $n=p*q$ and $e$ the public exponent. Here they are two "simple" numerical questions: If $p = 13$ and $q = 17$, what is ...
1answer
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### Difference between AES CMAC and AES HMAC?

Can someone elaborate on how 'signing' is done using AES- CMAC and AES-HMAC? HMAC Signing as I understand: Compute the HMAC( Hash the key and the input concatenated in a special way) Verification: ...
3answers
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Let $a$ be the plaintext message, $e$ be a small prime exponent (such as 11, 13, 17 etc.), $b$ be cipher text and $n$ be a large modulo (non-prime). Suppose that: $b \equiv a^e \space mod ... 4answers 760 views ### Are there any practical ways to transfer random data securely? The owner of the random.org service recommends that you do not use randomly generated numbers from their website in cryptographic keys. This makes sense, but it got me wondering. Is it possible to ... 2answers 97 views ### Why does the time complexity of RSA encryption and decryption depend on the size of the modulus? I would think that the time complexity would depend on the exponent, not the number of bits in the modulus. For example, if the exponent were 5, using the "naive" method, we would perform 5 ... 1answer 132 views ### Is H'(M)=5*M + 9*H(M) mod 2^n a secure hash? theorem: For arbitrary$i≥0$there exists$m,n≥0$such that$35+i=5m+9n$. We can use this theorem for constructing simple hash function. Let$M$be message and$H(M)$be its hashed value with ... 0answers 10 views ### A standard for multi-key provider encrypted data? I'm looking for a standard that standardizes data encryption with multiple and diverse key sources to represent a heterogene access structure and culture. The standard must offer the following ... 1answer 37 views ### What information should I send along with an AES ciphertext and is there any standard to do that? While using some crypto implementations I got to see that many times a comment is added to the ciphertext. For example: ... 2answers 60 views ### 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 ... 1answer 32 views ### Generating passwords from a master password using rules I realize it's generally considered bad idea to use the same password everywhere. I can use icloud / lastpass to generate my passwords for me, but that leaves me clueless when I don't have access to ... 2answers 115 views ### Is TLS secure against VM reset attacks? Is the TLS protocol secure against VM reset attacks? Background: Imagine running cryptographic software inside a virtual machine (VM). A VM reset attack is one where the the state of the software is ... 1answer 82 views ### Authentication protocol which detects client impersonation from another machine Consider following authentication protocol. Client ClientA knows some initial secret KeyA shared with server. Client also knows IP of legitimate server. Now when client wants to establish connection ... 0answers 31 views ### Point Multiplication Double-And-Add-Method [migrated] Double and Add Method If i have k=5, base poin G =(2,1) Is is double and method like this: 5.G = 2.(2G)+G How is the logic, is i want convert integer 5 to base 2, like this : 5 = 101 ... 0answers 31 views ### 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 ... 2answers 100 views ### Is there such a thing as a hash function with a fixed size input? That seems like it would be a useful primitive. Using it in place of a block cipher in the Merkle–Damgård construction would give a variable-length hash, maybe without the tricky business with related ... 1answer 91 views ### S boxes in Blowfish I know how the s-boxes in blowfish are initialized, but how is the output sorted, I mean which bit from the input goes where in the box ? and what do mean they have 256 entries if they are 32-bit ... 1answer 53 views ### How exactly does certificate based authentication work? I know the concept of key generation, as well as encryption and decryption using public and private keys. When it comes to authenticating a user by a server, in general there are three types: ... 0answers 38 views ### When we use permutation and random long sting to hide a message This question is modified version of this: Can we use merely permutation to hide a message? I have an English name$m=m_1 || m_2 ||..||m_{30}$. I pick a random string$d=d_1 ||..||d_{70}$I mix ... 1answer 711 views ### Is this DIY remote lock protocol secure? I need your advice on following scheme of exchange protocol between remote lock and key. I'm planning to use following algorithm: Key generates unique value that never repeats (in reality it's just ... 2answers 268 views ### How to determine the order of an elliptic curve group from its parameters? Let$\quad E:\; y^2 = x^3 + ax + b \quad$be an elliptic curve defined over a finite field$\mathbb F_q$where$q = p^n$,$a,b \in \mathbb F_q$and$p \neq 2, 3$. By Hasse's theorem we know that the ... 1answer 29 views ### Security for secret key of server in Identity-based Encryption In the key set up phase, server generates Pub=s.P, where s is the secret key.Then , it gives clients Pub,P as public parameters and pairing descriptions. Is it possible for clients to pre-compute r.P ... 1answer 168 views ### Simple digital signature example that one could compute without a computer? I am working on a document to explain Bitcoin to students. But I am having a hard time translating the principle described in §2 of the Bitcoin whitepaper in layman's terms. There is a great question ... 1answer 31 views ### Is it okay to send an encrypted key using XSalsa20-Poly1305, and send subsequent messages using ChaCha20-Poly1305? I am looking at a cryptographic protocol in a somewhat unusual environment: the communicating parties can share arbitrarily long secret keys over a secure channel. If forward secrecy is not required, ... 1answer 85 views ### Encrypting passwords I am using Android's keystore to generate a key which is protected by fingerprint authentication. This key can be used to encrypt any secrets. In my case, I want to encrypt a user password (which is ... 2answers 53 views ### Should the most significant byte of a cryptograhic key be nonzero? Is it safer to give a non zero value to the most significant byte (MSB) of a cryptographic key to ensure its length (not 31 instead of 32 for an EC private key on a 256-bit curve for example)? Or is ... 3answers 82 views ### How to convert RSA public keys to one key to use for text hashing? I've recently asked a question about RSA encryption. One of the things that I don't understand is the usage of two public keys. In the example on Wikipedia, a message is encrypted like: ... 0answers 47 views ### Why doesn't Morse Code try to achieve more compression? [migrated] I was working on the Morse Code article (wikipedia), when I noticed something strange. There might be a reason, but I can't find it. As you can see, the code symbol ... 3answers 145 views ### Are there reasonably secure online voting implementations e.g. for student council elections? Assume a university want to elect the student council. This was done via paper ballot and there is a need to reach more students for voting, so electronic voting is on the table. While researching ... 1answer 79 views ### Can we use merely permutation to hide a message? This question is related to this: When all shares of a secret are given to adversary as a permuted matrix I think the below question is a simplified version of that. Assume we have a 40-bit ... 3answers 330 views ### Parallel-resistant proof-of-work scheme? I am looking for a proof-of-work scheme which cannot be effectively parallelized. For example, in hashcash (and by extension bitcoin) you have some collision-resistant hash function$f()$, a target ... 0answers 41 views ### Vulnerabilities in SHA-1 [duplicate] Are there any known vulnerabilities in SHA-1 that can be exploited? Certain patterns that it shows? I recently performed a frequency analysis using 20,000 common words, and found nothing exploitable. ... 1answer 150 views ### Hamiltonian Path Zero Knowledge Proof using Commitments to a Series of Edges Commonly, Zero Knowledge Proofs based on the Hamiltonian Path or Cycle problems are given as follows: The Prover has a graph$G$, for which he knows a Hamiltonian Path (or Cycle).$G$is also known ... 1answer 21 views ### Do any Identity-based Encryption Systems support messaging between domains? Identity Based Encryption is covered under RFC 5091 (specifying the algorithm), 5408 (specifying data structures), and 5409 (using IBE with Cryptographic Message Syntax). However, every IBE ... 1answer 48 views ### RSA-OAEP: How does it work? I was looking at this image about OAEP: ..and was wondering what was happening here. 1) We have a data block DB which consists of: ... 1answer 149 views ### Why$e>n^{3/2}$prevents Wiener's attack Here's the Wiener's attack as I understand it. Suppose$n=pq$with$q < p < 2q$, and$d < n^{1/4}/3$where$ed=k\phi(n)+1$and$e < \phi(n). \begin{align*} n-\phi(n) ... 1answer 41 views ### Understanding the distinguisher for a PRG Given the following definition of a psuedorandom generator, I'm having trouble understanding what exactly the "distinguisher" D is outputting, and when? 0answers 54 views ### Q: Relevance of physical layer security to cryptography I found that there are quite some active research activities in a field named "physical layer security", with in particular some types of codes that apparently are different from those commonly ... 2answers 43 views ### Stateless proof-of-work system with 0-roundtrip time I'm designing an API. To avoid abuse, I need to rate limit the requests somehow, but I don't want to do it per user as it's very easy to create new accounts automatically. I think a proof of work ... 0answers 40 views ### Compressing AES [duplicate] I have this program which encrypts multiple files using AES then compress it using LZO into one single file. The problem is that the final file is almost equal in size than all the files which means ... 0answers 28 views ### Subset sum hardness Hardness of a subset sum probleme depends on it's density. Is it possible for a subset sum problem to be intractable even if it's density is less than 0.9408 ? 1answer 198 views ### What is the “compression function” in Merkle-Damgård? Is the "compression function" in Merkle-Damgård just a collision-resistant, one-way hash function but one that operates only on fixed size inputs? If so, is MD just a way to extend it to work on ... 1answer 51 views ### Finding a secret cipher given the key and known plaintext? Let x,y,k be plain text, cipher text and key respectively. Also suppose \operatorname{Enc} is the algorithm of encryption for block cipher with size n. So we have\operatorname{Enc}_k(x)=y$$... 0answers 19 views ### Security issues with self encryption? [duplicate] I'm thinking about the scenario of using a set of (potentially untrusted) hosts to store (potentially private) files. Think something like distributed backup where you contribute some disk space and ... 0answers 23 views ### Degrade in performance with SSL_OP_SINGLE_ECDH_USE? We have used SSL_OP_SINGLE_ECDH_USE when setting up our SSL_CTX . This seems to be causing a degrade in performance. I'm not able to find proper documentation for it except that it generates new ... 1answer 23 views ### Proof of Knowledge/Data Integrity using Hash Function Objective: A verifier wants to ensure that a block of data M is also known to a prover. The protocol should exchange little information and should not leak information about the value of M. Proposed ... 1answer 49 views ### Affine cipher and plaintext attacks Last year, a question concerning plaintext attacks was posted at Mathematics.SE: “Plaintext attacks: affine cipher”. I have no problem to see how to solve it when we are given two ciphertexts and ... 0answers 53 views ### history of non secret cryptograph Prior to the discovery of any specific public-key schemes, such as RSA, an existence proof was developed whose purpose was to demonstrate that public-key encryption is possible in theory. Consider ... 1answer 52 views ### How much information is revealed? Let Alice pick v_{1},m_{1}$$\in \mathbb{P} and Bob $v_{2}$,$m_{2}$$\in \mathbb{P}. Now Alice computes v_{1}$$^{m_{1}}$ and so Bob does $v_{2}$$^{m_{2}} Then Alice exchanges v_{1}$$^{m_{1}}$ ...

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