# All Questions

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### Is this a valid real-time authentication scheme?

The scenario in Alice/Bob/Cindy terms: Alice approaches someone she doesn't know, but thinks is Bob, and asks for some secret information. Bob doesn't know and doesn't trust Alice, but Alice says she ...
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### Computer appliance protocol

I build the computer appliance system that consist of software server and multiple controllers. The system I want to build must be pluggable in the sense that when I connect a controller C to the ...
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### Structure of encrypted office files

I try to find out the structure of encrypted office files (I'd like to implement something like a carving tool for finding certain encrypted file types). I try to understand the Office Document ...
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### How to properly format a PGP Message Block?

As sort of a follow up to my (currently) unanswered question, if it is correct, how do i properly calculate and format the packet that actually stores the data being encrypted? I thought I knew what I ...
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### Setting protocol parameters to achieve concrete security

Background One issue with modern security proofs is that they are usually asymptotic. In other words, such proofs are usually formulated as follows: For any polynomial-time adversary $\mathcal A$, we ...
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### Security of stream cipher based on Matrix multiplication GF(256) with randomized padding

The actual question is stated at the end of the text. Suppose you have a (grossly inefficient) confidentiality stream cipher with the following components: The keyed function ...
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### Why are the first 4-6 characters from ssh-keygen (RSA) the same everytime?

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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### How about a homomorphic integer sorting in a MPC context?

I want to implement the ATV-FHE scheme as described here https://eprint.iacr.org/2013/094.pdf and based on it I want to compare homomorphically a vector of integers. Would this be a valuable ...
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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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### 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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### How to check the signature without the predetermined function in ECDSA from openssl?

I'm developing a client for a two-factor authentication in Qt/C++ under openSUSE 13.2, 64bit. I've got the FIDO U2F Security Key from yubico/fido. I'm using the classes from openssl, in particular ...
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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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### Upper bound on r* on page 7 in the Scale Invariant Fully Homomorphic Encryption over the Integers paper

I was hoping to get some clarification on how the bound on r* was calculated (bottom of page #7). I'm trying to reproduce the results that have been shown, however I keep getting a slightly smaller ...
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### Are there any FHE-MPC schemes implemented?

I want to know if there are any publicly available multiparty computation schemes derived from fully homomorphic encryption schemes. An example would be the implementation of this scheme ...
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### Can you explain “k out of n” from the VCS paper by Naor and Shamir

I am new to visual cryptography. I’ve just read the paper “Visual Cryptography” by Moni Naor and Adi Shamir and I'm stuck on the chapter about the “$k$ out of $n$” construction. I am aware that, on ...
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I want to implement the ATV-FHE scheme as it is described here https://eprint.iacr.org/2014/039.pdf. How to generate the public key? Should I compute it as indicated in section 2 : $h(i) = ... 0answers 67 views ### Simplest way for strong cryptography in chat I am a beginner in cryptography and I am interested for several weeks on this for my chat application. Basically, it is a centralized model. All messages are stored and potentially visible by ... 0answers 72 views ### Pseudorandom Functions with different input and output lengths I am working on a problem found in my Cryptography textbook that goes as follows: Let F be a pseudorandom function such that for$k \in \{0,1\}^n$, function$F_k$maps$l_{in}(n)$-inputs to ... 0answers 38 views ### equation of mixcolumn in the aes field$c(y) = (03)y^3 + (01) y^2 + (01) y + (02)$Let$d = (01)b$in the AES field$GF(2^8)$. Provide exact equations for the bits$d_i, 0 \leq i \leq 7$, in terms of the bits$b_i$of b. Can ... 0answers 78 views ### Obtain Enigma key having a encrypted text and the plain text I have an Enigma message and I know what the original message is (a repetition of the same letter). Is it possible obtain the enigma key? I only know what kind of Enigma machine is it, but I need to ... 0answers 122 views ### What is the most secure encryption algorithm to encrypt my password database? The password manager I use lets me choose an algorithm from the list below in order to encrypt my password database: Microsoft Base Cryptographic Provider v1.0: RC2-128 RC4-128 DES-56 3DES TWO ... 0answers 38 views ### Fingerprint RSA public key in PKCS1 format I can't find anywhere details on how to fingerprint a public RSA key in PKCS1 format. There are a lot of examples on the internet on how to fingerprint an RSA public key in openssh format using ... 0answers 46 views ### Properly storing an E-mail Password I'm creating a messaging application that will use an outlook account to send an email. However, I'm unsure about the proper steps to store the e-mail password on the user's computer. Suppose I had ... 0answers 48 views ### Using a product of a series of curve25519 scalars as a private key There are a few systems like the GNU Name System and the Sphinx mixnet packet format that employ a series of curve25519 scalars all multiplied together as a private key. Are there any caveats to ... 0answers 45 views ### Construct points with the same discrete logarithm Assume we have an elliptic curve$E$with a Tate (or Ate,...) pairing$G_1 \times G_2 \mapsto G_T$Now the task is to find$g_1, g_1' \in G_1$and$g_2, g_2' \in G_2$such that the discrete logarithm ... 0answers 52 views ### Binary equation with encryption I'm trying to test the ring signature scheme proposed by Rivest Shamir and Tauman in the famous paper "How to leak a secret". One of the step consist in 4) the signer solves the following ring ... 0answers 84 views ### Best way to implement secure client/server communication in Python For a college programming assignment I have to implement a secure communication protocol between a server and one or more clients. Upon instantiation of the server program, a file is generated, call ... 0answers 30 views ### ABE Attributes Based Encryption I have a question . How the system know the attributes of the users to be after that as a public key in ABE? 0answers 18 views ### Does the length of the keyword used for keytable for Playfair matter? So for Playfair, we choose a keyword and put that in the keytable with no duplicates. Then, we fill the rest of the table with alphabets in order that are not in the keyword without 'J'. Does the ... 0answers 32 views ### Is it possbile to get same set of public and private keys twice while creating CSR ? If so where does the conflict happens !? I'm new to cryptography and please clarify if my below assumption is wrong. Lets assume, that two different banks creating CSR and they get identical public and private key pairs of RSA. In such ... 0answers 31 views ### How exactly is an enigma simulation software supposed to encrypt alphabets? EDIT: The model I'm trying to make is "Enigma 1". I learned initially about it from a book called "Code Book" and then looked at it in detail from its wikipedia page. The rotor wiring and other ... 0answers 79 views ### Is there specific name for simple XOR with ADD/SUB encoding? Several years ago a group of malware authors began using code like this: for (j = 0; j < length; j++) ptr[j] = (ptr[j] ^ VALUEA) - VALUEB; for encoding, and ... 0answers 28 views ### Are there any existing ASN.1 structures for transferring symmetric keys? Is there any reasonable ASN.1 standard to encode symmetric keys? I found PSKC, but it seems extremely heavy, and I can't quite find any library (e.g. bouncycastle) that would simplify using it. If ... 0answers 19 views ### LFSR, different bits from different initial states Suppose we useLFSR, which gives output$x_1x_2x_3\dots$from some particular initial state. From another state, different from previous, same LSFR though, it gives sequence$y_1y_2y_3\dots$... 0answers 36 views ### Symmetric stream cipher, is it known? I am thinking on a simple symmetric stream cipher. My first idea is the following: The key is a set of array of random bytes ($\underline{k}_1$...$\underline{k}_n$), whose length is$l_1$,$l_2$, ... 0answers 20 views ### Is there a system where I can securely store a secret value and compare a semi-ambiguous input to it? I have an idea for a new kind of mobile phone lock screen. When you first create your pattern, you put together a series of colors and compass directions from the following choices: Red, Blue, Yellow, ... 0answers 28 views ### Hash-values's length in identification schemes I am trying to understand the impact of applying a hash function in an identification scheme. It makes reference to the publication "On the length of cryptographic hash-values used in identification ... 0answers 45 views ### CPA-security proof of “l-padded RSA” encryption scheme The 10.9 exercise from the Katz-Lindell textbook creates a lot of confusion, let me state it here. Consider the following experiment for an algorithm$A$and a function$l$with$l(n) \leq 2n-2$for ... 0answers 46 views ### Ciphertext size of any CPA-secure public-key encryption scheme I know that, for any CPA-secure public-key encryption scheme, the size of the ciphertext after encrypting a single bit is superlogarithmic in the security parameter. That is, for$(pk,sk) \leftarrow ...
I am trying to understand the small subgroup attack on Diffie-Hellman. I will present the attack and try to explain why it works. Let be a group $\mathbb{Z}_p^*$ where $p$ is a large prime, so the ...
Having a large set of ECC public keys $P_i = n_iB$ on a fixed curve $E$ over a prime field, is there a way to determine if coefficients $n_i$ were generated using a bad PRNG? In other words, can a ...