2
votes
2answers
179 views

Is Functional Encryption about Access Control over encrypted data alone?

Informally, In functional encryption system , a decryption key allows a user to learn a function of the encrypted data. But all i see is that the function acts as an access control over the data. ...
2
votes
2answers
975 views

True random numbers generated by sensors

Is using sensor raw data a good way of generating true random numbers? I mean using an IPhone's gyro/accelerometer raw output as a base for generating an array of true random numbers.
1
vote
0answers
42 views

calculating beta for elgamal elliptic curves [duplicate]

Suppose we use elgamal elliptic curves for secure communication. Bob selects a prime $p$, an elliptic curve $E$, a point $\alpha$ on $E \pmod p$, and a secret integer $f$. Suppose that Bob has ...
0
votes
3answers
846 views

Is AES really used for Top Secret stuff?

I've read almost everywhere that AES-256 can be used for Top Secret material (in the US). Is it really used or is it some kind of decoy to hide the more advanced algorithm they might use ?
4
votes
1answer
579 views

Partial collisions for md5

Let $h$ be a bitstring and let $P(h, n)$ be the n-bit prefix of $h$. A partial collision of length $n$, for a hash function $H$ is a pair $(x,y)$, such that $P(H(x),n)=P(H(y),n)$. What is known about ...
0
votes
1answer
65 views

PKCS#11: Which CKK_types are supported by CKM_PKCS5_PBKD2?

On Solaris there's a function pkcs11_PasswdToKey() which is a wrapper of C_GenerateKey(). I made a simple test and it turned out ...
3
votes
2answers
371 views

Is there a way to do fair exchange between two parties who don't trust each other?

Let's suppose we have an Alice who knows a secret key A, and Bob who knows key B. Using their own keys, they each encrypt a message (Alice encrypts $m_A$, Bob encrypts $m_B$) with their own key, and ...
2
votes
1answer
301 views

What is a fair exchange scheme?

The Wikipedia entry Fair exchange has only two lines, and the only paper I can find describing such a protocol is extremely complicated. Is this a new field in cryptology or can someone give an ...
0
votes
1answer
78 views

Encrypting a broadcast channel

We're building a data service that is expected to go out through a single "fat pipe" channel. The design requirement says that the data in the channel must be protected by authentication and ...
6
votes
1answer
851 views

Using an MD5 hash as a password

Suppose Alice is using a password prompt that only accepts up to 32 characters for any particular password. Memorization of long strings of random characters is not one of Alice's strengths, so she ...
3
votes
1answer
271 views

Why is the discrete log problem easy when the exponent comes from a binomial distribution?

I read in http://epubs.surrey.ac.uk/7219/2/esorics06.pdf that in exponential El Gamal the discrete log problem for recovering $m$ from $g^m$ can be made tractable when $m$ is drawn from a binomial ...
4
votes
1answer
383 views

Trying to better understand the failure of the Index Calculus for ECDLP

So I'm going to give you guys my understanding and then if you would be so kind as to tell me where I'm off the mark (hopefully I'm not completely wrong). So basically the index calculus for the ...
3
votes
4answers
837 views

Increased CRC collision probability when adding bits to input message

The Scenario I have a message string I need to transport over a wireless network that may be unreliable. This message string is about 100 bits long, and is packaged with an 8-bit CRC. When the ...
1
vote
2answers
291 views

Why is the following RSA PRNG cryptographically secure?

One requirement states that the generator has to withstand the next-bit test. Consider the following PRNG, where we calculate next output $x_i$ via the formula $x_i = x_{i-1}^ e\mod n$. I can see ...
3
votes
3answers
2k views

Does encrypting twice using the same block cipher produce a security weakness?

If I use the output of a cipher, for example a block cipher such as AES and encrypt it again with the same algorithm, I read that this introduces weaknesses into the overall security of the system. ...
1
vote
1answer
270 views

Designing a security system from scratch with a custom protocol, suggestions needed

For the reasons I cannot go into details, I need to design my own protocol and overall security system which will be used to establish secure data exchange between various devices (most notably ...
5
votes
3answers
355 views

What kind of adversary is the cloud?

I just started some work on computation over encrypted data in the cloud. We're still in the early planning stages, and to really understand the kind of security we can offer I need to determine what ...
2
votes
2answers
460 views

Combining LFSRs for Stream Ciphers: Why do we need high non-linearity?

Linear Feedback Shift Registers (LFSRs) can be excellent (efficient, fast, and with good statistial properties) pseudo-random generators. Many stream ciphers are based on LFSRs and one of the possible ...
7
votes
2answers
312 views

Finding hash almost-collisions

A few months ago, XKCD posted a challenge to find a plaintext which hashed (using Skein 1024 1024) to a specified value. Inputs were scored based on the hamming distance between the hash of the ...
-2
votes
1answer
254 views

Cryptography based upon neural networks [closed]

I would like learn about use neural networks in cryptography. Is there any research literature on this subject? What should I do to learn about the subject? I am taking short course in neural ...
1
vote
1answer
323 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 ...
2
votes
2answers
132 views

Does knowledge of original file size provide a cryptographic weakness?

I'm encrypting files using an AES-256 CBC cipher and an SHA-256 HMAC every 64KB in the file. Since AES CBC needs an amount of input bytes to be divisible by 16, I use PKCS#7 padding to bring the ...
1
vote
1answer
141 views

Given a certain entrophy per character, how long should a passphrase be to guarantee key strength?

Given an entrophy of 1.5 bits per character, how long does a passphrase have the be to guarantee a key strength of a 128 bit random key? Assuming an alphabet of 26, one character takes log2(26)=4.7 ...
3
votes
1answer
954 views

Winzip AES256 vs PGP

If I use the AES256 option in Winzip to encrypt a file, is it any less safe or less secure than using pgp encryption?
2
votes
2answers
315 views

encrypt message with one digest 0-9 instead of 01-26

How much will the security of one-time pad decrease if I use one digit from 0-9 instead of 1-26? for example plaintext = "blue" and ...
3
votes
1answer
595 views

CBC-MAC , fixed length, all blocks returned

CBC-MAC, with fixed length message. Is it safe to return all ciphered blocks instead of the last? My intuition says it is less secure, since is gives an attacker more information. But how could one ...
3
votes
1answer
145 views

How to derive formulas for addition and multiplication in Jacobian coordinates

Is there a way to derive the formulas for point addition and multiplication on elliptic curves in Jacobian format by yourself? How could I have derived these formulas by myself?
4
votes
1answer
527 views

Recommended way of adding a pepper/secret key to password before hashing?

There have been several questions regarding password hashing here and on Security.SE. A "pepper" is sometimes mentioned – an application-specific secret key. The canonical answer on password hashing ...
0
votes
1answer
158 views

Weak One Way Function

Show The function $mult(a,b)$ is weak one way if the Integer Factorization Assumption is true. I am read the proof of this enunciated but I want know if I understand this in my way. The proof use the ...
2
votes
2answers
238 views

Many consecutive hashes to slow down brute force attack?

I've heard that hash algorithms like bcrypt are more secure because they take longer to complete, and therefore take much longer to prute force, without a noticable ...
4
votes
1answer
261 views

Injecting salt into PyCrypto KDF - useful?

I have noted some sources indicate that when using a KDF like PBKDF2 (alternatively) some advocate injecting the salt at the time of execution - like this: ...
0
votes
1answer
76 views

How to hash a structure that needs to include that hash's outcome?

so I am busy writing a TCP protocol in which I send a "packet header" with each packet of data. This packet header is a structure that contains information about the data that was sent. I want to ...
7
votes
3answers
3k views

What should be the size of a Diffie-Hellman private key?

I'm implementing the SRP-6 protocol, which relies on discrete logarithms for it's security (essentially Diffie-Hellman). The RFC documents state: The private values $a$ and $b$ SHOULD be at least ...
3
votes
1answer
182 views

Are digital signatures secure for signing lots of small messages?

I need to sign the numbers from 1 to 1 billion (literally this, it's not an analogy) using a digital signature and then send these numbers in a particular order to someone. The message is not private ...
1
vote
3answers
721 views

Implementing secret reconstruction in Shamir's Secret Sharing

I am trying to implement Shamir's secret sharing in C++. I have got the generation of shares working. However, I am very confused with the reconstruction of shares. I get the part on how three users ...
2
votes
1answer
212 views

Attacking AES/CCM with random IV

A cryptosystem is using AES-128 in CCM mode with random IV. Suppose an attacker capable of: asking the cryptosystem to encrypt a single plaintext as many times as he wants; and asking the ...
1
vote
1answer
187 views

Indistinguishability attack example

I want solve the next exercise. The author defined the experiment for the cryptosystem $\Pi$, the adversary $A$ and the security parameter $n$ as follows $\mathsf{PRIV_{EAV}}(\Pi,A,n)$ The ...
1
vote
2answers
355 views

Is there an algorithm for factoring N, which is just as simple as this one, but faster?

I found a simple algorithm for factoring semiprime numbers, you can read about it in Factoring Semiprimes and Possible Implications for RSA. It basically works like this: You reverse the digits in ...
1
vote
2answers
280 views

What's the difference between AES and earlier block ciphers?

Is there any difference except speed and key and block size from user's point of view? Or does AES have something that ideal block cipher doesn't have?
7
votes
2answers
394 views

How to derive two keys from one password

What is the best way to generate two independent symmetric keys from one user-entered password or passphrase? Would using both scrypt and pbkdf2 achieve this?
-1
votes
3answers
378 views

how much is secure to encrypt a password using itself (as encryption key) for storing it on a unsecure media?

I want to store a password securely on a storage (suppose in a usual unencrypted file). I use this password as AES key to encrypt itself (using AES-ECB) & then store it on the media (the password ...
1
vote
1answer
158 views

Elliptic Curve is DH function or PKI?

can we reuse same ECC key on TLS for long terms or it must be used just once? (i mean can we use ECC like RSA?) is there patent free ECC implementation ?
6
votes
2answers
480 views

Have I understood pedersen commitment correctly?

I want to do a one-sided integer commitment scheme. I.e. the whole process must be non-interactive where I at one point first publicly reveal some data and then at a later time reveal the committed ...
3
votes
1answer
340 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 ...
12
votes
2answers
1k views

What is the most practical fully homomorphic cryptosystem?

Craig Gentry recently gave the first fully homomorphic cryptosystem. Quite a bit of work has been done since extending his work. It seems, however, that no system is practical for real world use. ...
4
votes
4answers
353 views

Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash?

$H(s,r)=d$ is a function that hashes the secret string $s$ with a salt $r$, and returns a digest $d$. $r$ may be arbitrarily chosen and each $r$ returns a different $d$. For any $d$, $r$ is known from ...
2
votes
1answer
1k views

SHA256 HMAC brute force with chosen plaintext attacks

This is a follow up to Is It Possible To Reconstruct a Cryptographic Hash's Key I am using a SHA-256 HMAC function on a single-word input: sha256hmac(privatekey,word) = output. The private key length ...
2
votes
2answers
5k views

Decrypt digital signature using RSA public key with openssl

I have a digital signature that was created using the following algorithm: a SHA-256 hash of the body of the message is calculated. It is then signed using an RSA private key and the result is ...
4
votes
2answers
353 views

toy hash algorithm

I'm looking for a toy hash function, where the idea is to have high school students break (i.e. find a collision) a hash function by hand, in order to teach them how one way functions and hashing ...
2
votes
1answer
855 views

Encrypt a single file, chunk-by-chunk, each chunk using different key (AES)

Encrypt a single file, chunk-by-chunk, each chunk using different key. I am a security newbie (only took 2 security courses before) But currently I am using this encryption method for my Android ...

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