# All Questions

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### Security of a parallelizable block cipher mode

This is the scheme of a parallelizable block cipher mode of operation: $IV$ is the initialization vector. $BN$ is the zero-based index number of a block in a stream of data. $BT$ is the tweak that ...
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### Is part of an output of secure PRNG also cryptographically secure?

Lets say I generate 192 bits using some secure PRNG. If I now take first 128 bits is it equivalent of generating 128 bits in the first place? P.S. More practical application for me is that I need a ...
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### Secure blinding factor switching at malicious server-side (Switching in One Time pad)

Suppose all the values and operations are defined over a finite field $\mathbb{F}_p$ where $p$ is a large prime number. Assume all values are non-zero. Here $(z)^{-1}$ denotes multiplicative inverse ...
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### Algorithms for Cyrillic encryption

I am working on a cryptography application which is to be used in my country. I tried looking up to see if there are any algorithms that support the Cyrillic alphabet, but I found none. Do you know of ...
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### Block cipher not resistant to collisions

If we have the following block cipher, $E(k,m)$, which is a permutation, why is $f_2 (x,y)$ not collision resistant? $$f_2(x, y) = E(x, x \oplus y)$$
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### Extension of Diffie Hellman for many participants [duplicate]

How to do the Extension of Diffie Hellman Algorithm for Multiple Participants
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### Are there any practical methods of steganography?

There isn't a whole lot of information on steganography in the modern age, from what I can tell. A google search for steganography usually brings up a list of very impractical methods (e.g. tattooing ...
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### How does a non-prime modulus for Diffie-Hellman allow for a backdoor?

Recently someone found that a Diffie-Hellman modulus used in a unix tool (socat) was not prime. This led some people to shout "backdoor". What I don't understand ...
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### ABE Schemes - Access Structures & Performance

I'm reading this article, CP-ABE with constant-size keys for lightweight devices and in the Table 1, are there these access structure: Threshold, Tree, ($n$,$n$)-Threshold, LSSS, AND gates. I have 2 ...
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### Determining Involutary Keys in Substitution Ciphers

How would I be able to show that a key $K=(a,b)$ is involutary for an Affine Cipher over the integers modulo n? Similarly, is there a way to generalize the conditions for a key to be involutary for a ...
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### Key Exchange MAC encryption order of events? Problem

I want to write the sequence of events of how a MAC encryption works for documentation at work. For this purpose I have a document (given by my boss to me) in German and I am trying to translate it ...
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### When we use one time pad TWICE in two different ways

Assumption and Notations: All the values defined over the field $\mathbb{F}_p$, where $p$ is a large prime number. We denote multiplicative inverse of value $v$ by $v^{-1}$. All values are non-zero ...
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### Need help with PKCS#12 3 key triple des key expansion

I am trying to decrypt PKCS8-Shrouded Key Bag inside a PKCS#12 pfx file. The oid for the encrypted data is PbeWithSHAAnd3-KeyTripleDES CBC. Iteration count and salt has been provided within the pfx ...
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### How does RIPEMD160 pad the message?

For RIPEMD hashing algorithm on hardware I am not exactly getting how padding works So as per my understanding padding will work like explained below: For ripemd 160 message should be multiple of ...
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### What is the complexity to break second preimage resistance?

In the context an of hash functions we have three required properties. Preimage resistance Second preimage resistance Collision resistance. The bounds for Pre image collision is $O(2^{n-1})$, the ...
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### Classification of a stream cipher

How would one classify a stream cipher that is neither synchronous, nor self synchronizing? I would have thought asynchronous, but from all the sources I can find, asynchronous is synonymous with ...
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### Computing inverses in a binary field

Please suggest how can i solve the below question What is the inverse of {03} in GF (2^8) with the irreducible polynomial x8+x4+x3+x+1?
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### Elliptic curve ElGamal with homomorphic mapping

I am interested in ElGamal due to the fact that you can achieve some degree of homomorphic properties. I became interested in applying ElGamal to elliptic curves, and found this other question with an ...
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### Would a Rubik's cube be a trapdoor function?

It's easy to scramble a Rubik's cube but it is hard to solve, unless you know the algorithms for solving it. This makes it a trapdoor function, correct?
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### SHA1 collisions and the impact for ECDSA signatures

What will it mean for ECDSA using SHA1 when we have practical attacks breaking the collision resistance property of SHA1? [UPDATE] Added a bit more details to be clear. If $(r,s)$ is the ECDSA ...
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### Help with AES decryption with AES-NI Instructions

I am in process of implementing a AES 128 encryption in AVX2 assembly (using AES NI), and am having some problems with decryption logic. I have implemented the encryption sequence without any trouble. ...
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### Can an AES-256 key be generated for a large number of cribs? [duplicate]

I have a question in regards to Ransomware type viruses. I know that a number of these viruses use the AES-256 algorithm to encrypt user's files using a randomly generated key. I work in IT support ...
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### Multiparty encryption and decryption

Is it possible to send a single encrypted data message to multiple users so that the users receiving the message can only decrypt certain parts of the message, depending on their rights. For example, ...
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### Difference of full codebook and non full codebook

While reading a paper about Dynamic cube attack, I saw two terms: full-codebook and non-full-codebook attacks. What do they mean? What is the difference? Regards
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### Can one passively load-balance HTTPS traffic based on TLS SNI?

Is it theoretically possible to passively snoop into the HTTPS traffic, reliably detect whether or not TLS SNI is attempted to be used by the client, and, if it is, redirect the stream of the ...
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### Shamir SS: To associate shares to the public values $x_i$, when the shares are permuted [duplicate]

Suppose we use $(t,n)$ Shamir secret sharing as follows: We share the secret $\beta$ as $S=[s_1,..., s_n]$, where $X=[x_1,...,x_n]$ are the public values. For the sake of simplicity, lets assume ...
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### MAC algorithm difficulties

Can anyone explain to me why this is a bad idea? I cannot see anything wrong with it except that key K should be different for MAC and AES. MAC algorithm consists of letting the MAC of message M (...
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### total effective key length of the AESX-192

If I have a AESX-192 be a block cipher which is similar to DESX but has the DES being replaced by AES and the AES key size is 192 bits. How should I compute the total effective key length of the ...
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### how to define the Truncated Polynomial?

In NTRUEncryption, I seen the trucated polynimials, but I cannot understand the trunacated polynomial calculation. So, could tell me anyone How do we calculate the truncated polynomial?
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### Why does image size increase after steganography?

I am working on a steganography technique. Sometimes I need to send my steganographed images online to my friends but in the process the image gets compressed destroying all the data. I found ways to ...
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### Proving property of Modified Rabin Signature

I'm trying to prove one of the properties of the “Modified Rabin Signature Scheme”: If $gcd(x, n) = 1$, then $x^{\frac{(p−1)(q−1)}{2}} \equiv 1 \pmod n$ This is what I’ve got so far… Proof Let ...
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### Any stream cipher with avalanche effect?

As far as I know, all stream ciphers work by generating a stream of pseudo random bytes, and XOR it with the plain text to get the cipher text. It has the property that one bit change in the cipher ...
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### Is there a standard for hashing a list of strings?

I'm working in Python, and my goal is to hash a list of strings with a cryptographic hash function such as sha256. It would need the property that a change in any string, or the order of the strings, ...
201 views

### How to determine RSA block size for decryption

A message coded with RSA is broken up into blocks, then padded/transformed prior to encryption via modular exponentiation with the public key. However, the encryption result for each block is not ...
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I need to store passwords for a web application, and was looking at Bcrypt for a possible solution. After some research, it appears only the first 72 bytes influence the final hash output. While it is ...
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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 speaking,...
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### Exactly how bad is using 'mod' to clamp reduce numbers to a given range?

Let's say I'm naive and want to generate a random integer in the range [0, m), so I do the following: ...
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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 ...
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### 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: ...