2
votes
2answers
613 views

Security of Deterministic Encryption Scheme

A deterministic encryption scheme is a cryptosystem which always produces the same ciphertext for a given plaintext and key, even over separate executions of the encryption algorithm. Although we ...
3
votes
3answers
2k views

Export from US of crypto software with key-size > 56 bits still needs permission?

Elsewhere someone stated that he couldn't publish his crypto software on the Internet, because US Export Regulations require approval, if the key size is greater than 56 bits. I know that several ...
6
votes
4answers
930 views

What is the difference between a hash and a permutation?

As defined by Wikipedia a hash function is [...] any algorithm or subroutine that maps large data sets of variable length to smaller data sets of a fixed length. For example, a person's name, ...
2
votes
2answers
1k views

Even passwords are vulnerable to hash collision attacks?

As stated in this page large documents hashed using md5 maybe vulnerable to collision attacks. My question is even passwords of 6-30 character are vulnerable to such hash collision attacks? If yes, is ...
1
vote
1answer
186 views

Sensible usecase for restricting special characters in passwords? [closed]

Please See Why Not Allow Special Characters In a Password on Security. Several websites I've come across limit the allowed special characters for a password. My thought was, it should all be ...
1
vote
0answers
100 views

Secure modulus length for Diffie-Hellman [duplicate]

What is secure modulus length for modular exponentiation so that finding discrete log is computationaly impossible?
0
votes
1answer
99 views

reduces the coefficients of a modulo 3 on NTRU

i'm still don't understand about 'reduces the coefficients of a modulo 3' on NTRU tutorial $a = f*e = 3 - 7X - 10X^2 - 11X^3 + 10X^4 + 7X^5 + 6X^6 + 7X^7 + 5X^8 - 3X^9 -7X^{10} \pmod{32}.$ Note ...
2
votes
2answers
240 views

How to efficiently generate a stream of independent, but biased random bits?

Usually, in cryptography, one is interested in debiasing a stream of independent (true) random bits, and several algorithms exist to do this. What about the converse? Let's assume I have a stream of ...
6
votes
2answers
1k views

HRNG for One Time Pad

I have just advanced to the last grade of high school (in Denmark). In this year, we all get to do one big project, where we can build/invent/create whatever we want to "solve a problem". As I study ...
4
votes
3answers
313 views

Comparison: complexity measures vs. security

Assume that you want to compare several cryptographic primitives (say, encryption schemes), and choose one. You need to consider several complexity measures, such as the key length, encryption time, ...
1
vote
3answers
374 views

File encryption with one keypair?

I'm working on a program that uses an ECC keypair in a (password protected) PKCS12 file (.pfx) to encrypt files. I like this method because I think it will be higher security (using ECDH to negotiate ...
3
votes
1answer
309 views

How to compute the dot product on encrypted values?

Is there a practical homomorphic encryption scheme that can give reasonable execution time results in computing a dot product: $$a_1*b_1 + a_2*b_2 +a_3*b_3 +\ldots+ a_n*b_n$$ I imagine the scheme will ...
6
votes
4answers
254 views

Could a very long password theoretically eliminate the need for a slow hash?

Before I provide details, I want to clarify that I am not looking to implement this practically, but I'm only asking to get a better understanding. The way I currently understand it, we use slow ...
2
votes
2answers
123 views

Key Exchange and anonymity issue

If Alice (sender) sends a private message embedded with public key of the Bob(recipient) through onion network, so in this key exchange, does Bob remains anonymous to Alice as Alice knows about the ...
0
votes
1answer
89 views

RSA Certificate Owner and Issuer line meanings?

Apologies if this question is not in the right place. I've recently been looking through some RSA certificates ( specifically, Android apk .rsa files) and I'm having trouble deciphering the one and ...
2
votes
1answer
227 views

How large should a Diffie-Hellman p be if the messages are encrypted?

How large should the prime $p$ and generator $g$ values be in a Diffie-Hellman handshake if the messages are encrypted. If the key that encrypted the Diffie-Hellman messages becomes compromised, ...
0
votes
1answer
178 views

Encrypting and Decrypting a 19-digits long BigInteger

How can I take a maximum 19-digits long BigInteger and encrypt it with the following rules: The result must be based on digits and lower-case English letters ...
-1
votes
2answers
1k views

How does the rand() function in C work? [closed]

I want to know how rand() works (even when I don't provide any seed how it produces PRNs?) thanks!
2
votes
1answer
702 views

Brute forcing an HMAC

Given current technology such as GPUs and GPU cracking software I was wondering if anyone has an idea on how long it would take to brute force the key used to derive an HMAC?
8
votes
1answer
505 views

How would one crack a weak but unknown encryption protocol?

I asked a question on security.stackexchange, but was told it would be a better fit here: http://security.stackexchange.com/questions/32779/how-would-one-crack-a-weak-but-unknown-encryption-protocol ...
2
votes
3answers
136 views

How comparable is OFB to a one-time pad?

This is a rather trivial question, I just want to see if I get the concept right. In OFB you generate a pseudorandom stream based on some nonce, and you XOR that with the message to get the ...
33
votes
2answers
4k views

What makes a hash function good for password hashing?

Using a cryptographic hash to store e.g. passwords in a database is considered good practice (as opposed to storing them plaintext), but is subject to attacks on said cryptographic hash, assuming the ...
25
votes
5answers
8k views

Is Convergent Encryption really secure?

Recently a company called Bitcasa demonstrated a product of cloud storage. they indicated that they would use "Convergent Encryption" to secure your data and de-duplicate, essentially one copy of the ...
2
votes
0answers
27 views

Encryption with Deduplication [duplicate]

Suppose I have a file server running ZFS with file deduplication enabled, and I have two users that are sharing files: Alice and Carol. If Alice uploads huge_file.tgz to her www/ directory, and Carol ...
4
votes
1answer
142 views

Does Linear Cramer-Shoup have pseudo-random ciphertexts?

"Linear Cramer-Shoup" is defined on pages 4 and 5 of $\:$ eprint.iacr.org/2007/074.pdf . Are the ciphertexts in Linear Cramer-Shoup computationally indistinguishable from uniform under a ...
-1
votes
2answers
900 views

Where to store the private key and the public key in a communication protocol

I want to create a simple secured connection protocol over TCP/IP. I just want to connect from my mobile device to a server securely. Firstly I thought to use an SSL connection, but I changed my ...
1
vote
1answer
140 views

Selecting a large NUMS Safe prime

Suppose I want to use the following simple hash function. For a mesage $m$, take some public $a$ and prime $p$ and raise $a^m \bmod p$ (never mind the computational expense of this operation). This ...
3
votes
1answer
226 views

When confusion is applied during encryption?

I know confusion is used in cryptography to make it more difficult to identify any relationship between the ciphertext and the symmetric key. I want to know when the confusion is applied during ...
1
vote
0answers
229 views

computing inverses in truncated polynomial rings manually for NTRU encryption

Can someone explain how to find inverses in truncated polynomial rings manually (i.e. on pen and paper)? As an example from the tutorial: Example. Take $N=7$, $q=11$, $a=3+2X^2-3X^4+X^6$. The ...
7
votes
3answers
638 views

Is this encryption algorithm build from MD5 secure?

I'm being asked to use an encryption algorithm in my code, and besides the fact that I'm sure there will be implementation faults that lead to vulnerabilities, I also have concerns about the algorithm ...
1
vote
0answers
71 views

Can a LIRA system work with cjdns or other decentralized meshnets?

LIRA (Lightweight Incentivized Routing for Anonymity) is a scheme to incentivize participants in Tor to contribute resources, in particular bandwidth. The paper talks about it being used for Tor. Can ...
3
votes
1answer
781 views

BCrypt vs Key Stretching MD5

Ars technica posted an article about the latest in cyber warfare from the stuxnet crew. They key stretched MD5 by iterating it 10,000 times. It seems to be effective at handicapping HashCat's gpu ...
6
votes
2answers
553 views

Can I use a key-derivation-function as the hash function H in SRP?

In the Secure Remote Password Protocol, the verifier must be stored on the server. In the case of a server compromise, an attacker could obtain these verifiers. If nobody reused passwords, this ...
0
votes
1answer
188 views

Solving congruences using PARI

I'm having trouble finding info in the docs about how to solve a system of congruences. The closest I can find is 'matsolvemod' in here: ...
3
votes
1answer
309 views

Understanding a Blowfish cryptanalysis

I'm reading a cryptanalysis on Blowfish, and I've come across something that I don't quite get. Let's denote $$\delta = a \oplus a'$$ where a and a' are bytes that cause a collision in some S-box ...
5
votes
1answer
524 views

How to use GCM mode and associated-data properly

I'm currently migrating a project to use the bouncy castle GCM mode. I understand that using an authenticated mode removes the need for a HMAC, however I want to be sure that I'm using this mode ...
4
votes
1answer
635 views

What are the constraints for an IV using AES in CBC mode?

I'm designing a protocol for use into a VPN software. The VPN frames are encapsulated into AES-256 CBC encrypted frames. I understand that IVs must be uniquely used for each message encrypted with ...
-1
votes
2answers
362 views

Strong encryption done by hand [duplicate]

One of the distinguishing features of a good encryption algorithm, is that it is easy to encrypt, and hard to crack. Are there any that are easy enough for average folk to remember, and calculate by ...
2
votes
1answer
289 views

Encrypting many small messages (log-lines)

What would be the best way of encrypting small mesasages to be stored in encrypted form? (This sounds like somthing a textbook would cover, but I haven't found any references). Let's say I have many ...
1
vote
0answers
64 views

What is the significance of i^Q mod P = 1 and are there any special properties to be aware of?

I'm studying UProve, and one of the fundamental components of this technology is based upon the relationship between i^Q mod P = 1 Lacking a specific name I can call this mathematical ...
1
vote
1answer
171 views

Is there an efficient way to hide the encrypted plaintext length with a block cipher?

In block cipher modes of operation for encryption on input of a plaintext of $N$ blocks (We assume that the input size is always a multiple of the blockcipher mode: $N·16$ bytes) the size of the ...
2
votes
2answers
214 views

Zero-Knowledge Challenge-Responce Protocol

Good day to everyone. I am trying to implement an e voting system (just for reference -it is not important though-it is described at the Internet Voting Protocol Based on Improved Implicit Security ...
1
vote
1answer
337 views

Partial decryption of large file in AES-GCM (or other modes)?

Lets assume a 4gig file that is internally made of 1 million pages of 4k bytes each (page). the file was originally AES-GCM encrypted these pages (or entire file) lives across the internet i.e. ...
2
votes
2answers
1k views

Would a “Triple AES” (in the sense of how Triple Des works) serve for a dramatic increase in safety?

The system requires to be as paranoid as possible regarding security. One of the few contemplated changes to the current design is to use multiple encryption. First proposal was to use Serpent on top ...
1
vote
0answers
189 views

Method and explanation for calculating difference in speed between DES and RSA

Our assignment asks us to convert an RSA speed in bytes per second to a DES speed in bytes per second. Our professor's treatment of this ("RSA is 100 times slower than DES") seems insufficient. Could ...
2
votes
1answer
266 views

Is this method for exchanging private key using RSA sound?

Bob know's Alice's public key, and he wants to make sure he's connecting to the one which has that key. Furthermore, Alice wants to verify when she gets a connection from Bob who'll give his public ...
5
votes
4answers
1k views

Is SHA-256 secure as a CTR block cipher?

Generate a 256-bit random nonce. XOR it with a 256-bit reusable symmetric key. This is x. We represent numbers in simple binary instead of a counting function. ...
2
votes
1answer
93 views

How difficult is it to check if a group element is in a sub group?

I am just curious. We have a group $G$ and its subgroup $H$ with a generator element $h \in H$. How difficult is it to check for $x \in G$ that $x \in \langle h \rangle$? Is there a better way than ...
1
vote
1answer
288 views

What are some different cryptography methods?

Some of the most effective cryptography methods and algorithms are based of factoring large prime numbers (e.g. RSA). I'm curious whether there are some other cryptography methods. Somethings that is ...
5
votes
3answers
556 views

Finding roots in $\mathbb{Z}_p$

Is there an efficient way (algorihtm) to compute the solutions of the congruence $x^n\equiv a \pmod p$? $n\in \mathbb{N}$ $a\in\mathbb{Z}_p$ $p$ is a large prime number Note that: By efficient I ...

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