# All Questions

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### Block cipher detect padding behaviour

Suppose i have a crypto provider, i.e: i give that provider some plaintext bytes and get the encrypted result. I don't know the used encryption method, but i know ...
67 views

### Discrete logarithm hash function Exercises

I have a problem with this exercise: Let $G$ be a group of order a prime $q$ and let $g, h$, be two randomly selected elements of $G$, with $g,h\ne 1$. Consider the following hash function on ...
936 views

### Why do we implement a protocol?

In general, after we design a secure cryptographic protocol and make sure that it is efficient (e.g. through complexity analysis) we implement it. Questions: Do we implement it for proof of ...
66 views

### Encryption that could have multiple decryption key

The title looks strange but it is to deal with this situation. The data we have should only be accessible by our end user and data administrators, not anyone else. Ideally, each user should have ...
89 views

### System parameters in identity-based encryption

In IBE schemes, the system parameters are $(q, \mathbb{G}, F, \hat{e}, P, Q, T, H_1)$. I don't know $\hat{e}$. For example, in type A pairing… ...
100 views

### Best attack on double DES followed by XOR with third key

What is the best attack here? $E_k(m)=DES_{k1}(DES_{k2}(m)) \oplus k3$
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### How to correctly encrypt data with proper authentication using AES-256-CBC in php?

I have been using the openssl function for encrypting data with AES-256-CBC in php. I have been able to encrypt it using an unique IV (by generating with openssl_random_pseudo_bytes)for each new ...
67 views

### How to encrypt a number using AES?

How to do padding to input giving to encrypt using AES algorithm if the input is less than 128 bits? I need to encrypt a decimal number such as 10.
101 views

### Security of encrypting many short values with AES in GCM mode

We intend to encrypt certain values in a database which holds personal information such as addresses so as to prevent data leakage should the database be compromised. The intention is that each field ...
68 views

### Plaintext and ciphertext block sizes

How can you determine the largest plaintext block size and smallest ciphertext size if you used RSA with $n=59768553302699443$ using an alphabet with $27$ characters?
54 views

### Can BPS be used to encrypt a single integer, and restrict the output to a certain range?

As per specifications document, "We have the natural restriction that at least two characters must be ciphered, i.e. $b \geq 2$" where each character is represented by an integer of certain ...
107 views

### Finding the subgroup in isogeny-based cryptography

Isogeny-based cryptography is one of the newest post-quantum cryptography. Hardness of this system is based on finding isogeny between two elliptic curves. Also this is theorem: Elliptic curves ...
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### emulation of a one-time-pad based on multiple keys of running cipher

a one-time-pad cipher is one that uses a long "truly" random key that is as long as the plain text message. the key must be only used once. the key must not be derived from pseudo random number ...
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### EC Schnorr or Ed25519 Java card implementation [migrated]

I am trying to implement ED25519 in java card. Since there are many restrictions with respect to data types and sizes, its becoming tough to implement it, especially curve operations. Is there any ...
286 views

### Which cryptography technique does not increase the size of the plain data?

I want to encrypt some data(text/voice/video) but it is essential that the size of the cipher data remains the same as the plain data. Which techniques can I use, and how secure are these techniques ...
58 views

### How to split up $GF(2^{128})$ into smaller fields?

I've heard that it's possible to split up $GF(2^{128})$ into copies of several smaller fields like $GF(4)$ so as to make the math easier in some cases. How do you do that? I know how it works for ...
45 views

### How to generate fingerprint for PGP public key

I would like to know how to generate a fingerprint for an openPGP public key. To be clear, I'm interested in how the fingerprint is generated. I'm aware that I can get the fingerprint with a ...
85 views

### How does the Flickr length extension attack work?

I am currently reading about the length extension attack on Flickr (pdf) but not understanding what's going on there. ...
53 views

### Preserving Confidentiality and Integrity in a Public channel

Let's say I've got two people that are trying to talk in a public channel $A$ and $B$ and they want to preserve their confidentiality and integrity. Which cryptographic algorithm would they want to ...
45 views

### What happens if a nonce is reused in ChaCha20-Poly1305?

In ChaCha20-Poly1305, I know that reusing a nonce loses confidentiality and integrity for all messages encrypted with the (key, nonce) pair. However, is security affected for those messages encrypted ...
78 views

### Why are obfuscators generally defined to be probabilistic algorithms, rather than deterministic ones?

One possible explanation is, randomness is not going to hurt you, so might as well use it. Also, if we are hoping to prove impossibility, it only makes the result stronger. One other explanation is ...
24 views

### Setting up of polynomials for CP-ABE or KP-ABE Access Trees

My question I guess is how the selection is done. A brief explanation with a simple access tree will be very helpful.
125 views

### Full encryption of IDs, partial decryption with customer specific key

I am looking for a "partial" encryption/decryption algorithm. Let's say I am encrypting the following List of IDs (long values) $[1,2,3,4]$ (with a secret key) to an encrypted string $e$, I would ...
109 views

### Are there any successful preimage attacks?

Are there any successful preimage attacks on any serious cryptographic hash algorithm or at least on serious-looking non-cryptographic hashes (like MurmurHash)?
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### An MPC protocol from Elgamal is a good solution a homomorphic multiplication?

I want to compute a multiplication between many secret values and then distribute the result to everyone involved. For this, I thought about an MPC protocol built from Threshold Homomorphic Elgamal. ...
30 views

### Solving ADFGVX decoding with only ciphertext

Hi I'm new to the world of crypto and I'm trying to decode a message. I know it is encrypted using the ADFGVX cipher. I also have a ciphertext, but I have no other information given to me. My question ...
25 views

### Lagrange Coefficients & Polynomial in KP-ABE

My question is how these concepts relate in ABE. I am assuming that the attributes are used to share the master secret key and only a user with the right attributes for a particular access tree is ...
29 views

### What does this advantage statement signify?

In many Attribute Based Encryption papers I see this line... The advantage of an adversary A in this game is defined as Pr[b' = b] − 1/2. Can someone explain what it means to me? I assume it means ...
62 views

### Zero-knowledge proof for the product of additive Paillier ciphers

Suppose that Alice received the cipher values: $E(x_1), E(x_2), ..., E(x_n)$ that are encrypted using Paillier cryptosystem by $n$ entities with Bob's public key. Alice computes $E(\sum x_i)$ from ...
102 views

### Understanding ransomware – What makes plain-text-attacks or brute-forcing so hard?

Say I have four files. Two are completely unencrypted, while the other two are the exact same files other than that they have been encrypted with (apparently) the same public key (via a ransomware ...
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### collision resistant hash function

I have got through the collision resistant hash function,and I found it a little bit tricky to solve this question: Would you please help me solve it?
107 views

### Complexity using RSA

You encrypt a message using the RSA encryption system as $t^e \pmod n$ , where $t, e < n$ and $t$ is the numerical equivalent of the message. The message is written in a $27$-letter alphabet and is ...
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### Perfect Secrecy in Symmetric Key Systems

For symmetric key systems does perfect secrecy imply that all ciphers in the cipher space are equiprobable? I would think not, but I am finding it hard to come up with a counter example.
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### Using e-ID Number for Encryption and Digital Signing

I want to use e-ID number for performing Encryption/Decryption and Digital Signing - and in order to do so, I've opted a PGP-oriented approach which goes a bit like this. Generating PGP key-pair ...
19 views

### If you use an AES key just once, do you need to use IV, and/or pad the message? [duplicate]

I am developing an application that is going to use AES encryption. Since it is going to use a different key for every message encrypted... Would it be secure to use a constant IV? (i.e null bytes) ...
123 views

### Is F' a pseudorandom function when F is composed with G, a pseudorandom generator?

If $F$ is a pseudorandom function, is $F'$ also a pseudorandom function in the following: $$F'_k(x)=F_k(G(x)) \space \space \text,$$ where $G$ is $a$ pseudorandom generator? Also, does the other ...
82 views

### Why are there $2^{56}$ possible DES keys when there are 64 key bits?

On forums I read DES has $2^{56}$ possible keys so it takes x times to brute force it with hardware x. But there are 64 total bits of which are 8 parity bits. But those parity bits are a ...
33 views

### Symmetric encryption vulnerable when encrypting and decrypting are the same?

To my understanding, symmetric encryption is where encrypting and decrypting data can occur with the same key. It's possible for symmetric encryption that the encrypting and decrypting algorithms are ...
43 views

### Understanding the cryptography used in a file [closed]

I use a software that stores a raw binary file with information, and exports it to a text file. I have access to the data in the original binary file. You can find a sample of the contents of the raw ...
35 views

### Can blockchain be used in a comparable way to an encrypted SQLite database?

Further to a previous question of mine regarding multiple encryption levels in a SQLite database, I was asking whether it's possible for different users are able to decrypt only records of the ...
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### User key User Certificates [migrated]

Why would I find these in someone's security settings? I was going through my moms phone when I noticed some type of encrypted certificate. Figured out one of the two passwords required for access. ...
23 views

### This is bad. I don't know how to figure out the keyword and the keyletter [duplicate]

That may be bad, but I cannot crack any keyword messages!
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### Example of Projective Coordinates

Given the affine form of coordinates $(x,y)$ such as $(5,3)$, if I want to convert $(5,3)$ to projective coordinates $(x,y,z)$, should the form of point be $(5,3,1)$? It is triplet not a point, right? ...
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### Cycling hashing in PBKDF's and their limitations in strength?

I hope this question isn't too similar to one that's already been asked. I also want to point out I know part the answer already, AFAIK: I just want some expert input on this. Let's model a hash that ...
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### Convert projective to affine coordinates in ECC? [closed]

I am working with my project. I use projective coordinates but when I convert to affine coordinates, I can't get it. Can anyone help me? Projective Coordinates $(X,Y,Z)$ to Affine Coordinates ...
65 views

### Non-numeric Diffie-Hellman?

It may seem that this is a programming question, but I just need advice on the cryptography side of the question. I want to implement the Diffie-Hellman key exchange algorithm on iOS (since no free ...
39 views

Is there any difference in adding the plain text and key vs using xor instead(apart from the performance improvements)?
132 views

### Recommendation for lightweight algorithm for partial message encryption

I have designed a specific client and server program, which sends a file from client to server in 32k chunks. I want to encrypt only part of the data. More specifically: in each 32k chunk passing ...