# Questions tagged [encryption]

a transformation of information to make it unreadable to outside observers

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### Is this Correct, the existence of cryptography requires $UP \cap Co-UP \not\subseteq BPP$

Is this Correct, the existence of cryptography requires $UP \cap Co-UP \not\subseteq BPP$? Or does it require $UP \not\subseteq BPP$?
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### Asymmetric encryption puzzle

I came across this riddle link question: Your name's Graham. Bob wanted to send Alice a secret msg m. However, he was concerned about the security. He encrypted his message with a keyword that both ...
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### encrypt with permutation ciphers

I came across this question: You are given a permutation cipher defined by the bijection t: N -> N where, ...
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### Decryption of RSA

How can we decrypt an RSA message if we only have the public key? For example, Message: 21556870,12228498 Public Key: ...
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### How to perform matrix multiplication in Mixing Columns step of AES?

I am studying AES and trying to implement it. But I am having difficulty understanding the Mixing Column step. In that step we have to perform matrix multiplication between the state matrix and ...
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### Absolute limit in Huffman encoding

I have computed the average code length using the Huffman encoding. Now, I am asked to compare the average code length with Absolute limit but I ain't sure about the absolute limit here is :( Could ...
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### Two way communication using asymmetric encryption

According to image 1, for two way communication, each person needs to generate a pair of keys. However, according to image 2, only one pair of keys is required for two way communication. Thanks to ...
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### What happens when the xor inBlock chaining returns false?

"With block cipher, each plaintext block is XOR-ed with the previous ciphertext block and then encrypted, this is known as block chaining. This prevents identical blocks of plaintext from producing ...
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### Abusing ElGamal in order to attack a known encrypted text

I saw a very interesting question regarding Elgamal cryptosystem that I don't know its answer. It is really interesting and I would be very happy if you could elaborate on it and explain the tricky ...
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### learning the private key by reusing same random variable k in ELgamal

i wonder: if for some reason, someone, say alice, sends unencrypted messages to bob and signs it using elgamal signature, can oscar,the adversary, gain knowledge of the private key if alice reused the ...
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### equivalent sub-keys in DES encryption

i am trying to understand the DES cryptosystem and was wondering: what would've happened if all of the sub-keys were equal? does it reduce the security? can we actually find the keys if we know all ...
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### Period of sum of two periodic sequences

I was wondering, what is to be the shortest possible key using Vigenere encryption, if a text is ciphered one time with a key of length $i$ using Vigenere and second time with a key of length $j$ ...
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### Determining key from plaintext and ciphertext (Vigenère)

I was wondering why mathematically we can know the key in the Vigenère cipher if we know in advance a message and its encryption.
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### Can reducing NP problems to subset NP Problems leed to faster calculation of NP problems

Integers can be represented with a pair of smaller numbers. To achieve this I use 2 integer sequences. Integer Sequence 1 1,1,2,1,2,3,1,2,3,4... And Integer Sequence 2 1,2,1,3,2,1,4,3,2,1... This ...
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### Weaknesses arising from using same key in both channel directions

I came across the following question: Which of the following risk may arise, when same key is used to encrypt both directions of a communication channel, that are not present if using different keys ...
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### Perfect Probabilistic Encryption still requires key length about as long as message

Let $(E,D)$ be a probabilistic encryption scheme with $n$-length keys (given a key $k$, we denote the corresponding encryption function by $E_k$) and $n+10$-length messages. Then, show that there ...
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### RSA cryptosystem: Encrypting a signed message

I've started to read the section about the RSA cryptosystem in CLRS (page 958) and I don't understand the way it describes how to encrypt a signed message. If Bob wants to send a message $M$ to Alice:...
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### A tree-like data structure with rights delegation for distributed computing

Every actor can create a root node and delegate a right to add a child node. Every node contains name of its’ creator or who added it, and value S. Sum of all values S at the same level of the tree ...
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### Structure Preserving Continuous Hash Function

This question was originally posted on super user, but redirected here based on some suggestions. I am completely new with computer science, and not only recently did I run into the notion of hashing....
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### Why don't most websites I visit seem to use TLS with Ephemeral Diffie-Hellman?

When I click "view certificate," under Public Key Algorithm it usually says "RSA Encryption" or "Elliptic Curve Public Key." I assume this is the algorithm used to agree on a premaster secret (...
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### Why does Feistel encryption algorithm encode half block every time?

Why does Feistel encryption algorithm encode half block every time? What would happen if the entire block is encrypted in each step?
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### What is the point in hashing a value?

I apologise if this is not the right place for this question... I didn't want to ask on Stackoverflow or CodeReview as it will closed in minutes as "too broad". A client of mine is writing an API ...
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### Finding small-length RSA private key

I was wondering if there is an algorithm to derive $p$ and $q$ or is it simply trial and error? Consider the following RSA crypto-system with public key $(437,13)$. Since the numbers are so small, it ...
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### DES encryption - what happens if all sub-keys are the same?

What will happen if we change the DES encryrption algorithm to use the same key in each round ? How will it affect the encryption - will it only make the encryption same as decryption or are the more ...
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### fully homomorphic encryption with information-theoretic security?

An encryption algorithm with information-theoretic security is one which even with infinite amount of computation cannot be broken. That is, given only the ciphertext, no amount of computation can ...
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### How can a fully encrypted homomorphic program resist copying?

Let's say that we have an original homomorphically encrypted program with any possible code inside. What methods can it use to ensure that it is impossible to create an exact copy of it? Are there ...
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### Finding $d$ in RSA Encryption mathematically and by hand

I'm working on this RSA encryption problem and the catch is that it must be done by hand and mathematically. Let's say $p=11$, $q=13$ $$N=p \cdot q=11 \cdot 13=143$$ I chose $e$ to be relatively ...
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### Can I prove that I have x such that f(x) < c without revealing x?

I'm interested in applications to verifiable computing. Let's say Alice would like to find an x such that f(x) < c for some real-valued function f and some c of Alice's choosing, so she hires Bob ...
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### Can software itself be encrypted?

Imagine that I need to install my server in a remote location, so my algorithm can do something fancy (collocation of server at a financial exchange is how I thought of this question, but it applies ...
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### Given a string, is it possible to determine which hashing algorithm has produced it, if any?

Given a string, is it possible to determine which hashing algorithm has produced it, if any? For example, the MD5 hash of "string" is b45cffe084dd3d20d928bee85e7b0f21. Is it possible to determine ...
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### What are the flaws in this encryption algorithm?

Let $A$ be a randomly generated matrix. Let $I$ be the identity matrix. $\times$ is the matrix multiplication operation, $/$ is matrix division (multiplying the first matrix by the inverse of the ...
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### Is my interpretation of the SSL process correct?

Firstly, which of the two diagrams below is the more accurate or more well-explained (if both are accurate) representation of the SSL process (at a beginner's level)? Diagram 1 Diagram 2 Also where ...
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I am having a very difficult time trying to understand what exactly this question is asking. How many valid English plaintexts are there for the ciphertext message CJU using a length-3, one-time ...
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### Data Compression Algorithm for Less repetitive pattern (redundancy) [duplicate]

Context: Lossless Data compression (source coding) algorithms heavily rely on repetitive pattern (redundancy) Questions Which data compression method/algorithm deals with less repetitive pattern (...
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### If P=NP, are there cryptosystems that would require n^2 time to break?

If P does equal NP, will it still be possible do design a cryptosystem where the optimal cryptanalysis algorithm takes, say, the square of the time taken up by the legitimate encryption and decryption ...
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### Monoalphabetic Cipher Key

I am not sure how to get the key for the following Monoalphabetic Cipher question. This is a textbook question and I know the answer, but I juts dont know how they got the key. Question: Decrypt the ...
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### Encryption on a compressed file

Can I do encryption on a compressed file and again decompress the file after decryption to get the original data?
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### What if p and q are not distinct in RSA Crypto System? What could go wrong?

In RSA Crypto System we choose p and q such that they are distinct primes. Calculate n=pq and phi=(p-1)(q-1). Then e is chosen for public key (n,e) s.t. gcd(e,phi)=1
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### Issues in RSA setup

Suppose we have public key: $$n= 1015, e= 3$$ and private key: $$d= 635, p= 35, q= 29, \phi(n)= 952$$ For $m = 100$, we have $$c = m^e ~mod~n = 100^3 mod~1015 = 225.$$ To decipher this, let us ...
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### How to revert a carryless polynomial multiplication?

In the carryless multiplication of two polynomials in a 8-bit environment, is it possible to obtain the original 8-bit values from the result? As an example: \begin{align} (x^{11} + x^{10} + x^9 ) ...