All Questions
1
vote
2answers
122 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
230 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 ...
4
votes
4answers
197 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, ...
0
votes
2answers
212 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 ...
0
votes
1answer
91 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 ...
0
votes
0answers
80 views
Secure modulus length for Diffie-Hellman
What is secure modulus length for modular exponentiation so that finding discrete log is computationaly impossible?
0
votes
1answer
36 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
156 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 ...
5
votes
2answers
485 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
202 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, ...
0
votes
3answers
129 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
75 views
Perfectly hiding / binding commitment scheme
Let's say Alice and Bob are playing a game where Bob is trying to guess a number Alice has chosen.
Alice chooses a key $K$ and a number $N$ at random and performs $C=Commit(K, N)$ where $Commit(K, ...
2
votes
1answer
114 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 ...
2
votes
0answers
65 views
State of the art in zero knowledge proof compilers?
What is the current state of the art zero knowledge proof compiler ? I need one that can minimally handle double exponentiation by a known value E.g.
$$Pok\{(\alpha):h=g^{\alpha^b}\} $$
where b, ...
4
votes
4answers
197 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 ...
0
votes
2answers
91 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
42 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 ...
1
vote
1answer
88 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, ...
2
votes
1answer
104 views
Cracking an RSA with no padding and very small e
I have a project wherein i have to crack a given cipher text encrypted using RSA and have been given N and e. Can someone suggest an RSA attack using a very small exponent e(here e=3) and no padding?
0
votes
1answer
73 views
What are unified addition and differential addition in elliptic curve point arithmetic?
A lot of papers use these terms but I do not find a proper explanation of them. Can somebody tell the meaning / difference / intuition / application and if possible with an example.
0
votes
1answer
71 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
86 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!
1
vote
1answer
85 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?
6
votes
1answer
155 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
80 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 ...
26
votes
2answers
1k 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 ...
20
votes
5answers
6k 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
21 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
102 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 ...
-5
votes
1answer
32 views
decrypting unreadable string [closed]
Does anyone recognise this type of encryption? These are phone numbers consisting of 10 digits and starting with 0 but the encrypted text width is not 10 and does not start with the same character.
-2
votes
2answers
142 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 ...
15
votes
4answers
5k views
Google is using RC4, but isn't RC4 considered unsafe?
Why is Google using RC4 for their HTTPS/SSL?
$ openssl s_client -connect www.google.com:443 | grep "Cipher is"
New, TLSv1/SSLv3, Cipher is RC4-SHA
Isn't RC4 ...
1
vote
2answers
112 views
How to verify a number encrypted with an unknown key
Are there any crypto-constructions to do the following kind of "zero-knowledge proof":
Alice encrypts a set of numbers using some cryptosystem and a secret key (e.g. Paillier crypto-system).
Bob ...
1
vote
1answer
33 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
90 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 ...
0
votes
0answers
74 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 ...
6
votes
3answers
448 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
34 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 ...
0
votes
1answer
52 views
Efficient algorithm for remainder calculation over prime field for ECC implementation?
I am working on 224-bit elliptic curve cryptography. In this 224-bit * 224-bit multiplication results 448-bit output. I am reducing 448-bit into prime field range( prime number $2^{224}-2^{96}+1$) ...
3
votes
1answer
104 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 ...
4
votes
2answers
170 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
46 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
114 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 ...
1
vote
1answer
53 views
What is total key space in transposition algorithms
How we can measure key space in transposition algorithms? Should we specify the method, like rail fence ?
2
votes
1answer
181 views
Low Public Exponent Attack for RSA
I'm having trouble understanding the algorithm for finding the original message $m$, when there is a small public exponent. Here is the example I'm trying to follow (you can also read it in the 'Low ...
3
votes
1answer
87 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 ...
-2
votes
1answer
321 views
Is a steganographic technique which has a universal decoder novel/secure? [closed]
I've come up with an approach to steganography which needs review of both its cryptography and its math. There's a complete working implementation at https://github.com/bramcohen/DissidentX and the ...
1
vote
1answer
60 views
Proof of the standard pseudorandom generator + XOR encryption scheme in Goldreich
Reading Goldreich's Foundations of Cryptography II, I found this proof for the security of the common pseudorandom generator + XOR encryption scheme (Proposition 5.2.12 in the book):
Assume you have ...
3
votes
1answer
129 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 ...
0
votes
2answers
104 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 ...
