0
votes
0answers
3 views

Digital signature with collision

Let's assume a digital signature scheme such that there's a probability of 0.2 for two random messages to have the same signature. How can I exploit this sheme? I'm having difficulties in finding an ...
1
vote
0answers
5 views

How to decrypt ssl data on a considered to be compromised host

I have a package which I want to encrypt on my machine and send it to another remote machine. If I use asymmetric encryption, I must have the private key on the remote side. However, I don't trust the ...
0
votes
0answers
10 views

In RSA why is it better to have the private key as the multiplicative inverse mod phi(n)

Im studying the RSA algorith and the theory says to select d to be the multiplicative inverse of. $$ e \hspace{5pt} mod \hspace{5pt} \phi(n) $$ If we take this approach i know that it is important ...
0
votes
1answer
17 views

Chosen-Message-Attack RSA-Signature

I know that because of the multplicative property of RSA $s =(s_1s_2) \bmod n = (m_1m_2)^d \bmod n$ If Oscar wants to sign now an own message $m$ he chooses another message $m_1$ with ...
0
votes
1answer
16 views

How to brute force sqlcipher encrypted file with MD5 hash of a 7 bit key?

I have a database file that is encrypted using sqlcipher and MD5 hash. Sqlcipher to my understanding provides 256 bit AES encryption. The key is 7 bits long. How do I brute force it?
-1
votes
0answers
11 views

Packet Filter with Access Control List [on hold]

Can anyone explain how to read and understand row 1,2 and 3? My understanding for row 1 is : If any inside packets want to go to outside via any source port to destination port number 80 using HTTP ...
-3
votes
1answer
17 views

Is SHA-256 safe and difficult to crack? [duplicate]

I am trying to crack a SHA-256 hash but I am not sure how to approach this in an efficient way. The following is known of the original non-hashed content: 64 characters long only consists 0-9 and ...
0
votes
1answer
6 views

Is it possible to extract the decryption key from a locked android device

Is it possible to extract the decryption key from a FDE android device while the screen is locked but the phone is powered on. Is the data only safe when the phone is off ?
-3
votes
0answers
11 views

What kind of hash is in JSON of “Bing image of the day” [on hold]

I just found the json output of Bing image of the day, here it is: http://www.bing.com/HPImageArchive.aspx?format=js&idx=0&n=1&mkt=en-US I saw a "hsh" json object, and after I checked ...
2
votes
0answers
23 views

El Gamal special construction

The algorithm: I have to determine if the following algorithm is secure $p,q \in \mathbb{P}$ and $N = p \cdot q$ $e \in [2, N - 1]$ such as $gcd(e, N) = 1$ $r_1, r_2$ are random number in ...
0
votes
0answers
15 views

If I encrypt data with two different ECC Private keys, how secure is the result?

I'm wondering whether encrypting data with two different 256-bit ECC keys will result in a more secure encryption (minimum 384 bit equivalent) or will result in data that's effectively encrypted with ...
3
votes
1answer
14 views

Not-So-Simple Substitution Cipher?

The first go-to weakness when talking about substitution ciphers is frequency analysis. It assumes that there's a simple 1 to 1 character mapping between the plain text and the cipher text. Could a ...
0
votes
0answers
6 views

Key Management - general question [migrated]

I have a little question regarding the wonderful topic of "Key Management". Let's imagine I would use a service like AWS Key Management. According to their API documentation one can easily decrypt a ...
1
vote
0answers
29 views

Exponential cipher theory misunderstanding

Say I pick modulus $m=2633$ and key $e=13$ My private key $d$ is $13$ inverse in $\bmod 2632$ which I need to compute $(e,m)$ is my public key that anyone can use to encrypt message for me and then ...
2
votes
0answers
25 views

How to determine how good our generator of random numbers is [duplicate]

I am using SecureRandom class in java with default constructor and after i run it four times and each times i will take back 4 "supposed highly secured random numbers" . First run : -35 -43 -127 12 ...
1
vote
1answer
17 views

Size of Pre-Master Secret?

I am implementing TLS 1.2 and using cipher ECDHE_RSA_WITH_AES_128_GCM_SHA256 According to RFC 5346 Section 8.1 length of premaster secret will vary depending on key exchange method. So, what is the ...
2
votes
3answers
38 views

Why the exponent is a power of 2 in Ring-LWE?

I was reading some papers on Ring-LWE. I found almost all of them talk to choose the polynomial modulo $x^n+1$ where $n$ is a power of $2$. I did not understand why this condition is necessary?
-2
votes
0answers
18 views

Login page in Asp.Net -IMPROVEMENT [on hold]

I recently did a registration and login page in ASP.NET and I hashed the password using SCRYPT and saved it to the database. I feel like my project is not complete, I would like to add something ...
0
votes
0answers
15 views

What is the right strategy for me to develop a paid public wifi? [migrated]

I want to establish a paid public WiFi system. Here is my model of how it will work: I will install a public WiFi in my apartment. Using the WiFi system is paid but, voluntary. Those who pay me a ...
2
votes
1answer
36 views

multi key encryption

Is there an algorithm which allows using multiple keys for decryption? I had this scenario in mind: think about being a boss of a corperation which has an encrypted data base with loads of secret ...
7
votes
1answer
63 views

In the Quadratic Sieve, why restrict the factor base?

In the Quadratic Sieve, when factoring a number $N$, many descriptions and most implementations select as the factor base the set of small primes $p$ less than some bound $B$ restricted to having ...
2
votes
2answers
20 views

X509 certificate

I am studying the X509 certificate structure and here is an OpenSSL configuration file for generating the certificate. ("sth" just mean the actual string is removed) ...
-2
votes
1answer
40 views

Range of a number notation

If a number is chosen from $[-n,n]$ is that the same as being chosen from $\mathbb{Z}_n$? Since the operations are modular, what meaning does the negative part of the range have?
2
votes
2answers
61 views

Is this approach able to protect against padding oracle attacks?

I know that a protection against the padding oracle attack is to encrypt-then-mac. This is to avoid trying to decrypt a modified ciphertext, and leak information about padding errors. Suppose that we ...
0
votes
0answers
33 views

RC4 using a key without nonce's or IV's [duplicate]

I'm researching RC4 and I am trying to recreate a condition where RC4 is crackable. However I can't quite find the article/resource I need to find the weakness for an implementation like this ...
2
votes
2answers
30 views

How do I pick variable length passwords uniformly at random?

How do I pick passwords uniformly at random when given a min and max length? It seems that if I first pick a random length, then choose random characters, this procedure will choose shorter length ...
1
vote
1answer
54 views

Sieving the sequence $x^2-n$ to recognize b-smooth numbers

I am currently programming the quadratic sieve and have several literature books / papers and will take an example out of [1] for my question: [1] An Introduction to Mathemtaical Cryptography by J. ...
0
votes
0answers
25 views

Why $p-1$ needs large factors in discrete logarithm?

In discrete logarithm over cyclic group $\Bbb Z_p$ where $p$ is a prime or $p=q^n$ a prime power it is desired that $p-1$ needs to have large factors except for $2$. What is the consequence even if ...
0
votes
0answers
14 views

Why is a-symmetric so much harder to compute and why does it need such big keys? [duplicate]

I read that a symmetric encryption requires a way bigger key size to have the same security as symmetric encryption with smaller keys. But why is this, and why is it so much harder to compute?
22
votes
5answers
4k views

In RSA, how does the CPU deal with this huge modulus (8196 bits)?

Whilst I understand how the RSA algorithm works, I don't understand how the CPU operates when it needs to use the mod function with a huge number $n$, for example. ...
-1
votes
1answer
47 views

Fully Homomorphic Encryption over the Integers with Shorter Public Keys [on hold]

I want to implement Homomorphic scheme from the paper Fully Homomorphic Encryption over the Integers with Shorter Public Keys. I successfully implemented key generation by taking $\lambda = 2, \beta ...
1
vote
1answer
58 views

Scalar Multiplication for Elliptic Curve

Let $\mathbb{E}$ be the elliptic curve $y^2 = x^3 + 6x \text{ mod } 11$ and consider the point $P = (2, 3)$ on it. How do I compute $3P$? I have been able to figure out what $2P$ is, $2P = (5,10)$. ...
2
votes
1answer
60 views

Do I need to use a CSPRNG when creating salts for user accounts?

I'm not sure about the need of using CSPRNG to create salts for each user account. I found “Cryptographically Secure Pseudo-Random Number Generator in Qt/C++ (Cross platform)” (at StackOverflow) with ...
2
votes
0answers
44 views

How does the Number Field Sieve find the target number for Diffie-Hellman?

I have read some papers relating to the Number Field Sieve, but I could not figure out how this algorithm helps in Logjam, or even what is meant by the number field. What is this? What is meant by ...
3
votes
1answer
63 views

Can I change an encryption key used without decrypting first

I'm looking to solve for a little challenge I have in regards to changing the encryption on a message with asymmetric keys. I'd love to be able to swap the encryption to a different key in memory and ...
4
votes
1answer
55 views

Is this a good choice of a digital signature scheme?

This is a concrete instantiation of Rabin-Williams signatures. Specifically: The private key is 2 primes $p, q$. The public key is their product $N = pq$ and is approximately 3072 bits long. the ...
2
votes
1answer
32 views

Is this hash function almost XOR universal?

Consider this hash function: The key $K$ is $2^n$ bits. The message $M$ is divided into $2^n$ bit blocks. Arithmetic is performed modulo $2^{n+1}$. The hash is defined by: $ H_0 = 1 $. $H_{i+1} = ...
3
votes
1answer
20 views

Encrypt-then-MAC when encrypting data in multiple blocks

Given a stream cipher such as ChaCha20 and a MAC such as Poly1305, how do we handle data that comes in chunks in the encrypt-then-MAC scheme ? For example, the plaintext is 530 bytes long. We read ...
3
votes
2answers
240 views

CBC mode with same plaintext

I'm working on a file encryption system where multiple versions of the same file might exist. The filenames are encrypted using AES in CBC mode with a random IV and stored in metadata. Since versions ...
1
vote
1answer
26 views

Montgomery multiplication without final subtraction

I am looking for methods to avoid the final subtraction in Montgomery multiplication. I found this paper "A Cryptographic Library for the Motorola DSP56000 " (http://goo.gl/DHePEx) In this paper they ...
0
votes
0answers
21 views

Combining Elgamal encryption with the Elgamal signature scheme - Security issues?

For didactic (not real-world) purposes, I would like to teach other students the Elgamal encryption system and the Elgamal signature scheme. Are there known security issues when combining them? (i.e. ...
3
votes
1answer
46 views

DTLS 1.2 Key Expansion

I am using DTLS 1.2 and my cipher is TLS_ECDHE_WITH_AES_128_GCM_SHA256 According to RFC 5246(RFC 5246 key calculation) i am calculating keys but i can not find the size of these keys in this RFC. ...
0
votes
0answers
20 views

OpenSSL ECDSA verification [migrated]

Using ECDSA, how do you verify integrity of Data (D), Given the value for the following: Random number (r) Signature (s) ECpublic Key (K) In other words, Using OpenSSL command line, can I decrypt ...
4
votes
1answer
46 views

How to distributively compute a secret of that form $\mathsf{s=sk\cdot g^a}$?

Is there a standard way to distributively compute secrets shares such that any $t+1$ combination out of $n$ of them constructs the secret $\mathsf{s=sk\cdot g^a}$, for a generator $\mathsf{g} \in ...
6
votes
2answers
289 views

How can a poor RNG impact security?

I have read this article and I do somehow "understand" that the reduction of entropy makes it easier to guess. But how exactly can the security be reduced by that? Can you give a concrete scenario, ...
0
votes
1answer
86 views

Derive $x$ when given $g,g^x$ and $g^{(1/x)}$?

If an adversary has access to the generator g of a group G and is given access to $g^{x}$ and $g^{(1/x)}$, will it make it any easier to derive the value of $x$ compared to when he had access to only ...
11
votes
1answer
1k views

Why can an elliptic curve private key be 1?

I often see in papers (e.g. this one) that for an elliptic curve with generator point $G$ and order $n$ the private key $d$ can take on any integer value in the range $[1, n)$. When $d = 1$ the ...
1
vote
2answers
44 views

Why is padding the plaintext with a random string before encryption worse than OAEP / PKCS#7?

I'm reading about OAEP / PKCS padding used for PGP to turn deterministic algorithms like RSA from deterministic encryption to probabilistic encryption (randomness in the resulting output). That way ...
3
votes
1answer
39 views

Master keys' strengh

If I am going to use a master key to generate a session key, why the master key must be "stronger" than the session key? Like should not be distributing 128-bit AES keys derived from a 56-bit DES ...
1
vote
1answer
21 views

Use MDC to provide origin authentication for public key encryption?

Why can't we use an MDC (“Manipulation Detection Code”, as also handled in this Q&A) to provide origin authentication when using public-key encryption?

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