1
vote
2answers
1k views

decrypt AES without whole key

Is there a way to decrypt a AES cipher Text which was encrypted twice with some key, when having a part of this key? (the last 3 bytes of the key are missing)
0
votes
1answer
313 views

What is the complexity of the Square attack against the reduced 4-rounds 128-bit Rijndael variant?

I'm looking at a square attack against a reduced version of AES-128 with only 4 rounds (with block and key size of each 128 bit). I have a set of 256 plaintext-ciphertext block pairs. What is the ...
-1
votes
1answer
384 views

Recovering SHA1 knowing 2/3 of the hash generated

I am doing some research on cryptography, so I generated a random string of numbers resembling a creditcard number(according to MOD10 validation) and encrypted it with SHA1 algorithm for encryption. ...
2
votes
2answers
228 views

Subgroups generators with respect to group generators of composite order

If I have a group $\mathcal{G}$ of order $N=npq$ and subgroups $\mathcal{G_n,G_p,G_q}$ of order $n$, $p$, $g$ respectively and if $g$ is a generator of $\mathcal{G}$ why then $g^{nq}$ is a generator ...
9
votes
3answers
225 views

Does chaining random number generators lead to loss of randomness?

Assume you chain random number generators, i.e use the result of one PRNG as the seed for the next. What kind of errors can that introduce in the randomness of the numbers generated?
2
votes
4answers
771 views

Would this simple encrypted chat program be feasible using One Time Pads?

Lets say I want to build a simple chat program that has encryption that is impossible to crack for anyone, even a theoretical government with a massive super computer. Ok here's my idea. You'll need ...
1
vote
5answers
2k views

How easily could this be cracked?

I'm looking into encrypting files like this: First, I SHA-1 hash the password. Let's say the password is something normal like "hello123", and then hashed. Then I encrypt a file using this: ...
3
votes
1answer
237 views

Seed a PRNG with random data and a password

I'd like to combine a random key file with a password to generate a secure seed for a CSPRNG. The key file is assumed to have very high entropy, but the password will be whatever the user provides. ...
2
votes
2answers
368 views

Why is a non fixed-length encryption scheme worse than a fixed-length one?

I have the following definition (highlights by me): An (efficient secret-key) encryption scheme $(Gen,Enc,Dec)$, where $Gen$ and $Enc$ are PPT algorithms and $Dec$ is a Deterministic Polytime ...
-1
votes
1answer
209 views

Because the algorithm is known, it is no longer a trade secret [closed]

In wiki about RC4 cipher they said, Because the algorithm is known, it is no longer a trade secret. Is that mean the cipher text will be more secure if the algorithm unknown to the attacker. ...
-2
votes
2answers
398 views

How weak/strong is this hand cipher? (updated) [closed]

In simple steps here the system: first write text. hand cipher then divide into groups of three letters: han dci phe r then choose a key consisting of three letters, for example ...
3
votes
1answer
189 views

Impact of algorithms for factoring using elliptic curves over $\mathbb{Q}$

Recently a few papers have appeared that describe a new approach to factoring, using elliptic curves over $\mathbb{Q}$. See, e.g., Factoring integers and computing elliptic curve rational points, ...
3
votes
1answer
381 views

Is frequency analysis a useful tool against encryption by multiplication?

If I transform natural plaintext by: making each letter two decimal digits, considering the whole as a decimal number; multiplying by the key (some integer constant), giving the ciphertext; would ...
0
votes
1answer
497 views

Matrix Multiplication of Bits

How to multiply the matrix of bits example I have two 4x4 matrices of bits ...
4
votes
1answer
175 views

What authentication techniques can be used by QKD Systems?

As it is well known that QKD systems are prone to simple MITM attacks… what authentication techniques are being used in QKD to prevent those attacks in practice?
39
votes
5answers
13k 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
745 views

Key derivation from a random seed

The main problem is to use a block cipher to generate a random key. I would like to generate 256-bits key which can be as random as possible. I generate it in the following way: Pick a plaintext ...
2
votes
1answer
4k views

What exactly is addition modulo $2^{32}$ in cryptography?

EDIT: I've been confusing this the whole time. What I've been wanting to say this whole time is addition modulo $2^{32}$ not addition modulo 32 as the question originally said. Thanks for pointing ...
2
votes
1answer
776 views

Shamir Secret Sharing - Threshold decryption implementation

I'm implementing Shamir secret sharing in C using OpenSSL. I've split up the private key and I'm ready to start decrypting the original message using some of the shares. I set $n = 5$ (5 shares ...
5
votes
2answers
3k views

Is AES in CBC mode secure if a known and/or fixed IV is used?

I have a need to encrypt credentials for a third-party app used by a secured internal app. Over on ITSec.SE, I was helpfully shown a scheme to encrypt the third-party credentials based on a hash of ...
1
vote
1answer
108 views

Is secure remote snap possible?

Scenario: We have a central server $S$. We have a number of peripheral servers $P_i$ We have some individuals $U_j$ A given individual may be "known" to one or more peripheral servers. Each ...
1
vote
1answer
114 views

Conditional entropy

Suppose I have a file with random binary strings of the same length in each line. If I'm computing the conditional entropy $H(Y|X)$ where $Y$ is the variable string of fixed length $l$ which is ...
3
votes
3answers
155 views

The meaning of “scheme”

This question is a bit different from other questions here, but I think it is suitable to correctly understand the terminology of cryptography. Consider the following two sets of terms: Encryption ...
1
vote
1answer
2k views

How can I implement the “Multiplication Modulo” and “Addition Modulo” operations in IDEA?

I am currently working on IDEA (International Data Encryption Algorithm), and I don't know how to perform Multiplication Modulo and Addition Modulo. This is how IDEA operates: IDEA operates on ...
2
votes
2answers
227 views

How to construct a variable length IND-CPA cipher from a fixed length one?

Suppose we can construct a secure (IND-CPA) encryption scheme for fixed length messages. I was wondering if there is a natural way to extend this construction for messages of variable length such that ...
3
votes
4answers
601 views

Is there a symmetric-key cryptography based on key establishment techniques?

Most of the current key exchange techniques are based on public-key cryptography. Are there any key exchange/establishment techniques based on symmetric-key cryptography too? In my setup primary ...
4
votes
3answers
3k views

Hill Cipher known plaintext attack

I know a plaintext - ciphertext couple of length 6 for a hill cipher where its key is a [3x3] matrix. Based on what I've read and learned, to attack and crack keys of [n x n], if we know a plaintext ...
1
vote
1answer
425 views

Key Scheduling of International Data Encryption Algorithm (IDEA)

How to perform the key scheduling in International Data Encryption Algorithm (IDEA), I took a research but I can't understand how to perform it, "further groups of eight keys are created by rotating ...
3
votes
1answer
490 views

Are there secure stream ciphers that cannot be parallelized?

Are there any stream ciphers (or a deterministic random number generators, that should work as well I guess?) that cannot be parallelized? So for example if I seed it with a specific value, and then ...
1
vote
1answer
1k views

Traditional DES scheme in Unix crypt function

In a security context course, we need to reproduce the old DES hashing scheme in the crypt program, on an old unix system. I am actually reading the crypt page on wikipedia: The traditional ...
0
votes
1answer
272 views

Ways to protect flash player against MITM diffie hellman

We have a server, which sends content encrypted with AES128 or RC4 to a flash client over an insecure HTTP channel. The encryption key is negotiated using Diffie Hellman. The problem: Diffie ...
2
votes
1answer
356 views

Increase number of rounds for SPN and Feistel ciphers

Read a post on Schneiers blog (and again 2011) about increasing the number of rounds for AES from to "AES-128 at 16 rounds, AES-192 at 20 rounds, and AES-256 at 28 rounds" to raise the security. ...
3
votes
2answers
410 views

Can I save space for short messages by using encryption with private key instead of a signature?

Let's say I have a message, a Private Key and a Public Key. Normally if you want to see if the message is unaltered and is from the sender you would have the message part + signature part, which you ...
1
vote
1answer
350 views

password entropy calculation

I have the following password with a prescribed format: it starts with a fixed value of 111, followed by 2 random lower case letters and then lastly a random digit: "111""2 Random lowercase ...
2
votes
1answer
115 views

Is it safe to hold key file's hash in application

Title says all; Is it safe to hold key file's hash (MD5 or SHA1) in application (hard-coded)? Thanks
2
votes
0answers
200 views

How do I encrypt with the private key? [duplicate]

Possible Duplicate: RSA encryption with private key and decryption with a public key This wording is creeping everywhere (e.g. there): "I encrypt with the private key" and even sometimes, ...
3
votes
1answer
814 views

What happens to the entropy of a password when you hash it?

For example, if the entropy of a password is 30 bits, what is the entropy of the password when you hash it with MD5?
9
votes
1answer
1k views

What's the difference between a Key Derivation Function and a Password-Hash?

It seems to me that anything that was sufficiently good as a KDF would work just fine as a password hash, though the reverse might not be true. Are there considerations specific to password-hashing ...
4
votes
3answers
377 views

Is there a public key semantically secure cryptosystem for which one can prove in zero knowledge the equivalence of two plaintexts?

If Alice encrypts two messages $a$ and $b$, such that $x=E(a)$, $y=E(b)$. Can Alice prove (without revealing $a$, $b$ or the private key) that $a = b$? Obviously the proof must not be too long and it ...
1
vote
2answers
155 views

Encryption algorithm which produces comparable results for substrings

I am wondering is there any encryption algorithm available which produces comparable string. I am expecting the output as shown below. ...
1
vote
1answer
115 views

Using pairings to verify an extended euclidean relation without leaking the values?

Let $P_i(x)$ be polynomials $i=1,...,n$, $s$ some value, and $g$ a generator of a group $G$ where the discrete logarithm is hard. Assume a prover wants to convince a verifier having access to the ...
2
votes
2answers
194 views

RSA keys finding when messages are known

I have this situation, where, in a game, people send messages to each other (game moves etc...) These messages need to be encrypted, and should only be readable by the destination person. I am using ...
1
vote
1answer
1k views

AES encryption for images [closed]

Why AES based encryption is not recommended for encrypting images with high redundancy in their content? for example, the encryption of an image which shows a bird in the blue sky. Most of the pixels ...
1
vote
2answers
135 views

Quantum key distribution and OTP

Why is QKD usually associated with OTP ? QKD is about key distribution, where as OTP is about using key as long as message to get perfect secrecy, can any one explain how they are related ?
3
votes
1answer
613 views

What's the main difference between Pohlig-Hellman and RSA?

Both Pohlig-Hellman and RSA perform encryption and decryption by exponentiation modulo some integer ($p$ prime for PH, $n$ composite for RSA). They both use a key $e$ as the exponent to encrypt a ...
2
votes
2answers
687 views

Avalanche noise RNG for one-time pad use

I came across this little HRNG widget and was really intrigued as I have been looking for a decent but afordable source for truly random bits to use in a one-time pad. The question is, would a HRNG ...
4
votes
2answers
2k views

Perfect security definitions

In my notes, there are 2 definitions of perfect security: "For $M \in \{0,1\}^m$, define the distribution $D_M$ on strings as follows: to choose a random member of $D_M$, choose a random $K \in ...
2
votes
4answers
723 views

One time pad key exchange

If two parties want to communicate with securely using OTP , how is the key ( that is lengthy as plain text) shared with other party for decryption ?
5
votes
4answers
437 views

How is it possible to parallelize a hashing function to crack an iteratively hashed password?

Suppose I have an algorithm that relies on multiple iterations of a hash function like SHA1 to slow down an attacker trying to bruteforce a hash. ...
2
votes
1answer
1k views

One-time pad and zero key

I'm doing some exercises before my exam, and I am stuck with task number 4 in this file: http://www.cs.umd.edu/~jkatz/crypto/f10/hw1.pdf Could you help me with this task? When using the one-time ...

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