# Tagged Questions

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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### Algebraic cryptanalysis on public-key schemes

The book Algebraic Cryptanalysis (Bard, G.V.; Springer, 2009) speaks about transforming a symmetric scheme into a system of polynomial equations and solving these equations to break the scheme. But ...
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### Altering the message space of Paillier

Is there any obstacle if we restrict in Paillier the plaintext values to be drawn from $\mathbb{Z}_N^*$ instead of $\mathbb{Z}_N$ as the original scheme indicates.
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### What use is the signature without the message under RSA?

Using RSA for signing & verification, is there a way to take a valid signature for an unknown message, and for someone else to generate a valid signature using a different key against the valid ...
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### Toy public/private key example open to frequency analysis?

I've been looking at an online Public Key Encryption Example (summarized below). It seems to me that choosing cyphertext = plaintext^(key) mod n would produce the ...
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### Decrypted RSA 64 bit encryption using modulus and private exponent. Got gobbledygook [Verify] [closed]

As part of an experiment, I was given a .enc RSA (encrypted) file that was most likely generated using openssl. I had the public key, I factored it, obtained the private exponent, and decrypted the ....
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### How to decrypt a '.enc' file that has been encrypted with RSA using a public key? [closed]

While solving a CTF challenge online, I came across a situation where I needed to decrypt a '.enc' file that has been encrypted using someone's public key. The ...
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### DSA: Authentication and assurance of authorized parties

When dealing with Asymmetric cryptographic algorithms, usually the sender of the critical data asks the listener for a Public Key for encrypting the data and sending them. This is while the private ...
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### Device intercommunication with commands

I want to create a smart house project, and in that I bought some simple RF modules. Those modules don't support crypto out of the box but I want to connect them to a device that I would program to ...
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### How does ECDHE_RSA key exchange mechanism work?

Using Wireshark, I found these data exchanged with google.com over TLS: Client Hello possible cipher suites and possible curve types (eg. secp256r1) sent Server Hello cipher suite selected ...
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### Do asymmetric signatures require constant-time verification?

To avoid a timing attack, HMAC signatures are usually compared in constant-time (every byte is compared, and the results aggregated). Is the same necessary for asymmetric signature algorithms such ...
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### Deterministically generate a RSA public/private key pair from a passphrase (with sufficient entropy)?

Is it possible to deterministically generate public/private RSA key pairs from passphrases? Would giving the (key generating) algorithm data made from key-stretching the passphrase (instead of a ...
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### Why not send the identity in the encrypted message?

I've recently started reading the book "Applied Cryptography" written by Bruce Schneier. Quite a few times I stumbled across protocols who send the identity outside of the encrypted message (see step ...
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### Given only RSA public key parameters, what security issues might I be able to detect?

I'm attempting to automate some checks against a large list of .NET assemblies, and want to include a test to see if the RSA parameters used to sign the assemblies are sensible. However, since I don't ...
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### RSA Decryption from samples

I am working on a project for a client which involves decryption of data which was encrypted using RSA. The system in question retains three independent copies of each data-set, but each copy is ...
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### What does variable-base point/scalar multiplication mean in ECC?

I am a bit confuse about the term, variable-base point/scalar multiplication, in Elliptic Curve Cryptography. What I have understood so far. It means that the base or point on EC is variable/unknown. ...
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### RSA key pair generation and FIPS 140-2, 186-4

I need to securely generate RSA key pair and I need to have access to private exponent in order to process it further. What's the difference in FIPS 140-2 and FIPS 186-4 in connection with RSA key ...
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### Can the same random number be used in encryption and signing?

In several public key algorithms, the person running the algorithm must generate a random number (that's separate from the key). Can this random number be the same for an encryption and a signature? ...
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### Security implications of public nonce

I'm planning to use the crypto_box() functions of Nacl (or, alternatively, the crypto_box_easy() functions of libsodium) to ...
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### What work goes into creating new asymmetric cryptosystems?

It's seems to me that it is rather easy to create a symmetric encryption algorithm (though not necessarily a secure one). They're a dime a dozen. The number of asymmetric encryption algorithms ...
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### Multiple private keys for single public key

I am currently working in security in mobile ad-hoc networks. I have several clusters, and I want to send some data encrypted with its public key, from the cluster head to the cluster members. I ...
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### RSA Key composition

I'm trying to figure out how it is composed the RSA key. Follow what I know : For a RSA 2048, 2048 is the bit length of the module. The public key is composed by $(e,M)$ when $e$ is the public ...
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### ECDSA Public key randomness

If I have a 256bit ECDSA public key how likely is it that the same public key will be generated by another person. Will it take on average 2^256 tries to generate same public key? If I xor the first ...
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### Strategies for securing public key storage on software

I´m kind of new in this so I apologize in advance for any noob mistakes (feel free to point it out) We are developing a licensing mechanism on our software, and I would like some advice. The ...
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### Client-Server application how can data be encrypted so only clients can read it

Requirements We are building a system that connects multiple clients through a restful API. It allows the creation of groups of trusted clients. We need a way to store group shared data encrypted ...
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### Using Permutation polynomial to compute a MAC

Is the following MAC secure? For a block $y_i$ in a file, we defined a MAC as follows: $Mac_i:PRF(k,i) \cdot g^{y_i \cdot r_i} \bmod p$. Where $p$ is a prime number, $g \in \mathbb{G}$,$PRF(k,i)$ is ...
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### Any ideas on login using Digital Signature PKCS

I am curious to know about possibility to login into web application just by identifying the user based on digital signature in usb token. any reference in this direction?
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### x509 Certificate Signature

I've got a question about the signature of a CA. As I understand, the CA takes the public key of the client and signs it with his own private key by using "md5WithRSAEncryption" (like explained here: ...
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### RSA: how does it work and how is it more secure than symmetric systems

Before starting the question, I know that RSA is naturally a key exchange cryptosystem rather than an antilogarithm fully implemented for encrypting and decrypting data since the length of the secret ...
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### What is contained in a RSA key file? [duplicate]

Consider a RSA key file like this: ...
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### Converting a number to a member of a multiplicative cyclic group

I am currently trying to make an implementation of the ElGamal encryption for educational purposes. As I understand it, when using the encryption with multiplicative cyclic groups, one generates a ...
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### Practical Attack on RSA

Currently I am designing an RSA based application, and I am thinking of how long should the key be in order to be secure against attacks. I know that RSA 4096 bit key can be recovered using Sound ...
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### Smart Card - Entropy during on-board public key generation

I know that some (advanced) smart cards or tokens allow generating key pair directly on the device, but I wonder how the card can gather entropy during the process. Any idea ?
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### What is the SSL private key file format?

I was researching about how to encrypt with RSA. I understood everything but not the format of the private keys. In the phpseclib (RSA in PHP), you can import your private key (private.key format) ...