# All Questions

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### Montgomery Exponentiation - selecting input value R for a given BigInteger

I have Montgomery exponentiation working, but it's working quite slow. I suspect there are two reasons for this - I implemented it bit size instead of word size (I didn't realize at the time that ...
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### How do unkeyed hash functions (for MDCs) provide security?

Unkeyed hash functions are, by definition, hash functions computed without a key. SHA-1 is an example. MDCs (message digest codes) are a subclass of unkeyed hash functions. How are unkeyed hash ...
3k views

### Why are these specific values used to initialise the hash buffer in SHA-512?

I'm reading the book Network Security Essentials written by William Stallings. To create a message digest with SHA-512, we have to go through some steps: append padding bits. append length ...
109k views

### Difference between stream cipher and block cipher

I read that A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. (...
14k views

### How does a client verify a server certificate?

As far as I know, when I request a certificate from Verisign (for example), and after they approved that me is me, they create a certificate (for me) which contains the digital signature and public ...
405 views

### Why do we need Hash by key?

(i'm just trying to find what am I missing...) Assuming John have a clear text message , he can create a regular hash ( like md5 , or sha256) and then encrypt the ...
210 views

### On the fly key generation for cloud storage objects

Cryptographic key management involves storing and managing encryption decryption keys , certificates , signatures etc . If the requirement is just to store and manage object specific keys ( say ...
2k views

### Is it possible to crack a blowfish 256 bits encrypted file? [closed]

I have a file of my own that I have encrypted using Coder-Abi. Now, I can just remember part of this password. So, would it be possible (much easier) for me to crack it knowing only part of it? I can ...
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### Can there be two hash functions without common collisions?

Is there a way to prove/create (or are there known hash functions) two hash functions that never have the same collision? I mean, like provable in way that someone who took one cryptography class in ...
6k views

### Explanation of the Decision Diffie Hellman (DDH) problem.

I'm extremely new to crypto, and very much inexperienced. Lately I've been reading about the Diffie-Hellman key-exchange methods, and specifically about the computational diffie-hellman assumption vs. ...
3k views

### How can I break a Vigenère cipher with partial plain text?

I have an exercise to do regarding Vigenère encryption. I've got the cipher text: WNZTNVIEEGTJYKRRWYUELWNZTNV and a partial plain text: ...
949 views

### Why does HOTP use such a complex truncate function?

In the HOTP protocol after calculating a 20 byte hash it is truncated to 4 bytes. For this first an offset is calculated (low-order 4 bits of the last byte) which determines the four bytes to be ...
1k views

### Does gpg's symmetric encryption keep information about the filename?

If I encrypt a file with gpg -c file.zip, and change the filename. Is there any way I can restore the filename as I used as a password. or the name encrypted to?
148 views

### Can you use the same key to provide a signature and a MAC?

Apologies if this is a stupid question, but I'd be grateful if someone could provide an answer. I'm assuming that you need to use different keys to provide these functions, is that correct?
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### Proving that a scheme is not IND-CPA-secure

Suppose I want to prove that a given symmetric encryption scheme is not IND-CPA secure. The first thing I do is to define a specific adversary that attacks the scheme. How can I proof neatly, (using ...
1k views

### CPA Secure Chosen plaintext scheme

The example for IND-CPA secure schemes given is generally: for a random r, Enc_k(m) =(r|| E_k(r) XOR m) where E is a PRF But does the role of r and k really matter--i.e. isn't this equally ...
85 views

### IND-CPA security scheme [duplicate]

Possible Duplicate: CPA Secure Chosen plaintext scheme The example for IND-CPA secure schemes given is generally: for a random r, $Enc_k(m) =(r|| E_k(r)$ $XOR$ $m)$ where $E$ is a $PRF$ But ...
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We have a multiplicative cyclic group $G$ with generators $g$ and $h$, as in El Gamal. Assume $G$ is a subgroup of $(\mathbb{Z}/n\mathbb{Z})^*$. There are two parties, Alice and Bob: Alice knows: $... 0answers 130 views ### Computer appliance protocol [closed] I build the computer appliance system that consist of software server and multiple controllers. The system I want to build must be pluggable in the sense that when I connect a controller C to the ... 2answers 137 views ### Distinguish messages Since i cannot comment on questions, I give my own: If a secret key encrypt algorithm can encrypt messages of arbitrary length and the encrypt algorithm is probabilistic then: suppose the adversary ... 1answer 598 views ### How can I prove in zero knowldege that an ElGamal shuffle is correct for a special setting? [closed] In a special ElGamal encryption scheme, every user has an ElGamal encryption key-pair using the same cyclic group$G$and generator$g$. The system has a special function :$$\mathrm{ShuffleEncrypt}... 1answer 148 views ### Initialization value (IV) bit error When a bit error happens in an IV, what would the result of decrypting a cipher-text be in the different encryption modes like ECB, CBC, CFB, OFB, and CTR? 4answers 705 views ### Is an RSA variant with public exponent$e=f+(p-1)\cdot(q-1)$safe (for$f$random in some small interval)? Alice is going to use some existing RSA scheme (employing industry-standard signature or/and encryption with proper padding), which she can't change, or influence beyond her choice of key. In this ... 1answer 1k views ### Can ElGamal encryption and ElGamal signatures be used together sharing the same key-pairs? I'm working on a encryption system where each party can store exactly a single ElGamal private key in a device. This is a hardware limit. The system must be expanded to support signatures and ... 1answer 879 views ### Crack SHA1 hash code Given a set of SHA1 hash code, if we knew a portion of the plaintext for each particular hash code, and also the remaining portion of the plaintext is a constant value for each hash code. Is it ... 2answers 2k views ### Optimising Pollard's Rho algorithm for large semi-primes I have programmed an implementation of Pollard's Rho factoring algorithm using C++ and the GMP library. It is reasonably fast with large numbers, however I haven't implemented any form of cycle ... 5answers 2k views ### “Padless” One-time-Pad encryption I have just read about the perfect security of an OTP encryption and what came to my mind was that what if the Pad used for encryption/decryption did not have to be transported separately from the ... 2answers 2k views ### Real world use cases of Multi-Party Computation Most of the research papers give imaginary applications of multi-party computation. Either they talk about Yao's millionares' problem or two or more corporates willing to compute some intrustion ... 1answer 7k views ### Why, or when, to use an Initialization Vector? i'm trying to figure out when an Intialization Vector (IV) should be used. There are anecdotal reports that WEP was broken because of weak IV's. It's also claimed that if two pieces of plaintext are ... 2answers 2k views ### Why is an Encrypt-and-MAC scheme with deterministic MAC not IND-CPA secure? I'm preparing myself to exam, but I have a lot of troubles with rigorous proofs. It's the task from two-years ago exam. At the bottom I reminded one definition. Let$(Gen_E,Enc,Dec)$be an IND-CPA-... 2answers 1k views ### Modifications of CBC-MAC I'm preparing myself to exam, but I have a lot of troubles with rigorous proofs. This post is very long, but this is because I remind here 2 long definitions. At the beginning I want to remind the ... 1answer 319 views ### Messages of different lengths and one-time computationally-secret I'm preparing myself to exam, but I have a lot of troubles with rigorous proofs. Let$\Pi=(Gen,Enc,Dec)$be an efficient secret-key encryption scheme that is not fixed-length. That is, for any$n$... 2answers 1k views ### Is there an algorithm for factoring N, which is just as simple as this one, but faster? I found a simple algorithm for factoring semiprime numbers, you can read about it in Factoring Semiprimes and Possible Implications for RSA. It basically works like this: You reverse the digits in$...
I think I roughly understand how the RSA alorithm is working. However, I don't understand why we need the $N$, which we use as a modulus, to be $pq$ for some large primes $p, q$. I vaguely know it ...