All Questions
3
votes
4answers
198 views
Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash?
$H(s,r)=d$ is a function that hashes the secret string $s$ with a salt $r$, and returns a digest $d$.
$r$ may be arbitrarily chosen and each $r$ returns a different $d$.
For any $d$, $r$ is known from ...
2
votes
3answers
157 views
Is it possible to work out the hash algorithm from a list of known message-hash pairs?
For example, in my situation I know hash(20) = 486e9638177faf1f34e49910491b77af.
I also know the hashes for all values from 0 to 20.
Is it possible to work out the ...
0
votes
3answers
86 views
Padding of Original Message in MD5
what should I do if my original message is greater than 64 bits? what is the process of padding in MD5?
As described in Internet Security: Cryptographic Principles, Algorithms and Protocols, page ...
2
votes
1answer
150 views
SHA256 HMAC brute force with chosen plaintext attacks
This is a follow up to Is It Possible To Reconstruct a Cryptographic Hash's Key
I am using a SHA-256 HMAC function on a single-word input: sha256hmac(privatekey,word) = output. The private key length ...
1
vote
2answers
368 views
Decrypt digital signature using RSA public key with openssl
I have a digital signature that was created using the following algorithm: a SHA-256 hash of the body of the message is calculated. It is then signed using an RSA private key and the result is ...
3
votes
2answers
171 views
toy hash algorithm
I'm looking for a toy hash function, where the idea is to have high school students break (i.e. find a collision) a hash function by hand, in order to teach them how one way functions and hashing ...
1
vote
1answer
99 views
Encrypt a single file, chunk-by-chunk, each chunk using different key (AES)
Encrypt a single file, chunk-by-chunk, each chunk using different key.
I am a security newbie (only took 2 security courses before)
But currently I am using this encryption method for my Android ...
1
vote
1answer
197 views
What does the expression $1^n$ mean as a function argument?
In a paper about predicate encryption or attribute based encryption, the setup function is mentioned with the $setup(1^n)$ or $setup(1^l)$. I want to know what is meant here.
Is it multiples of ones ...
-1
votes
2answers
55 views
Book Didactic Security Notions [closed]
I want study more about cryptography reading papers, but I have a problems to understand few concepts: security notions, random oracle model, IND-CCA, ... etc. I began study this concepts by internet: ...
1
vote
2answers
137 views
Is it worth applying a MAC on data in a HSM?
I'm building a basic HSM out of an Arduino, and am using the following scheme to store data:
Master symmetric key $k_m$ stored in firmware (secure bit set to prevent trivial extraction).
Secondary ...
2
votes
1answer
99 views
Why does key generation take an input $1^k$, and how do I represent it in practice?
In my lecture, the lecturer said:
Let $K$ be the key generation algorithm. Given a security parameter represented in unary, $1^k$, $K(1^k)$ will output a keypair $(pk; sk)$, known as the public ...
3
votes
1answer
136 views
Secure encrypt-then-sign with RSA
I understand that when you want to encrypt and sign data with RSA the generally recommended approach is sign-then-encrypt.
However, I have encrypted data that I need to sign, to prove the author of ...
5
votes
1answer
100 views
Proof that padded RSA is CPA-secure
I'm referring to page 383 of J. Katz and Y. Lindell's Introduction to Modern Cryptography. The book presents a padded RSA:
${\bf Key Generation:}$ same as Textbook RSA (given security parameter ...
0
votes
0answers
54 views
Perfect secrecy Proof
A symmetric cryptosystem have perfect secrecy $\iff$ for all
distribution over the messages space, $M$, and for all $m0$, $m1$ (messages) and any ciphertext $c$,
$$P(c|m0) = P(c|m1).$$
I got ...
3
votes
2answers
224 views
Is it generally possible to employ brute force methods when the encryption scheme is not known? Why or why not?
Lets say you are presented with an encrypted string of bits or text, and no other information. What would be necessary before you could apply brute force methods to decrypting the string? How would a ...
-1
votes
1answer
39 views
Sematically Secure McEliece
I am read the Lemma 2 (pp13) in the paper [1]. Here a make a question Why "for any $Hash_z$ and any $Gen$"? The author of paper reply
The reason why "for any $Hash_z$ and any $Gen$" is that if ...
1
vote
1answer
67 views
How to compare two datasets „anonymously”?
Ok, I hope this question makes some sense because I am not so sure how to word it any differently… Imagine the following situation:
There are 10 defined colors (blue, orange, yellow etc.)
There are ...
0
votes
0answers
35 views
Interesting game: decipher the following sequence [closed]
Decode the following sequence:
KROWE
MOHSI
HTDEY
OJNEI
The original cipher is printed in such matrix format
0
votes
1answer
45 views
chaining rsa with ecies
In an answer to a previous question it was suggested that one way to protect your asymmetrically encrypted AES-256 keys, from say a solution to prime factorization, would be to chain asymmetric ...
1
vote
2answers
78 views
standard way to authenticate parties with public keys without certificates
Is there some standard protocol for mutual authentication of two parties when there is possibility to use public key cryptography? The problem is that two parties can use public key cryptography but ...
1
vote
1answer
41 views
Adversary Two Stages
I am read this paragraph and I have a doubt.
"An adversary to PKC $\Pi$ is given by two probabilistic polynomial
time algorithms, $A = (A1; A2)$. In the first stage, the "find"
stage, the ...
1
vote
1answer
81 views
Secure order preserving hash function
Is there a construction of an order preserving hash function that keeps the preimage property of a crypto hash function? By order preserving hash function (OPHF) i mean for $x<y$ then $OPHF(x) < ...
3
votes
2answers
109 views
what is the difference between Scrypt and PBKDF2
After reading these two resources I am wondering am I getting all the differences between Scrypt and PBKDF2 or am I missing something?
As far as I understood, the similarity is:
both are using ...
4
votes
1answer
154 views
Quality of randomness on a Linux system with haveged
Has anyone checked if using haveged, a Linux daemon which uses the HAVEGE algorithm, changes the non-deterministic properties of the random data from ...
1
vote
1answer
50 views
Counter Mode: static IV but different keys [duplicate]
Given we are using AES counter mode, suppose we randomly generate several keys, all of them are using same IV (say, zeros). Does this lead to any security issue?
I know that in CTR mode, same key-iv ...
0
votes
2answers
85 views
risk of attacker decrypting RSA ciphertext without public or private key
As I describe in my previous question I am trying to decide if it's worth it for me to use the Offline Private Key Protocol in creating some long term private archives, instead of just going with a ...
2
votes
0answers
91 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 ...
3
votes
1answer
50 views
Chosen ciphertext insecurity in an ElGamal variant
I'm trying to prove something and if I can show that there is a simple way to calculate $(g^a \bmod p)^k$ if I know both $g^k \bmod p$ and $g^a \bmod p$, then (I think) it will help me prove it, but ...
0
votes
1answer
48 views
Certificate == signed public key
For primes p and q used to create a keypair, I understand that the following operation is used to create a signature :-
M ^ d (mod N)
where d and N have their usual meanings and M is the message. In ...
6
votes
3answers
2k views
How to generate a list of unique random strings?
How would you generate a list of distinct random strings, where all strings are alphanumeric and have a fixed length?
1
vote
1answer
138 views
Simple example for CP-ABE (Ciphertext policy attribute-based encryption)
I'm currently working on Ciphertext Policy Attribute-Based Encryption (CP-ABE). So far I'm only using it with a basic understanding how it actually works. Now I want to understand it a bit better, but ...
1
vote
0answers
37 views
Parallelizing AES GCM in BouncyCastle [migrated]
I'm using AES-GCM with the BouncyCastle provider in Java and it works fine. The NIST documentation seems to suggest that GCM is an algorithm that allows parallelization -- however, on looking at the ...
4
votes
3answers
218 views
McEliece Cryptosystem Implementations
Are there any current implementations (language irrelevant) of the McEliece Cryptosystem?
I have been hunting around all day, and yet, have only found a few mathematical equations pertaining to the ...
2
votes
1answer
162 views
Are there any practical implementation of a homomorphic hashing or signature scheme?
A homomorphic hash function is a function $H : A \to B$ between two sets with some algebraic structure $(A, *)$ and $(B, \star)$ such that
$H$ is collision resistant, i.e. it is hard to find $x \neq ...
-4
votes
1answer
96 views
Indistinguishability CPA and CCA2
I am study security notions in two lecture and I am cofusing with the concepts IND-CCA and IND-CPA2. About IND-CCA in wikipedia say
Indistinguishability under chosen ciphertext attack/adaptive chosen
...
-1
votes
0answers
36 views
Adaptative chosen ciphertext security McEliece
I am read the paper and I found the follow Lemma2 (pp. 13)
Suposse that there exists, for any $Hash_z$ and any $Gen$, an algorithm $A$ ...
My question is Why "for any $Hash_z$ and any $Gen$"? Are ...
-1
votes
1answer
40 views
Determine the Identity of Caller [closed]
Let me explain my situation.
I have a provider which is made up of a registration page and a web service. Online vendors (clients) register with the provider by providing details such as username, ...
1
vote
0answers
67 views
Question about proof of knowledge defintion?
I am just reading the "soundness"-definition for proofs of knowledge by Bellare / Goldreich. A proof of knowledge is a proof between a prover $P$ and a verifier $V$. $P$ convinces $V$ to know a secret ...
0
votes
1answer
90 views
RSA - Ecrypting the same data with the same public key = same ciphertext?
If an adversary knows my public key and guesses what was my plaintext, can he test for it somehow?
The most obvious way is encrypting the guessed plaintext with my public key and the same parameters ...
1
vote
0answers
61 views
Is OAEP reversible?
Given nothing more than some integer $m =$ OAEP($M$), is it possible to recover the original plaintext $M$? In other words, without being given the hash functions or the random string used for ...
-1
votes
0answers
58 views
Tools to assist learning cryptography in real world [closed]
Hey I am new to cryptography and I wanna see how cryptography works in the real world. Suggest me some tools which will help get me a better idea about cryptography. (If it were networking I would ...
3
votes
1answer
81 views
implementing long term archive encryption
Let's say I want to create private archives for the long term (e.g. more than 30 ). The archives' sizes could be anywhere from 1 GB to 30 GB. As far as I understand I could go down two ways:
The ...
1
vote
1answer
74 views
How can a key pair be derived from an arbitrary hash?
If I correctly understand the concept of a "brain wallet" in BitCoin, you start with a passphrase, generate the hash of the passphrase, then somehow derive a public / private key from that to use as ...
2
votes
4answers
188 views
Tamper-proofing log files
Problem Overview
I want to securely store log files so the contents are secret, and they can't be modified without detection.
The files will be encrypted using authenticated encryption (AES in GCM ...
3
votes
2answers
191 views
What is the best way to store sensitive information on the client?
I am very inexperienced with cryptography and would like some advice on how to store some sensitive information on client machines.
I have some information stored in a central database on my server. ...
1
vote
2answers
190 views
Does key authentication in SSH protect from MitM?
How do cryptography in SSH(2) work (secure channel establishment, authentication)?
Does key authentication in SSH(2) protect from MitM by providing authentication and integrity and why?
1
vote
0answers
57 views
Algorithm digipass-like
I'm pretty sure there are famous algorithms for my problem, but I don't have enough knowledge in this field, so I need your lights.
Here si the problem :
Alice sends key X to Bob.
Bob has an ...
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 ...
0
votes
1answer
96 views
Is this scheme a provably fair random number generation?
I have thought up a method for generating random numbers between a client and a server which I hope is fair:
The client and server decide on a range in advance, $0$ trough $n-1$.
The server ...
4
votes
1answer
185 views
repeating-key xor and hamming distance
I read that to break repeating-key xor you can do the following: try a keysize $n$ and compute the hamming distance between the first $n$ bits of the encrypted string and the bits $n+1$ to $2n$ of the ...
