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Questions tagged [cryptography]

Questions about the construction and analysis of protocols and algorithms for secure computation and communication (including authentication, integrity, and privacy aspects).

Cryptography studies techniques for secure computation and communication, i.e. secure storage and manipulation of information, both theoretical and practical.

The purposes of cryptography include confidentiality (leading to ), data integrity (leading to functions), authentication and more. Cryptography often relies on algorithms based on mathematical operations to transform data. Cryptography also includes the study of secure communication protocols.

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Why hasn't there been an encryption algorithm that is based on the known NP-Hard problems?

Most of today's encryption, such as the RSA, relies on the integer factorization, which is not believed to be a NP-hard problem, but it belongs to BQP, which makes it vulnerable to quantum computers. I wonder, why has there not been an encryption…
Ken Li
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How to verify number with Bob without Eve knowing?

You need to check that your friend, Bob, has your correct phone number, but you cannot ask him directly. You must write the question on a card which and give it to Eve who will take the card to Bob and return the answer to you. What must you write…
Joe
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Can a public key be used to decrypt a message encrypted by the corresponding private key?

From what I have seen about usage of a pair of public and private keys, the public key is used for encrypting a message, and the private key is used for decrypting the encrypted message. If a message is encrypted by the private key, can it be…
Tim
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Meaning of: "'If factoring large integers is hard, then breaking RSA is hard,' is unproven"

I was reading CLRS and is said: If factoring large integers is easy, then breaking the RSA cryptosystem is easy. Which makes sense to me because with the knowledge of $p$ and $q$, it is easy to create the secret key which the knowledge of the…
Charlie Parker
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What is an extremely basic asymmetric cipher that I can present at the pub?

I'm trying to explain the basics of Bitcoin to my parents. One of the core components of bitcoin, is signing transactions to make sure your identity can't be impersonated, and thus the need to explain a simplified asymmetric cipher. What's an…
Ryan Leach
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Why is encrypting with the same one-time-pad not good?

To encrypt a message $m_1$ with a one-time-pad key $k$ you do $Enc(m_1,k) = m_1 \oplus k$. If you use the same $k$ to encrypt a different message $m_2$ you get $Enc(m_2,k) = m_2 \oplus k$, and if you perform Xor of the two ciphertext you get $$(…
Ran G.
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Given RSA, why do we not know if public-key cryptography is possible?

I was on wikipedia on list of unsolved computer science problems and found this: Is public-key cryptography possible? I thought RSA encryption was a form of public-key cryptography? Why is this a problem?
Namster
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How does an operating system create entropy for random seeds?

On Linux, the files /dev/random and /dev/urandom files are the blocking and non-blocking (respectively) sources of pseudo-random bytes. They can be read as normal files: $ hexdump /dev/random 0000000 28eb d9e7 44bb 1ac9 d06f b943 f904 8ffa 0000010…
Adam Matan
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Could quantum computing eventually be used to make modern day hashing trivial to break?

Simply put, if one were to build a quantum computing device with the power of, say, 20 qubits, could such a computer be used to make any kind of modern hashing algorithm useless? Would it even be possible to harness the power of quantum computing…
hakusaro
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Why did RSA encryption become popular for key exchange?

This is a soft question. I don't know a lot about cryptography or its history, but it seems like a common use for RSA is to do key exchange by encrypting a symmetric key to send a longer message (e.g., the description of iMessage here). Isn't this…
Louis
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Are asymptotic lower bounds relevant to cryptography?

An asymptotic lower bound such as exponential-hardness is generally thought to imply that a problem is "inherently difficult". Encryption that is "inherently difficult" to break is thought to be secure. However, an asymptotic lower bound does not…
Micah Beck
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How to practically construct regular expander graphs?

I need to construct d-regular expander graph for some small fixed d (like 3 or 4) of n vertices. What is the easiest method to do this in practice? Constructing a random d-regular graph, which is proven to be a.a.s. an expander? I also read about…
user2145167
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Is it possible to create a "Time Capsule" using encryption?

I want to create a digital time capsule which will remain unreadable for some period of time and then become readable. I do not want to rely on any outside service to, for instance, keep the key secret and then reveal it at the required time. Is…
Micah Beck
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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 algorithms? Do any such algorithms already…
S.LAKSHMINARAYAN
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Break an authentication protocol based on a pre-shared symmetric key

Consider the following protocol, meant to authenticate $A$ (Alice) to $B$ (Bob) and vice versa. $$ \begin{align*} A \to B: &\quad \text{“I'm Alice”}, R_A \\ B \to A: &\quad E(R_A, K) \\ A \to B: &\quad E(\langle R_A+1, P_A\rangle, K)…
Gilles 'SO- stop being evil'
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