# All Questions

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### efficient authentication with broadcast encryption?

(I am not a crypto expert, so I apologize if the terminology I use is incorrect.) Suppose I have the following simple broadcast encryption scheme for securely sending content to $n$ authorized ...
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### Future-Proof Versioning and Validation

I am working on a library (using standard primitives: AES256 CTR; HMAC with SHA256; PBKDF2 with SHA256, 128 bit salt, and 10000 rounds) to encrypt and decrypt data, given a password. The encrypted ...
7k views

### How to use RCON In Key Expansion of 128 Bit Advanced Encryption Standard

I have a question about RCON here is my illustration... this is the 128 bit key.. ...
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### What are the cryptographic assumptions in the Dolev Yao model?

In the Dolev Yao model for interactive protocols, the cryptographic primitive (encryption, for example) is considered as a blackbox. Does blackbox here mean that the primitive is to be considered CPA ...
237 views

### Can Add and Multiply On Cipher Text achieve all operations?

A Fully homomorphic encryption scheme needs to support an evaluate function that can do add and multiply operations on cipher text. Can we do all kinds of complex operations on cipher text like ...
431 views

### Blind quantum computing and fully homomorphic encryption

I am somewhat familiar with current research on fully homomorphic enryption schemes and their possible application to Cloud computing. I've just noticed (somewhat late) that a marketing-savvy group ...
17k views

### Best way to reduce chance of hash collisions: Multiple hashes, or larger hash?

I would like to maintain a list of unique data blocks (up to 1MiB in size), using the SHA-256 hash of the block as the key in the index. Obviously there is a chance of hash collisions, so what is the ...
169 views

### Security model for privacy-preserving aggregation scheme.

Suppose that $S=(E,D)$ is an additively homomorphic encryption scheme. Now I want to design a protocol $P$ such that given inputs $x_1,x_2,..,x_n$, the adversary $A$ (who can decrypt) can only learn ...
751 views

### On-the-fly computation of AES Round Keys for decryption?

The usual implementation of AES first computes all the Round Keys sequentially starting from the key, and stores them in RAM for later uses. However, when enciphering a single block with a key that ...
1k views

### AES-GCM and its IV/nonce value

I was reading about the differences between the GCM and the CBC more here and I have a follow up doubt on the same. In the CBC mode the person who performs the encryption is the one who provides the ...
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### Why is it important that phi(n) is kept a secret, in RSA?

Why is it important that $\phi(n)$ is kept a secret, in RSA?
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### What would the Internet be like without public-key cryptography?

The functioning of the Internet as we know it nowadays depends very heavily on public-key cryptography, including several key root systems that depend on its asymmetric properties. But what would it ...
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### True 128bit secured password length?

assume we say 128bit keys are secure so a 16 character long password is safe, but if i even use lower case and upper case letters with numbers and special characters on my keyboard its about 80 ...
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### AES 128bit round level recommendation

we don't want change standard just after every attack, now for AES 128bit blocks i think 16 round is secure, but what is best round amount ? we had attack for 7 round then 10 round and it going so on ...
946 views

### Why the following attack in common modulus RSA works?

Suppose that i share a common modulus $N$ with two users $u_1$ and $u_2$ with public, secret keys ($e_a,d_a$), ($e_b,d_b$). Why then $gcd(V,e_b)=1$ where $V=d_a*e_a-1/$W and $W=gcd(e_b,d_a*e_a-1)$ ...
164 views

### Does zero-padding the secret in Shamir's sharing scheme increase security?

When performed in binary Galois fields $GF_{2^n}$, Shamir's threshold secret sharing scheme produces shares that are each the same bit-size as the secret. Though the scheme is "perfectly secure" in ...
1k views

### Double Encrypting with two different keys

In terms of security, would it be MORE or LESS secure to take, say, an RC4 output (or Serpent) or other, that is encrypted with one key, and to encrypt that output with AES (using a different key)? ...
552 views

### creating a small number from a cryptographically secure random string

i'm trying to figure out the best way to generate a cryptographically secure random number between 0 and 200 from a cryptographically random string of bytes (ie. read from /dev/urandom or some such) ...
136 views

### What is an easy method of proving that a cryptographic oracle attack is possible?

Lets say an application is using CBC mode without authentication. The application is generating cipher text. This cipher text can be manipulated by the attacker and then transmitted back to the ...
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### phi(P*Q) = (P-1) * (Q-1)

I was trying to understand RSA when I encountered the Euler Function. I do understand this: $\phi(P)$, where $P$ is a prime is $P-1$. However it seems that for a number $N$ such at $N=P\cdot Q$ where ...
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### Is there a big RSA Data Set

Is there a big dataset containing RSA public $(n,e)$ and private keys $d$ and primes $p,q$ where $n = p.q$ ?
2k views

### Galois fields in cryptography

I don't really understand Galois fields, but I've noticed they're used a lot in crypto. I tried to read into them, but quickly got lost in the mess of heiroglyphs and alien terms. I understand they're ...
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### Necessity for finite field arithmetic and the prime number p in Shamir's Secret Sharing Scheme

Shamir's original paper (PDF, 197kb) describing a threshold secret sharing scheme states: To make this claim more precise, we use modular arithmetic instead of real arithmetic. The set of ...
330 views

### How do unkeyed hash functions (for MDCs) provide security?

Unkeyed hash functions are, by definition, hash functions computed without a key. SHA-1 is an example. MDCs (message digest codes) are a subclass of unkeyed hash functions. How are unkeyed hash ...
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I have been wondering about the options available for managing passwords. However, they all seem to fail if the master password is compromised (which isn't a big surprise). On one hand you have ...
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### Polynomials and efficient computability

In public key crypto, the popular definitions of security (CPA, CCA1,2) depend on PPT adversaries. I'm trying to understand why adversaries should be PPT. It's clear that adversaries should be at ...
As I know, in Paillier cryptosystem, the encryption $c$ of a message $m$ is calculated as $c=g^m r^n \bmod n^2$. Now, I am wondering if I can derive $g^m \bmod n^2$ given that I know $c$, $r$, and ...