# All Questions

374 views

### Is there any research about cryptography on nondeterministic Turing machines?

I know it's a highly theoretical topic, but I was wondering if there was any research out there about what cryptography would be like assuming that we had access to nondeterministic Turing machines. ...
302 views

### Is this method of deterministically using CBC secure?

I'm trying to design a deterministic encryption scheme to enable searching in untrusted databases with the smallest possible IV overhead on ciphertexts. I know it's very bad practice but unfortunately ...
235 views

### Swapping Key and IV in AES? Safe?

I have an application where I want to be able to send an encrypted file, and then mete out "keys" that allow the receipient to decrypt the file from a certain point to the end of the file. Actually, ...
357 views

### Is asynchronous perfect forward secrecy possible?

DH key agreement protocols require the participation of both parties, so are only suitable for synchronous connections. Is it possible to implement PFS in a fashion usable for asynchronous protocols, ...
724 views

### Compare two hashes with different salt

I'm storing the salted hash of a credit card number in a database. What I'd like to be able to do is determine if two different entries in the same database correspond to the same credit card. ...
1k views

### Why is SRP not widely used?

SRP seems to be a very good password authentication protocol, compared to any other things used now. So why is there no popular implementations, or even no working secure implementations? I tried to ...
537 views

### Attacking historical ciphers methodology

It's more a theoretical question of how would you approach it. All you know about the ciphertext it's was generated with a historical cipher. The ciphertext appears to be random, BUT it's divided into ...
216 views

### Is it safe to assume Salsa20 to be a PRP?

Often in security proofs a certain block cipher is assumed to be a pseudorandom permutation or PRP. I wonder if this goes for stream ciphers as well, and specifically for Salsa20. If limit ourselves ...
608 views

### Client and server using same SSL certificate - any issues?

I'm working on software where multiple components will communicate with each other using SSL. There would be one central component acting as a server, which would also require the clients to present ...
191 views

### Which MAC scheme is quantum resistant?

will HMAC survive Grover algorithm ? if not then which MAC scheme provide post-quantum security ?
454 views

### Understanding a Blowfish cryptanalysis

I'm reading a cryptanalysis on Blowfish, and I've come across something that I don't quite get. Let's denote $$\delta = a \oplus a'$$ where a and a' are bytes that cause a collision in some S-box ...
355 views

### What crypto system allows for 3 parties: Party 1 who makes an assertion, Party 2 mutates the assertion, Party 3 validates it

I'm looking for the cryptographic equivalent of a Drivers license where the issuer can be verified, the issuer doesn't need to know who you showed the drivers license to, but also allows ...
2k views

### Would a “Triple AES” (in the sense of how Triple Des works) serve for a dramatic increase in safety?

The system requires to be as paranoid as possible regarding security. One of the few contemplated changes to the current design is to use multiple encryption. First proposal was to use Serpent on top ...
509 views

### Security of Pohlig-Hellman exponentation cipher?

I am looking into implementing Pohlig-Hellman exponentation cipher and I would like to know how secure that algorithm is? I am guessing it's security relates greatly to the prime number used in it. ...
413 views

### Can I combine two PRNGs to make use of more seed data?

I have 320 bits of seed data (actually 512 bits of data with 320 bits of entropy, derived from a Diffie-Hellman shared secret and nonces). The PRNG I am using at the moment is the android version of ...
553 views

### Implementations of Ntru TLS

Has anyone come across any implementations of Ntru TLS? I'm working a project for uni that does quantum secure encryption. It relies on a mix of Ntru & AES, but I can't find an implementation of ...
881 views

### What is “Blinding” used for in cryptography?

What does "blinding" mean in cryptography, and where do we usually use it? Can you describe a sample implementation?
125 views

### Security implications of using constant string to verify AES key?

I know that you can use for example CBC-MAC to verify data integrity and thus the password upon decryption. But would there be security implications of just adding a constant string to the plaintext ...
978 views

### What happens to the entropy of a password when you hash it?

For example, if the entropy of a password is 30 bits, what is the entropy of the password when you hash it with MD5?
420 views

### Exact mathematical definition of simulation based security?

I've been trying to understand cryptographic protocols and how to define their security. The problem is that while I can understand what the intuitive definition says, I have trouble understanding how ...
821 views

### is this RSA private key valid?

is this RSA private key valid? First, here's the RSA private key in question: ...
346 views

### A discrete-log-like problem, with matrices: given $A^k x$, find $k$

Let $p$ be a large prime; we will work in $GF(p)$. Let $A$ be a $n\times n$ matrix. Also, let $x$ be a $n$-vector and $k$ a positive integer. Suppose we are given $p$, $A$, $x$, and $y$. The goal ...
1k views

### Implementing AES encryption for firmware distribution system

I need to implement an AES encryption for firmware distribution system. I have a bootloader that can decrypt various AES variants (ECB,CBC,CTR). When I approached this I found few issues that aren't ...
575 views

### RSA algorithm's license free or paid?

I checked RSA's patent application, which was registered in 1983. As patents don't last more than 20 years, it seems to me it should be free. But my friend said to use RSA I have to buy a license from ...
560 views

### How can I solve the discrete logarithm modulo 2q+1 if I can solve it in the subgroup of order q?

As part of my cryptography course I came across an exercise that neither me or my friends could figure out. The problem statement is as follows: Let $p$ be a large prime of the form $p = 2q + 1$ ...
323 views

### Does Identity-Based Encryption actually solve any problem?

Identity based encryption schemes [*] seem to have great potential in high-latency Delay-Tolerant and mobile, ad-hoc networks since they apparently seem to avoid the need for key negotiation and ...
156 views

### Proof of work scheme with one and only one correct answer

I need a problem that has one and only one solution, such that the solution is hard to find but easy to verify. I need to be able to generate the problem deterministically from a random seed. So ...
249 views

### Is this streamable combination of encryption and MAC secure?

I want to combine encryption and MAC. For encryption I use AES-256 with CBC and PKCS5Padding. For MAC I use HmacSHA512. I use the Encrypt-then-Mac approach (calculate MAC over the ciphertext and ...
279 views

### LFSR dynamic mutation

In normal LFSR, the state is a function of the initial seed, taps positions and time, nothing else. I've seen a modification of LFSR that works like this: ...
971 views

### Efficient and stateless anti-forgery method

One use of an anti-forgery token is to prevent Cross-Site Request Forgery (CSRF) attacks. The attacker doesn't need to sniff the wire in order to carry out a CSRF attack. This attack relays on the ...
157 views

### Does security under ROM imply exactly what?

I'm not sure I understand really the implications of proofs of security in the random oracle model. Does a proof of security in ROM translate to a reduction of security of the crypto-system to the ...
670 views

### How can I generate a one-time password?

Through my reading about the generation of one-time password (OTP) I found that there are many algorithms that generate OTP and they are either based on time such as TOTP or based on mathematical ...
600 views

### Can one group the SHA-256 outputs depending on partial inputs?

Is it possible to predict a hash key based on half key? Let's have some example: I have 100000000 hash results, and they are generated by either ...
2k views

### How to attack a general polyalphabetic cipher?

I am able to decrypt vigenere cipher text using the index of coincidence and chi test. However out of interested how do you go about attacking ciphertext that was encrypted using a mix alphabet ...
538 views

### How can one break a monoalphbetic substitution chipher at pseudorandom text?

Does anybody know how to break monoalphbetic substitution cipher, if it is applied to some pseudorandom text (for example to some surrogate key filed in a database)? Let us assume that we have only ...
234 views

### How common are SEED certificates outside of Korea? When is support required?

I'm looking at the SEED algorithm and would like to know if this is still in common usage. Can anyone tell me when I would need to implement this standard, either as a client or a server?
56 views

### Why does HKDF use HMAC(salt, key) instead of HMAC(key, salt)?

I've been looking over the HKDF specs (RFC 5869), and something I noticed is that in the key and salt are reversed: ...
47 views

### Is it possible for a simulator to always abort?

In this paper of J. Baron, et al. (PDF), simulator always aborts the ideal functionality before client outputs its pattern-matching result obtained from the ideal functionality. (See bottom of page ...
72 views

### How to compare performances of lattice-based and pairing-based IBE schemes

I try to compare the performances (cost of Enc, Dec, ... size of keys, ciphertexts, ...) of IBE schemes using lattices (LWE hardness assumption) or pairing (Diffie-Hellman hardness assumption). I've ...
122 views

### Is there a way to prove equality of plaintext that was encrypted using different public keys?

Imagine if Alice encrypts message to Bob (using public key $P_{bob}$) and then Bob encrypts the same message to Carol (using $P_{carol}$). Is there a way for Bob to prove that: $P_{carol}$ was ...
50 views

### Designing Secure Multi-Party Computation Sub-Protocols Based on Homomorphic Encryption

When designing SMPC protocols using secret-sharing, it is a common approach to compose a protocol from several sub-protocols (each proven secure under the formal definition of security w.r.t. ...
46 views

### State level “Weak Diffie-Hellman” working for SRP too?

I've read about the "Weak Diffie-Hellman" attack (paper, website), where a resourceful entity like a state can pre-compute values for known primes to aid solving the discrete logarithm problem for ...
117 views

### How have MD5 constants been determined?

For instance, the per round shift amounts, ...
91 views

### Are shorter password hashes safe?

How much can I shorten a password hash before it begins to impact security? Suppose that I use PBKDF2 (correctly salted) with HMAC-SHA256 as the PRF. From what I've seen, it's typical to use 256 ...
74 views

### Adding two RSA private key

We've constructed a new private RSA key from two known private RSA keys $Priv_1$ and $Priv_2$ as follows: $$p = next\underline{}prime\left(\frac{Priv_1(p) + Priv_2(p)}{2}\right)$$ and q = ...
140 views

### POODLE attack on TLS 1.2

The POODLE attack uses the way block ciphers in CBC mode are decrypted in combination with the packet's padding to determine some byte's value. From what I understand, even TLS 1.1 can be vulnerable ...
140 views

### Randomness in Public Key Encryption

For a scheme to be at least CPA secure, one has to randomize the scheme. In case of a secret key encryption scheme, the used randomness will typically become part of the cipher text as in the case of ...
110 views

### Proving set membership in less than log(N) bandwidth

I have an unchanging set of unique items, and I want my server to show one of those items to a client, then cryptographically prove that the item is part of the set. How I do this currently: ...