# All Questions

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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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### Elliptic curve point addition with $Z_1 = Z_2 = 1$

Elliptic curve point addition and point doubling operations using Projective and Jacobian coordinates require fewer field multiplication operations when considering $Z$ coordinates of input points ...
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### How is a public/private key pair generated from a Diffe-Hellman key exchange?

I very well understand the DH key exchange. But what is the public key and private key at the end of the exchange. For example: Alice and Bob agree to use a modulus $p = 23$ and base $g = 5$ (which ...
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### Why not use OAEP for signatures?

There seems to be a distinct lack of implementations of RSA signatures using OAEP to pad the digest. Other than the old, funky, padding schemes, it's PSS all the way. PSS seems to have the whiff of ...
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### Computation indistinguishability questions

The definition I have is: Two probability ensembles $X = \{X_n\}_{n \in \mathbf{N}}$ and $X = \{Y_n\}_{n \in \mathbf{N}}$ are computationally indistinguishable if for every probabilistic ...
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### Is succinct verification of an arbitrary data transformation theoretically possible? Is it feasible? How?

Given input string S, and transformation (i.e. computer program) T, is it possible to provide a succinct proof that another binary string S' is identical to the output T(S)? By "succinct", I ...
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### Why a key pair is used in OpenSSL ECDSA_sign or ECDSA_do_sign instead of only private key ?

If the digital signature in ECC (ECDSA) is made using a private key, why is the OpenSSL library using ECKEY pair as an input parameter instead of the private key only ?
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### How can AES + cycle walking be used as an FPE?

I'm a bit confused after reading several articles that seem to provide conflicting information. Basically I was wondering if AES by itself can be used as an FPE? And if so is this something ...
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### Practical benefit of using a KDF?

I see that KDFs should be used in systems that rely on PSKs, but what practical benefit does it provide? For instance, suppose the system relies on a shared secret for AES encryption and decryption. ...
83 views

### Extend OTP on random data?

If Alice and Bob both start with a shared OTP $P_0$, which is 256-bytes long, and Alice wants to send a 512-byte message, would it be secure to send the first 256 bytes with standard OTP (...
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### Forward-secure static-ephemeral ECDH key agreement protocol

The question is whether the following simple key agreement protocol design has good security properties, and how it can or should be improved. Assumptions Alice is a persistent entity with a static ...
85 views

### Are shorter password hashes safe?

How much can I shorten a password hash before it begins to impact security? Suppose that I use PBKDF2 (correctly salted) with HMAC-SHA256 as the PRF. From what I've seen, it's typical to use 256 ...
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### AES S-Box Affine Mapping - how to do it?

For the byte substitution layer in AES how do we determine B(i)? I follow how to use the multiplicative inverse table and using that to get B`(i), but how do I actually calculate the affine mapping? ...
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Are there any algorithms/protocols that let you produce a specific length of random tokens/bits in such a way that you can cryptographically determine which 2 or more tokens/bitstrings where produced ...
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### How does DSA provide non-repudiation in proving a document was properly displayed (not altered or displayed incorrectly)?

I am building an iOS app that allows the user to sign a document served to them by a web server. So that page of the app simply has a document in the top pane, and at the bottom pane, a place to sign ...
900 views

### Switching between AES and a highly performant algorithm

I want to de/encrypt several binary files rather frequently on a normally battery-powered device. My goal is to use AES-256 when the device is AC-powered and a highly performant algorithm when it's ...
48 views

### Using related pair of cryptographic keys to open/seal plaintext

I am using curve25519xsalsa20poly1305/fn.seal from sodiumoxide crate written in Rust language which is a wrapper over libsodium ...
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### Lower bound of key space size with relaxed perfect secrecy

My apologies, I'm quite new to cryptography. If we relax the definition of perfect secrecy such that for cyphertext $c$, messages $m_0$ and $m_1$, and constant $E$: $P[c|m_0] \le E * P[c|m_1]$ ...
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### Generating a 30-32-bit PRP

I want to generate a small 30 or 32 bit PRP. I am going to use that PRP in an open web service and anyone will be able to get any number of plaintext-cyphertext pairs. I've found an algorithm called ...
50 views

### El-Gamal signature with two messages

Alice uses an ElGamal signature with base the group $Z^*_{107}$ and parameter $g=3$ of order $q=53$.The private key of Alice is some $x \in \{0,1,.....,52\}$ and the public key of her is $y=10$. To ...
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### How to understand RSA Proofs of correctness at Wikipedia

From Wikipedia(https://en.wikipedia.org/wiki/RSA_%28cryptosystem%29#Proofs_of_correctness): $m^{ed}$ ≡ $m$ $\pmod{q}$ $m^{ed}$ ≡ $m$ $\pmod{p}$ then $m^{ed}$ ≡ $m$ $\pmod{pq}$ Question: if $m^{ed}$ ...
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### In FUSE while using openssl's AES_256_CBC getting an error “digital envelope routines:EVP_DecryptFinal_ex:bad decrypt” and the filesystem crashes [migrated]

I run the filesystem with the -d flag, everything goes well, except when a read operatio is to be performed, then the filesystem crashes with the error "139701621372568:error:06065064:digital envelope ...
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### Do Curtmola et al.'s IND-CKA1/2 security definitions protect against search pattern leakage?

In the article Searchable Symmetric Encryption: Improved Definitions and Efficient Constructions, Curtmola et al. propose adaptive and non-adaptive (indistinguishability and simulator-based) security ...
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### Picking a nonce in the context of CCM (CTR with CBC-MAC) mode

After having read the RFC 3610: Counter with CBC-MAC (CCM) and the Wikipedia article on CTR mode, I'm not sure how a nonce should be picked. The RFC says: A nonce N of 15-L octets. Within the ...
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### How do I tack on “additional data” to libsodium's public-key authenticated encryption?

I note that libsodium provides AEAD when using symmetric cryptography. And yet, under section 8 (Public-key cryptography) there are no public-key methods described which allow for additional data ...
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### Is it possible to perform a rudimentary asymmetric key encryption algorithm by hand?

I have at least three, distinct motivations for having a simple way to use asymmetric key encryption by hand or without modern computers. First, I was recently demonstrating to a 12-year-old how ...
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### Why should $a,b < N$ for Montgomery Reduction?

In Montgomery reduction, when calculating $a \times b \mod N$, it is required that $a \lt N$ and $b \lt N$. I think $0 \le T \lt N \times R$ is enough for the Montgomery Reduction. Rationale: Let ...
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### If I know the hash function that produced a hash key, how easily can I generate an input to hash to the same key? [duplicate]

The title is pretty descriptive of my question. As an example, say I use the MD5 hash function to create a hash key of the word "hello": 5d41402abc4b2a76b9719d911017c592. There are two questions I ...
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### Are the values of Tate and Ate pairing the same?

Assume we have a Baretto Naehrig curve over $GF(p)$ and a field extension $GF(p^{12})$ given by a minimum polynomial. Let $G \in GF(p)$ and $Q \in GF(p^{12})$ from the trace 0 subgroup. Do then the ...
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### One time pad- XORing or substitution

I have come across 2 versions of otp while reading it and want to confirm it. they have taken a plain text consisting of alphabets where they are shifted and replaced by alphabets itself. XORing of ...
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### What do we need to know in terms of plaintext/ciphertext, and how can we decrypt all ciphertext? [duplicate]

Let's say we have a cipher stream that we know has a period of 150-200bit in length. We don't know anything else about the stream cipher. If we wanted to decrypt all cipher text, what else do we ...
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### Quantum circuit to implement McEliece cryptosystem

I've come up with the proposal of implementing McEliece cryptosystem with the help of quantum gates and quantum programming, creating a circuit to do the encryption based on the McEliece original ...
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I have just read about the perfect security of an OTP encryption and what came to my mind was that what if the Pad used for encryption/decryption did not have to be transported separately from the ...
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### Describe how you can estimate/find $g^{a^3}$ without estimate any discrete logarithm

Given odd prime number $q$,cyclic group G of order $q$,one generator $g$ of group and one element $y=g^a$(but not the $a$).Still you are given an algorithm $A$ who as input $g^a$ gives as output ...
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### Compact digital signature for noisy data

The question asks for a signature scheme, with a public verification procedure, that is tolerant to minor alterations of the signed data during its transmission [possibly in analog form over some ...
195 views

### Authenticating code securely and generating a certificate

I'm working on a product with strong security. Essentially there is an App, downloaded from the iTunes App Store. When the App starts up, it generates a strong RSA keypair and stores it in the ...
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### How is wpa2 authentication structured

I would like to understand the cryptography behind Wpa2. As far as I know, there are 4 messages on the 4-way handshake and three of them are values concatenated with hashes. First, I am trying to ...
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### Zero-Knowledge Proof of Subgraph Isomorphism

I'm trying to find a Zero-Knowledge proof of subgraph isomorphism in the following scenario: Alice and Bob both know about graphs G1 and ...
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### Why can't we use a hash with no collision to compress data reliably?

According to wikipedia, a hash maps digital data of arbitrary size to digital data of fixed size. For all practical measures, a hash is a unique signature of a big chunk of data. But there is such a ...
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### Quantitatively measure the private information revealed by a matrix?

Suppose I have a private matrix $A$ that I want to conceal. I do the matrix multiplication $A^\prime =AK$ and $A^\prime$ is public. How can I quantitatively measure "how well" the matrix $A$ is ...
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### Is Encryption without knowing the input directly possible at all?

I know the question is rather unusual, but let me clarify what I'm searching for: There is one person, lets call her $Alice$. $Alice$ has $n$ plaintexts $t_1..t_n$ and $n$ public keys $pk_1..pk_n$ of ...
25k views

### Possible ways to crack simple substitution ciphers

We had a quiz in class today where we had to break the ciphertext with the key given, but not the algorithm. Suffice to say that I wasn't able to decrypt it within the allotted time of 12 mins and ...
233 views

### Is ChaCha20 alone sufficient for securing data-at-rest?

I wonder if ChaCha20 alone is sufficient for securing files stored on a disk or should Poly1305 should be used along with it? AFAIK, Poly1305 is used to secure the authentication channel, but for ...
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### What was the first MD5 collision ever constructed?

We all know that MD5's collision resistance is severly broken. But when thinking of "random" strings with great cryptographic importance I've come up with NIST's curve seeds and MD5 collisions. But ...
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### Challenge-Response Phases in IND-CPA

The IND-CPA game has two challenge-response phases A key is generated by running $Gen(1^n)$ and challenger selects a bit b {0,1} uniformly at random. Adversary gets input $1^n$. Can query the oracle ...
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### Elligator-2 against curves over Fq, q mod 4 = 3

It appears that the conditions for applicability of Elligator-2 against many of the SaveCurves curves, where $q \mod 4 = 3$ will inevitably poke a hole in the bit-string set over $(0, 1, .. (q-1)/2)$. ...
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### What is the meaning of the overlapping region on OTP-perfect secrecy diagram?

I'm new to cryptography. I want to ask about OTP-perfect secrecy diagram like figure below: On the overlapping region (middle), notated by R(X;Y;Z). R can be calculated by I(X;Y) - I(X;Y|Z). That ...
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### Signing and verifying message consisting of several parts

Given a message $m$ that consists of several parts. we want to be able to verify individual parts or a combination thereof. The simplest solution is independently signing each part. But since ...
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### Stern identification scheme to signature scheme

I'm trying to understand the comment in the page 103 of the book Postquantum Cryptography. about convert the identification Stern procedure for signing. Can you write an example of signature of this ...
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### Do random oracles are called even by honest users in the PAK protocol [BMK00]?

In the full-length proof of PAK scheme, the authors made extensive use of random oracles to extract (guessed) password from the messages generated by the real world adversary for the ideal world ...

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