4
votes
1answer
715 views

Recent attacks on RSA

At Blackhat 2013 this week, there was a talk saying RSA is (essentially) doomed in the near future. networkworld.com ~ “Black Hat: Elliptic curve cryptography coming as smarter algorithms threaten ...
4
votes
2answers
405 views

CTR mode nonce with aggressive key rotation policy

I'm writing a small encryption library which will be required to use the shortest possible amount of overhead for describing ciphertexts. I want to use CTR mode with a 6-byte random nonce. Normally ...
4
votes
1answer
881 views

Why is asymmetric encryption IND-CPA?

According to Wikipedia, the IND-CPA game is: The challenger generates a key pair PK, SK based on some security parameter k (e.g., a key size in bits), and publishes PK to the adversary. The ...
4
votes
3answers
167 views

using elliptic curve point multiplication as a key stretching method

My friend came up with the following idea: assuming we agreed on curve parameters, use the following algorithm for key stretching/derivation from user-entered password. Pad the ascii representation ...
4
votes
2answers
3k views

How does SafeNet MobilePASS generate passwords?

We use MobilePASS at work but the latest version of the android client seems to be buggy so I wanted to have a go at implementing the algorithm myself. You can download the client to play with here: ...
4
votes
2answers
372 views

It is reasonably safe to leave a SSH private key with a 30+ character passphrase public?

I'm curious about exploring uses for a distributing a single RSA private key with a passphrase instead of trying to secure the private key and distribute the public key. With the private key (and ...
4
votes
2answers
714 views

Is this hand cipher any more secure than the Vigenère cipher?

I know that inventing one's one crypto always sucks, but the problem is that hand ciphers are usually very insecure very slow. This is an attempt to make a relatively secure, keyable, and ...
4
votes
1answer
753 views

Are RSA signatures deterministic?

If I sign the word HELLO with the mechanism "NONEwithRSA" with the same private key, do I always will have the same signature? A Java example always return ...
4
votes
1answer
1k views

Hashing passwords with a salt - why use different salt for everyone?

Given a database where we have usernames and passwords, we want to secure users' passwords by hashing them. We should not use only username and passwords in this hash, as someone having data from ...
4
votes
1answer
262 views

How much do we trust KEA1 Assumption?

Let $$(g,h=g^s,q)$$ be a tuple such that $g$ is a generator for a group $\mathbb{G}$ of ord $q$ and $s$ is uniformly random in $\mathbb{Z}_q$. The KEA1 Assumption saies that for any adversary $\...
4
votes
1answer
191 views

Why does Rijndael with a 256-bit block require the bottom two rows to be shifted one more space than usual?

From Wikipedia: For a 256-bit block, the first row is unchanged and the shifting for the second, third and fourth row is 1 byte, 3 bytes and 4 bytes respectively. Why the change? Is there any ...
4
votes
1answer
238 views

How can we determine if two discrete logarithms are equal?

Let $p$ be a prime number, and let $g_{1},g_{2},...,g_{n}$ be $n$ generator of $Z^{*}_{p}$. We have a list $y_{1},y_{2},\dotsc,y_{n}$ of elements in $Z^{*}_{p}$ such that for every $i\in \lbrace1,2,\...
4
votes
3answers
177 views

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 ...
4
votes
2answers
833 views

Attacks on the RSA Cryptosystem

I was reading some articles about attacks on RSA system and I wonder about some generalization of the following theorem. Theorem (Coppersmith). Let $N=pq$ be an $n$-bit RSA modulus, where $...
4
votes
1answer
761 views

Exhausting the entropy of a hash function

In the case of password storage, consider the following: I have an idea that one can exhaust the entropy of input to the MD5 function by using a 128 bit random value as the password (indeed, any hash ...
4
votes
4answers
361 views

How to prove membership of a list without disclosing the list members?

I'm designing a messaging system where the sender A sends a message m with a signature s to n Receivers. A Receiver Ri should then be able to prove to a Verifier V that he is one of the receivers of ...
4
votes
3answers
3k views

Is it safer to encrypt twice with RSA?

I wonder if it's safer to encrypt a plain text with RSA twice than it is to encrypt it just once. It should make a big difference if you assume that the two private keys are different, and that the ...
4
votes
2answers
419 views

Real world use cases of Multi-Party Computation

Most of the research papers give imaginary applications of multi-party computation. Either they talk about Yao's millionares' problem or two or more corporates willing to compute some intrustion ...
4
votes
2answers
2k views

Homomorphic cryptosystems in RSA

Hopefully Crypto can help me understand homomorphic cryptosystems. I'm designing a high score server for a game I made, and because of facets in the language i'm using, the player would be able to ...
4
votes
2answers
4k views

Secure private key storage

I'm developing application in Java that has to store RSA keys in software for foreseeable future (that is, at least 10 years). The two most common standards of storing private keys are PKCS12 and JKS ...
4
votes
2answers
864 views

Additive ElGamal cryptosystem using a finite field

I'm trying to implement a modified version of the ElGamal cryptosystem as specified by Cramer et al. in "A secure and optimally efficient multi-authority election scheme", which possesses additive ...
4
votes
4answers
1k views

Signature and Timestamp for Long Term Document Archival Question

I have a PDF document intended for long-term (many years, maybe decades) archival which I would like to digitally sign with my personal certificate to ensure its integrity. As far as I understand, I ...
4
votes
2answers
212 views

Addition step in MD5 implementation

In MD5, there are four rounds. After every round, why do we need to add the computed $Q$ values to the initial values and then take this value as input to the next round. For example after the first ...
4
votes
4answers
2k views

Assistance Cracking Classical Cipher

Below is the cipher text I am trying to break and as you can see its rather short which is why I am having so much trouble. ...
4
votes
1answer
810 views

Correctly generating cryptographically secure pseudorandom numbers (in C)?

I am learning about cryptography, purely for my own interest. As part of that, I am implementing a simple one-time-pad toolset in C. Naturally, I need to generate random numbers to create the pad, ...
4
votes
2answers
1k views

Order Preserving Encryption for Numeric Data Values

How can I ensure order of encrypted data i.e., Enc(m1) < Enc(m2) where m1 < m2, and all messages are integer values. I have gone through Order Preserving ...
4
votes
1answer
725 views

Can I secure my key by XORing it with a hashed password?

I'd like to build a simple password-protected symmetric key system. The key-creation process in my system operates as follows: The system creates a 256-bit key purely at random. The user chooses a ...
4
votes
1answer
669 views

Differential cryptanalysis - breaking the last round of FEAL4?

I've been trying to learn cryptanalysis. I've come across this resource which proved very helpful: http://theamazingking.com/crypto-feal.php So far I've been almost successful in breaking FEAL4 ...
4
votes
3answers
3k views

How can I create a fixed length output in my hash function?

I've been recently looking into the creation (and theory) of hash functions, however I just can't figure out how to turn a message into something of a fixed length. At the moment, my theory of a hash ...
4
votes
1answer
1k views

Encrypting (CBC) identical files with same key and different iv, is it less secure?

I would like to learn more about cryptography. Let's say you encrypt multiple files with CBC encryption using the same key, but each file has unique (and pseudo random) iv. Does this weakens the ...
4
votes
1answer
310 views

Protocol for Randomized Oblivious Transfer?

If we define Oblivious Transfer as following: Alice inputs $(x_0,x_1) \in F^2$, where $F$ is a field, and Bob inputs $b\in\{0,1\}$, then Alice gets a dummy output(for which she knows nothing about b),...
4
votes
1answer
232 views

Is the last step of an iterated cryptographic hash still as resistant to preimage attacks as the original hash?

Considering a cryptographic hash, such as MD5 or SHA2, denoted by the function $H(m)$ where $m$ is an arbitrary binary string, there is a lot of material available that deals with potential weakness ...
4
votes
1answer
3k views

How to practically find solutions to a discrete logarithm?

Are there any ongoing or current practical attempts to solve instances of the discrete logarithm problem of the order of magnitude used in cryptographic applications, for example with a 256 bit ...
4
votes
1answer
180 views

How should I read these three-dimensional (pseudo)random-generated numbers representations?

Visiting this site: http://lcamtuf.coredump.cx/oldtcp/tcpseq.html#phase I found graphical representations of random-generated sequence numbers, but I can't figure out how to interprete the plots. ...
4
votes
1answer
203 views

PBKDF2 PRF: HMAC or just hash?

I understand the way $\textrm{PBKDF2}$ can be used to 'stretch' a password/passphrase that does not provide full entropy. My question: how does one choose the PRF? A common choice is any well-known ...
4
votes
1answer
44 views

Can you decrypt with 1 round key?

If one has the last round key in an SP or Feistel network (the 16th round key in DES for example), would that someone be able to decrypt the rest of the encryption?
4
votes
1answer
58 views

Is this a good choice of a digital signature scheme?

This is a concrete instantiation of Rabin-Williams signatures. Specifically: The private key is 2 primes $p, q$. The public key is their product $N = pq$ and is approximately 3072 bits long. the ...
4
votes
3answers
110 views

Is this Diffie-Hellman key exchange variant vulnerable to man-in-the-middle attacks?

As we all know, the Diffie-Hellman key exchange protocol without authentication is vulnerable to a man-in-the-middle attack. And if we use STS(Station-to-Station protocol) instead, it would be secure. ...
4
votes
1answer
42 views

Generate a random number $r \in \{1,2, \dots , k\}$, where $k$ is not public and is distributedly held

A set of parties can securely generate a random number $r \in \{1,2,3,\dots, k\}$ when $k$ is publicly known. However, can we generate the random number $r$ if $k$ is not publicly known and is held by ...
4
votes
2answers
148 views

Help with breaking AES reduced to 2 rounds

I was studying Rijndael AES proposal, and its cryptanalysis. I started with writing code to break AES reduced to 2 rounds. I am using 128 bit key size and 128 bit block size. 2 round AES means the ...
4
votes
1answer
63 views

Does length-prepending stop length-extension attacks?

Does length-prepending stop length-extension attacks on Merkle-Damgård hash functions, assuming that the length is checked?
4
votes
1answer
99 views

Exactly how bad is using 'mod' to clamp reduce numbers to a given range?

Let's say I'm naive and want to generate a random integer in the range [0, m), so I do the following: ...
4
votes
3answers
134 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 ...
4
votes
1answer
104 views

Could we share a secret key using the Birthday problem?

I stumbled upon this code on-line, it seems obvious to me that generating all 2^40 keys in trivial time can always be achieved as can hashing each and everyone of them, using the SHARED Salt that "Eve"...
4
votes
1answer
153 views

Is finding collisions in a part-hash not often enough a bad problem?

My situation: I've been working now for a couple of months on my own unique hash function, I've changed it many times and had two main versions but I won't bore anyone with the details of my work; at ...
4
votes
1answer
250 views

Difference between Hardware implemented algorithm and software implemented one?

In the articles about cryptography I see the words Hardware implemented and Software implemented. I'm curious to know what is the difference between them? In the other words, even in the computer ...
4
votes
2answers
122 views

Is there such a thing as a hash function with a fixed size input?

That seems like it would be a useful primitive. Using it in place of a block cipher in the Merkle–Damgård construction would give a variable-length hash, maybe without the tricky business with related ...
4
votes
2answers
108 views

Is the RC4 keystream random or not?

I have cause to use the PRNG half of RC4, but am seriously confused as to the randomness of its output. (Only the PRNG half.) On the one hand, there is a large body of literature finding ever ...
4
votes
1answer
105 views

Using HMAC as a nonce with AES-CTR encrypt-and-MAC

Suppose I use the following encrypt-and-MAC construction: $E(k_1, k_2, n, m) = E_\text{AES256-CTR}(k_1, n, m) \| \text{HMAC-SHA256}(k_2, m)$, where: $k_1$ and $k_2$ are 256-bit keys $n$ ...
4
votes
2answers
200 views

Public key crypto instead of hashing passwords in a database

Suppose I want to keep a client's password database secure (not in plaintext), while still having access to the plaintext password. Now I generates a public/private keypair. Then I send the public ...

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