# All Questions

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### 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 ...
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### 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 ...
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### 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 ...
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### On composition of encryption schemes

If we composed (as in function composition) multiple CPA-secure encryption schemes would the result also be CPA-secure?
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### 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 ...
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### 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 ...
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### Which of the following hash functions is collision resistant?

Let $H: M \to T\$ be a collision resistant hash function. Which of the following is collision resistant: $1.H'(m) = H(m \big\| m)$ $2.H'(m) = H(m) \big\| H(0)$ $3.H'(m) = H(m) \oplus H(m)$ ...
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### System of linear congruence, not relatively prime [migrated]

Consider we have the following set of congruences $$x\equiv b_i \pmod {m_i}$$ for all $1\leq i\leq d$. $m_i$'s doesn't have to be relatively prime, so the Chinese remainder theorem doesn't work here. ...
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### How to get the (x,y) from the EC point in the pubkey of an ECDSA certificate?

I have this certificate, and I have the bitstring of the pubkey that should represent Q = (x,y) = [d]P. How can I get the x part? (The curve is the NIST B-163)
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### 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 ...
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### 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?
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### Protocol for signing and timestamping documents for e-archive system [migrated]

we are building system for storing PDF file for long term in database. Before we store PDF file in database we have to: ...
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### Practical Uses for Timing Attacks on Hash Comparisons (e.g. MD5)?

Consider the following code snippet: ...
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### 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: ...
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### 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 ...
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### 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 ...
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### Is there a tangible benefit in keeping the number of PBKDF iterations secret and not storing them with the ciphertext?

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 ...
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### Security of a custom non linear hill cipher

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, ...
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### DH with AES - is that an acceptable security scheme over UPnP?

I'm trying to devise a good security scheme to protect comms over Wi-Fi. Right now, I have 2 devices that already communicate via UPnP but I need to secure this communication. I've come up with this: ...
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### what is the public information in Elliptic curve cryptosystems [closed]

Currently my knowledge about Elliptic curve is quite limited to the textbook and I don't know how a practical Elliptic curve cryptosystem works. I read an example about key exchange using Elliptic ...