# All Questions

265 views

### Post-quantum authenticated encryption

It is well-known that the Grover's algorithm reduces cryptographic strength of symmetric ciphers to a square-root - e.g. AES-256 becomes only 2128 strong. However, these statements are always made ...
130 views

### Rsa Key Exchange Protocol

Moderators: Maybe this question will be move to other sub-forum. My question is Anybody know some link with a tutorial of how implement the RSA key exchange protocol using java for chat comunication?
131 views

### Can spatial filters be used to factor composite numbers?

$Z=(N-XY)^2$ is a surface with absolute minima ($0s$) anywere $Y=N/X$. I know this question is naiive, but shouldn't it be possible to apply a lossy compression filter to this function which ...
106 views

### RSA_DH vs ECDH implementation

In ECDH protocol is possible, naturally, to use the same algorithm for calculate a secret key for both communication parties (Alice and Bob for example). It is possible to design also a same algorithm ...
351 views

### Building a combined encryption scheme from two encryption schemes that's secure if at least on of them is secure

Any thoughts on how this can be done? Let $\Pi_1 = (\mathrm{Gen}_1, \mathrm{Enc}_1, \mathrm{Dec}_1)$ and $\Pi_2 = (\mathrm{Gen}_2, \mathrm{Enc}_2, \mathrm{Dec}_2)$ be two encryption schemes for ...
123 views

### Do hand-based hash functions / MACs exist?

Is there research on a hash function or a MAC that can be executed without computers? I know there's a lot of history on hand-based ciphers and the OTP can easily be excecuted by hand. (By using ...
69 views

### Are digital signatures using preprocessed paper pads possible?

My idea was, given that a computer could produce some sort of lookup tables on paper, could then a human use those lookup tables to create a system for digital signatures? This was really an idea for ...
90 views

### What are some hashes that do not exhibit the avalanche effect?

I do not know much about cryptography. But, I'm in a situation where I need a hashing method that does not exhibit the avalanche effect. I tried Googling but couldn't find anything.
72 views

### Could this alternative hash based MAC construction be as, or even more secure than an HMAC?

Begin hashing (Key||Message) Encrypt the hash state (or some part of it, such as the first 128 or 256 bits), with the key. Add the encrypted hash state to the hash ...
132 views

### Inversion Free Direct Conversion between Twisted Edwards (X,Y,Z) and Montgomery (X,Z)

The Wikipedia page for Montgomery curves shows how to convert points on a twisted Edwards curve to and from points on an equivalent Montgomery curve. However, their description and the original ...
3k views

### What is the difference between online and offline brute force attacks?

I read some papers saying a certain scheme is secure for offline brute force attacks, but vulnerable to online brute force attacks. I wonder the difference between the online and offline brute force ...
45 views

### Does failure of indistinguishability of encryptions imply lack of CPA-security?

Suppose we have an encryption scheme that fails the indistinguishability of encryptions test. That is, given messages $m_0, m_1$ and a ciphertext $c_b = Enc_k(m_b)$, where $b \in \{0, 1\}$, the ...
75 views

### On composition of encryption schemes

If we composed (as in function composition) multiple CPA-secure encryption schemes would the result also be CPA-secure?
141 views

### Is there an asymmetric One-Time-Pad? [duplicate]

Is there something like a perfect asymmetric crypto-algo? Is there proof that there must be one or not. From a logical point of view it seems to be possible to design such algorithm if your keysize is ...
98 views

### Chosen-plaintext attack

Let F be a block cipher with 128-bit block length. Consider the following encryption scheme for 256-bit messages: to encrypt message $M=m_1∥m_2$ using key $k$ (where $|m_1|=|m_2|=128$), choose random ...
199 views

107 views

### How to securely store data? / How to securely encrypt a file? [closed]

How do I safely encrypt a file? or formulated differentely: How do I safely store the data of my application in a file? Note: Using established tools isn't an option for this. The tasks need to be ...
338 views

### Is the HMAC construction really neccessary for a fixed length message?

The HMAC Construction is defined as: Hash( (Key XOR opad) || Hash((Key XOR ipad) || Message) ) This was designed because a simpler construction such as: ...
579 views

### Is One Time Pad considered Chosen-Plaintext Attack Secure?

If we're considering Chosen-Plaintext Attack setting, then the adversary has access to the Encryption Oracle right, and we know that OTP is only considered secure if we use the key only once. How ...
89 views

### How do I verify a reset token if the token is hashed in DB?

I've been searching around the web for best practice how-to on secure password reset system (email with reset url), and the idea to hash the token in the database, which I did not implement at first, ...
92 views

### CCA secure or not?

Let $F : \{0,1\}^n \times \{0,1\}^n \rightarrow \{0,1\}^n$ be a PRF. And let the encryption function be $Enc{_k}(m) = r || (F{_k}(m) \bigoplus r \bigoplus 0^n)$ , where r's value is random. Is this ...
184 views

### Computational indistinguishability with Example

Based on computational indistinguishability definition no PPT algorithm can distinguish $X$ from $Y$, where $X=\{X_n\}_{n \in N}$ and $Y=\{Y_n\}_{n \in N}$ are ensembles of probability ...
58 views

### Proposed unidirectional authentication scheme

I've been looking around for a way to authenticate a client to the server and deliver a message, but in a unidirectional fashion - that is, the client sends messages to the server, but the server ...
76 views

### How do we know one-way functions can be iterated?

Suppose we have some one-way function $h$. Without further specification of $h$, how can we be sure that we can define another function $g(n) = h(h(n))$? That is, how do we know the range of $h$ is a ...
1k views

### Modifying a [CRC32] checksum protected file [closed]

I dont want to ask for help without showing how far I came by myself, so this is what I have so far: The file of interest is an encrypted/obfuscated .bin script, which cotains information on certain ...
337 views

### Encryting data with password for local storage using libsodium

I want to encrypt data that is stored on the hard disk using a user-entered password. For this I look into the sodium docs and all I can find for symmetric encryption is authenticated encryption. ...
88 views

### Negilible Function $\frac{1}{n!}$

Is $\frac{1}{n!}$ a negligible function where $n$ is a security parameter? Application: I have a vector of n>100 elements. I permute it and give it to an adversary. The adversary can break it if it ...
83 views

### Does the plaintext have to be at least as long as the key in Polyalphabetic substituion cipher

In polyalphabetic substitution cipher, if we have a key $k$ (a string of letters) of length $n$ does the plaintext $m$ (also a string of letters) have to satisfy $|m| \geq n$? Also, side question, ...
177 views

### Authenticate a short message with redundant encryption instead of using a MAC?

I have a short message that’s exactly 128 bits of length. The message serves as a time-limited authorization token for a database. It contains a permission profile ID (64 bits) and an issue timestamp (...
110 views

### AES process candidates implementations and usages

There were fifteen candidates in the AES selection and I suppose that Rijndael wasn't the single good algorithm. I know that Camellia is an evolution of E2 candidate and can be used as symetric ...
67 views

### Ephemeral Diffie-Hellman in a symmetric crypto context

I'd like forward secrecy in a case where the two sides have a shared long-term symmetric key. I intend to use AES in GCM. It looks like ephemeral Diffie-Hellman is the best way to generate the session ...
163 views

### How do we guarantee plaintext is coprime in RSA?

The specifications for RSA state: $P^{\phi(N)} \equiv 1 ~mod~N$ if and only if $P$ and $N$ are coprime. Here $P$ is the plaintext and $N$ is the product of two suitable primes $x_1, x_2$. My question ...
334 views

### Calculating XOR Key Sizes

I've been playing around with the Matasano crypto challenges (cryptopals.com). I had a couple false-starts on the challenge that has you creating a program to calculate the key size of a XOR encrypted ...
383 views

### Elliptic curve brute forcing

I have elliptic curve of equation $y^2 \equiv x^3 -x$. And the coordinate of points $Q$ and $P$. I want to solve $Q=[k]P$ (where $k$ is the unknown) by testing all possible $k$. Is this the right ...
50 views

### what is recommended x length in Diffie Hellman algorithm

According to rfc documents http://tools.ietf.org/html/rfc2412#appendix-E.2 & http://tools.ietf.org/html/rfc3526#section-3, it has defined the prime and the <...
47 views

### Commitment scheme to share money

I have such problem: party $X$ has an amount of money $M$, which it needs to share with $n$ other parties. Every week the amount of money is different. Let say not, that I am a party A, which is one ...
199 views

### Why doesn't this defeat RSA?

Apologies for the obviously ridiculous question but I need to know where I'm going wrong here. For RSA, we compute $n=pq$ for primes $p$ and $q$. We then choose an $e$ such that $gcd(e, \varphi(n))=1$...
629 views

### Why cant Public Key Encryption be perfectly secure? [duplicate]

I would be very grateful for any help. I cant figure out why (probabilistic) public key encryption schemes can never provide perfect secrecy? Any Ideas? Excerpt: In contrast to the private-key ...
2k views

### Meet-in-the-Middle attack on 3DES [duplicate]

How can I apply the Meet-in-the-Middle attack to the 3DES algorithm, and why does the literature say that 3DES is more secure than DES?
242 views

### Recovering key length from transposition block cipher ciphertext

First of all I would like to say hello to all StackExchange users. So far I've been stalking the forum for some time, now it's time to leave the shadows. In my master thesis I'm writing about ...
34 views

### Outsourced Multiparty computation proof in Ideal world

I need to know in an outsourced two party computation where honst $A$ and $B$ outsource their private and secure data to a malicious server, why we need to design a simulator that interacts with an ...
123 views

### How to exchange authentication keys

I am trying to create a secure Bluetooth Low Energy connection. The peripheral server (the main device other people would connect to) would need to service many clients (users) and due to hardware ...
235 views

### Generate a list of valid ASCII codes

I have read many pages about encryption but I am really stuck on this. For access control to a building I want to print a 10 character ASCII code on a ticket (as a barcode). The barcode reader (...
806 views

### Practical Uses for Timing Attacks on Hash Comparisons (e.g. MD5)?

Consider the following code snippet: ...
125 views

### SHA3 Keccak, absorbing data to the state

I'm working on implementation of keccak algorithm and have one problem. Tell me please, takeing an example "test": will state (before block permutations, but after padding - pad10*1) look like this: {...
327 views

### where should private and public keys be stored? [closed]

I am kind of confused whether should a newly generated key pair be stored in a keystore or in just a normal plain text file? From what I read from online source, there are serveral explainations ...
49 views

### Role of trusted party in the Ideal model in Malicious case

Imagine there is a protocol supporting outosurced multi party computation. There are three parties involved in the protocol: client $A$, client $B$ and a server. Client $A$ and $B$ send their private ...
Suppose I have a local file and it is encrypted with a symmetric key derived from a strong passphrase, unique salt and $x$ number of PBKDF (e.g. Scrypt) iterations. I am interested in keeping this ...
Initially let me state that this is an atypical type of cryptographic encoding. First I break up my key into three parts $k = {W,\overrightarrow{b},\alpha}$. the $W$ is simply a $n \times n$ matrix, \$...