# All Questions

416 views

### Entropy calculation

I calculated the entropy for "variable", "property" and their concatenation "variableproperty" (with e.g. the Shannon Entropy) and got $2.75, 2.5$ and $3.33$ respectively. These are all entropy per ...
46 views

### How to implement oblivious transfer where probability of the receiver getting secret is exactly 1 out of 4? [duplicate]

As we know that oblivious transfer is done with probability of 1 out of 2. But how to implement it with probability of 1 out of 3 or 1 out of 4??
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### application of algebra to cryptography [closed]

I have taken some courses on algebra like abstract algebra, linear algebra etc. I want to know more on the application of algebra to cryptography.I don't know if anyone can be of help. thanks.
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### How to decode a (Vigenère?) substitution cipher? [closed]

I am trying to decode a message that I suspect has been encrypted with a Vigenère cipher: Utl zluuocp zsc rvzu hbcp ztvibfz vrabzz utl ilry. Utlal flal cb fbaiz, qszu zbsciz. Utvu cllihl vhzb dvil ...
1k views

### Confusing XOR notation in textbook sample solution

I am preparing for the midterm exam in my Data Security class so I am trying to read and understand the textbook's exercise questions. The sample solution at the end the textbook makes this problem ...
59 views

### How does Blom's scheme work?

How does Blom's scheme work? Is the polynomial expression of it a normal one? A few additional questions: How big does the prime modulus need to be? Can it be bigger than the size of the derived ...
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### Store hashed password using Digest authentication

Digest Authentication is something like this: Server sends client a nonce. Client hashes username and password with nonce. Client sends hashed value back to server. Server validates hash. However, ...
54 views

### Difference between Constructor and Destructor terms

I was thinking that in the formal model (or symbolic model?) the destructor terms were used to model processes that could abort generating some errors or something like that. But then, I realized that ...
281 views

### How to check that you got the right key when brute forcing an encryption?

How do you know when you have the right key when brute-forcing? Let's say that they test the right key. They then have to check that the decrypted text makes sense. To do so, they can test whether ...
59 views

### Decrypting messages crypted with one time pad (used more than once)

I have a stream cipher with a key (that is unknown to me) that is used more than once. Messages are ciphered in this way: ciphered_message = message XOR key Let'...
80 views

### Decrypting SHA-256 with SHA-256 as input?

Is there a way to somewhat decrypt a SHA-256 hash, provided that the initial string was also a SHA-256 hash? If not, could anyone link me to a rainbow table containing some of the $16^{64}$ possible ...
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### One time pad (OTP) perfect secrecy with different key space

Let say $K_{0} = \left \{ 0,1\right \}^n$ $K_{1} = K=\left \{ 0,1\right \}^n$ \ $0^n$ $[b\leftarrow \left \{0,1 \right\}, k \leftarrow K_{b}:b=1|k \neq 0^n]$ --- (1) Key is chosen using ...
54 views

### ARC4random – ChaCha-based LibreSSL PRG

Here is arc4random.c from the LibreSSL pseudorandom generator as used in OpenBSD. It works as follows: Take 40 "truly random" bytes from OS (PRG seed); Use the seed as key and IV to generate 1024 ...
30 views

### Does TLS have to provide both confidentiality and authenticity?

I'm working on a project that uses OAuth. In the OAuth RFC it says many times that communication between the authentication server and the client should be with the use of TLS. However, in the project ...
32 views

### 3 digit code at the end of a changing magstripe (maybe hash?) [closed]

Myself and some fellow students are doing some "Independent Security Research" on campus and trying to crack the campus paid parking system to better understand how it might work. The system works ...
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What is it about the architecture of GPG that requires me to enter the password of the secret key when I export it with "gpg --export-secret-keys"? I would expect that gpg can just copy the relevant ...
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### Can a list of hashes with property $X_{n+1} = \operatorname{SHA-256}(X_n)$ loop?

This is the situation: I have a list of hashes where each hash is always the SHA-256 of the one above it. If I keep going down the list it must at some point loop since there can only be $16^{64}$ ...
53 views

### Is the complexity of Pollard rho for discrete logartihm really the modulus?

so I'm reading everywhere that the Pollard Rho for Dlog's complexity of $g^a \pmod{n}$ is $\mathcal O(\sqrt{n})$. Shouldn't it be $\mathcal{O}(\sqrt{q})$ with $q$ the order of $g$?
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### Could a key derivation function be used to encrypt data with a weaker key?

To allow the key to be more easily remembered. Example: To encrypt a hard drive without having a remember a key with 128 bits of entropy. From what I find they seem mostly used to hash passwords so ...
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### Hash + Salt v. Hash + Encryption

I was wondering if using a salt with a hash is more effective than hashing and encrypting your hash.
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### Difference between hierarchical and multi level secret sharing

In this paper, the definitions are simply as follows. Multi-Level secret sharing: Higher level participants can help lower participants to get the secret. Compartmented secret sharing: Each level ...
172 views

### How easily could this method of cryptography be cracked? [closed]

Background A couple of years back, in my naivety, I dreamt up a method to encrypt information. The result can be seen on my website, I evidentially thought it was pretty neat and I decided to mix it ...
132 views

### What is a branch number in the PAEQ algorithm?

I'm reading an article about authenticated encryption algorithm - PAEQ. It uses a permutation function AESQ, which has transormations from AES. And there is written about MegaSBoxes, which contain 4 ...
61 views

### Named Elliptical Curve parameters

Are named curve parameters always the same? I know this may be a stupid question however I think this is the case. For example the secp256r1 is defined in this documet http://www.secg.org/SEC2-Ver-1.0....
77 views

### What is the fastest modular reduction algorithm available?

I have been browsing for the fastest and most efficient modular reduction algorithms and came across quite a few. But the one in A Fast Modular Reduction Method (2014) by Zhengjun Cao, Ruizhong Wei ...
453 views

### Applicability of birthday attack to AES brute force

Is the following snippet from a recently published cryptography book correct? EDIT: Expand the snippet from the book to make the context (symmetric key search) more clear. You can apply this to ...
96 views

### Understanding WPA2 authentication in details

As far as I know, WPA2 authentication consists of a 4-way handshake. The first message is from the AP, containing the ANounce, a random number generated by the AP, The second message is from the ...
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### Why do the subexponential algoriths for the DLP not work for the ECDLP?

Elliptic curve cryptography is much more secure for the same parameters because attacks that work on the DLP do not work on the ECDLP. Why do the attacks fail in the latter case?
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### k(k-1)/2: Combinations and the Birthday bound

Disclaimer: I'm new to cryptography. Background: I'm reading Cryptography Engineering by Ferguson, Schneier, and Kohno, where, in Chapter 2, the authors write this: Question: What is $k(k-1)/2$ ...
32 views

### How do we calculate DHKey using A's public key and B's private key?

I have 2 set of public/private keys. I would like to know how I can calculate DHKey. e.g: ...
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### Are there any historic examples of breaking the one-time pad because of a careless key re-use? i.e simple attack on a multiple-time pad?

It doesn't have be using a truly random key, (it could be the Enigma) but the attack must be based on XORing two parts of ciphertext.
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### Is it okay for a non-cryptographer to submit crypto code to an open source probject? [closed]

I am not a cryptographer, nor do I claim to be. However, at times I have wanted to contribute to open source cryptography projects such as libsodium. The maintainer of libsodium is a cryptographer, ...
196 views

### Implementing hmac+bcrypt in bash script

I was recently reading Mozilla's Secure Coding guidelines and found their password storage method interesting. They combine an hmac hash of the password with bcrypt which allows for added security ...
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### DES_cblock and Key Recovery

I'm trying to get my hands on the key that OpenSSL uses for a DES encryption. The key is based off a random number generator with a seed taken as parameter. The code used is below, provided as part ...
41 views

### What information about the ciphertext can an attacker discern if a key is used too many times?

Suppose that I use Blowfish or XTEA (both secure block ciphers with 64-bit block size) in CTR mode. In these cases, I need to change the key after $2^{16}$ blocks to avoid leaking information with ...
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### Are timing attacks exploitable if each key is used only once?

Suppose that one performs (for example) AES-GCM encryption and decryption using an algorithm that is vulnerable to timing attacks, but each key is used only once. Is it still possible to perform a ...
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### What does the comma mean in m, c := E(K, m)

Disclaimer: I'm a beginner to cryptography. Background: I'm reading Cryptography Engineering by Ferguson, Schneier, and Kohno, where the authors use this image: Question: What do the commas mean ...
116 views

### Differential Cryptanalysis for Hash functions

How is differential cryptanalysis used to attack hash functions? I've been struggling through a couple academic papers and thesis's on the subject but they all sound like greek to me. Does anyone know ...
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### C implementation of GEA/3 [closed]

Can somebody please point me to an implementation of GEA/3 (an encryption algorithm) in a high-level language (preferably C/C++)? I looked for a while but could only find block diagrams and ...
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### How would the opponent defraud this secure communication?

I am new in the cryptography and the information security problems, I try to understand things as much as I can and I know it's really complicated in the beginning. Considering the communication ...
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### Does a stream cipher provide perfect secrecy?

From WAR10CK here: If I actually do create a machine using RC4 or AES-CTR and have a TRNG continually feed it a constant steady stream of random bits. Provided that the stream of bits is purged ...
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### Difficulty to find any preimages $a$ and $b$ for given $V$ so that $H(a) \oplus H(b) = V$?

Suppose I have the values $a$, $b$, and $V = H(a) \oplus H(b)$ with inputs $a \neq b$ and $H$ being SHA-224. None of these values are a secret. How secure is it to assume that no other possible values ...
105 views

### Shouldn't a signature using ECDSA be exactly 96 bytes, not 102 or 103?

Attempting to use openssl to create a signature is confusing on several levels: If I'm using it to sign a hash that I've already created (HMAC-SHA-384-192, specifically), a. why must I specify ...
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### Historical algorithm which is frequency analysis resilient

Long ago, I've read an article on wikipedia which describe a cipher algorithm, used by kgb (if I remember well) with the property that all ciphered letters has the same (or nearly) probability to ...
40 views

### What do associated data and message numbers mean?

I'm reading an article about authenticated encryption and don't understand some inputs to an encryption function. They are associated data, public and secret message numbers. Can you explain me their ...