# All Questions

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### How can I decode a Hill Cipher without a key? [duplicate]

On my exam, we had to solve this problem: message matrix is: [20, 19, 14; 17, 0 10] and the ciphertext matrix is: ...
174 views

### What's the entropy of the output for Tiger Encryption?

I'm tasked to apply tiger encryption to my string(key). Does anyone have any idea how to find the entropy of the hex output by tiger encryption?
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### Using beginning of TLS ecnrypted data to reveal secret [duplicate]

I have the following question about TLS security: Assume TLS-PSK protected HTTP with AES256-CBC cipher. When a TLS connection is established, client sends some encrypted data, where the plaintext is ...
2k views

### Obtaining the key length from the ciphertext of an auto-encipher

I'm revising for a cryptography exam at the moment and I'm having some problems with a question. The question looks for the key length of a cipher given that the word "earthquake" appears in the ...
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### Spurious keys filter [closed]

Is there any possible way to filter a key space in a crypto-system to get a set of keys that decrypts out a valid text (text that is in same language or readable)? I tried a random approach and got ...
454 views

### Frequency analysis of transposition ciphers

Can we do frequency analysis of transposition ciphers? If yes, please tell me the procedure and I would be happy if you provide me with some links related to it. I heard that it can be done using ...
457 views

### Security of RSA for paranoids with padding?

RSA for Paranoids (RSAP) (in cryptobytes v3n1), also known as Unbalanced RSA, is a variant of RSA proposed in 1995 by Adi Shamir, as a mean to increase the RSA public modulus size while keeping ...
87 views

### Cryptography systems based on NP complete problems

There is any cryptography system that have a good reputation based on NP complete problem? I read about the knapsack, but it was cracked.
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### Which homomorphic encryption scheme has the same size of plaintext space and ciphertext space?

I want to pre-compute the result for all possible ciphertext of a homomorphic encryption. Is it acheivable? Is there a fully homomorphic encryption scheme that has the same size of plaintext space ...
225 views

### Key construction in the Full Cramer-Shoup cryptosystem

The Full Cramer-Shoup Encryption scheme needs to choose six random numbers from $\mathbb{Z}_q$, which we denote them by $x_1,x_2,y_1,y_2,z_1,z_2$. Then the scheme hides these random numbers in the ...
119 views

### How can I generate a brief (100) stream of random numbers, not using the computer or throwing dice? [closed]

I am looking for a method of making random numbers that is off-line and is also independent of specialist equipment like a Geiger counter.
395 views

### CVE-2014-1295 (Apple's Secure Transport and Triple Handshake Attack)

Apple is releasing some updates today. They have not published the bulletins yet, but they are available here: https://gist.github.com/FredericJacobs/11189734. In the writeup for CVE-2014-1295: ...
121 views

### Can a zero knowledge proof (or MAC) be generated from human-typable input?

Edit: My goal is to create a voting scheme that doesn't require a lot of crypto infrastructure to be deployed on the client side. Smartcards, and tokens are impractical for these purposes. I ...
440 views

### Extracting only the entropy

(I cleaned up the question a bit, It was previously titled "extracting entropy without hashing" .. thus some of the comments / answers) I'd like to extract the entropy of data without injecting ...
90 views

### Is less security required for a short stream cipher than for the AES enciphering of very long messages? [closed]

Criticisms of a cipher system such as 'the ciphertext from one message must be indistinguishable from the ciphertext of a second message" surely only apply when there are very large amounts of ...
22 views

### Cryptography — with a semi-priveleged user in the middle — to prevent request-tampering with another server

I'm working on a chat server for a mobile app I am writing. I would like to use a different application server for non-chat related operations and another application for chat operations. I would ...
123 views

### Encryption mode with chained algorithms

Suppose that I want to encrypt some data using two different algorithm chained (e.g. AES and Serpent) and harnessing the benefits of CBC. I can do: ...
349 views

### Are the MD5 constants an S-Box?

I noticed that several hash algorithms, in this case MD5, defines some constants. MD5 defines 64 constant values (the shift count per round) which also gets expand (via sine) to a summand for a ...
546 views

### What are the key differences between the draft SHA-3 standard and the Keccak submission?

I just noticed that on the NIST website there is a PDF with a draft of the SHA-3 standard (i.e. FIPS 202) (marked as "new", and seemingly the page was last changed on April 7, 2014). Previously it ...
198 views

### “Fine grain Cross-VM Attacks on Xen and VMware are possible” and OpenSSL 0.9.7

Here's a paper that just made its way into Cryptology ePrint Archive: Fine grain Cross-VM Attacks on Xen and VMware are possible!. Its a good survey of past attacks and presents a couple of new ...
3k views

### Use cases for CMAC vs. HMAC?

Both can be used to verify the integrity of a message. Assuming you have the needed primitives available to you (i.e. the code space of needing both a cipher and a hash function isn't prohibitive), is ...
102 views

### Why does computing g^a * g^{-a} with the PBC library result in zero?

My example code is as follows: /* * Example 1 * 1) Calculate g^a * 2) calculate g^{-a} * 3) multiply g^a * g^{-a} * */ Note: here ...
100 views

### Definition of one-way functions: randomly-chosen point vs every point

Notation: $\Sigma^k$ is the set of $k$-strings on the alphabet $\Sigma$; $\Sigma^\ast$ is the set of all finite dimensional strings on the alphabet $\Sigma$. In the context of computer-science a ...
3k views

### How to calculate Modulo using a standard calculator for a one time pad encryption

I wrote a simple python script to preform one time pad encryption I would like to be able to decrypt the ciphertext by hand. In my script I use mod 26. This is how I was shown to calculate a mod ...
1k views

### Leaving authentication data blank less secure for AES GCM?

The AES GCM example for Ruby / OpenSSL shown at: http://ruby-doc.org/stdlib-2.0/libdoc/openssl/rdoc/OpenSSL/Cipher.html has a GCM example of the following: ...
3k views

### Which algorithms are used to factorize large integers?

Even if RSA decided to cancel the Factoring Challenge, it seems that some teams keep working on it. According to Wikipedia, RSA-768 has been factored in late 2009. What are the current large integer ...
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I'm conducting an experiment dealing with differences in padding across different aes operations for my Intro to Crypto class, and the question says OpenSSL uses PKCS5 padding, and that I need to ...
1k views

### Security of N bit HMAC

Lets say that I am using 128 bit HMAC. How many operations are needed to find a "non secure" message. Is a birthday attack possible?
164 views

### Why do we need extra hashing if we could use simpler scheme?

Lets say that we use init vector IV, key K and HMAC key H. Message is M. Mode of operation is CBC !!! We usually encrypt this way: ...
98 views

### Is a tweakable block cipher still considered deterministic in nature?

"a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called blocks, with an unvarying transformation that is specified by a symmetric key. - Wikipedia " But with a ...
119 views

### Cryptography Implementation in software

I am trying to implement a password manager in C and I had a question about the proper steps in implementing the crypto. I looked at some implementations, google talks on crypto and what the standards ...
82 views

### MAC security and adversaries with memory

The usual definition of MAC security is (roughly) that there is no "efficient" adversary that has non-negligible advantage in the "MAC game" where the MAC game is as follows: the challenger chooses a ...
177 views

### Is there a reason that the N value in scrypt is limited to powers of two?

I have been looking at the scrypt hashing algorithm and am confused as to why the N value, which determines the overall cost, is limited to powers of two. There are ...
416 views

### Relation between key size and PRNG state size

Some (supposedly) cryptographically secure PRNGs have an internal state of only 160 bits or less. When the algorithm is otherwise properly designed, that seems like enough to generate a 128 bit key ...
211 views

### Is it necessary to use single hash function for signature generation and HMAC?

I am trying to figure out if we need to use single hash function across all the places in a ssh session. i.e – signature generation/verification and HMAC calculation/validation. Is it possible to use ...
102 views

### Cryptographic library quality [closed]

I've been working on a project that will require secure communication over the internet, so I've been thinking of TLS 1.2. After looking around I chose Botan but then I thought about using a more ...
335 views

### What does it mean when two encrypted files, encrypted with the same method but with different keys, when xor'd together, produce a repeating pattern?

Let's say I have fileA and fileB, both encrypted somehow, but in the same method with different keys. If I xor them together, they will make a repeating pattern of 32 bytes length. What does that ...
407 views

### One time pad: why is it useless in practice?

The symmetric cryptosystem One time pad (OTP) seems to be very beautiful since it is perfectly secret according to Shannon. Many books, however, point out that the main drawback is that one must ...
135 views

### Is ElGamal still unforgable if the adversary knows r

Is ElGamal still unforgable if the adversary knows the value of $r$ that was calculated by $g^k$? When we do sign the message $msg$, we calculate $r = g^k$. As we know the regular ElGamal ...
230 views

### Using Stream Cipher as CSPRNG

Are there any guidelines or justification for converting a stream cipher algorithm for use as a CSPRNG? For instance, lets say I wanted to convert one of the eStream portfolio Profile 1 (software) ...
114 views

### Is there a method to combine encryption and ring signing?

There's the well-known question on whether to sign or encrypt first. However, if instead a message is to be ring-signed such that either recipient could be the actual signer, is there any scheme that ...
89 views

### Do Chosen-Chiphertext attacks only apply to public-key cryptosystems?

Do Chosen-Chiphertext attacks (CCA-1/2) only apply to public-key Cryptosystem?
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2k views

### Turning a cipher into a hashing function

This is theoretical question. I'd like to know if it's possible (and what are eventually the consequences), not that I'm going to do it in one of my projects. ;) The first hashing functions created ...
98 views

### Existence of a map $\phi:\mathbb{Z}_{N^2}^* \mapsto \mathbb{F}$

Is there a map between the group of $\mathbb{Z}_{N^2}^*$ where $N$ is a composite number , a product of two equal size secure prime numbers $p$ and $q$ and a finite field $\mathbb{F}$, such that for ...
82 views

### Composing hashes and/or MACs

Suppose I have n hash/MAC functions, of which n-1 are extremely insecure, leaving only one that is in fact cryptographically secure. If I use all of them on a message and the XOR them together the ...