All Questions
3
votes
5answers
828 views
Galois fields in cryptography
I don't really understand Galois fields, but I've noticed they're used a lot in crypto. I tried to read into them, but quickly got lost in the mess of heiroglyphs and alien terms. I understand they're ...
3
votes
2answers
272 views
are CFB and OFB really meant for streaming?
CFB, OFB and other modes are meant for streaming and don't require padding. Are there still limitations such as the text needs to be greater than key length?
3
votes
3answers
405 views
What is the relation between Discrete Log, Computational Diffie-Hellman and Decisional Diffie-Hellman?
How are the three problems Discrete Logarithm, Computational Diffie-Hellman and Decisional Diffie-Hellman related?
From my understanding, since the Discrete Log (DL) Problem is considered hard, then ...
3
votes
2answers
840 views
Brute forcing CRC-32
I'm working on a cryptosystem which uses IDEA. The designer made the mistake of including a CRC-32B hash of the password unencrypted in the header, so that the system can quickly reject bad ...
3
votes
3answers
303 views
How can two different passphrases unlock the same content?
I have heard that in enterprises it is common that the IT admin has a master passphrase that can unlock any content that an employee encrypts.
It would be temping to think, that the IT admin's ...
2
votes
0answers
96 views
LT codes with Homomorphic hashing
I have been working on a project implementing LT codes with Homomorphic hashing (inspired from http://blog.notdot.net/2012/08/Damn-Cool-Algorithms-Homomorphic-Hashing and ...
2
votes
3answers
424 views
Hash function in PBKDF2
From this excellent answer I learned (correct me if I am wrong) that when writing a block cipher with say key size 128 bit, one has to pad the password given (variable size) so that it becomes exactly ...
2
votes
1answer
193 views
Padding for the TEA
Sorry if this is a noob questions, but I finally figured out how to implement the Tiny Encryption Algorithm in C++.
My question is what to do about padding the key and the plaintext? I know that ...
2
votes
1answer
229 views
DGK Cryptosystem Encryption Speedup
Following @poncho's nice clarification of the RSA speedup here, let's see if I'm able to do the same in the case of the DGK cryptosystem:
We have pk = (n, g, h, u), sk = (p, q, $v_p$, $v_q$) which ...
2
votes
1answer
578 views
What is the difference between CBC and GCM mode?
I am trying to learn more about GCM mode and how it differs between CBC. I already know that GCM provides a MAC which is used for message authentication. From what I have read, and seen code ...
2
votes
1answer
608 views
Sending KCV (key check value) with cipher text
I was wondering why it is not more common to send the KCV of a secret key together with the cipher text. I see many systems that send cipher text and properly prepend the IV to e.g. a CBC mode ...
2
votes
1answer
242 views
Families of public/private keys in elliptic curve cryptography
I'm looking for a related key scheme for elliptic curve cryptography. The basic idea would be that there would be a master public key and a master private key. From the master public key, you could ...
1
vote
2answers
200 views
Are there any tools for modelling and analysis of Cryptographic protocols?
Iam designing few cryptographic protocols and am new to it.
Are there any well known tools that can be used to model/design these protocols ?
And also verify/Analyze their validity ?
If not a tool , ...
1
vote
2answers
364 views
Why is asymmetric cryptography bad for huge data?
I've been told that asymmetric cryptography requires that the message to be encrypted be smaller than its key length.
Why is this?
I know about hybrid encryption, which uses symmetric encryption to ...
1
vote
2answers
160 views
Is it possible to match encrypted documents using user-defined search terms?
Suppose I am storing a number of encrypted documents in a database. I would like to make it possible to identify the subset of documents whose contents match user-specified search terms without a) ...
1
vote
1answer
918 views
Cipher Feedback Mode
I can't understand what CFB really is. It said in Wikipedia that CFB is same as CBC, but I find that CFB is more difficult than CBC.
Can someone explain to me how CFB works. Such as how ...
1
vote
2answers
726 views
How to perform file encryption using 128-Bit AES?
I am confused, how can I encrypt a file using 128 Bit Advanced Encryption Standard?
Do I need only to encrypt the file name and it's content or is there something that I need to do to encrypt it? Is ...
10
votes
4answers
234 views
Do parts of a hash carry the properties of the entire hash?
When I need to generate unique id's based on some information hashing is typical choice. However, sometimes that id needs to be of a particular size. I've seen a lot of schemes (HMAC-MD5-96 in SSH, ...
10
votes
3answers
1k views
At the current time, is SHA256 the de facto standard for strong cryptographic hashes?
At the current time, is SHA256 the de facto standard for strong cryptographic hashes?
From what I am seeing of more sites utilizing it, I would suppose the answer to this is yes, but would like to ...
10
votes
3answers
543 views
Is CBC really dead?
I developed a p2p-app in C# which sends and receives encrypted text messages (50KB). For encryption, my app uses AES 128 bit in CBC cipher mode. For each message it uses a new randomly-generated IV.
...
10
votes
3answers
441 views
Could one construct a cipher that is secure for friendly parties to use but insecure for hostile parties?
Consider the situation of a nation state (Blue) at war with another nation state (Red). Blue wants to deploy a secure cipher that blue currently can not break, but they are considered that Red could ...
10
votes
4answers
475 views
About Cryptography in a Character Language
Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it.
Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem ...
9
votes
4answers
424 views
Can you make a hash out of a stream cipher?
A comment on another question made me wonder about something:
Assume you're on a rather constrained platform — say, a low-end embedded device — with no built-in crypto capabilities, ...
9
votes
1answer
345 views
Does unbalancing a feistel cipher always improve security? Does it improve security at all?
So according to wikipedia unbalanced feistel ciphers provide greater provable security. Specifically, they state:
The Thorp shuffle is an extreme case of an unbalanced Feistel cipher in which one ...
8
votes
1answer
207 views
Is Encrypt+HMAC stronger than AEAD?
There are a few posts that I've come across that seem to infer that using regular encryption and a MAC might be better than using the newer AEAD (ie: AES/GCM) modes.
...
8
votes
2answers
764 views
Can two different pairs of RSA key have the same modulus?
Can $n=pq$ be part of two different pairs of RSA keys?
If such keys exist, say $(e_1,n)$ and $(e_2,n)$, how are they related?
What will be the security concerns for the two users?
8
votes
3answers
372 views
Is it reasonable to assure that p-1 and q-1 aren't smooth?
I came across the requirement that, in RSA, $p-1$ and $q-1$ shouldn't be smooth, shouldn't consist of lots of small factors. Therefore my question:
How complicated is it to check whether $p-1$ is ...
8
votes
1answer
279 views
Can one efficiently iterate valid bcrypt hash output values?
bcrypt is an intentionally slow hash algorithm. In my last protocol idea, I wanted to use it to expand a password and then only transfer the bcrypt-hashed password.
An efficient attack on this would ...
8
votes
2answers
509 views
Salting when encrypting?
I was attending a database encryption session at a developers conference. The presenter (who was a published author on the subject) said that MS SQL Server did not support salted hashes in the ...
7
votes
4answers
1k views
What is the effect of the different AES key lengths?
How does a changing key length affects the ciphertext, not only in case of AES, but in general? I know that the key spaces become much larger and the number of rounds in case of AES changes, but is ...
7
votes
4answers
427 views
How do you find a cheater in Shamir Secret Sharing?
If there are 4 people involved, and every two of them should be able to know the secret (the polynomial is just a line) and you are given f(x) and x for each of those people, and you know one of them ...
7
votes
2answers
546 views
Measuring entropy for a ciphertext only attack
When bruteforcing a password (e.g. the common attacks on DES), where you have ciphertext only, you need a way to assess whether a decrypted plaintext is the right one. I believe the EFF DES machine ...
7
votes
3answers
294 views
Is key size the only barrier to the adoption of the McEliece cryptosystem, or is it considered broken/potentially vulnerable?
A recent paper showed that the McEliece cryptosystem is not, unlike RSA and other cryptosystems, weakened as drastically by quantum computing because strong Fourier sampling cannot solve the hidden ...
6
votes
2answers
236 views
Attacks of the MAC construction $\mathcal{H}(m||k)$ for common hashes $\mathcal{H}$?
Consider a common practically-collision-resistant Merkle–Damgård hash function $\mathcal{H}$ (e.g. SHA-1, RIPEMD-160, SHA-256, SHA-512). We define a Message Authentication Code $\mathcal{C}$
$$(k,m) ...
6
votes
1answer
205 views
A fair peer-based coin-flipping protocol?
I found this question on the game programming site and was intrigued. I came up with an answer off the top of my head but I'm no cryptanalyst so it is probably not water-tight.
This is how my idea ...
6
votes
1answer
161 views
Useful pairings for cryptography
I've recently looked a bit at pairing based cryptography and I was wondering what properties the groups involved should have in order to be useful for cryptographic purposes? Has anything more exact ...
6
votes
1answer
300 views
Exposing RSA private-key data… bad?
We know that exposing $p$, or $q$ or $\phi(n)$ results in trivial attacks on RSA since they allow us to factor $n$ and to compute the private exponent $d$.
In OpenSSL (and most RSA implementations) ...
6
votes
1answer
580 views
AES and perfect ciphers
I'm taking a crypto class this semester and after learning the definition of a perfect cipher. I started wondering how this definition applies to AES.
Obviously AES isn't a perfect cipher, since the ...
6
votes
4answers
920 views
What is the difference between a stream cipher and a one-time-pad?
A (synchronous) stream cipher is an algorithm which maps some fixed-length key to an arbitrary-length key-stream (i.e. a sequence of bits): $C : \{0,1\}^k \to \{0,1\}^{\infty}$.
This key-stream is ...
5
votes
1answer
116 views
In textbook RSA with low public exponent, how big does a random message needs to be?
Assume RSA with a public modulus $N$ of $n$ bits, a small odd public exponent $e$, plaintext $M$ a random non-negative integer less than $2^m$ for some integer parameter $m$, with $M\mapsto C=M^e\bmod ...
5
votes
2answers
159 views
Is there any research on the problem of making a number more memorable to humans?
Authentication protocols often rely on humans dealing with large numbers correctly. For example, when a user logs into a remote machine using SSH for the first time, she is prompted to confirm that ...
5
votes
1answer
278 views
Necessity for finite field arithmetic and the prime number p in Shamir's Secret Sharing Scheme
Shamir's original paper (PDF, 197kb) describing a threshold secret sharing scheme states:
To make this claim more precise, we use modular arithmetic instead of
real arithmetic. The set of ...
5
votes
1answer
215 views
Why does SRP-6a use k = H(N, g) instead of the k = 3 in SRP-6?
I've been reading up on the Secure Remote Pasword protocol (SRP). There are a couple different versions of the protocol (the original published version being designated SRP-3, with two subsequent ...
5
votes
1answer
199 views
Does TLS use RC4-drop[n]?
According to many sources, the first few (n) bytes of the RC4 keystream are strongly biased, and therefore should be discarded before using the keystream to encrypt anything; this precaution is ...
5
votes
1answer
2k views
HMAC vs MAC functions
I've read definitions of MAC and HMAC, but can't say I've completely grasped the differences.
What are principle differences?
When to use one and when the other?(Typical Use Cases)
5
votes
3answers
186 views
Cipher for Product Registration?
I'd like to implement a "Product Registration" scheme into my software where upon initial use, the User must enter a key to unlock it.
The key they enter is compared to their encrypted Username.
...
5
votes
1answer
117 views
Is there any general description language for block ciphers
In page 9 (16 in the PDF) of Final report of European project IST-1999-12324(NESSIE) it is mentioned that:
The NESSIE project is also developing a new generic tool to analyze block
ciphers with ...
5
votes
1answer
138 views
Hermitian curves introductory references
Could you give me some reference to start on Hermitian Curves. Some papers or textbooks would be perfect, and please mention if it's math inclined or comp.sci. inclined.
I've only seen hermitian ...
4
votes
1answer
187 views
A situation where security by obscurity might be the best solution - or am I wrong?
Imagine the following situation. There is an application for a microcontroller-driven system. The profits are made from manufacturing and selling the device itself. However, it is expected, that the ...
4
votes
1answer
203 views
AES timing attacks
I'm just interested in cryptography, so please don't expect me to be an expert. ;) I recently read about AES cache timing attacks and found it very interesting. I read the article Cache-timing attacks ...
