2
votes
1answer
415 views

Is it feasible to break Diffie-Hellman key exchange when the implementation uses a poor-quality PRNG?

I've come across an implementation of DH in Java that uses the Random class to generate the secret integer value $a$, as shown in in Wikipedia's description of the ...
2
votes
2answers
181 views

Does it take brute force to find a pair of plaintext and ciphertext that each follow a certain condition, given an AES encryption key?

Suppose that I have an AES key $K$, and I'm instructed to find a plaintext such that the first 32 bits of the plaintext are some string of bits $S_1$, and the last 32 bits of the ciphertext once the ...
2
votes
1answer
2k views

AES key length greater than 256 bits - is it dangerous to do an implementation outside of the standard?

I'm making a digital-asset manager written in ActionScript 3, it will be used to manage highly sensitive information. I'm using the AS3Crypto library (which has quite a good reputation) to implement ...
2
votes
2answers
1k views

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 ...
2
votes
2answers
256 views

Speeding up partially known plaintext preimage recovery attack on MD5

Suppose there are three messages A, B and C of different length, that are 16 DWORDs in ...
2
votes
2answers
254 views

What cipher mode is suitable for independantly decryptable short messages?

I'm building a small VoIP client which transmits encrypted audio data via UDP, and some packet loss is expected, so each packet will need to be decrypted separately even if others in the stream don't ...
2
votes
3answers
357 views

Safety of DSA key parameters sharing

I'm looking for a solution to use in a context where I need to be able to generate new asymmetric key pairs quickly (using a widely recognized algorithm, and EC-DSA is not applicable). It sounds like ...
2
votes
2answers
358 views

Why is a non fixed-length encryption scheme worse than a fixed-length one?

I have the following definition (highlights by me): An (efficient secret-key) encryption scheme $(Gen,Enc,Dec)$, where $Gen$ and $Enc$ are PPT algorithms and $Dec$ is a Deterministic Polytime ...
2
votes
2answers
265 views

The security of data traffic over both SSTP & SSL

Suppose you're connected to a VPN server via an SSTP VPN, and then you're trying to sign in to your webmail account that is already secured with an SSL certificate, e.g. hushmail.com. However, it ...
2
votes
1answer
157 views

Why do we need extra hashing if we could use simpler scheme?

Lets say that we use init vector IV, key K and HMAC key H. Message is M. Mode of operation is CBC !!! We usually encrypt this way: ...
2
votes
2answers
964 views

How to decrypt this RSA-like cipher

Suppose we have a RSA-type Modulus $n = pq$ with $p,q$ prime. We also pick a random public exponent $e$ with $\gcd(e,\varphi(n)) = 1$ and compute the private exponent $d$ with $de \equiv 1 ...
2
votes
2answers
497 views

Are there standards for transport layer security using only symmetric keys?

I have an embedded system with AES-128 implemented in hardware. There is very little flash/RAM and the (8-bit) CPU runs relatively slowly. Public key crypto isn't a viable option. This system is ...
2
votes
2answers
977 views

Is RSA in a ECB-like-mode safe for bulk encryption?

Let's say I would like to communicate with my friend using asymmetric/public-key encryption, e.g. RSA. (Note: I do realize that in practice this is done through an intermediate symmetric key, but ...
2
votes
2answers
270 views

Is the “Signed on date” of a PGP Signature Verified?

I noticed that PGP signatures have a "signed on" field. I was wondering if this is "trustworthy" information and if so how is it accomplished. For example, I can't see how it could be trusted ...
2
votes
2answers
4k 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 ...
2
votes
2answers
318 views

Simple xor cipher extension

Probably the simplest cipher is the xor cipher with a single integer. One can extend this to use more than one integer by several means. I'm wondering if there is any benefit to doing more than this: ...
2
votes
5answers
464 views

What is the flaw in this model for homomorphic encryption?

Imagine a Field Isomorphism $g : \mathbb F1 \to \mathbb F2$ given by some $g(x)$ Assume a client is planning to outsource his computations to server, translates every possible $x$ as $g(x)$ and ...
2
votes
1answer
858 views

Is RSA of a random nonce with no padding safe?

Consider the following protocol: Bob has a private RSA key $B_{priv}$, and Alice knows the public key $B_{pub}$. Alice wants to send confidential messages to Bob (no integrity intended). To send a ...
2
votes
1answer
157 views

Does an attacker gain an advantage if they have the initialisation vector?

Does an attacker gain any advantage by knowing which initialisation vector was used for encryption?
2
votes
2answers
50 views

Checking both Quadratic residuosity and Jacobi symbol simultaneously and efficiently

I have to randomly generate a number $u$ such that $u \in J(N)-Q(N)$ where $J(N)$ denotes the set of elements less than $N$, whose Jacobi symbol value is equal to 1; and $Q(N)$ denotes the set of ...
2
votes
1answer
71 views

Are post-quantum cryptographic ciphers *also* secure if the P=NP conjecture holds true?

First of all, I only understand the P versus NP debate on a rather shallow level as I am not a computer scientist. So perhaps the answer to the question is straightforward but if not, I would be ...
2
votes
2answers
241 views

Which attacks are possible against raw/textbook RSA?

The PKCS#1 standard defines multiple padding schemes for signature generation/verification (EMSA-PSS and EMSA-PKCS1-v1_5), and encryption/decryption (EME-OAEP and the less safe EME-PKCS1-v1_5). ...
2
votes
3answers
529 views

How can a Vigenère cipher be broken if the message is short?

I was watching a Stanford lecture on Vigenère cipher and in it the professor said that – to break the cipher – we assume the length of the key is known. We then break the cipher into groups of this ...
2
votes
2answers
259 views

What curve and key length to use in ECDSA with BouncyCastle

I'm developing a client/server system in Java which is not interacting with third party software, so I don't have to worry about compatibility. At a certain point, I need the client and server to ...
2
votes
1answer
94 views

Empty message Hash length

I am learning Hash Algorithms and in a presentation (PDF) about these algorithms I read that: Length of $h(m)$ much shorter then length of $m$. Now my question is: What if we talk about an ...
2
votes
2answers
189 views

Password-based encrypted key storage?

RFC 5959 specifies the encrypted storage of private keys based on PBKDF2-HMAC-SHA-256 and AES encryption (in ECB mode). Is there a particular reason AES is used, instead of simply XORing the key ...
2
votes
1answer
73 views

Familiar key signing over video call

Say that a friend of mine and I have both generated new PGP keys, and we want to use a video call to verify each other's new keys because we live on opposite sides of the planet. While we could both ...
2
votes
2answers
149 views

Zero-knowledge proof of a product

I have non-negative integers $x,y,z$. I'm going to give you commitments $C(x),C(y),C(z)$ to them. Then, I would like to prove in zero knowledge that $xy=z$. I can choose the commitment scheme to ...
2
votes
1answer
179 views

One time pad: why is it useless in practice?

The symmetric cryptosystem One time pad (OTP) seems to be very beautiful since it is perfectly secret according to Shannon. Many books, however, point out that the main drawback is that one must ...
2
votes
2answers
222 views

Does AES-128 have the same strength as AES-256 with a padded key?

When I use the same 128-bit key for AES-128 and AES-256 with a known/public padding for the latter, is there some weakness in AES-256 that is not present in AES-128 with essentially the same key? ...
2
votes
3answers
224 views

How can we sign a contract digitally between two parties?

I'm trying to sign a contract with another party, without using handwritten signatures. I've just read through Applied Cryptography, but there doesn't seem to be a protocol that can solve this ...
2
votes
3answers
195 views

Why can't hashes be reversed with toffoli gates?

This is a follow up question to this question: Crack cryptographic hash functions using Toffoli gates?. Suppose there is a hash function $$H(x)=y,$$ where $x$ is the input of the function and $y$ is ...
2
votes
2answers
208 views

What does “Worst-case hardness” mean in lattice-based cryptography?

In the wiki page of Lattice-based Cryptography the "Worst-case hardness" is defined as below: Worst-case hardness of lattice problems means that breaking the cryptographic construction (even with ...
2
votes
2answers
189 views

Random numbers for Rabin-Miller primality tests

I've implemented a Rabin-Miller primality test fuction following Wikipedia and the book Applied Cryptography. Now I'm using it for generating primes with a string seed. The book suggests the following ...
2
votes
2answers
1k views

OpenSSL PBKDF2 documentation

I'm using OpenSSL 0.9.8 on OSX Maverick, and I want to use a PBKDF but I can't find any doc/man about it on OpenSSL's website. I'm working on QCA, an OpenSSL wrapper for Qt, and when I arrived there ...
2
votes
2answers
114 views

What is the impact of different modes on pseudo randomness of AES?

AES in CBC-MAC mode is proven to be good PRF . But is there any study done on other modes ? how good is ECB mode as PRF ?
2
votes
2answers
107 views

Speed up modular exponentation with fixed base and modulus

Can someone explain, how $a^x \mod N$ can be speeded up, when $a$ and $N$ are known constants? How big is the gain and what resources are needed? ...
2
votes
2answers
116 views

How does secret sharing solve the partial exposure problem?

I have been trying to understand how secret sharing methods like Shamir's secret sharing solve the problem of a share revealing information about the secret. I guess there are some random numbers ...
2
votes
1answer
155 views

Elliptic Curve Factorization: Why are elliptic curves suited for this kind of task?

Currently I'm working on a presentation of a paper that talks about the factorization of large numbers. In the paper elliptic curves are presented as a way to factorize large numbers. After hearing a ...
2
votes
3answers
252 views

OFB or CTR mode without an IV

I'm reading the SSL specs here. The interesting thing seems to be that RC4 is not re-keyed with a new IV for each message. The stream cipher state is simply carried over to the next record. Why is ...
2
votes
2answers
262 views

What does the linear assumption over bilinear groups mean?

In the abstract of "Cryptography with Tamperable and Leaky Memory", at the end of the 3rd paragraph, the authors say: In both schemes we rely on the linear assumption over bilinear groups. What ...
2
votes
1answer
279 views

What kind of machine can generate 100-400 digits prime number? RSA

The 64 bit machine's biggest number can be 2^63 (9,223,372,036,854,775,807). So what kind of a monstrous machine can generate 100-400 digits prime number? I probably don't understand something, so ...
2
votes
2answers
255 views

Is scrypt a hashing function, encryption, or PBKDF?

I have installed a python implementation of scrypt and noticed it has several functions, both an encrypt and hash function. Yet Wikipedia lists it as being used for PBKDF instead of PBKDF2. Is scrypt ...
2
votes
2answers
311 views

Openssl & RSA : how many public exponents are possible?

I would like to know if it is possible to set the public exponent we want with "OpenSSL RSA" or if it is really limited to 2 possible exponents: 3 65537 If yes, why ?
2
votes
2answers
351 views

Please punch holes in this authentication scheme

Folks, I am undertaking a crypto course in Uni. Our professor asked us to try and design some authentication schemes which are resistant to active attacks. This was just a in class exercise to get us ...
2
votes
2answers
692 views

Estimating bits of entropy

Best ways to estimate average bits of entropy in a byte stream. I know there are different statistical tests out there (NIST, Dieharder, etc), which all do different ways of analyzing entropy. What ...
2
votes
1answer
114 views

Is it dangerous to use plaintext as IV in CBC?

Is it dangerous to use plaintext as IV in CBC? For example: I want to encrypt $p1$, $p2$ with CBC. However I use $p2$ as the IV for CBC purposes. What would be the risk of not using a IV based on ...
2
votes
3answers
193 views

How to renew keys for a statically-encrypted database

Preamble Before I ask this question, please accept that I fully understand that 'rolling your own' is never a good idea, and that I am a relative n00b. The only reason I am doing this the way I am is ...
2
votes
3answers
373 views

Identification of correct plaintext after decryption in Rabin cryptosystem

In the Rabin cryptosystem, decrypting a message can produce four different outputs, of which only one is the correct plaintext. How can one know which of the outputs is the correct one?
2
votes
2answers
206 views

Alice's forgetful banking

Alice has a bank account number, but has forgotten which bank it is for. There are 4 banks, run by Bob, Carlos, David, and Eve. She could find out by going to all of the banks and asking if they have ...

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