3
votes
2answers
119 views

CSPRNG that cannot be used as random extractor

Are there any specific CSPRNG's that cannot be used as randomness extractor within a TRNG? In other words, can you simply provide enough entropy gathered from a good entropy source as seed in any ...
4
votes
1answer
162 views

Implementing CD serial key system

I am trying to create a system where to unlock the application one needs to enter a serial code. I have read many articles on the theme but there are two problems bugging me. One is, If I have a ...
16
votes
9answers
2k views

Why not the one-time pad with pseudo-number generator

I am very new to cryptography (so be kind), but I have a question that may seem silly. If the one-time pad is the perfect cipher and impossible to crack, why would the following algorithm not be one ...
6
votes
1answer
88 views

The REACT transform and Replayable CCA

Before stating my questions, let us recall the REACT transform [OP01], which enables to construct a CCA-secure hybrid PKE scheme, $\varepsilon'_{pk}$, from an OW-CPA PKE scheme ...
0
votes
1answer
201 views

Vigenère cipher: Security when key length and plaintext length are the same

I have read the Vigenère cipher is secure as long as the key length is the same as the length of the data to be ciphered. Is this true in cases where the same key is used multiple times. In that case ...
1
vote
1answer
149 views

Whitening and randomness extraction

The Von Neumann randomness extractor is explicitly mentioned in the randomness extractor page of Wikipedia, but strangely enough that page does not contain any reference to whitening the results of an ...
2
votes
0answers
100 views

Understanding Lattice based cryptosystem [closed]

I have heard that there is one paradigm of Public key cryptosystem, called Lattice based crytography. Further, its security claims are such that it will not be affected even if the quantum computers ...
2
votes
1answer
233 views

Adding points on Elliptic Curves

How do we add the integer points $P=(-1, 4)$ and $Q=(2, 5)$ on the elliptic curve of the form $y^2=x^3+17$ ?
5
votes
1answer
235 views

Elliptic Curve Cryptography

I have been trying this for a while. But I couldn't get it. How can I determine the point of intersection of the tangent line at (0, 0) on the curve $y^2 + y = x^3 + x^2$ ?
4
votes
1answer
144 views

Compression function to guarantee randomness of one time pads?

First of all, I would like to note that I am not building my own crypto. I am simply curious and would like to learn. What I am wondering is this: if we are worried about randomness of something ...
-6
votes
1answer
92 views

RSA Key and RSA Security

Is there any relationship between the RSA keypair standard and RSA security? Keypair: http://en.wikipedia.org/wiki/RSA_(cryptosystem) Security: http://en.wikipedia.org/wiki/RSA_Security If not, ...
2
votes
0answers
84 views

Can you help me understand this “Message Modification by Meet-in-the-Middle” related to Tiger Hash? [closed]

I found this paper that explains how to attack a reduced round tiger hash, but I'm having a hard time understanding some of the mathematical notation. Could someone explain the section "Message ...
0
votes
1answer
132 views

How to implement a Cryptographic Pseudo Random Function

I need three pseudo random functions which all take a seed S (t-bits long). The functions should map from $d$ bits to $d$ bits $d$ bits to $1$ bit $d$ bits to $t$ bits $d$ is $10$ bits $t$ is ...
2
votes
2answers
696 views

Digital Signature using symmetric key cryptography

Generally digital signature is a public key cryptography concept.But it needs high overhead. So is there any publication or link available where 'digital signature using symmetric key' has been ...
3
votes
1answer
295 views

True Random Number Generator by milliseconds per keystroke (TRNG-Kms)

The simplest way to generate truly random numbers for OTP keys is to measure the time in milliseconds between each keystroke on a keyboard. The randomness depends on the user typing in various speeds. ...
2
votes
1answer
71 views

Should I calculate two CMACs over two different strings that share common bytes?

I am trying to secure my RF communication in which one RF package consists of two separate segments. I need to make sure that one segment of one packet is not replaced with a valid segment of some ...
37
votes
5answers
10k views

Is AES-256 weaker than 192 and 128 bit versions?

From a paper via Schneier on Security's Another AES Attack (emphasis mine): In the case of AES-128, there is no known attack which is faster than the 2128 complexity of exhaustive search. However, ...
3
votes
2answers
553 views

Reversing DJB2 Hashes

I have some spare time, and a few hundred DJB2-hashed values sitting around. I thought I'd try to do something "useful" and invert DJB2, such that I could calculate the plaintext of the hashes (which ...
1
vote
0answers
173 views

Rainbow tables and blowfish

I'm thinking of implementing rainbow tables for a specific blowfish problem, but I have trouble thinking of the proper way to apply the original paper (and its application to hash functions) to ...
2
votes
1answer
653 views

Probability that two different files can have same MD5 hash for both an unencrypted and encrypted version (AES256, same key between files)

This math is way over my head, and don't even know if it can be done, but here are the assumptions: Take unencrypted File [Fu] of variable length, but assume 100KB as an example. Let File [Fe] be ...
2
votes
1answer
190 views

Why must we hash lot of times in rainbow tables?

I don't get the utility to chain and reduce in generating rainbow tables. Hash functions are made to not have collisions, or at least chain only 2 times so we don't return to the concept of the hash ...
2
votes
2answers
253 views

Idea for user/pass hashing to prevent rainbow tables, would it work?

I'm very new to cryptography (and security in general, for that matter), but I had an idea that I'm sure is very flawed, but is worth asking. If a computer user, online account, etc, needs to verify a ...
3
votes
2answers
145 views

Merkle Tree High Tree

I'm reading Merkle tree signatures and I like know if the security of the that scheme depends, also, of the high of the tree or only depends that hash function is resistant to collision?
3
votes
1answer
118 views

Why is Lamport-Diffie secure?

Why is Lamport-Diffie secure? I note that there is a demonstration based on onewayness (in the book postquantum cryptography). But a one way function is not sufficient to ensure that it can not infer ...
3
votes
2answers
430 views

How does using salt reduce rainbow table attack?

I could create a rainbow table for a particular salt value and still create a successful attack similar to one without salt value.
2
votes
2answers
447 views

Is rainbow table attack applicable to any hash?

Typically when rainbow tables are discussed MD5 hash is used as an example. It's not quite clear whether this attack is specific just to MD5 or to a certain subset of hashes or to just any hash ...
6
votes
1answer
4k views

Example Rainbow Table Generation

I have read up on rainbow tables and think I understand the idea behind them. However, I find that it would be better for me to actually attempt to implement a (very basic) rainbow table generator in ...
5
votes
1answer
118 views

How can I find the order of the group that an elliptic curve is defined over?

I have a Weierstrass elliptic curve ($y^2=x^3+a \times x+b \mod p $) How can I find the order of the group itself? I have seen Mathematica has a GroupOrder[] ...
2
votes
1answer
117 views

Generalized Merkle Signatures, SHA-3 and Sakura

Sakura specifies a tree hash mode for e.g. Keccak (SHA-3). Are there any reasons to use the Sakura tree hash mode in a Generalized Merkle Signature Scheme? It seems to me Sakura primarily solves a ...
1
vote
1answer
4k 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 ...
2
votes
3answers
112 views

Does 3-DES take approximately the same time to encrypt 1B and 1KB?

I am using 3-DES to encrypt some data in Java. I decided to run some tests and plot some graphics to decide how to optimally pack data for its encryption (see how encryption/decryption time behave ...
4
votes
1answer
954 views

AES mixcolumn stage

I'm studying AES, and am having problems with the "mixcolumn" stage. I read about finite fields, but I still don't know. How do I construct $GF(2^8)$? ...
0
votes
2answers
113 views

Elliptic curve group over a prime finite field $F_p$

If $p$ is a big prime, and the elliptic curve $E$ is defined over $F_p$ by the equation $y^2=x^3+ax+b$ where $a,b\in F_p$. The point on $E/F_p$ together with the infinite point $\mathcal{O}$ form a ...
-1
votes
0answers
81 views

Calculating periodicity of a pseudo random number generator (Middle-square method) [closed]

Below is a Python code that calculates the periodicity of the middle-square method for a given 4 digit-numbers: ...
2
votes
0answers
55 views

Where is the OID and ASN.1 specified for AES_CMAC?

I have a requirement to implement AES_CMAC as the authentication algorithm in a CMS library. I just can't seem to find the OID and the ASN.1 definition for it. I expected to see the OID under ...
0
votes
1answer
126 views

Is MAC-then-encrypt with stream ciphers secure?

While thinking about this recent question about a hash-then encrypt design, I reread the MAC-encrypt vs. encrypt-MAC question and noticed this answer quoting a paper showing that MAC-then-encrypt is ...
1
vote
1answer
97 views

Adding an IV to an AES implementation

I have an embedded project that requires AES, I seem to have found an AES implementation that fits the bill here: http://www.literatecode.com/aes256, but it does not include an IV. Are there any ...
1
vote
0answers
89 views

Universal hashing techniques based on matrix multiplication

The article here says below, about a universal hashing technique based on matrix multiplications. "However, there is a little known method based on using a random matrix. It has lots of advantages - ...
4
votes
1answer
362 views

Is there a generic attack on encrypted CRC32 when used as a MAC?

I am examining a protocol that uses CRC32 as a MAC (see note 1) the weaknesses of this method but I would nevertheless like to see if it is just weak or actually relatively easily to break. The ...
2
votes
2answers
198 views

Why is MAC using nonce+message+hash(nonce+message+identifier) not the standard?

Given that we know that nonce + message + hash(nonce + message + mac-key) all inside the encryption makes it possible to detect if any bits of the message have been ...
-1
votes
1answer
80 views

Textbook RSA with exponent e=3 [duplicate]

I remember $e=3$ is a bad choice for textbook RSA. But I can't really recall why. What happens if I choose $e=3$? Can I retrieve the message by simply calculating $\sqrt[3]{c}$? Why can't I do that ...
2
votes
1answer
111 views

Secure way to store fixed size string of digits

I need to store a string of digits (0-9) of a fixed size in my database securely. I've already performed a rainbow table attack on a plain hash, and it broke it in a matter of seconds. One option I ...
1
vote
1answer
132 views

Which algorithm do you recommend for practical use to generate unique passwords for each website?

So, in my imagination there is this simple algorithm with only one master password, let's say a prime number 'p', which I have to remember. Using only the address of the website and p, the algorithm ...
3
votes
2answers
251 views

Reordering non-block-aligned parts with DES in ECB mode

I was given a ciphertext file which was encrypted using DES in ECB mode. It is known that the plainttext that was encrypted has the following form: Each line of text consists of a payroll followed ...
5
votes
3answers
636 views

RSA with modulus n=p²q

I've been wondering what would happen if the RSA modulus would be $n=p^2q$ instead of $n=pq$. I feel like there is an obvious security flaw (as there is with most modifications). Would this choise of ...
1
vote
1answer
95 views

Cryptography: Oblivious Transfer with at most one transfer?

I am trying find a transfer method for this problem. Suppose Alice and Bob both has access to a function $\text{F}$ with two parameters. $\text{F}(x,y)$ randomly returns $p$ to alice and $q$ to Bob ...
1
vote
1answer
159 views

MAC using a modified CBC mode of operation

I'd like to know whether the following modified CBC-MAC is secure (given a fixed IV). Source: an old course on Network Security Are there some obvious attacks I can't see?
5
votes
1answer
2k views

Advantages using Diffie-Hellman or Elgamal

For what kind of usage should we prefer using Diffie-Hellman in order to exchange keys instead of Elgamal, and most important why should we use one or the other? I do not see a clear difference ...
4
votes
2answers
156 views

Spoofing protocol nonce

Amy and Betty have a shared key $k$, and the protocol below is to provide a mutual authentication for both Betty and Amy. A sends B : $n_a$ B sends A : $n_b \;\|\; E(k, n_a)$ A sends B : $E(k, n_b)$ ...
2
votes
4answers
755 views

RSA with composite numbers

It has been said that RSA uses a modulus product of two prime numbers for security reasons. But does RSA even work correctly if we allow composite integers instead? I think that the answer is "NO". ...

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