# All Questions

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### Proper way of encrypting large files with AES-256-GCM

I would like to encrypt and decrypt files with AES-256-GCM. I need a solution that works with Java, PHP and JavaScript. In practice, users can upload, download files via web browser and/or desktop app ...
36 views

### Is using Fiat-shamir Heuristic safe?

What I'd like to do is to have the Prover store a value x where x remains hidden. From x, I'...
76 views

259 views

### Is it reasonable to consider that an encryption scheme must be invertible?

I am in a dispute regarding a test question in an exam. The question is something like that: What would happen if one were to use RSA with $n=100$ and $e=13$ to encrypt a message $m$? a) You would be ...
58 views

### Calculate share for a specific value of $x$ in Shamir's Secret Sharing scheme

I am currently reading the book Cryptography: Theory and Practice (4th edition) by Stinson and Paterson and going through the exercises. I have reached the Threshold schemes section and am currently ...
46 views

### Practical Threshold Signatures

In Practical Threshold Signature it says that there is a requirement in which the RSA public exponent $e$ must be more than $l$. $l$ in this case is the number of players in the threshold group. My ...
59 views

### Combining algorithms for password storage?

Is it more secure to combine algorithms for password hashing, for example, Scrypt, Bcrypt, SHA-3, etc.? If an attacker wants to use dedicated hardware, he would need one for each algorithm. However, I ...
57 views

### How to derive a private/public keypair from a random seed?

For RSA, my understanding is that large primes are found randomly to build a private/public keypair. If a static seed can be used in that random search process, it should be possible to derive the ...
960 views

### Does there exist a hash function which has a fixed point? [duplicate]

Does there exist a hash function $h$ and an input $x$ such that $h(x) = x$? I believe this is a special case of collision-resistance. I imagine such a case is guaranteed not to be possible (otherwise ...
23 views

### How to handle IKEv1 and IKEv2 diffie-hellman shared secret bits length not multiple of 8?

In Cryptographic Algorithm Validation Program for KDF document, here, page 9, it says "If a field size that is not a multiple of 8 bits, the shared secret is represented by a string of bytes that ...
132 views

### Negative numbers with additive ElGamal

I'm trying to implement Private Set Intersection using the additive ElGamal public key encryption system. The code I've written can encrypt and decrypt numbers using the ElGamal system, so far so good....
35 views

### What's the difference between compromise and corruption in MPC?

What is the difference between corruption and compromise? Are Compromising a user and ...
70 views

### How convert Point on Curve into AES key?

I'm playing around with ECC, trying to encrypt traffic between my webapp and backend (its not a product, im trying to learn and understand more about ECC) I managed to generate keys, successful run ...
102 views

I hear a lot about AEAD encryption (GCM, EAX, ...). Why is it unsecure (it seems to be as AEAD exists) to do do that: Imagine a block of data, hash it, append the hash to the data, then encrypt the ...
33 views

### What can be said about the order of short Weierstrass curves with $a=1$ and $b=0$?

I am looking at a elliptic curves of the form $E:y^2=x^3+x$, i.e. short Weierstrass curves wtih $a=1$ and $b=0$, defined over a field $\mathbb{F}_p$ with $p$ being a safe prime. Somewhat interestingly,...
30 views

### What kind of approach can I use to reverse the following encryption scheme?

I'm trying to reverse an encryption scheme without success. I have several pairs of ciphertext + plaintext and found the encryption method to be something like this: ...
23 views

### Stable coin values [migrated]

(not sure this is the right stack exchange to ask crypto-economics questions, if not please direct :) ) Why are stable coin values not exactly the value they're pegged against? For example as of this ...
84 views

### If $((p-1)*(q-1) -1)$ divisible by $e$ ($e$ is odd number) , then $\text{gcd}(e,(p-1)*(q-1)) = 1$

If $((p-1)(q-1) -1)$ divisible by $e$ ($e$ is odd number) , then $\text{gcd}(e,(p-1)(q-1)) = 1$. ($p,q$ are prime numbers ) Is this true, if yes why, if not why not ?
172 views

### How to convert random bytes into blocks of numbers without weakening entropy?

Let's say you have a, or several random byte(s) 0-255 and would like to derive numbers 0-9 for a One Time Pad (OTP) of them. What would be a non-naïve approach to represent 256 possible values onto 10 ...
31 views

### Attack on Many Time Pad

I have 7 ciphertexts that have been XORed with the same key. I'm writing a program to figure out what the plaintexts are and possibly maybe recover the key. I have managed to convert the HEX ...
5 views

### Use of encrypting data in transit over a HTTPS connection [migrated]

I find certain APIs (that provide sensitive information) using algorithms like ECDHE with X25519, on top of the already ...
63 views

### RSA Encryption that only my family could decrypt

I was doing some thinking about what would happen to my computer and passwords if I fell off the earth tomorrow. I wanted to have a message that only they would have the ability to decrypt and I ...
52 views

### Can two traitors safely reveal themselves to each other?

Suppose Alice is a traitor and suspects Bob might also be a traitor. If Bob is a traitor, she would like to conspire with him together, if not she doesn't want to reveal that she is a traitor. Is ...
23 views

### Authentication using a nonce

For an authentication problem imagine the following scenario with two devices: 1 server 1 client. The client starts a conversation with the server and sends him his public key. All public keys in ...
47 views

### Pohlig-Hellman on ECDLP over extension field $\mathbb{F_p}^6$

Suppose there is an elliptic curve $E$ in form $y^2=x^3+b$ defined over $\mathbb{F_p}$, where $p$ is large prime. #$E(\mathbb{F_p})$ is also a large prime but #$E(\mathbb{F_p})\ne p$. ECDLP on this ...
47 views

### How to design number of rounds in my cipher?

I have specific cipher which is built from permutations. It has or should has few rounds. If we do 2 rounds it is like 2 permutation composition. 3 rounds are like composition of 3 permutations and so ...
100 views

### Using SHA2 as random number generator?

I need to generate secure pseudorandom numbers in an embedded system. Instead of having to port some RNG library, could I just use an existing SHA2 function as a random number generator (n-th random ...
43 views

### What if the CSR is intercepted in transit while sending it to CA?

I was studying PKI the other day and I had a serious thinking about the following question. What is CSR? - It is a block of encoded text that is given to the CA when applying for an SSL certificate ...
61 views

### Why cant we use AES to hash a password, instead of a hash function?

AES needs a key in order to work, hashing-algorithms do not. Suppose we have a fixed key for AES, why don't we simply hash our keys with that? As with hash functions, AES has arbitrary input size, ...
23 views

### Range Proof without trusted set-up

If verifiers are public blockchain nodes, which might leak information. Does this imply that range proofs such as Boudot's and Lipmma's (squaring based) approach could not be applied, unless a trusted ...