# All Questions

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### plaintext distribution ,perfect secrecy cryptosystem

I have been trying to solve the next problem: let pi be a perfect secrecy cryptosystem with specific plaintext probability distribution. How to prove pi will have perfect secrecy for all other ...
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### Collision resistant hash functions

Let (Gen1,H1) and (Gen2,H2) be two hash functions. Define (Gen,H) so that Gen runs Gen1 and Gen2 to obtain keys s1 and s2, respectively. Then define Hs1,s2(x)=Hs1(x)||Hs2(x). a) Prove that if at ...
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### Many single-time use keys encryption scheme (not Shamir's Shared Secret)

I'm looking for encryption scheme with the following properties: There's a sequence of keys that can be used to decrypt the message Strictly only one key from the sequence is required to decrypt the ...
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### How does GPG verify succesful decryption?

How does GPG (or other programs using the OpenPGP file format) verify that it has succeeded with decryption (for symmetrically encrypted data)? Is something appended to the clear text so there exist ...
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### Zero Knowledge Example using discrete log

I've been exploring Zero Knowledge Proofs and while the classic cave example by Jean-Jacques Quisquater makes sense, I find the discrete log example problematic. Since the Verifier is given p, g and ...
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### Is it possible to find out whether a number is greater than another number without knowing the numbers?

Is it possible to construct a zero knowledge proof that one encrypted number is larger (or not) than another encrypted number without releasing the values of either numbers?
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### Key generation in Digital Signature Algorithm

In DSA, one needs to generate two primes $p$ and $q$ such that $q$ is $256$-bit and $p$ is $3072$-bit and $p-1$ is a multiple of $q$. Question: How to generate such $p$ and $q$. Attempt: First, use ...
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### CCA security of a system that splits messages and encrypts each packet

Propose a symmetric key based crypto-system for implementing a secure email system. This system is based on AES and CCA secure. Suppose that you have to encrypt a large message and that this message ...
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### Checking both Quadratic residuosity and Jacobi symbol simultaneously and efficiently

I have to randomly generate a number $u$ such that $u \in J(N)-Q(N)$ where $J(N)$ denotes the set of elements less than $N$, whose Jacobi symbol value is equal to 1; and $Q(N)$ denotes the set of ...
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### OpenSSL RSA same plaintext but different ciphertext

how OpenSSL RSA work ? I generate public (n,e) and private key (n,d) then I encypted a file by : ...
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### Why sorting is needed for Meet-in-the-middle attack

The weakness of double DES, E (k1, E(k2, M)) is that Meet-in-the-middle attack is possible. When performing Meet-in-the-middle ...
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### What is the most computationally efficient way of generating pseudo-random permutations?

I have an application in which I need to create up to J randomly shuffled-copies of an array of length N. Then I will have millions or even billions of iterations such that, in each iteration, I will ...
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### Are all block ciphers FPE (format preserving encryption)?

A FPE preserves the format. There is a correspondence between the format of the ciphertext and the plaintext. Given that a message in some binary enconded format can easily be divded in some ...
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### Implementation and Testing of SRP-6a

I have been wracking my brain trying to develop a functioning implementation of SRP-6a in Python to use with a 3rd-party API that claims to use SRP-6a with $N=$ 2048-bit prime and generator of $2$. ...
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### How many of primitive point on the elliptic curve?

In elliptic curves for cryptography, I know $nG=O$, where $G$ is a base point represented by $G=(x_g,y_g)\ on\ E(F_P)$, where $n$ is Order of point $G$. For example, $P(0,6)$ is a primitive point on ...
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### Complexity to find an preimage of a hash function

Suppose a hash function based on Merkle-Damguard construction. Its compression function is given as $H_i=E_{m_i}(H_{i-1})$, where $E_{m_i}()$ denotes the encryption of an ideal block cipher with ...
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### Perfectly secret cipher can leak about the key?

As defined by Shannon, a cipher is perfectly secure if ciphertext leaks no information about the plain text. Under this definition, can ciphertext leak something about the key? Are there any ciphers ...
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### Key derivation for AES-Encryption key and HMAC

Okay so I have two options for how I would derive two keys for AES encryption and HMAC they are as follows: Run BCrypt on a users passphrase and then use the bouncy castle implementation of HKDF in ...
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### How can CBC-MAC be secure when message length is fixed?

I was doing some self-initiated knowledge gathering about digests, signatures and hmacs and I ran across the fact that you can use CBC as a MAC, but if the message size is not fixed; then it is not ...
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In RSA, the $p$ and $q$ should be randomly generated, and they are the same size. The difference between $p$ and $q$ should not be small. Suppose that $u=|p-q|<20$ and $p \times q ... 2answers 119 views ### What is primary security in ECC? I read the following paragraph in a book about Elliptic Curve Cryptography, but didn't understand it: The primary security in ECC is the parameter$n$; where$n$is Order of point$G$, that is$n$... 1answer 130 views ### Best practices (if any) to add metadata to encrypted files I am encrypting files using AES/CBC/PKCSPadding, with 256 bits long keys. So far the first 16 bytes of each encrypted file are the IV. I would like to store some more information regarding the ... 1answer 167 views ### Verifiable, Distributed Secret Santa Assignments My family was planning a secret santa and I thought about how I might write a little web app to dole out these secret assignments. But the downsides to this approach are obvious: there's no way for ... 2answers 121 views ### Strange MAC algorithm I'm working with a payment provider which uses the following algorithm for signing messages: The merchant is securely given a secret key k, known to them and the ... 1answer 171 views ### What is the use of segments in Cipher Feedback Mode (CFB) In NIST SP800-38A: Recommendation for Block Cipher Modes of Operation CFB can be used with a parameter s - the size of a data segment in bits - which determines the ... 0answers 324 views ### Consequences of AES without any one of its operations Suppose AES-$128$. There are$4$operations in AES's encryption, they are SubByte, Shift Row, MixColumns and AddRoundKey. Question: If I remove one of the following opearations, what will happen to ... 2answers 56 views ### Condition on Vector Boolean Function to be Bijective Suppose the vector boolean function be$\begin{align} f:F^n_2 \longrightarrow F_2^n \\ (x_1,\dots ,x_n) \longrightarrow (x_2,\dots x_n,g) \\ \\ g:F^n_2 \longrightarrow F_2 \\ (x_1,\dots ,x_n) ... 1answer 533 views ### AES with weak keys Suppose that the key generation is insecure; each key bit is independently generated, but the value of each key bit is '1$' with probability$0.90$. These keys are used in AES. What is an efficient ... 2answers 320 views ### RSA-KEM construction "Create a random key for CCPA2 scheme, encrypt m, send it along with a PK encryption of the symmetric key. " Say in the process, the KDF is HMAC, then: get a random key (128-bit) C1 = encrypt ... 3answers 117 views ### How a file is encrypted? I thought encryption algorithms(like AES) are used to encrypt texts,so how a file or folder(containing pictures,videos,...) is encrypted?What is encrypted exactly? 3answers 5k views ### What is the most secure hand cipher? By "hand cipher", I mean a symmetric cipher for which encryption and decryption can can both be performed with a pencil on graph paper, consuming about 10-20 seconds per character by a proficient ... 3answers 143 views ### Are modified implementations of cryptographic algorithms a good idea? Are modified implementations of standard cryptographic algorithms better that the standard versions? Since the algorithm is not standard, is it harder to break? Is it safer? 1answer 53 views ### CSPRNG's (ELI5) What's the purpose of multiple seeds/ regenerating sources of entropy (i.e. /dev/random)? Why is there a need to generate more than one high entropy seed? It would seem like if the original source of entropy is high quality, than some 512 bits should be more than enough to seed a CSPRNG ... 0answers 61 views ### Permutation parity after cycle-walking Let$E$be a random even permutation of the set$\{0\dots n-1\}$. We construct a permutation$P$of the set$\{0\dots m-1\}$, for some$m\le n$, using cycle-walking; that is, computing$P(x)$is as ... 5answers 559 views ### Can hashing become encrypting? hashing some data and encrypting data share some similar attributes. Surely this is why "Difference between encrypting something and hashing something" has been asked. The point of this question is ... 2answers 239 views ### Using bcrypt derived keys for encryption? Sorry for this ignorant question but I am currently weighing up the advantages and disadvantages of BCrypt over PBKDF2 and from what I have read BCrypt is considered more secure but from what I have ... 1answer 606 views ### what are the security benefits of LUKS? Aremed with only basic crypto knowledge, I'm trying to understand the security benefits of LUKS over plain-mode dm-crypt. Hopefully this is a good place to ask. Given the LUKS header contains ... 1answer 51 views ### Why has SSL v3 not yet been upgraded to a higher version with stronger principles, what are the dependencies and issues to do so? SSLv3 is almost 18 years old; why has it not been updated to a more advanced version with stronger rules and structure. What have been the issues and dependencies in doing so till now??? 1answer 116 views ### RSA: Fermat-Euler Theorem - coprimality of message and modulus [duplicate] I understand the implication of the Fermat-Euler theorem and how it applies to RSA and the detailed explanation by user Ninefingers at What is the relation between RSA & Fermat's little ... 1answer 455 views ### IPsec authentication and encryption lgorithms I am looking for the implementation of IPsec security algorithms for both authentication and Encryption. Is there any open source implementation available for the same ? And if someone supposed to ... 2answers 312 views ### Can RSA signing be implemented on top of RSA encryption/decryption? I need to use RSA-SHA256 signing. Unfortunately not all Microsoft CryptoAPI providers support that. It's possible that I might get a handle to a CryptoAPI provider that can just encrypt/decrypt with ... 1answer 75 views ### ASCII input to HMAC weaker than raw bytes? In various cryptographic programs I've written (most have been toy problems, but this current one is not) I have taken a slothful approach to performing a message digest (hash) of numerical input. I ... 1answer 184 views ### Why is this cryptosystem insecure? Can someone help me see the flaw in this cryptosystem? Note: This is homework and it is due today at 1:30pm. An answer before that is not expected; I'd just like to understand what the flaw is. ... 2answers 944 views ### Attacks of the MAC construction$\mathcal{H}(m||k)$for common hashes$\mathcal{H}$? Consider a common practically-collision-resistant hash function$\mathcal{H}$(e.g. SHA-1, SHA-256, SHA-512, RIPEMD-160), perhaps based on the Merkle–Damgård construction as are the first three. We ... 1answer 58 views ### TLS - self encrypting to avoid tls setup, secure? I am in doubt : of course one can create a tl connection between a client and a server. But what if the client sends self encrypted with, say, blowfish, data to the server? The goal is that the ... 1answer 102 views ### Random Permutation polynomial I need to know, please: (1) Is there anyway to pick uniformly at random permutation polynomial in a field of prime order? (2) Are there many permutation polynomials in a field? (3) In a finite ... 0answers 36 views ### Why HMAC=H(message|key) is unsafe? [duplicate] As mentioned in wikipedia, HMAC=H(message|key) is not safe, but I don't know the reason. Anyone can tell me why? or give me an example? Thank you in advance. 1answer 90 views ### What is “electronic” in electronic codebook (ECB)? This is an honest question regarding terminology. I believe that I understand why one mode to use a block cipher is called a codebook. But I wonder why electronic as a work appears here? Is it fair ... 1answer 85 views ### What does$\mathbb Z_p^*$contain? I have a prime$p = 7$and was tasked to select a random value in$\mathbb Z_p^*$in my signature scheme. What does the full range of$\mathbb Z_p^*$contain in this case? Is it$\{0...7\}$or ... 1answer 365 views ### How can we prove that two discrete logarithms are equal? Suppose there are two elements$a = g^x$and$b = h^x$, where$g$and$h$are generators in$Z^*_p$and$p$is a large prime. How can we prove that$a$and$b\$ have the same discrete logarithms with ...

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