An asymmetric cipher is an encryption scheme using a pair of keys, one to encrypt and a second to decrypt a message. A special class of asymmetric ciphers are public-key ciphers, in which the encrypting key need not be kept secret to ensure a private communication.

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Correct method to encrypt data so that it can be decrypted only by Alice and Bob

I need to build a system that stores encrypted transactions. Transaction involves always two parties. Payer and payee. Both must be able to download encrypted transaction from server and decrypt it ...
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When is each key used when encrypting an email using OpenPGP?

When you send an email using PGP to encrypt emails, is the recipients public key used to encrypt the email, or is your private key used? Are they both used? At what points do each of the four keys ...
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344 views

What is the strength of unpadded RSA?

I would like to use unpadded RSA for homomorphic encryption in a toy P2P game, for things like fair coin flips and shuffling. How many bits of security does unpadded RSA have, in relation to its key ...
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How to avoid a chicken and egg scenario with encrypting passwords?

I am working on application that allows users to upload files containing company data and then share those files with a list of other users that have specific roles within the system. I want to ...
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305 views

implications of SSH server key compromission when authenticating users against a public key

Friday we had a disagreement with a colleague of mine about the implications of SSH server key compromission The question was stated as this : What could a hacker do while provided with the SSH ...
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Is it possible to pick your Ed25519 public key?

Is it possible to generate an Ed25519 keypair that has a very similar public key as another keypair (fooling a casual visual comparison) or is this as hard as solving one of SHA-512 or the discrete ...
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561 views

How to jusify {e,n} is a valid RSA public key?

I have a two set of key {36,77} and {50,117}, How can validate if it is a valid RSA public keys? Is there any simple method?
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490 views

Additive ElGamal cryptosystem using a finite field

I'm trying to implement a modified version of the ElGamal cryptosystem as specified by Cramer et al. in "A secure and optimally efficient multi-authority election scheme", which possesses additive ...
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354 views

Is truncating a hashed private key with SHA-1 safe to use as the symmetric key for AES for data at rest?

I realize this is mixing the purposes between asymmetric and symmetric crypto, but I was wondering if it is safe to use a hashed, truncated private key (asymmetric) as the symmetric key for encrypting ...
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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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3answers
145 views

Is it possible to ensure security with zero pre-shared information?

Is it possible to secure a communications channel against both passive (sniffing) and active (injecting / MitM) attackers without either legitimate party knowing any pre-shared information? I know ...
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Is this a correctly formatted PGP session key packet?

RFC 4880 may be full of information, but it can be incredibly vague at times, so im looking for someone who actually knows the answer to this. Given this public key: ...
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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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Is this design of client side encryption secure?

I want to build a secure file storage web application. Users should be sure that server doesn't know how to decrypt files so encryption should take place at client side (i.e. in Javascript) and TLS ...
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162 views

Does a high exponent compensate for a low degree of certainty?

If a RSA certificate is created with a low degree of certainty, does the value of the exponent compensate for this? I'm asking because certain implementations of key generation software hide the ...
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949 views

How to decrypt this RSA-like cipher

Suppose we have a RSA-type Modulus $n = pq$ with $p,q$ prime. We also pick a random public exponent $e$ with $\gcd(e,\varphi(n)) = 1$ and compute the private exponent $d$ with $de \equiv 1 ...
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638 views

What is the correct value for “certainty” in RSA key pair generation?

I'm creating an RSA key pair in Bouncy Castle and need to specify an int value for certainty. This Stack Overflow answer says it is a relative test for how prime the values are. There is another ...
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Impacts of not using RSA exponent of 65537

This RFC says the RSA Exponent should be 65537. Why is that number recommended and what are the theoretical and practical impacts & risks of making that number higher or lower? What are the ...
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How much computing resource is required to brute-force RSA?

It's been over 30 years since Rivest, Shamir and Adleman first publicly described their algorithm for public-key cryptography; and the intelligence community is thought to have known about it for ...
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610 views

Is it possible to use the RSA algorithm, or a variant, for software licensing?

I'm almost a newbie about RSA encryption, so my question could it be banal. I'm beginning to create a licensing system for my software. I need to distribute a crypted license file to many customers. I ...
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172 views

How can the Schmidt–Samoa cryptosystem uniquely decrypt large messages?

Suppose I choose $p=7$ and $q=11$. This gives a public key of $p^2·q = 539$. However, decryption occurs using a modulus of $p·q=77$. If a person chooses to encrypt $500$ using my public key, how ...
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755 views

GPG/PGP Verification of Revocation

How does one verify a key revocation? After revoking a key and sending the revocation to MIT's keyserver, I noticed that the key is listed as such: ...
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What's the fundamental difference between Diffie-Hellman and RSA?

What is the difference in the purpose of DH and RSA? Aren't they both public-key encryption?
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131 views

In S/MIME, are the same certificates always used to sign and encrypt messages?

My assumption is that S/MIME almost always utilizes certificates as follows: My certificate can be used to allow people to encrypt messages and send them to me. My certificate (the same ...
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113 views

How can I protect against the failure of a block or symmetric cipher?

Can I protect against the failure of a block or symmetric cipher by chaining different techniques together? If so what implementation details should I be aware of? Are some combination of ciphers ...
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903 views

Is RSA in a ECB-like-mode safe for bulk encryption?

Let's say I would like to communicate with my friend using asymmetric/public-key encryption, e.g. RSA. (Note: I do realize that in practice this is done through an intermediate symmetric key, but ...
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When to prefer exchanging cryptographic certificates over exchanging only public keys?

Let's think of the following case: A group of peers want to exchange messages with each other. They use public-key cryptography to sign and encrypt messages. Anyone with any name can join the ...
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RSA/DSA: Wouldn't it make sense to sign using decoding the data hash?

Why is encoding using the private key used for signing? Wouldn't it make sense to keep the premise, that private is for decoding and public is for encoding? i.e. create a hash and threat it as a ...
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Algorithm to securely exchange identities

Say four people each have a public/private key pair that they can use to encrypt or sign messages. They have an anonymous way to post messages such that the others can see them. Malicious entities can ...
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What is the harm if I publish an encrypted RSA private key publicly?

What is the harm if I publish an encrypted RSA private key publicly? Or in this case, what is the harm if I publish many thousands or millions of them? Assuming that the private key is encrypted ...
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DGK Cryptosystem Key Generation and Decryption Issues

I detailed here the DGK (Ivan Damgård, Martin Geisler and Mikkel Krøigaard) cryptosystem, and I managed to get it to work, most of the time... The BIG problem that I am facing at the moment is that ...
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367 views

ECIES protocol - what does the || operation mean?

I am studying elliptic curves problems, which also includes study of related protocols such as ECIES. A there is a problem I don't understand operation $||$. What this operation mean? Some stuff is ...
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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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DGK Cryptosystem Encryption Speedup

Following @poncho's nice clarification of the RSA speedup here, let's see if I'm able to do the same in the case of the DGK cryptosystem: We have pk = (n, g, h, u), sk = (p, q, $v_p$, $v_q$) which ...
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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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Why DSA cannot be used for encryption?

Here it is mentioned that DSA cannot be used for encrypt. But Both RSA and DSA can be used to generate public and private keys, right? (Or am I wrong?). Then why can't I use the DSA public key to ...
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RSA reencryption scheme

Is there such an algorithm that would allow to upgrade the encryption of a RSA-encrypted message without revealing the plaintext to the untrusted user? Basically, such an algorithm would probably be ...
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Public Key Encryption with forward secrecy

Is there a known non-trivial system with plausibly secure public key encryption where: 1. $\;\;$ the $\:$ time $\:$ function is efficiently computable 2. $\;\;$ [pubkey,privkey] $\:$ pairs are ...
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Do public/private pairs work both ways?

Public/private key algorithms such as RSA encrypt a message with a private key but only decrypted with another (mathematically related) key. Can the public key decrypt the messages encrypted with ...
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Generating Random Primes

Although this has been extensively discussed around here, I'm curious whether my approach makes sense, or I should just stick to "the standard version". I'm implementing some homomorphic encryption ...
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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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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 ...
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565 views

Is this authenticated one-way communication protocol secure?

I am looking to see if this one-way communication protocol is secure. Assume Alice wants to send Bob a message (and doesn't need Bob to reply in the same session/channel - think email). Bob knows ...
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532 views

How can one sign with NTRU?

I am looking at implementing NTRU, but I noticed that while the encryption/decryption algorithm seems to be mature and well-documented, there is comparatively little information about how to sign ...
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Security equivalence proofs for breaking RSA

It is my understanding that while a practical solution to the factoring problem will definitely break RSA, it has never been proven that the security of RSA is equivalent to factoring. In otherwords, ...
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536 views

Can RSA encryption produce collisions?

In RSA, a message is encrypted by $m^e \pmod N$. $N$ is the modulus, $m$ is the message and $e$ is the public exponent. (I know that $m$ should not be greater than $N$.) My question is, can $m^e$ be ...
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How does a chosen plaintext attack on RSA work?

How can one run a chosen plaintext attack on RSA? If I can send some plaintexts and get the ciphertexts, how can I find a relation between them which helps me to crack another ciphertext?
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How are the primes used to generate RSA keys?

I am confused about how keys in RSA asymmetric encryption are generated and what the implications for open communications are. Textbooks say the one-way function is merely two primes (with some ...
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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 ...
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Which encodings have |encoding key| >> |decoding key|?

I'm looking for an encoding scheme that requires a very large encoding key E (>10MB) and suffices with a relatively small decoding key ...