# All Questions

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### Is it possible to attack RSA with a WalkSat derivative?

We consider a large $n$-bit number $N$. We want to find a factor, if it admits any. For $m$ taking values from $1$ to $n$, perform the following three steps (actually, for each $m$, perform many ...
253 views

### What is 'key agility' in relation to symmetric-key encryption?

I sometimes see, in discussions of symmetric ciphers, reference to the 'key agility' of a particular algorithm. It seems to be related to the difficulty of switching encryption keys, but I don't ...
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### The exact definition of a symmetric encryption

I have doubts for the definition of the decryption algorithm $D(.)$. I think I've already seen that the decryption returns a plaintext $M$ on input the key $K$ and $C=E_K(M)$. I have also seen thet ...
297 views

### Why does WPA2-PSK use 4-way handshake instead of 2-way handshake

I don't seem to grasp why is the handshake for PTK in WPA2-PSK 4-way and not 2-way. Isn't it enough to just have these two steps: First party sends Nonce1 to the other party Second party sends to ...
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### How do you prevent the risk of damage to the security of Fully Homomorphic Encryption?

In this paper on pg. 1248 in the "preprocessing phase" section the authors say: In the preprocessing phase, the parties run a (standard) MPC to collectively generate a key pair (pk,sk) for the ...
137 views

### Design requirements for a keyed hash function?

I am actually preparing for a cryptography exam, and I found a question asked by the professor in the class, below are the lines that I copied from the slide : Example: Let $Ek$ be the encryption ...
268 views

### Tamper-evident audit logs

I want to be able to detect tampering in a log file that is under the control of an untrusted party. I am not interested in secrecy, only the integrity of the contents in the file - so I want to ...
256 views

### Ciphers in CBC mode reveal place of change in plaintext

Theoretically, when using a symmetric block cipher in CBC mode, the current block is dependent on the previous block. Suppose one plaintext is encrypted using CBC, and then one bit of it is changed, ...
193 views

### Best group if one wants the discrete log problem to be hard?

Suppose one is implementing a cryptographic scheme over a group where one needs the discrete logarithm to be hard - what is the recommended group to use? I'm looking for a group where calculations are ...
219 views

### How can Shamir's method for secret sharing work in the GF(256)?

I have been reading about Shamir's secret sharing, and maybe i completely misunderstand something, but the field is meant to be a prime. However, when i look at the implementations of this algorithm, ...
466 views

### A key-derivation function that is as strong as the stronger of PBKDF2 and scrypt

I want a key stretching system that's as strong as the stronger of scrypt and PBKDF2. The consensus now is that scrypt is by far the better system, but that might change if in the future, a weakness ...
569 views

### Adding and multiplication in jacobian coordinates

How can I derive formulas for adding 2 points and multiplication by a scalar in Jacobian coordinates $(x,y) = (\frac{X}{Z^2},\frac{Y}{Z^3})$ over an elliptic curve?
125 views

### Upper bound Linear Feedback Shift Register

It is clear that when we have any output stream $x_0,x_1,...$ produced by linear feedback shift register than this output has to be periodic. Now I was wondering if we can find an upper bound for ...
131 views

### password generation: random length or max length

In short: Is it cryptographically stronger to have a known length password of the maximum length allowed, or a random length password somewhere in the range of the longest length possible? Does the ...
478 views

### Timing attack on modular exponentiation

It is known that computing $a^x \bmod N$ takes $O(|x| + \mathrm{pop}(x))$ multiplications modulo $N$, where $|x|$ is the number of bits of $x$ and $\mathrm{pop}(x)$ is the number of $1$ bits (Hamming ...
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### What is the impact of the Goppa code distinguisher on the CFS and McEliece?

What is the impact of the distinguisher for the high-rate Goppa codes (as published in "A Distinguisher for High Rate McEliece Cryptosystems") on the CFS signature scheme and the McEliece/Niederreiter ...
256 views

### Shamir's Simple Sharing Scheme - preventing partial recovery of data

In SSSS, if you recreate the original secret with one of the decoder inputs being slightly damaged (e.g. one or two chars incorrect), you receive a slightly damaged version of the original secret. So ...
228 views

### Is the following symmetric design secure?

Assume: $O$ be a reversible random permutation oracle on a finite set and $O^{-1}$ the inverse permutation (pretty much equivalent to a random permutation: What is the difference between a bijective ...
453 views

### Why are the initial states of hashes functions (like SHA-1) often non-zero?

There is already a question asking "Why initialize SHA1 with specific buffer?" and my question follows on from this: Why are the initial states of hash functions often non-zero? For most, I have ...
997 views

I developed a p2p-app in C# which sends and receives encrypted text messages (50KB). For encryption, my app uses AES 128 bit in CBC cipher mode. For each message it uses a new randomly-generated IV. ...