# All Questions

541 views

### Perfect Forward Secrecy with Pre-shared Key

I've recently been looking at how to do perfect forward secrecy on a unidirectional connection (server can only push messages to client, client cannot respond). What I've come up with is the idea of ...
116 views

### Modular protocol design

What does modular protocol design mean? Why does TLS not have modular protocol design? What protocols have modular design? (IPSec, SSH)
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### RFC 3526 - What does pi mean?

In RFC 3526 there are a series of primes listed as standard parameters used for Diffie-Helman. The primes are list in two formats. One is the long format, where the number is given in hex. For ...
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Ok - i will try to ask my question as clear as possible. Im getting a little deeper into the RSA-cryptosystem. At one point i'm a little confused. We have a plaintext $x$ and ciphertext $y$, with $x ... 1answer 398 views ### Prime factorization of RSA modulus Consider RSA public-key encryption with public modulus$N=3953$. Suppose we know that the public keys$e_1=337$and$e_2=23$correspond with the decryption information$d_1=3385$and$d_2=2663$. ... 1answer 133 views ### OTP sequence uniqueness Is it possible to use Time-based One Time Password (TOTP) algorithms to generate tokens from multiple channels (where a channel is defined as a "user" using a certain shared key$K_c$for channel ... 0answers 150 views ### Safe and computationally efficient way to verify a curve25519 identity? A client identifies itself as a curve25519 public key. The server wants to verify the client owns the associated private key. Is there a safe and computationally efficient way of doing so? Which ... 1answer 416 views ### crypto design with AES256 MODE OFB I have the following APP Design key store (contains 1:n user payload as 1:n key store entries) key store salt (is used to extend to key store password to 256 bits, stored in clear text) key store ... 1answer 2k views ### Certificate signature with SHA-1 and RSA: where do 1888 bits come from? Plenty of X.509 Certificates use "PKCS#1 SHA1 with RSA encryption" as the Certificate Signature Algorithm for generating a 2048-bit signature. The SHA-1 hash function generates a hash value of 160 ... 1answer 173 views ### Pseudo random permutation for arbitrary size domains Popular block ciphers like AES or Twofish are keyed pseudo random permutations on the domain$\{0,1,\dots,2^{k}-1\}$with$k\in\{128,192,256\}$or similar. I'm interested in pseudo random ... 1answer 196 views ### simple multiplication in GF(8) I am trying to do multiplication in the GF($2^3$) defined by the irreducible minimum binary polynomial$X^3+X^2+1$. I want to multiply$A(x) * B(x)$where$A(x) = x$and$B(x) = x^2$. The ... 1answer 132 views ### If its possible to derive the public key from a private key, why can't we go in reverse? I'm looking at source code for BitcoinJ that derives a public key from the private key. ... 3answers 383 views ### How can we sign a contract digitally between two parties? I'm trying to sign a contract with another party, without using handwritten signatures. I've just read through Applied Cryptography, but there doesn't seem to be a protocol that can solve this ... 1answer 670 views ### Proofs by reduction and times of adversaries I have some difficulties to understand, when we construct a reduction, how we determine the time for the constructed adversary to break a target security property. In general these details are not ... 1answer 210 views ### ElGamal with elliptic curves I It is very interesting to see @tylo's answer on ElGamal with elliptic curves. Instead of mapping the message to the elliptic curve point it just reduces an elliptic curve point to its$x$coodrinate. ... 3answers 288 views ### Why can't hashes be reversed with toffoli gates? This is a follow up question to this question: Crack cryptographic hash functions using Toffoli gates?. Suppose there is a hash function $$H(x)=y,$$ where$x$is the input of the function and$y$is ... 2answers 240 views ### Current standard for hash function security parameter? When designing a hash function, what should be the secure input and output lengths? In particular, what lengths do I need for collision resistance? 1answer 274 views ### Is this approach to generating a “random” number from a sha512 hash effective? I'm looking to run a small raffle but i'd like people to be able to verify that the number chosen was fair. I know some sites use a "Provably Fair" system to achieve this where user input is combined ... 2answers 98 views ### Should tweak be unique per message? Since tweaks are like IV,Salt for block cipher inputs . Should they be unique for each message ? If they are to be unique per message then what is the advantage of giving tweak directly into the ... 2answers 591 views ### Signature based on public key cryptography and forgery In the definition of existential unforgeability, there is no detail about the following questions. In general, can we suppose that a signer is also a possible adversary ? When generating a signature, ... 2answers 464 views ### If attacker knows salt and hash, how is salt effective? Maybe I'm missing something, but if my salt and hashed/salted password are stored in the same place, how is salting any more secure than just hashing? Does this just rely on attackers not knowing how ... 4answers 166 views ### Is there a standard way to extend the output of a hash? Say I have a hash function, a state-of-the-art hash function that we would all approve of that produces a fixed number of bits (n) as output. (I'll call it HASH.) Is there are a standard way to ... 0answers 160 views ### Hash function that is efficient on GPU and poorly suited to ASIC I have such requirements to the hash function in my project: operating on 32-bit data, not mathematically breakable (built on existing hashes known to be secure), free of branching, as much as ... 0answers 61 views ### What does the notation$1^n$stand for? [duplicate] I'm seeing this all over my Katz and Lindell book and I don't see where they introduced it. 2answers 110 views ### Needed: signature over a collection of hashes I'm looking for some kind of crypto-based data structure that will allow me to produce a signature over a set of hashes. Call this a SignedSetOfHashes. I want to be ... 1answer 225 views ### a doubt in Rijndael's key expansion sizes I've often heard/read that AES key sizes 256 & 192 would be weaker than 128 or not stronger as expected from the size increase, but I've never seen a proof. How does one proof the strength of a ... 3answers 546 views ### RSA assumption and cryptography The RSA assumption states that it is hard to find$m$, given$c = m^e \bmod{n}$,$e$, and$n$(for appropriate choice of$n,e$). Suppose that there exists an algorithm,$D(c,e,n)$, that finds$m$in ... 2answers 434 views ### How can I simulate and measure brute force hacking using RSA? I want to implement brute force hacking using RSA. How can I simulate and measure time duration? What software (or software-based simulator) would be needed? 1answer 132 views ### Hashing a uuidv4 + time to create a unique id First, I am no crypto expert although I am doing the MOOC. I have an item which has as id: uuidv4 When I create this item: I create a new uuidv4 check if it already exists and if not, I store it ... 1answer 877 views ### Is the random data at the end of my decrypted message supposed to be there? This is my PKCS#5 & AES/ECB Python 3.3 code: ... 3answers 458 views ### One-time digital signatures Are there digital signatures for which, given two documents signed by the same key, one could derive the key? With such one-time signatures, one may be able to design a cryptocurrency based on ... 2answers 633 views ### Safe elliptic curve point addition using projective coordinates: How do I tell if the points are the same? I am trying to implement elliptic curve point addition in hardware for NIST p256 and p384 curves. I have noticed the following issue with the suggested NIST routines: Consider routine 2.2.7 of ... 1answer 165 views ### How to determine which type of crypto is used in a text file? [closed] is there a tool or a way to determine which type of crypto is used in this type of text: ... 0answers 224 views ### Prove preimage resistance property Let$ f : \{0,1\}^* \rightarrow \{0,1\}^n$and$ g : \{0,1\}^* \rightarrow \{0,1\}^m$.$h$is created by concatenate f and g:$ h : \{0,1\}^* \to \{0,1\}^{n+m}$with$h(x) = f(x) || g(x)$. How do I ... 1answer 137 views ### AES CBC encoding of decomposable OAuth2 tokens We're building a stateless RESTful API that relies on the content of OAuth2 tokens to identify users and what they have access to. Users authenticate with other means and are given a token, which they ... 1answer 241 views ### Unique, anonymous online authentication Is there a way to authenticate users online without knowing their real identity yet preventing them from registering twice? If not, what is the least amount of information you would need? If it helps, ... 1answer 87 views ### Collecting entropy from a stream to a file I have a potentially infinite stream of data originating in some physical phenomena that may expose random behavior (due to quantum effects, thermal noise, the butterfly effect in chaotic dynamical ... 1answer 305 views ### Generating Diffie-Hellman parameters efficiently I am working on an Android project for school and I am supposed to do a DHKE (Diffie Hellman Key Exchange). Everything works well. The problem is that it takes a lot of time (really a lot) to ... 1answer 433 views ### What are some restrictions when converting Montgomery Curves into Weierstrass Curves? I want to represent a Montgomery Curve (curve25519) in Weierstrass form as a personal exercise. After doing some math and referencing the conversion equation at ... 1answer 134 views ### Diffie-Hellman explicit key confirmation Suppose I wanted to add explicit key confirmation to Diffie-Hellman key exchange, would the following scheme be secure? Alice selects a random$a$and sends$g^a \mod p$to Bob Bob selects a random ... 4answers 2k views ### What is a Non-Interactive Zero Knowledge Proof? I understand the concept of a Zero Knowledge Proof thanks to the easy to understand analogy of Alibaba's cave. However, this seems to require interaction between the verifier and the other party. I ... 1answer 333 views ### Help in understanding exactly how lattices used as one way functions for hashing I am doing a cryptography course via long distance and we have been given an assignment which is based on lattice-based cryptography. I have spent the majority of the past week sifting through papers ... 0answers 77 views ### For$e(g, d) = c $, can we compute$d$, given others Given $$e(g, d) = c$$ where,$e$is bilinear pairing function chosen by the user/attacker, the values of$g$and$c$are known$g, d ∈ \mathbb{G}_1$,$c$depends upon the$e$can we somehow ... 1answer 589 views ### Encrypting a payload for transmission over HTTP. AES256 with PBKDF2 I've done some reading about implementing AES256 and deriving a key from a password. If I understand correctly: I want to generate a new salt (for the key) and a new IV (for the encrypted message) ... 1answer 653 views ### Known vs. Chosen (plaintext and ciphertext) and RSA I want to know whether I can break RSA (textbook) with known plaintext/ciphertext attacks. Known plain-text Let$c$be the ciphertext and$m$the plaintext. $$c = m^a \mod n$$ Since all I have is ... 1answer 116 views ### Do both client and server need to implement OpenSSL protections to protect from CBC attacks In regards to the protections implemented in OpenSSL 0.9.6d (described in section 2. http://www.openssl.org/~bodo/tls-cbc.txt). My understanding is that the mechanism prepends an empty record when ... 1answer 87 views ### Key change and key derivation There are two ways for key change: key update, and Re-keying. Which of these methods use a KDF (Key Derivation Function) for getting new keys? 1answer 132 views ### Why does a one-time-pad key have to be at least as long as a message? [closed] I am studying one time pad and find it hard to understand. What happens if the key is one bit or 100 bits shorter than the message? 2answers 1k views ### Securely send public key from client to server in RSA? I have a client and a server communicating with messages encrypted using RSA. Initially, the client knows server's public key, but the server does not know the client's public key. Is it possible to ... 1answer 311 views ### One-way permutation over a small interval? I am wondering what concrete computable functions we know that are a permutation over an integer interval of parameterizable size$s$, for relatively small$s$starting circa$2^{64}\$, to perhaps ...

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