# Tagged Questions

Analysis of individual security aspects of a cipher or algorithm, not the security of a cipher or algorithm in general (which would lean towards “algorithm-design”).

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### Find plaintext of RSA by solving extended euclidean algorith for two encrptions with two different exponents for same plaintext

This is my homework question (but I am not asking the answer to it): Suppose two users Alice and Bob have the same RSA modulus n and suppose that their encryption exponents eA and eB are ...
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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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### Key Derivation Function (KDF): Can a key derived from KDF be considered as a secure key?

Consider a case where we have a master key MS that is used in pseudo-random function to generate a set of pseudo-random values. Then we use key derivation function to derive a key from each of pseudo-...
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### How to compare the efficiency of public key cryptosystems, i.e., RSA vs El Gamal?

As part of my Mathematics degree I'm taking a Discrete Mathematics module which partially covers Public Key Cryptography but does not at all enter it in depth. I'm currently working on a project that ...
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### Is it OK to use a secret IV as key when creating keyed MD5 checksum for fixed-size data?

Is it OK to use a secret IV as key when creating keyed MD5 checksum for fixed-size data? Because data size is known, attacker cannot append anything. And I think it's hard to raise chosen-plaintext ...
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### Factorizing N to derive D [duplicate]

So I have the value of N. I have to use this value of N to find the value of D. N = 265291078722948385089717069136983657793 Now I read about RSA algos and found out that ...
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### Relationship between exponent and modulus in RSA (as RSA properties as listed in X.509)

In one of my assignments, I had the following question (Please read on. Not a homework assignment :) ): X.509 (1998 version) lists properties that RSA keys must satisfy to be secure. One such ...
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### Calculating Phi(M) and D for RSA decryption given p,q,e

I've been given a message in 4 digit numbers but it doesn't seem to work when I use my calculated values for phi(m) and d. If I'm correct phi(m) = (p-1)(q-1) correct? And d is given by ed = 1mod(phi(m)...
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### Which precautions to protect against side-channel attacks on ARX ciphers?

In recent crypto there has been a trend to design ciphers using only the ARX set of instructions - i.e. additions (modulo $2^{32}$ or $2^{64}$), rotations (by a fixed constant) and XORs, examples ...
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### Why can't I reverse a hash to a possible input?

I'm going to provide “proof” why a hash function can be reversed, and I hope you can tell my why I'm wrong So, a hash function can be implemented as a series of logic gates. All logic gates can be ...
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### Cracking a hash which descends to a single value after repeated hashes

Assume I have a hash function (such as the one below) and a cipher, and I know that if I repeatedly hash any string over and over again, it will eventually descend to one unique value. Is there any ...
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### Difference between SHA1 and SHA1HMAC? [duplicate]

Can anyone let me know the difference between SHA1 and SHA1HMAC algo?
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### Is this a correct way of use symmetric encryption?

If I want to encrypt some block of text in AES, but at the time of decryption I don't have a way to check if the symmetric key is the correct one. So I: Generate a randomKey, and use that to encrypt ...
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### How to calculate LOW(b) and HIGH(b) in spritz cryptography

I try to implement spritz cryptography like this paper . But I don't know how to calculate LOW(b) and HIGH(b). In paper page 7. ...
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### Finding key of AES in ECB mode

How many plaintext-ciphertext pairs would be needed to guess with probability, say $p=0.5$, the key $k$ of AES under ECB mode? Most of the articles that I found tried attacks on single plaintext-...
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### Developing algorithm for detecting plain text via frequency analysis

First post on Crypto SE, so I've attempted to search the term "English" and looked through the first 10 pages of results, to see if this was a common question, I couldn't find a suitable answer. I'm ...
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### how to decrypt hidden text under numbers? [closed]

got a question as follows - how to decrypt below in to letters knowing that crypting language is 469 and it base ofcourse on mathematic. 46781! 9063376290 3222011 677 80322429 67538 14805394, 6880326 ...
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### How do we solve Shammir 2,3 sharring scheme where arithmetic is modulo?

I am trying to get secret S and I have 3 values (2,2)(6,3)(4,9) and know that modulo is 13. I tried using Lagrange basis polynomial and got 8 but I am not sure if that is right and also I cannot find ...
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### Find plaintext from two ciphers encrypted with a one-time pad and the same key

I'm studying for a cryptography test and I came across this question: Assume a language has three letters: A=1111, B=0011 and C=1010. Two words in this language are encrypted with a one-time pad ...
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### Can Grover's Algorithm be combined with a meet-in-the-middle attack?

We all know and love the meet-in-the-middle attack which basically makes double encryption pointless using a time-memory-tradeoff. Now there was recently the recommendation by the NSA to use double ...
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### What is the most an adversary can do in the context of public key masquerading?

Suppose two parties A and B are using a public-key encryption scheme in order to communicate securely. A and B both have a public/private key pair: $$A: (P_A, S_A), B: (P_B, S_B)$$ and A already ...
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### One Time Pads and reuse of the decrypted message?

How is it insecure when Alice encrypts a message with a One-Time-Pad to Bob, and Bob then uses the decrypted message from Alice as the next One-Time-Pad? For example: Alice sends Bob a message ...
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### Carrying out a known-plaintext attack on a cipher [closed]

Previously posted here. Someone suggested I could post it here to get some answers. I am trying to solve a CTF challenge, here. Basically we're given message and key inputs to a cipher and its ...
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### Output of using the same key in all 3 boxes of 3DES

I am trying to figure this one out: What would be the encrypted output for an input x if the same key K is used in all three boxes of 3DES? The encryption scheme for 3DES with three different keys, ...
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### How to overcome issues with big ciphers? E.g. with small vs. big Vigenére Ciphers

I understand that bigger ciphers show more recurring patterns and therefore should be easier (or at least faster) to decrypt as you'll have a better statistical analysis, being However if we use a ...
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### Memory-hard proof-of-work: are they ASIC-resistant?

Is a memory-hard proof-of-work scheme necessarily resistant to speedups from custom ASICS? Background: Bitcoin uses a proof-of-work scheme based on SHA256 hashing. The scheme is compute-bound. ...
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### Data Switching and Information Leakage

Hypothesis: Let $z,a$ be uniformly random elements of a field $\mathbb{F}_p$ where $p$ is a large prime number. Also, let $(-z)$ be additive inverse of $z$. I have a fixed secret value $x$. I mask ...
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### Deciphering “easy” ciphers without hints

I've been keen on IT Security for a long time now and I've learned a lot about networking & security. However trying a "decryption challenge" I'm lacking what I think is basic encryption/...
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### How to clock a linear feedback shift register?

I have some trouble understanding what happens when a LFSR is clocked. I have tried many examples but I do not get the correct result. Can anyone point out my errors? I have the following polynomial: ...
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### Is there any benefit of changing the AES-CCM encryption key periodically even if the Nonce space is not exhausted yet?

I would like an answer that could be sustained by a mathematical proof. Let's say I'm using AES-CCM to encrypt a lot of messages, and I always use a different nonce. I know that when the nonce space ...
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### Weakness of SRP after server compromise

From SRP section 3: The host stores user passwords as triplets of the form { <username>, <password verifier>, <salt> } Password entries ...
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### Weaknesses of RFC6628

In RFC6628 section 2.2, they say: w = The password remembered by the user. W = The password verifier registered in the ...
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### Is it possible to find a Mersenne Twister seed given only the first output?

Suppose I have a Mersenne Twister seeded with some unknown value, and I’ve been given the first output. Given only this first output, and no other information, is it possible to derive the unknown ...
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### Avalanche effect sample size

With a fixed key size – key has 128 bits, while block size is 8 byte – how do I calculate how many different keys and texts I have to test for an cryptanalytic statistics study? Differently worded: ...
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### Hypothetical encryption technique, is it secure?

This to me, sounds like a great idea, but with all rosy-tinted ideas it needs some grounding in reality. Here are the steps of the hypothetical system (with the hypothetical part highlighted): User ...
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### What is a “freestart collision”?

In their work on SHA-1 collisions (cf. the EUROCRYPT-2016 paper “Freestart collision on full SHA-1” by Stevens, Karpman, and Peyrin) Stevens et al show that they are able to generate "freestart ...
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### Profile Complexity LFSR

I try to understand how Complexity of an LSFR calculated. For example I have a bit sequence 110101 and it's profile 1 1 2 2 3 3. Is there any easy way in order to produce the profile?
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### How to break a columnar transposition cipher?

I'm currently studying for a cryptography exam. I've been given ciphertext that has been encrypted by a columnar transposition cipher. I've been given no shift key length or key word, the only thing I ...
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### 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 ...
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### What is the difference between CPA and IND-CPA?

I am writing a paper and I got comments from the reviewer that I don’t really understand. Is there any difference between a CPA (Chosen Plaintext Attack) and IND-CPA (Indistinguishability under ...
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### Quantum on hash

Perhaps this has been answered before. Grover's algorithm should result in a 256 bit hash being complexity 128 bits to crack. I was wondering, what if you had a 512 bit hash , and xor'd the lower 256 ...
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I read this post from Ben Tasker describing how to take control of a powerline network. Since I am interested in security, I tried to do the same thing, to see if it was possible. I used Devolo ...
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### Do $v_1=\alpha\cdot r_1$ and $v_2=\alpha\cdot r_2$ leak information about $\alpha$
Please consider we have finite field $\mathbb{F}_p$ for large prime number $p$. We have a fixed field element $\alpha$. By $r_i\leftarrow \mathbb{F}_p$ we mean we pick $r_i$ uniformly random from the ...