1
vote
1answer
220 views

Key sizes for discrete logarithm based methods

I have a question regarding the key generation process of methods that are based on the discrete logarithm problem. This site gives some good insights, but I don't fully grasp it I think: ...
2
votes
1answer
145 views

Isn't the structure of a potential plaintext of a ciphertext generated by a one-time pad dispositive, cryptanalysis-wise?

That is, if I generate every possible result, and analyze the structure against, for example, a dictionary, won't that identify which of the solutions in the domain is most likely the correct one? I ...
2
votes
3answers
237 views

Requiring a “supervisor” key pair and a “user” key pair to decrypt multiple-recipient messages

I've been toying with some encryption scenarios recently. One of the hard ones I came across is a multi-party system. So we have Bob -- The person who sends the message (and knows it's recipients) ...
3
votes
1answer
449 views

Hill-cipher, disordered alphabet

I am going to apply a simple substitution cipher to my input, then encrypt the result with a Hill cipher. How can this be broken, in a chosen-plaintext threat model? In other words, instead of the ...
-1
votes
2answers
187 views

Perfect Secrecy, two Definitions

I'm reading the proof of the implication "Def 2.1 $\Rightarrow$ Def 2.4" in these slides about Adversarial Indistinguishability and Perfectly-Secret Encryption. I have a doubt in the slide 10. Here it ...
1
vote
1answer
139 views

Do I need to keep a 64-bit version number secret?

Assume the following structure: version | iv | ciphertext | HMAC(version | iv | ciphertext | ..., key) The version starts out ...
3
votes
1answer
314 views

How much data can I encrypt with AES before I need to change the key in CBC mode?

In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of $1/2^{32}$, we have to change the key after $2^{48}$ blocks are encrypted. It seems that ...
2
votes
2answers
363 views

What are the potential security impacts of using CRAM-MD5 for Emails, when not using an SSL connection?

Background: My current server-provider tells me it's no problem to store the passwords in plain-text in the database, saying he has to do so because they use CRAM-MD5 for email authentication. But ...
11
votes
1answer
602 views

Is Wikipedia's table about SHA-2 collisions correct?

I was looking a Wikipedia article on SHA-2, and the "Comparison of SHA functions" table seems to indicate that SHA-2 is less secure than SHA-1. Is this true, or is the table wrong / misleading? ...
0
votes
1answer
132 views

What key length is required to keep simple keyed “hash” secure?

In a previous question, I described a particular keyed "hash" that mapped a 5-digit input code into a 5-digit output code. It used a 8-bit key which is very insecure - more than 99% of the time, you ...
0
votes
2answers
144 views

Physical Level Encryption

What types of algorithms that are capable of signing a message are out there that run on a physical level, e.g. lacking the infrastructure of a standard PC, no memory, processor or motherboard in the ...
2
votes
1answer
144 views

Is this algorithm secure?

I recently found this site, proposing a hashing algorithm for passwords. They describe the following: pad the password on both sides with SHA1(email) to ...
2
votes
2answers
657 views

Is Base64(SHA1(GUID)) still unique like the original GUID?

Basically what the title is; GUIDs are unique by design. If you run the GUID through SHA1 and then Base64 the hash, will the resulting string have the same guaranteed uniqueness as the GUID, or not?
-1
votes
2answers
308 views

Why this k parameter is in unary in adversary PPT algorithm? [duplicate]

While reading some text on cryptography, I found that algorithm $A$, that the adversary (called Eve by convention) runs to break the cipher message, needs two parameters, candidate message ($y$) and ...
3
votes
1answer
253 views

Alternatives to HMAC + CBC?

I'm looking at using HMAC + CBC. The combination looks like this: ciphertext = AES256(text, k1) data = HMAC-SHA256(iv | ciphertext, k2) | iv | ciphertext Where: ...
2
votes
1answer
389 views

How does OAEP improve the security of RSA?

The heart of OAEP algorithm used for RSA encryption are the cryptographic hash functions $H$ and $G$. Does everybody (so also an adversary) know these functions? If YES: How does it help the ...
1
vote
1answer
145 views

Is this a sensible cryptographic protocol intending to reduce the impact of compromised security?

I'm in the process of designing a cryptographic protocol which will reduce the impact an attacker will have if they gained root access to a server storing data. The basic crux of it is that keys ...
1
vote
2answers
504 views

Generate fixed length cipher text from arbitrary length plaintext

Using an encryption algorithm like AES, is it possible to generate a fixed length cipher text no matter how long the plain text becomes?
2
votes
2answers
284 views

Generating IV in TLS 1.2

I'm writing a TLS 1.2 implementation (in C#) using a custom protocol. My question is how to generate the IV attached to each message, I don't want to use RandonNumberGenerator because I've heard it ...
2
votes
2answers
342 views

SHA-1:Is there any mathematical result that gives us the minimum number of 1's in a 160-bit SHA-1 hash output?

Is there any mathematical result that gives us the minimum number of 1's in a 160-bit SHA-1 hash output? What is the probability that a 160-bit SHA-1 hash output contains at least 128 1's?
3
votes
1answer
290 views

Understanding Feldman's VSS with a simple example

I'm trying to understand Feldman's VSS Scheme. The basic idea of that scheme is that one uses Shamir secret sharing to share a secret and commitments of the coefficients of the polynomial to allow the ...
2
votes
2answers
170 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
763 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
41 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 ...
1
vote
3answers
711 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 ?
3
votes
1answer
496 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
54 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
322 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
199 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
73 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
734 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
229 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
347 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
714 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
255 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 ...
2
votes
3answers
1k 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
248 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 ...
6
votes
3answers
336 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
355 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
281 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
202 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
300 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
129 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
139 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
782 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
235 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
464 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 ...
2
votes
1answer
126 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?
3
votes
1answer
439 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
137 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 ...

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