Questions tagged [public-key]

An asymmetric cipher is an encryption scheme using a pair of keys, one to encrypt and a second to decrypt a message. This way the encrypting key need not be kept secret to ensure a private communication. Similarly in public key authentication, the verification key can be public and the signing key private.

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How are timestamps verified?

You put an input and the hash value comes as an output then when someone puts the input the hash function it is applied to see if it is the same hash original value is stored in some database , that ...
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Why do we need asymmetric algorithms for key exchange?

In SSL protocols, both symmetric and asymmetric algorithms are used. Why is it so? The symmetric algorithms are more secure and easier to implement. Why are asymmetric algorithms usually preferred in ...
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Is it possible to validate a Public Key in RSA?

If I have a 1024-bit number, and someone is telling me that it is in fact a valid RSA public key, is there any way I can quickly validate that it is indeed so (without cracking RSA)? (I suppose I am ...
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File format of an $2048$ bits RSA public key

From here, there are $14$ bytes specifying the file format of the key. I am still quite confused about the format bytes. For example, the following public key 30 82 01 0a 02 82 01 01 00 8e a3 d1 c7 9c ...
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What happens when a root CA has its private key compromised?

What happens when a root CA has its private key compromised? Then all children in the tree are compromised too? And then all certificates are compromised? What needs to happen then? Related: - How ...
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Is there a public key semantically secure cryptosystem for which one can prove in zero knowledge the equivalence of two plaintexts?

If Alice encrypts two messages $a$ and $b$, such that $x=E(a)$, $y=E(b)$. Can Alice prove (without revealing $a$, $b$ or the private key) that $a = b$? Obviously the proof must not be too long and it ...
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Difficulty of breaking RSA for a given key size

Is it true that breaking a 1024-bit RSA key is as difficult as breaking a 128 bit symmetric key (e.g. AES)? I know that breaking a RSA key is equivalent to factoring the modulus $N$. To factor it, you ...
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Using same modulus for RSA

I know that there exist some attack when using same modulus. Can two different pairs of RSA key have the same modulus? RSA cracking: The same message is sent to two different people problem But ...
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Symmetric vs. Asymmetric cryptographic approaches to data security

I know the basic differences between Symmetric vs. Asymmetric cryptography, but I'd love to know more details: Exactly why is the asymmetric approach slower than the symmetric? Why does it make use ...
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Given enough RSA signature values, is it possible to determine the public key value?

Given enough RSA signature values, is it possible to determine which public key is required to verify the values? Could there be enough information to establish which key is required? Is there ...
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Why should the RSA private exponent have the same size as the modulus?

Consider the generation of an RSA key pair with a given modulus size $n$ and a known, small public exponent $e$ (typically $e = 3$ or $e = 65537$). A common method is to generate two random primes ...
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Can an RSA private key have several public keys?

Ok, so my cryptography lecturer in University posed this question at the end of the RSA key generation lecture as a brain teaser. I have been thinking about this and I think I have come up with a way ...
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Computing p and q from private key

We are given n (public modulus) where n=pq and e (encryption exponent). Then I was able to crack the private key d, using Wieners attack. So now, I have (n,e,d). My question is, is there a way to ...
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Alice answering Bob without knowing the question

I'm a student and I don't know how to solve the following problem : Alice is proposing a 10 questions list to Bob. Bob choose only 1 question and wants to know the answer without Alice knowing the ...
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Why is it impossible to find the private key from the public key?

From what I understand, based on the article A Beginner’s Guide: Private and Public Key Cryptography Deciphered, the public key is generated by performing $n$ tangent plus mirroring operations from ...
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Algorithm Design for only Mutual Information Sharing

Bob and Alice each have a bit string they want to keep private. They each want to know what the bitwise AND of their two strings would be without telling the other or anyone else listening to their ...
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Is it possible to perform a rudimentary asymmetric key encryption algorithm by hand?

I have at least three, distinct motivations for having a simple way to use asymmetric key encryption by hand or without modern computers. First, I was recently demonstrating to a 12-year-old how ...
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Can Paillier ,RSA or any other schemes be used for universal re-encryption like elGamal?

ElGamal, RSA, and Paillier cryptosystem have homomorphic property, and can be used fro re-encryption purposes. I want to use the encryption to re-encrypt ciphertext(as in proxy re-encryption but ...
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How do I encrypt with the private key? [duplicate]

Possible Duplicate: RSA encryption with private key and decryption with a public key This wording is creeping everywhere (e.g. there): "I encrypt with the private key" and even sometimes, "I ...
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Knowing pre-encryption data, can we find the private key CryptoWall used?

I'm just repurposing a question already asked about Cryptolocker for CryptoWall: "If we know exact contents of some of the files prior to them being encrypted, would it be feasible to use those ...
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RSA encryption/decryption process is not working correctly

I'm trying to apply the RSA cryptosystem to encrypt a byte M=72, using predefined modulus n, public key exponent ...
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ElGamal: Multiplicative cyclic group and key generation

Here on the ElGamal wikipedia page http://en.wikipedia.org/wiki/ElGamal_encryption Alice generates an efficient description of a multiplicative cyclic group G, of order q, with generator g. How ...
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What is Identity-Based Encryption (IBE) and why is it “better”?

Most CS/Math undergrads run into the well-known RSA cryptosystem at some point. But about 10 years ago Boneh and Franklin introduced a practical Identity-Based Encryption system (IBE) that has ...
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Is 128-bit security still considered strong in 2020, within the context of both ECC Asym & Sym ciphers

Given that much of our ECC crypto primitives provide “only” 128-bit security when defined over a 256-bit curve due to pollard-rho, is it then still safe in 2020 to consider 128-bit security safe for ...
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What is a trapdoor permutation?

Can anyone explain to me what a trapdoor one-way permutation is? Is RSA a trapdoor one-way permutation? Context: I was reading about ring signatures. On page 560, it describes steps to implementing ...
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Is it bad to expose the public key?

I am studying about blockchain and have been focusing on encryption protocols that allow authentication. The digital signature of public and private key seems solid, I have focused on the algorithm ...
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Can two different pairs of RSA key have the same modulus?

Can $n=pq$ be part of two different pairs of RSA keys? If such keys exist, say $(e_1,n)$ and $(e_2,n)$, how are they related? What will be the security concerns for the two users?
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What's the difference between rsa_pss_pss_* and rsa_pss_rsae_* schemes?

I'm trying to understand the difference between rsa_pss_pss_ and rsa_pss_rsae_. The picture shows the extension of TLS1.3's ...
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Is there an intuitive explanation as to why only the private key can decrypt a message encrypted with the public key?

I have just learned about using PGP/GPG for email encryption and one thing bugs me: How is it possible that a message encrypted with somebody's public key can be decrypted only with that person's ...
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Is it safe to reuse a ECDSA nonce for two signatures if the public keys are different?

We denote the s value of an ECDSA signature $(r, s)$ on a message $m$ as: $s=\frac{H(m)+xr}{k}$ Assume two ECDSA signatures sharing the same nonce $(r, s_1) , (r, s_2)$ on two messages $m_1, m_2$, ...
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Is key size the only barrier to the adoption of the McEliece cryptosystem, or is it considered broken/potentially vulnerable?

A recent paper showed that the McEliece cryptosystem is not, unlike RSA and other cryptosystems, weakened as drastically by quantum computing because strong Fourier sampling cannot solve the hidden ...
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Families of public/private keys in elliptic curve cryptography

I'm looking for a related key scheme for elliptic curve cryptography. The basic idea would be that there would be a master public key and a master private key. From the master public key, you could ...
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Why is Diffie Hellman used alongside public keys?

I just read this post here: Why do we need asymmetric algorithms for key exchange? that asserts that in public key cryptography when asymmetric keys are used to secure communications, for parties to ...
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Why is RSA usually limited to messages up to 1 block [duplicate]

I'm wondering why RSA encryption usually is only used for messages that fit into one block. For larger messages hybrid encryption in combination with symmetric ciphers like AES seem to be the solution ...
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Alice trusts Bob only when Bob trusts Alice

some story first: Alice and Bob both have public/private key pairs. Now Bob wants Alice to sign his public key id. Alice agrees but only when Bob signs the public key id of her. Is this something ...
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Why ECDSA has its form?

According to Wikipedia, if Alice wants to sign some message, she computes $s = k^{-1} (z + r d_A)$ then sends $(r, s)$ to Bob. I don't understand why they use this particular formula $s = k^{-1} (z + ...
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Diffie-Hellman Key Exchange with Authentication: Man-in-the-Middle query

I have been reading up on MiTM attacks, and the prevention of them using public key certificates. Recently I learnt about Diffie-Hellman Key Exchange with Authentication, and how it uses signed ...
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Which public key encryption scheme is re-randomizable? How can I re-randomize an encryption scheme?

Which public key encryption scheme is re-randomizable? Is there any library for re-randomizable encryption scheme? If not, how can I re-randomize a given public key encryption scheme? I cannot find ...
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How is a public key actually used to encrypt something?

One thing cryptography articles never seem to explain is how the message actually gets encrypted. You get this long-winded lecture on number theory which ends with, Ta Da! and we have a public and ...
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Encryption with private key?

we normally always encrypt by public key and decrypt with private key. If i encrypt with private key, then its still secure as normal PKI ? i mean known-plain-text will not take private key on the ...
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risk of attacker decrypting RSA ciphertext without public or private key

As I describe in my previous question I am trying to decide if it's worth it for me to use the Offline Private Key Protocol in creating some long term private archives, instead of just going with a ...
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Definition of Short Authentication String

I am studying for my cryptography final and I passed by the following term: short authentication string. Please, can anyone explain it to me, thank you very much.
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Knowing pre-encryption data, can we find the private key CryptoLocker used?

One of our users seems to have been hit by CryptoLocker. As a result, he has a hard drive full of encrypted files. The ransom-ware claims to have used public key encryption on the files. If we know ...
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Exposing RSA private-key data… bad?

We know that exposing $p$, or $q$ or $\phi(n)$ results in trivial attacks on RSA since they allow us to factor $n$ and to compute the private exponent $d$. In OpenSSL (and most RSA implementations) ...
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Modular exponentiation on calculator for textbook RSA

How do you encrypt $51$ with public key $(n,e) = (91,23)$ I understand that $c = 51^{23} \bmod 91$. How can I calculate the result on a calculator?
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Why are key lengths in asymmetric algorithms typically longer than key lengths in symmetric algorithms?

Apologies if this is a duplicate question. I did search prior to posting :-) I think it may be somewhat related to my other question here but I think it's different enough to warrant a separate ...
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Random Masking of Padded RSA Ciphertext through homomorphism

I had asked a question related to this before: Oblivious Decryption: Decrypting with a private key, without knowing the message @rikhavshah has an answer, which I would like to discuss the security ...
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Do any one-key-of-many cryptographic schemes exist?

I'm pretty sure I understand how public/private key cryptography works. Anybody can encrypt a message using a well-known public key, but only the person who holds the private key can decrypt it. My ...
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Generation of strong primes

It seems that this is pretty difficult to find large (above 1024 bits) strong primes, or at least such primes $p$ where $(p-1)$ has a very large prime factor. Is there any information regarding the ...
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Do known-plaintext attacks exist for public key encryption?

In asymmetric ciphers we publish the public key for anyone, which means an attacker can encrypt any message they want and compare the ciphertext and plaintext without communicating with the owner of ...