# All Questions

313 views

### DGK Cryptosystem Encryption Speedup

Following @poncho's nice clarification of the RSA speedup here, let's see if I'm able to do the same in the case of the DGK cryptosystem: We have pk = (n, g, h, u), sk = (p, q, $v_p$, $v_q$) which ...
3k views

### Cipher Feedback Mode

I can't understand what CFB really is. It said in Wikipedia that CFB is same as CBC, but I find that CFB is more difficult than CBC. Can someone explain to me how CFB works. Such as how ...
433 views

### RSA finding the inverse of the public exponent

I have a very basic doubt in RSA key generation and its usage. In RSA key generation you choose two large prime numbers of a very large order. Then you multiply them.(eq $p \cdot q = N$) Now, ...
179 views

### SSL-like protocol with public-key hard-coded in the client

Imagine a SSL-like protocol that instead of using certificates signed by a trusted CA, has the server's public key hard-coded in the client. My question is: what happens if the server's private key is ...
171 views

My question is related to PBKDF2. I am unsure how to validate user entered password with already existing hashes and salts. I have heard of the so called "length-constant" comparison which basically ...
80 views

### secret sharing of product of 2 numbers

Does evaluating the products of shares $a_jb_j$ give a secret sharing of $ab$? I think it's a no, but I am not sure on how to prove it.
158 views

### Testing if a password for an eCryptfs mount is valid [closed]

I'm using "eCryptfs" on several directories on my dedicated Debian server. I mount them manually via mount -t ecryptfs [dir] [mountpoint] But after some weeks ...
229 views

### One-Time Pad Distribution of Keys Between Two Parties

Consider the One-Time Pad (OTP). Suppose two parties, A and B, generate a completely random secret key in a real-life meeting, and they keep this secret. Now A wants to send some message to B using ...
278 views

### Derive a key from public data using a secret master key

Users in my system will be given a device that contains a 128-bit AES user key. The user key will be derived from 64 bits of public data related to the user, which is unique but very predictable (i.e. ...
297 views

### definition and meaning of semantic security

I'm taking coursera cryptography course. The definition of semantic security is hard to understand. I tried to slightly restate it (the word "efficient" was in the original definition). Do I get it ...
300 views

### What does $(\mathbb{Z}_n^*)^2$ mean?

In a paper they write once, $(\mathbb{Z}_n^*)^2$. Is this the group of quadratic residues or is it something else? Here the theorem: Under the strong RSA assumption, given a modulus $n$, along with ...
89 views

### Timestamping services authorities, utilizing Linked, hybrid and other than PKI schemes

What are well respected Cryptographic Timestamping authorities utilizing more sophisticated than just PKI (RFC 3161) schemes, like Linked, Linked and signed, distributed hash trees (maybe P2P?), ...
74 views

### Question Error Correcting Codes

Let $C$ be a code over the finite field $GF(2)$ with generator matrix $G$ and parity matrix $H$. Let $e+C=C'$ be a coset of code $C$. Let $S$ be a non-singular matrix and $H'=H\times S$. Finally, let ...
260 views

### Can a salt for a password hash be public?

For my understanding salts in password hashes are used to prevent the precompuation of plaintext$\to$hashvalues (rainbow tables). I know from different threads, it is not necessary to keep the salt ...
274 views

### Open source implementations of Symmetric Searchable Encryption and Order Preserving Encryption [closed]

Are there open source implementations of SSE and OPE? Can anyone please point to sample codes, if available. EDIT If cryptDB is not an option, what other options are available? (Indeed, these ...
692 views

### How can I find the prime numbers used in RSA?

I got this question in a local hacking event, but I couldn't solve it. Problem Statement ---- Continuing their snooping habit, NSA kept bugging Alice's communication. Resorting to the age old ...
268 views

### How do I choose the best 3*3 s-boxes?

I've read about 20 papers about the criteria for choosing best $n \times n$ s-boxes. Now, I've got two problems: How to generate $2^{24}$ s-boxes, and how to test them considering the criteria. ...
826 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. ...
590 views

### Is Encrypt+HMAC stronger than AEAD?

There are a few posts that I've come across that seem to infer that using regular encryption and a MAC might be better than using the newer AEAD (ie: AES/GCM) modes. ...
300 views

### Do parts of a hash carry the properties of the entire hash?

When I need to generate unique id's based on some information hashing is typical choice. However, sometimes that id needs to be of a particular size. I've seen a lot of schemes (HMAC-MD5-96 in SSH, ...
606 views

### Could one construct a cipher that is secure for friendly parties to use but insecure for hostile parties?

Consider the situation of a nation state (Blue) at war with another nation state (Red). Blue wants to deploy a secure cipher that blue currently can not break, but they are considered that Red could ...
764 views

### Why was the winner of the AES competition not a Feistel cipher?

The winner of the AES competition has a structure that does not qualify as a Feistel cipher, as explained in answers to this recent question. However, most many of the AES candidates, and all 3 out ...
638 views

### Why RSA can't handle numbers above 76?

I'm going to encrypt the characters Zhu, and decrypt them using RSA. I'm using the public key $\{e, n\}$ and private key $\{d, n\}$. The values of $e$, $d$ and $p$ ...
1k views

### Is 80 bits of key size considered safe against brute force attacks?

I came across KATAN Family of Ciphers for small domain input blocks . They cipher arbitrary block lengths 32,48,64 but their key size 80 bits only. Is 80 bits of key size considered safe with ...
2k views

### Can two different pairs of RSA key have the same modulus?

Can $n=pq$ be part of two different pairs of RSA keys? If such keys exist, say $(e_1,n)$ and $(e_2,n)$, how are they related? What will be the security concerns for the two users?
370 views

### Can one efficiently iterate valid bcrypt hash output values?

bcrypt is an intentionally slow hash algorithm. In my last protocol idea, I wanted to use it to expand a password and then only transfer the bcrypt-hashed password. An efficient attack on this would ...
917 views

### Salting when encrypting?

I was attending a database encryption session at a developers conference. The presenter (who was a published author on the subject) said that MS SQL Server did not support salted hashes in the ...
335 views

### Security of a security protocol for key exchange, using symmetric-key cryptography

This is an exam question: $A \to B$: Alice, $nonce_A$ $B \to A$: $nonce_B$, $E_{k_{AB}}(nonce_A \| k_1)$ $A \to B$: $E_{k_{AB}}(nonce_B \| k_2)$ Alice and Bob both compute $k_s$ = $k_1$ xor $k_2$ ...
285 views

### Encrypting files with known headers

If I encrypt a file of a known format that has a lengthy header (e.g. an XML Excel file), does that render the encrypted file susceptible to a "known plain text" attack? In other words, if the first ...
319 views

### Why initialize SHA1 with specific buffer?

SHA-1 is initialize with a specific buffer: h0 = 0x67452301 h1 = 0xEFCDAB89 h2 = 0x98BADCFE h3 = 0x10325476 h4 = 0xC3D2E1F0? Why?
475 views

### Why restricting SHA3 to have only two possible capacities?

I just read the presentation slides of John M. Kesley (from NIST) for his invited talk at CHES 2013 about SHA-3 and learned that NIST is going to standardize Keccak with a possibly modified padding ...
783 views

### Is it possible to distinguish a securely-encrypted ciphertext from random noise?

Say I have a bunch of data encrypted with a secure block cipher (such as AES). An attacker has unlimited access to this encrypted data. The attacker doesn't know whether the data is encrypted or if ...
922 views

### Related-key attacks on AES

According to Wikipedia: Related-key attacks can break AES-192 and AES-256 with complexities $2^{176}$ and $2^{99.5}$, respectively. What are the requirements for these attacks (i.e how many ...
444 views

### Is it reasonable to assure that p-1 and q-1 aren't smooth?

I came across the requirement that, in RSA, $p-1$ and $q-1$ shouldn't be smooth, shouldn't consist of lots of small factors. Therefore my question: How complicated is it to check whether $p-1$ is ...
262 views

### Is there an algorithm to find the number of intersections of two sets?

Suppose both I and my friend have a set of integer numbers. We want to know the number of common elements in our two sets but without knowing elements of the sets of each other. So I don't want my ...
207 views

### Will varying plaintext compensate for a fixed initialisation vector?

This is a follow-up question to Relative merits of AES ECB and CBC modes for securing data at rest. I need to store encrypted Personal Account Numbers (PANs) in a database. The only encryption option ...
5k views

### HMAC vs MAC functions

I've read definitions of MAC and HMAC, but can't say I've completely grasped the differences. What are principle differences? When to use one and when the other?(Typical Use Cases)
351 views

### Exposing RSA private-key data… bad?

We know that exposing $p$, or $q$ or $\phi(n)$ results in trivial attacks on RSA since they allow us to factor $n$ and to compute the private exponent $d$. In OpenSSL (and most RSA implementations) ...
1k views

### Why programming languages don't provide simple encryption methods?

In c#, there is a namespace called System.Security.Cryptography and there are many classes in this namespace, which help developers encrypt and decrypt data. ...
163 views

### Is there any general description language for block ciphers?

In page 9 (16 in the PDF) of Final report of European project IST-1999-12324 (NESSIE), it is mentioned that: The NESSIE project is also developing a new generic tool to analyze block ciphers ...
630 views

### Does AES have any fixed-points?

Is there any pair of 128-bit strings M and K such that AES$_K$(M) = M? If yes, how do I go about (efficiently) finding such a pair?
979 views

### Understanding CRC

There are zillions of articles describing CRC. What can I read to (more deeply) understand what's really going on? Both from an algebraic perspective and a bit-manipulation perspective, I'd like to ...
116 views

### Are there groups where the computational Diffie Hellman problem is easy but the discrete log problem is hard?

I know that there are elliptic curve groups, used in pairing-based cryptography, where the decisional Diffie Hellman problem (ie. given $g$, $g^a$, $g^b$ and $c$, determine if $c = g^{ab}$ is easy but ...
991 views

### Why hash the message before signing it? Digital signature with RSA

Here's a method to sign the message with RSA: Why hash the data before signing it? Why not sign the whole message? It'll save time if you sign the hash value, but I've heard there are also security ...
429 views

### Would RSA make sense if we used no computers?

I was recently wondering - would RSA be useful if we brought it to, say, medieval times? Could you choose the keys so that you could encrypt/decrypt messages quite easily, but factoring the private ...
1k views

### Will our app be FIPS 140-2 compliant if we use our own AES algorithm implementation?

We are in the processing to understand if our software applications is FIPS 140-2 compliant or not. Currently in our application, we are using our own implementation of AES algorithm. AES is a FIPS ...
987 views

### What is the progress on the MIT LCS35 Time Capsule Crypto-Puzzle?

Ron Rivest posed a puzzle in 1999. MIT LCS35 Time Capsule Crypto-Puzzle. The problem is to compute $2^{2^t} \pmod n$ for specified values of $t$ and $n$. Here $n$ is the product of two large ...
363 views

### Why does SRP-6a use k = H(N, g) instead of the k = 3 in SRP-6?

I've been reading up on the Secure Remote Pasword protocol (SRP). There are a couple different versions of the protocol (the original published version being designated SRP-3, with two subsequent ...
### In RSA, rationale for prime $p$ with $p-1$ having prime factor $u$ with $u-1$ having large prime factor?
In the 1978 RSA paper, it is recommended, among other things, to choose primes $p$ such that $(p-1)$ has a large prime factor $u$. This was motivated by Pollard's p-1 algorithm. Further, the authors ...