2
votes
3answers
504 views

reverse of md5sum

This might be out of ignorance, I apologize, but how complex of a problem might it be to generate a file of size N whose md5sum is X? For example, ...
2
votes
2answers
499 views

How exactly are GPG keys mathematically related, and prime numbers inside of GPG general

Before anyone suggest it, I've yet to pick up "Applied Cryptography" I'm planning on picking it up the next time I visit amazon. I know that the keys are somehow mathematically related, and I know ...
2
votes
2answers
193 views

Is there a way to systematically calculate the public exponent $e$ in RSA?

I'm learning RSA in one of my classes and we were given a problem: $p = 5$, $q = 11$ I have done the following steps: $n = 5 \cdot 11 = 55$ $\phi = (5-1)\cdot(11-1) = 40$ I know that to ...
2
votes
2answers
121 views

Is there an oblivious decryption scheme?

Alice has $K$; Bob has $E(K, m)$; Is there such a scheme that enables Alice decrypts $E(K, m)$ without knowing $m$, and Bob gets $m$ ?
2
votes
3answers
318 views

RSA assumption and cryptography

The RSA assumption states that it is hard to find $m$, given $c = m^e \bmod{n}$, $e$, and $n$ (for appropriate choice of $n,e$). Suppose that there exists an algorithm, $D(c,e,n)$, that finds $m$ in ...
2
votes
2answers
138 views

Can anyone tell me the name of this cipher please?

Can anyone tell me the name of this cipher please? I know it's a simple substitution cipher, I just don't know the name of it. Cipher Key: help Cipher alphabet: ...
2
votes
2answers
142 views

Why is it that researchers prefer finding secure software solutions over secure hardware solutions?

My understanding is crypto researchers tend to prefer solving a problem in software without the need for secure hardware. Why is this? What is the big problem with using secure hardware if the ...
2
votes
2answers
1k views

What happens when a root CA has its private key compromised?

What happens when a root CA has its private key compromised? Then all children in the tree are compromised too? And then all certificates are compromised? What needs to happen then? Related: - How ...
2
votes
3answers
373 views

Can ECB mode really leak some characters?

In an Ars Technica article about Adobe's use of 3DES in ECB mode, there's a paragraph The passwords, according to password expert Steve "Sc00bz" Thomas, are encrypted in a mode known as ECB. That ...
2
votes
3answers
426 views

What is the difference between a HMAC and a hash of data?

On a recent question it became apparent that there's a significant difference between an HMAC of input data and a hash of input data. What exactly is the difference between an HMAC and a hash of a ...
2
votes
3answers
7k views

Where i can find an AES implementation in python?

Where i can find a "reviewed" version of AES implementation in python. Not an API like PyCrypto whereby you can call AES algorithms in a single line.
2
votes
4answers
1k views

Is my pseudo-random initialization vector secure?

How can I know if I am generating a secure pseudorandom initialization vector? Currently I am planning to generate a pseudo-random initialization Vector using current date and time - is this secure ...
2
votes
3answers
169 views

How do I generate a number for a lottery and later proves its existence

I want to create a lottery that works like this: I choose a secret number A in the range [0:999] and publish an object B. People must try to guess the number A to win. When somebody wins, I want to ...
2
votes
3answers
203 views

Any weakness when sha256'ing output of sha256, repeatedly?

Is it significantly easier to reverse a sha256 hash if you know that the input was a 32 byte output of sha256? I assume for most outputs of sha256 there will be zero, one or two possible 32 byte ...
2
votes
2answers
208 views

In RSA, why does $p$ have to be bigger than $q$ where $n=p \times q$?

In openSSL – during RSA key generation – if $q$ is bigger than $p$, they exchange them. Why is that?
2
votes
2answers
217 views

Choice of MAC and handling it correctly

I have recently learned that Encrypt-then-MAC is recommended when using symmetric encryption. I have two closely related questions related to that, which is why I am posting them as one: Looking ...
2
votes
2answers
122 views

How can one parallelize tasks in CTR-AES for maximum performance?

From what I have read it seems like one of the purported benefits of using CTR mode AES, is that it can be parallelized to a greater degree, or maybe more easily than the other block cipher modes, ...
2
votes
3answers
491 views

Problems with using AES Key as IV in CBC-Mode

I'm a pentester and currently analysing a web application which are using some strange encryption scheme. The point is: They encrypt using AES-128, generate a (not cryptographic secure) key and use ...
2
votes
3answers
158 views

Frequency of letters change by the length of the texts?

In terms of the frequency of letters, how is it possible to have different frequent letters when the length of the text I'm analyzing is shorter? At the moment, I'm comparing the frequencies of a ...
2
votes
2answers
729 views

Why is a 2048-bit public RSA key represented by 540 hexadecimal characters in X.509 Certificates?

In the public certificate, an RSA public key specified as 2048 bits long is represented by 540 hexadecimal characters. Converted to base-2, this yields 2160 bits, 112 more than the stated 2048.
2
votes
2answers
243 views

How does this happen in RSA malleability?

I don't understand how the $E(m)$ turns into $E(mt)$. I mean, I don't know how does that transformation happen and how does the equation occur. $$E(m) \cdot t^e \bmod n = (mt)^e \bmod n = E(mt)$$ ...
2
votes
2answers
532 views

How does one deal with a negative “d” in RSA?

I'm trying to write my own RSA implementation using the textbook approach — which I know is not optimal — of picking primes $p$ and $q$, then computing Euler's totient $phi$, then randomly picking an ...
2
votes
1answer
2k views

What is the meaning of “trapdoor” in cryptography?

I do not really understand the meaning of a "trapdoor" in cryptography, so here are my questions: What is the meaning of trapdoor and how can I convert a word or string using a trapdoor in ...
2
votes
2answers
474 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 ...
2
votes
2answers
326 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 ...
2
votes
1answer
641 views

Can we replace the XOR operation in DES with some other operation?

Can we replace the XOR operation in the DES algorithm with some other operation? If so, does it work for both encryption and decryption?
2
votes
2answers
2k views

Is RSASSA-PKCS1-v1_5 a good signature scheme for new systems?

Is RSASSA-PKCS1-v1_5 a good signature scheme to recommend that people use in new systems? Is it believed to be secure and represent the state-of-the-art in RSA-based signatures? I understand that ...
2
votes
1answer
644 views

ABC Conjecture's Impact on RSA Encryption

A recent proof of the ABC Conjecture has been released by one Shinichi Mochizuki. Now, I'm not well versed in mathematics but it would appear that this proof implies that finding prime factors could ...
2
votes
2answers
299 views

Is the Blum Blum Shub PRNG suitable to create initialization vectors?

Is it a good idea to use a Blum Blum Shub Generator to create my initialization Vector (IV) in AES-OFB (Output Feedback Mode)?
2
votes
1answer
3k views

How can I convert a DER ECDSA signature to ASN.1?

I having trouble verifying an ECDSA signature signed using client side javascript with Java/BouncyCastle. The javascript signing function source: ...
2
votes
2answers
146 views

What, cryptographically speaking, is a “long message”?

I have read about long message attacks on some cryptographic hash functions. However, I don't quite understand what is being referred to as a "long message". Also, do long message attacks only apply ...
2
votes
2answers
212 views

Would xoring 2 independant AES CTRs to produce p-rand introduce vulnerabilities?

Assuming $$AES_{k_1}(C_1) \oplus AES_{k_2}(C_2)$$ where AES is used in CTR mode, $C$ is a 128 bit incrementing counter, keys and counters for both transforms are random as well as unique, and the ...
2
votes
3answers
209 views

How do I produce a stream of secure random numbers from AES-Counter mode?

I have just made my own program to encipher, using AES in counter mode, and have validated it using NIST data. So I know I have done it properly. I have read that AES-CTR can produce a stream of ...
2
votes
1answer
166 views

Is $H(x) = x^2 \bmod p$ pre-image resistant, second pre-image resistant and/or collision resistant

I have the function $H(x) = x^2\bmod p$ , where $p$ is a prime of length n bits and this function maps to the message $x$ to a n-bit hash value $H(x)$. I need to find out if it is pre-image ...
2
votes
2answers
626 views

Can a salt for a password hash be public?

For my understanding salts in password hashes are used to prevent the precomputation of $plaintext \to hashvalues$ (rainbow tables). I know from different threads, it is not necessary to keep the ...
2
votes
1answer
1k views

What is the difference between CSPRNG and PRNG?

Is there performance differential between them? For example: we use PRNG for key generation which is very expensive and CSPRNG for IV/nonce in block ciphers which is fast? What is the difference ...
2
votes
1answer
57 views

are ideal hashes possible to create?

In a hash function, you map an input of arbitrary length to an output of finite length such that the relationship is one-to-one (or at least that's what you are trying to achieve). Hence, isn't it ...
2
votes
2answers
103 views

Could someone reuse client certificates?

To my understanding, if a server “cert+key” (RSA) is compromised, than the SSL/TLS network traffic can be decrypted. What about a client “cert+key” (RSA)? Could someone reuse client certificates? ...
2
votes
2answers
317 views

what is the state of the art algorithm to encrypt credit card data

We have a business requirement to keep credit card data. What is today's (Nov 2013) state of the art algorithm to encrypt credit card data that will be saved on disk? Additionally, I'd appreciate ...
2
votes
2answers
467 views

Certificateless cryptography

While reading "Certificateless Public Key Cryptography" by Author Sattam S. Al-Riyami and Kenneth G. Paterson, they have considered generation of private keys by a Key Generation Center (KGC). If the ...
2
votes
2answers
326 views

Difference between collision resistance and target collision resistance

For a hash function, what's the difference between Collision Resistance and Target Collision Resistance?. I understand the definition of hash function collision resistance, but I don't know about ...
2
votes
1answer
182 views

How did they factor RSA-704?

I don't understand the 'Wiedemann algorithm' works. Can someone explain the factoring of RSA-704 in an easy way?
2
votes
2answers
374 views

Secure use-cases of block cipher with 64-bit block size

In what cases can we use a weak block cipher like DES ? More precisely, Are there specific situation in which a weaken block cipher can still be used, for instance for certain types of plaintext ?
2
votes
2answers
510 views

Why RSA uses {d,n} as private key instead of {e,n}?

While studying the RSA algorithm I referred to some books and some sites such as RSA (wikipedia) and all of them chose {d,n} as the secret (private) key and release {e,n} as the public key but as d ...
2
votes
4answers
702 views

random number generator 10-side dice alternative

A lot of sites (e.g. Dirk Rijmenants') refer to 10-side dice to generate random number for one-time-pad. I was thinking about new ways to generate random numbers, letters and passwords. here what I ...
2
votes
2answers
877 views

Keys required for cryptography

I am reading an article on cryptography at following location: http://www.entrust.com/resources/pdf/cryptointro.pdf Historically, encryption systems used what is known as symmetric cryptography. ...
2
votes
2answers
663 views

Berlekamp-Massey algorithm: case when sequence length is less than double the length of the LFSR

Suppose that we have a sequence of $N$ digits which is produced by a Linear Feedback Shift Register (LFSR) and the shortest such LFSR is of length $L$. A very important tool in cryptanalysis of stream ...
2
votes
2answers
2k views

what kind of hash function can provide a short hash and be collision resistant?

When you try to connect via SSH, you see a signature which is short but I heard it is even stronger than sha256. It is perhaps stronger because it uses more rounds. Is there a hash function or a ...
2
votes
3answers
1k views

LFSR for small numbers with large periods

I want to generate a few random numbers using an LFSR. However, the LFSR output depends on the number of taps, so for a large period I use large (relative) number of bits. This causes the numbers to ...
2
votes
2answers
660 views

Benefit of combining classical substitution ciphers with modern cryptography

I know that a simple monoalphabetic substitution cipher is considered extremely weak, on account of linguistic frequency-analysis attacks. However, assume the following: cleartext is encrypted ...

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