# All Questions

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### How to build an electro-mechanical public key cipher machine?

It is generally assumed that asymmetric encryption schemes were invented in 1973 at GCHQ in Britain and, independently, in 1976 at the MIT. Imagine, if the abstract idea of having a public key and a ...
614 views

### What does “running in polynomial time” really mean?

I'm currently learning private-key cryptography. I've been able to see that perfect secrecy is achievable if no assumption is made about the computational power of the attacker. However, perfect ...
1k 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 ...
980 views

### Homomorphic (encrypted) comparison to an integer

When working with an additive homomorphic encryption scheme (say Pallier's), is there an efficient way to get the encrypted value of a comparison test to an integer value (I realise that an ...
403 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 ...
1k views

### Can one reduce the size of ECDSA-like signatures?

Using $n$-bit ECDSA, a signature has a size of $2·n$. It is possible to recover the public key from this signature, which shows that there is a publicly visible redundancy in the signature. Is ...
870 views

### Predicting PRNG given some of its previous output

I a have a question about PRNGs and this is my very first experience with them. I have the following generator that takes a 56-bit seed $p$ during initialization and then chooses both $X$ and $Y$ ...
967 views

### AES and perfect ciphers

I'm taking a crypto class this semester and after learning the definition of a perfect cipher. I started wondering how this definition applies to AES. Obviously AES isn't a perfect cipher, since the ...
93 views

### Can cycle finding techniques reduce the memory usage of the MitM attack against 2DES and 3DES?

A 2DES like cipher $c=E^{(2)}_{K_2}(E^{(1)}_{K_1}(p))$ where both halves have an $n$ bit key is vulnerable to a meet-in-the-middle attack. Meet-in-the-middle using a big table Create a table ...
518 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 ...
461 views

### Why are cryptography algorithms not exported to certain countries?

There have been strict laws about the export of crypto software to certain countries. I can understand the intent, but never gauged the fact that there is nothing stopping these countries from ...
413 views

### Testing the Fortuna random number generator?

I have implemented the Fortuna random number generator as described in chapter 10 of "Practical Cryptography" (Ferguson and Schneier, Wiley, 2003), the result can be found at ...
812 views

### Is FIPS 140-2's “Continuous random number generator test” practical?

Section 4.9.2 of FIPS PUB 140-2 specifies, amongst other things, a "Continuous random number generator test." Here are the relavant bits: If each call to a[n] RNG produces blocks of n bits ...
6k views

### What is the difference between MAC and HMAC?

In reference to this question, what are the "stronger security properties" that HMAC provides over MAC. I got that MAC requires an IV whereas HMAC doesn't. I also understood that MAC may reveal ...
2k views

### Generating Random Primes

Although this has been extensively discussed around here, I'm curious whether my approach makes sense, or I should just stick to "the standard version". I'm implementing some homomorphic encryption ...
746 views

### Is ISAAC considered secure as a cryptographic random number generator?

The ISAAC site claims it is secure, but I wasn't aware of any trusted 3rd party sources that verified its security.
6k views

### Is Blowfish strong enough for VPN encryption?

I'm looking at an OpenVPN connection between two sites configured to use 128 bit Blowfish in CBC mode, and trying to figure out how to assess the strength, but I just don't know enough of the maths. ...
282 views

### Is HMAC-MD5 still secure for commitment or other common uses?

MD5 collisions have been out for some time. In spite of this, HMAC-MD5 is still secure for authenticating data1. This illustrates a strength of the HMAC construction, it does not require that the hash ...
1k views

### Does collision resistance imply (or not) second-preimage resistance?

I've seen contradictory results. Sometimes hash functions are collision-resistant but not necessarily second-preimage resistant. I've seen this kind of things in papers from Bart Preneel: “Security ...
250 views

### Fast hashing into elliptic curve

Is there a fast algorithm for mapping $n$-bit numbers $s$ (for fixed $n$) into a cyclic subgroup of an elliptic curve (over a finite field) in which the Discrete Logarithm Problem is hard? By fast, I ...
479 views

### Keccak padding implementation, and bit ordering

I am trying to implement Keccak in Java for my school work. And I am getting stuck in two places: On the summary page the padding is shown as P = M || 0x01 || 0x00 || … || 0x00 P = P xor ...
394 views

### Deterministically combine more than one source of entropy

What is the proper/canonical way to do this? For example, $0 < r1 < 1$ and $0 < r2 < 1$. Presuming uniformly distributed probabilities for the two, combining/averaging them is going to ...
446 views

### Slow one-way pseudo-random permutation?

I'm looking for a slow one-way pseudo-random permutation; or in other words a block cipher $E_K: P\in\{0,1\}^b\mapsto C\in\{0,1\}^b$ with moderate block size $b\approx 64$ bits, wide key $K$, ...
2k 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 ...
2k views

### How can I remove my personal data from my PGP public key?

According to this Q&A-discussion it is possible to remove all personal data (name and mail address) that is attached to a public key. What steps do I have to follow in order to remove all ...
2k views

### Is there a hash function which has no collisions?

Is there a hash function which has no collisions? To clarify: it would be some function which would produce variable-length output, and never produce the same output for differing input. It would ...
1k views

### How can an S-Box be reversed?

So, as an exercise to greater understand some things about cryptography, I decided to try and implement GOST 28147-89, which uses eight 4x4 S-Boxes. I can grasp the concept of putting 4 bits through ...
360 views

### Why doesn't CTR mode require blocking?

I've been reading a bit about block cipher modes and I have a relatively straightforward question regarding CTR. In essence, I was hoping you guys would be kind enough to validate my understanding of ...
244 views

### Finding where I am in a linear recurrence relation

Suppose I have a linear recurrence relation $$a(n) = c_1 a(n-1) + \dots + c_k a(n-k) + d,$$ where the constants $c_1,\dots,c_k,d$ are given and the initial values $a(0),\dots,a(k-1)$ are given as ...
404 views

### Does TLS use RC4-drop[n]?

According to many sources, the first few (n) bytes of the RC4 keystream are strongly biased, and therefore should be discarded before using the keystream to encrypt anything; this precaution is ...
620 views

### Security equivalence proofs for breaking RSA

It is my understanding that while a practical solution to the factoring problem will definitely break RSA, it has never been proven that the security of RSA is equivalent to factoring. In otherwords, ...
484 views

### Is there some way to generate a non-predictable random number in a decentralised network?

Is there a way to generate a random number with given restrictions: It will be used in a decentralised network with a big number of peers (no central authority to generate it) Its generation should ...
409 views

### Threshold Signatures for RSA and DSA

I have a few questions relating to threshold signatures: a scheme where $n$ participants hold a key share and any $t$ of them can conduct a protocol using their shares that results in a valid RSA or ...
1k views

489 views

### Where could I submit my algorithm?

I have come up with an encryption algorithm and I am looking for a place to possibly publish it. Any ideas?
853 views

### What are the constraints for an IV using AES in CBC mode?

I'm designing a protocol for use into a VPN software. The VPN frames are encapsulated into AES-256 CBC encrypted frames. I understand that IVs must be uniquely used for each message encrypted with ...
780 views

### Should the secret key of Shamir's secret-sharing algorithm be interpreted byte by byte?

Should the secret message of Shamir's secret-sharing algorithm be interpreted and processed byte by byte? Interpreting it byte by byte makes it easier to process, but in case one of the shareholders ...
351 views

### Is SHA-1 still practical secure under specific scenarios?

It is conjectured that SHA-1 has been broken from the "research" perspective but no in real world. That is that there is an algebraic attack that explores weaknesses on its algrebraic construction. ...
154 views

### How to generate a key using any m passwords out of total n?

My application requires an AES-256 key K for some secure operation. In order to avoid saving this key in application, I have implemented following scheme: There ...