# Questions tagged [cryptography]

For questions about cryptography as it relates directly to quantum computing. Not for general cryptographic methods.

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### Clarifying Mosca's Inquality Theorem - parallel vs additive

I wonder if anyone might have some insight to Mosca’s Inequality "Theorem". It states to when users need to be worried about quantum computers factoring traditional cryptography. Mosca’s theorem ...
77 views

### Calculating power of a quantum computer — RSA

As discussed in this question, the expected security of 1024-bit RSA is 80-bits: NIST SP 800-57 §5.6.1 p.62–64 specifies a correspondence between RSA modulus size $n$ and expected security strength ...
39 views

### GHZ - measuring particles

I'm referring to an earlier question. It involves secret sharing based on the different measurement directions 3 people i.e Alice Bob and Charlie do. Now there is a block in the referred paper which ...
71 views

### Does a 5-partite state with these entanglement properties exist?

Is there a 5-partite state such that if two parties each hold 2 shares of the state, without knowing what shares the other party holds, they can create a Bell pair between them and know which qubit in ...
82 views

### Grover's algorithm and RSA from Nielsen

Nilsen states that one can define a function for the oracle in the Grover algorithm, which is constructed as follows. So there is a number $m$ that consists of $p$ and $q$ (both primes) $m = pq$. Now ...
65 views

### Reversible crypto systems for use in the Grover's algorithm

I'm working on some papers (here and here) that use the Grover algorithm to crack krypto systems like AES and SHA. I had already asked a first question here. Now, however, a new question has arisen ...
199 views

### Grover's algorithm – DES circuit as oracle?

In the literature before me, the quantum oracle of the Grover algorithm is shown as a function, in which a sign change is made possible $|x\rangle\rightarrow(-1)^{f(x)}|x\rangle$. I have read that it ...
344 views

### Grover's algorithm: number of searches

I would like to start my question with a quote: If an encrypted document and its source can be obtained, it is possible to attempt to find the 56-bit key. An exhaustive search by conventional means ...
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### Consequences of Grover's algorithm

I want to look more closely at the consequences of the Grovers algorithm. As is well known, the algorithm provides a quadratic improvement compared to classical search algorithms. Specifically, I ...
97 views

### Quantum teleportation of a state, from one of two bases

I'm watching Christian Schaffner's talk on quantum position-based cryptography (link here) and have a question about a particular application of teleportation. At about the 16:40 mark, he seems to ...
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### What is the difference between quantum key distribution and quantum key exchange?

Both the words 'Quantum Key Distribution' and 'Quantum Key Exchange' are used in quantum cryptography in the problem of a secure key distribution for encryption/decryption between parties. There are ...
130 views

### What does teleportation have to do w/ XOR linked lists? [closed]

I am trying to wrap my head around the correlation between repeating teleportation & XOR linked lists. I understand that: "the quantum equivalent of the one time pad (i.e. XORing a message with ...
60 views

### How are nonlocal games used in device-independent quantum cryptography?

As I understand it, device-independent quantum cryptography enables you to safely perform cryptographic operations without necessarily trusting the quantum device on which they are performed. Nonlocal ...
390 views

### Density matrix after measurement on density matrix

Let's say Alice wants to send Bob a $|0\rangle$ with probability .5 and $|1\rangle$ also with probability .5. So after a qubit Alice prepares leaves her lab, the system could be represented by the ...
132 views

### BB84 protocol over LiFi

The question is already explained in the title, but I'm curious regarding the possibility to adopt the BB84 protocol over LiFi (in theory). Could it be possible or are there some limitations, such as ...
120 views

### How to encode a qubit in standard basis?

I wonder what the steps for encoding a qubit in a certain basis are (you can give your answer in terms of the standard basis for simplicity). Example and a little explanation about steps would be ...
61 views

### Comprehension questions on quantum cryptography especially BB84

I have recently read a lot about the BB84 protocol, I have used three primary sources, the original work, a QK book, and a diploma thesis. My questions refer to the photons sent by Alice, the base of ...
84 views

### Lesser qubit computer doing the parts of Shor's against e.g., RSA-2048 sized prime

After posting this question to Physics, it became pretty clear I should have posted here. So: How might a (e.g.) 72-bit crypto-relevant quantum computer attack RSA-2048? Bonus: how might that be ...
349 views

### BB84 attack with entangled qubits example

BB84 attack with entangled qubits example Hi, I'am interested in an attack for BB84 protocol with entangled quibits. Lets say Alice sends a qubit $x$ in state $\left|1\right>_x$ to Bob and Eve ...
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### Quantum attack on hash functions

The line of questioning is inspired by the pick one trick in Section 4 of the PDF version of the paper Quantum Attacks on Classical Proof Systems - The Hardness of Quantum Rewinding (Ambainis et al., ...
270 views

### Can we speed up the Grover's Algorithm by running parallel processes?

In classical computing, we can run the key search (for example AES) by running parallel computing nodes as many as possible. It is clear that we can run many Grover's algorithms, too. My question is;...
139 views

### How can Shor's algorithm be used to crack 32 bit RSA Encryption? [closed]

I have this as my minor project and plan to use Shor' Algorithm for factorization to figure out the $p$ and $q$ factors for cracking. Although I understand the theoretical part, I am having some ...
49 views

### Lattice based cryptography vs BB84

Am I correct in thinking that post-quantum cryptography such as lattice-based solutions run on classical computers are resistant to quantum attacks (as opposed to RMS), whereas quantum key ...
153 views

### How many qubits does it take to break a 10 characters password?

Let's assume we developed a hashcat-like programs for quantum computer. How many qubits we need to find the correct hash (WPA, MD5,...) from a 10 characters password make from upper, lower & ...
150 views

### How many bits do Alice and Bob needs to compare to make sure the channel is secure in BB84?

I was trying to self-study qmc by reading the Quantum Computing A Gentle Introduction book, in section 2.4 it talks about the quantum key distribution protocol BB84. After (I thought) I understood it ...
51 views

### Are non-secret-based quantum money mini-schemes susceptable to Jogenfors' “reuse attack?”

Aaronson and Christiano call public-key or private-key quantum mini-schemes $\mathcal M$ secret-based if a mint works by first uniformly generating a secret random classical strings $r$, and then ...
188 views

### Can quantum money be reliably “burned?”

One of the novel features of Bitcoin and other cryptocurrencies is that coins can be irrefutably "burned" or destroyed, by creating a transaction to send the money to a junk burn address. Thinking ...
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### What measures can be taken against attacks on cryptosystems by quantum computers other than just classifying research?

If quantum computers advance to the point where they can defeat RSA, DSA, SHA (and really all existing classical public key encryption or and authentication) then it appears that it would be ...
123 views

### What classical public key cryptography protocols exist for which hacking is QMA complete or QMA hard?

Such a public key cryptosystem would be "quantum safe" in the sense that quantum computers cannot efficiently solve QMA hard problems.
170 views

### How to justify post quantum encryption security?

Is there some definition or theorem about what a quantum computer can achieve from which post-quantum cryptographic schemes (eg lattice cryptography, but not quantum cryptography) can justify their ...
310 views

### Time Entangled Quantum Blockchain

This answer cites a paper[$\dagger$] which purposes a quantum blockchain using entanglement in time. "The weakness is that the research only presents a conceptual design." - QComp2018 How could a ...
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### Is quantum cryptography safer than classical cryptography?

Quantum computing allows us to encrypt information in a different way compared to what we use today, but quantum computers are much more powerful than today's computers. So if we manage to build ...
195 views

### BB84 Protocol Alice Choice to Bob

The image is taken from this link. Here Alice is using random bases to encode 0 or 1. After the process is completed, Bob has similarly polarized photons as Alice. These polarization can be any of ...
233 views

### 'Rectilinear' and 'Diagonal' Basis in BB84 Protocol

A lot of the tutorials on BB84 protocol talks about these two measurement bases, 'Rectilinear' or 'Vertical-Horizontal' and 'Diagonal'. I understand that it is possible to create a physical device ...
427 views

### Quantum computing and blockchain technology

It is popularly stated that quantum computing could destroy and disrupt blockchain technology completely. How is quantum computing a threat to blockchain technology?
364 views

### Does quantum computing threaten blockchain?

As per Wikipedia, blockchains are a way to maintain "a continuously growing list of records, called blocks, which are linked and secured using cryptography [... and] inherently resistant to ...
136 views

### How can we reliably know if a key size is still safe to use as new quantum computers are created?

I've heard that quantum computers pose a major threat to 1024 bit and possibly even 2048 bit RSA public-private key cryptography. In the future however, bigger size keys will probably become at risk ...
386 views

### Rigorous security proof for Wiesner's quantum money

In his celebrated paper "Conjugate Coding" (written around 1970), Stephen Wiesner proposed a scheme for quantum money that is unconditionally impossible to counterfeit, assuming that the issuing bank ...
340 views

### How would Blockchain technologies change to survive a post-quantum world? [closed]

Reading this entertaining piece of a QC enthusiast mining bitcoins with a Quantum Computer (although efficiently mining bitcoins with the current state of QCs is far-fetched, it is quite possible to ...
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### Will post quantum crypto come soon enough? [closed]

The main reason to start with Post Quantum Crypto (PQC) right now is because creating strong crypto, good implementation and accepted standards takes very long. Right now, most PQC is in the 'crypto' ...
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### Quantum Bitcoin Subdivision

Background Recently I was reading the article "Quantum Bitcoin: An Anonymous and Distributed Currency Secured by the No-Cloning Theorem of Quantum Mechanics" which demonstrates how a quantum bitcoin ...
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### Advantage of quantum key distribution over post-quantum cryptography

Post-quantum cryptography like lattice-based cryptography is designed to be secure even if quantum computers are available. It resembles currently employed encryptions, but is based on problems which ...
208 views

### Are there any encryption suites which can be cracked by classical computers but not quantum computers?

Are there any encryption suites that can be cracked by usual computers or super computers, but not quantum computers? If that's possible, what assumptions will it depend on? (Factorizing big numbers, ...
276 views

### Blind quantum computing — generic structure variable selection

Background Recently I came upon a research article entitled Experimental Demonstration of Blind Quantum Computing. Within this research article, the scientists claimed that - through the proper ...