# Tagged Questions

A reference request is a request to be provided with a source from documentation, official papers, and specs related to one or more specific algorithms or cryptographic procedures. Open ended literature recommendation requests are off topic!

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### Lowest number challenge scheme

Suppose Alice knows a secret number $a$, and Bob knows a secret number $b$. Is there a simple way for Alice and Bob to know who has the lowest number, without Alice & Bob exchanging their numbers ...
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### Difference between “ECDH with cofactor key” and “ECDH without cofactor key”?

I need to use “ECDH with a cofactor key” for generating symmetric key. I have a fair idea on how ECDH works, but I don’t understand the cofactor part. What is the difference between ”ECDH with a co-...
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### Choosing primes in the Paillier cryptosystem

In the first step of key generation phase in Paillier cryptosystem given here. It's given that $$\operatorname{length}(p) = \operatorname{length}(q) ) \implies \operatorname{gcd}(pq,(p-1)(q-1))=1$$ ...
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### Malicious KGC attack in the identity-based authenticated key exchange protocol

There are two users $A$ and $B$ who are in different domain. Let's assume that $A\in KGC_1$ and $B\in KGC_2$. If they want to establish secure session, then they must run an identity-based ...
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### Adi Shamir's secret database of all primes

I was going through these presentation slides (PDF) on Crypto 2013. It summarizes the paper, Factoring RSA keys from certiﬁed smart cards: Coppersmith in the wild. In the last slide, it was ...
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### Public and Private key encryption in simple math

Can anybody show me (or point me to resource) how to generate public key and private key with simple math? Steps which can be reproduced in a simple calculator – so the message, keys all can be ...
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### Client authentication on limited hardware

I'm developing a authentication and authorization protocol for a Bluetooth device. The device should communicate with an Android app and needs to be able to authenticate the app during the connection ...
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### Encrypting text file line by line with stable encryption

I want to store files in a secure manner (i.e. no one should be able to decrypt the contents of the file without the secret key) but I also want a diff tool to be able to show me which lines changed (...
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### Is there a standard way to use a nonce with HMAC?

Many protocols use HMAC on messages that include a nonce, but they don't seem to do it in a consistent way. For example, in OAuth the nonce is in the middle of an URL-encoded key-value string, like ...
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### LFSR Output Sampling for Berlekamp-Massey

Looking at the use of Linear Feedback Shift Registers in cryptographic algorithms, I have learned that the Berlekamp-Massey algorithm can be used to find the (shortest) LFSR that generates a given ...
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### What is the quantum-resistant signature scheme with the smallest signature + pubkey?

After playing cat and mouse with an number of quantum-resistant signature schemes and coming up with nothing with small enough signatures, I'm turning to Crypto.SE. I need a quantum-resistant ...
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### Is there a deterministic number generator that allows me to share some (not all) numbers I've generated using a smallish seed?

I was just wondering, is there a type of deterministic number generator that allows me to share some (not all) numbers I've generated using a smallish seed? I have pairs of random numbers and need to ...
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### Description of signatures with message recovery (as in ISO/IEC 9796-2 and EMV Signatures)

I'd like to get an overview of how the signatures with message recovery work, especially in case EMV and other smart card systems. Is there a nice overview available without being required to read the ...
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### Where do I truncate the bits in SHA 256 hash algorithm?

I'm implementing SHA-256 in PHP using strings. Where do you truncate in the algorithm? There are a lot of places where there's binary addition happening so the number of bits in $W(i)$ keep ...
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### Elliptic Curve based blind signature implementation

I want to use Elliptic Curve based blind signature scheme for my research. There is no proper implementation of ECC-based blind signatures. Can someone describe to me which things I need to follow ...
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### Can I create a fixed length output from a fixed length input?

I use AES256 for various purposes in my app. However, the purpose I am now trying to achieve is the following: Input is for example AU00000001 and I would like a ...
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### Theory for pre-paid debit card: card-to-card transfer [closed]

In Sweden in the late 90's they deployed a electronic prepaid cash system system, and if I remember correctly you could transfer money between cards without online access. Wikipedia states that the ...
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### One Time Pad for large changing files

i wondered what the security implications are if I do the following. If I had a large file encrypted with a OTP and want to change only a few bytes in the plaintext. what security vulnerabilities do I ...
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### Bilinear pairing

I am working on Efficient Construction of Pairings which are being realized by Miller's algorithm. In this algorithm the basic steps are point doubling and line function computation point addition ...
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### Update to “Cryptographic Right Answers”

I'm a big fan of Colin Percival's "Cryptographic Right Answers" post. This was written in 2009, which is a long time ago in Internet years. Is the advice still valid or, if not, can someone point me ...
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### Blowfish vs. Twofish regarding power consumption

If I wanted to use Blowfish or Twofish to provide security on a device where power consumption is crucial. Regarding power consumption, which one would win? Generally, which algorithms are known to ...
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### Looking for crypto with derived keys?

I am looking for cryptographic algorithm or combination of them. Basically I need an algorithm, which can generate private key and for this key can be computed multiple keys based on some seed. So a ...
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### Computing youngest person among 3 while keeping ages private

I already found a protocol to find out who is richer (older) between two parties, but Is there any protocol to find the youngest person among 3 parties, without revealing actual ages?
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### Is knowing the private key of RSA equivalent to the factorization of $N$?

Given the RSA modulus $N$ the fastest method to factor it is of sub-exponent order. But, now if I know the private key $d$ of RSA, does that mean I can factor $N$ efficiently?. It intuitively seems ...
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### Can 64-bit “PRINCEcore” practically be brute forced?

There is a cipher called PRINCE proposed in ASIACRYPT two years ago. See the paper: “PRINCE – A Low-latency Block Cipher for Pervasive Computing Applications” The cipher divides the 128-bit key into ...
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### How to implement a Cryptographic Pseudo Random Function

I need three pseudo random functions which all take a seed S (t-bits long). The functions should map from $d$ bits to $d$ bits $d$ bits to $1$ bit $d$ bits to $t$ bits $d$ is $10$ bits $t$ is ...
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### Pseudocode for constant time modular exponentiation

I'm looking to implement modular exponentiation (for RSA) in constant time, but most of the examples I've found are more mathematical descriptions of the operations. Are there any references with ...
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Two or three years ago, I was reading something interesting, but then I lost the paper. The paper described building secure and fast P2P networks (not like tor or I2P). Unfortunately, I only remember ...
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### Is it possible to track down copies of WW2-era codebooks?

I've got an academic curiosity regarding some of the old Japanese naval codes used during World War II specifically. I'm wondering if any of the codebooks from this era have been made available, or ...
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### Decrypting a small Message using RSA with a Private Key [closed]

If I have a private key of (43, 341). What would be the steps I need to take to decrypt a small message using RSA? I have looked ...
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### How do they implement MAC in TLS?

I recently read questions about "MAC then Encrypt/Encrypt then MAC" which made me curious about how they actually use the algorithm. MAC only calculated once either case (according to what I read). ...
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### Wrong Test Vector for HKDF with HMAC-SHA256 [closed]

I have implemented HKDF by myself and it seems to work good. I managed to replicate all the test vectors listed here but... except this one: ...
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### What is the Geometric Generalised T' Method?

This page by Nicolas T. Courtois mentions Geometric Generalised T' Method. It is described as an advanced geometric algorithm, never published, for finding extra linearly independent equations at ...
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### Use cases for “online” authenticated encryption?

What is the the point of an "online" mode for an authenticated cipher? I understand what "online" means in this context. However, I have trouble coming up with applications that would benefit from ...
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### Side-channel attacks against ECDH for Weierstrass normal form curves

I hear a lot about why Montgomery curves are used in ECC, and one reason is that the same algorithm can be used to do both point addition and doubling (this is not true for the Weierstrass normal form)...
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### Encryption mode with chained algorithms

Suppose that I want to encrypt some data using two different algorithm chained (e.g. AES and Serpent) and harnessing the benefits of CBC. I can do: ...
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### Elliptic Curves of different forms

Looking at http://safecurves.cr.yp.to/ to find a safe curve, I find that most curves described here are of a different form from that generally used. In Bouncy Castle, for example, ...
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### Where did Martin Hellman acknowledge Ralph Merkle's contribution to public-key cryptography?

I am trying to find the article to reference when discussing the Diffie-Hellman key exchange. According to Wikipedia Hellman said that The system...has since become known as Diffie–Hellman key ...
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### Is it meaningful to consider the leakage of master key of KGC?

In the Identity-based cryptography, the user's private key in the systems is delivered by the key generation center(KGC). My question is : Is it meaningful to consider the leakage of master key of KGC?...
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### Digital Signature using symmetric key cryptography

Generally digital signature is a public key cryptography concept.But it needs high overhead. So is there any publication or link available where 'digital signature using symmetric key' has been ...
Multi-prime RSA is now a well known technique (described here): it uses $k>2$ distinct secret prime factors in the public RSA modulus, with the advantage that, using the CRT, we can gain a speed ...