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74 votes

Password cracking: What if attacker is lucky?

"lucky" is not a property of the attacker. There's no "lucky" attacker nor "normal" attacker. They both have the same probability (low, very low) to guess the key. You can decrease the probability at ...
Gianluca Ghettini's user avatar
42 votes

Password cracking: What if attacker is lucky?

Note: This answer assumes that by "lucky" OP meant "able to remove X% of valid answers", because I believe that was intent. Of course you can't measure luck ;) And if he is very lucky, say 90% ...
axapaxa's user avatar
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34 votes
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Can Shannon's entropy be used to determine if a file is encrypted?

You are likely going to have both false positives and false negatives if you try to use Shannon entropy for this. Many compressed files would have close to 8 bits of entropy per byte, resulting in ...
otus's user avatar
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32 votes
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A website that identifies an RNG from its output

A colleague of mine told me about a website that, given a sufficient quantity of output from an PRNG, had been able to deduce which application the PRNG was from. As you correctly identified this ...
SEJPM's user avatar
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29 votes

A website that identifies an RNG from its output

One tool that tries to do this is untwister. It's almost certainly not the tool you were thinking of, though, as it cannot determine if the output came from OpenSSL specifically. It can determine ...
ChrisInEdmonton's user avatar
25 votes
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Password cracking: What if attacker is lucky?

So if at each bit he has a 50% chance, that means that 1 bit is actually only half bit. And if he is very lucky, say 90% chance, that means that 1 bit is actually only 0.1 bit.So in face of a very ...
Chris Dodd's user avatar
24 votes
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How hard is it to guess a 8 digit PIN?

If a computer is doing the selection of PIN numbers, then you would be very lucky indeed to guess a PIN in three times. The entropy - assuming that all numbers are valid - is of course $\log_2{10^8} \...
Maarten Bodewes's user avatar
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23 votes
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Calculating entropy within xkcd 936: Password Strength

I don't get nearly the amount of entropy stated in the comic. Interestingly enough the reasoning for the entropy rating are actually justified in the comic by the little boxes which each represent 1 ...
SEJPM's user avatar
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19 votes
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Where do Windows applications get entropy from?

Update: Since I wrote this post, CryptGenRandom has been deprecated. Apparently it is now recommended to use BCryptGenRandom ...
otus's user avatar
  • 32.2k
19 votes

Relation between entropy and min-entropy

There is min-entropy, Shannon entropy, and max entropy. (Plus a few more definitions, but let's only focus on these.) All of these measures are greatest, for a given number of outcomes, when each ...
Future Security's user avatar
17 votes
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XOR a set of random numbers

Assuming that $b = 2^k-1$ for some positive integer $k$, XORing two (or more) numbers in the range $[0,b]$ will indeed yield a number in the same range. If the numbers are random, uniformly ...
Ilmari Karonen's user avatar
16 votes
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Using 32 hexadecimal digits vs equivalent 16-character string password

They are both equal. Passphrase security is based on the amount of entropy that the passphrase contains. In your case, both of your pieces of data are only different in the encoding. The actual ...
dusk's user avatar
  • 1,175
15 votes

Password cracking: What if attacker is lucky?

I'm not sure what you're trying to understand and if the other answers cover it, so I'm trying a different approach and interpret your question like this: What if an attacker guesses the right ...
David Foerster's user avatar
14 votes
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Password entropy much lower than entropy of encryption keys. Why is this acceptable?

I know that humans would find it impossible to maintain a 128 bit password -- however, I wonder if there is some technical reason why a 52 bit password would not be as weak as a 52-bit encryption key ...
otus's user avatar
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13 votes
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Extracting randomness from highly-biased RNG

If the source as 3 entropy bits per 1024 source bits, constructing a 128-bit seed requires $\lceil128\cdot 1024/3\rceil=43691$ source bits at least. The "XOR 350 bits with jump 128 bits" translates ...
fgrieu's user avatar
  • 142k
13 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Following my comment, and even though it's a bit different from what you ask: I really enjoyed the story here where the use of an incorrect pseudorandom number generator led to the arrest of members ...
Geoffroy Couteau's user avatar
12 votes
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How is the quality of a password calculated and what does it mean?

How much entropy is enough? For a password, something around truly 96-bits of entropy is enough. After all password usually go through some slow password hashing which increases the work load for ...
SEJPM's user avatar
  • 46.1k
12 votes
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How much entropy do you need for a secure certificate generation?

The answer to "how much entropy" is always "128 bits". The tricky point is that the term "entropy" is very often misused. In general terms, the situation is the following: A computer is a ...
Thomas Pornin's user avatar
12 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Actually, bank and expenses fraud is identified for the very opposite reason. Many of human societal numbers begin with a small digit, with number 1 appearing as the leading significant digit about 30%...
Paul Uszak's user avatar
  • 15.6k
12 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Since you mentioned you're also interested in crypto examples, here's one from probably the most famous cipher machine in history: Enigma. Most of Enigma's key settings were distributed in a keysheet ...
Robert Williams's user avatar
12 votes
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Would sending audio fragments over a phone call be considered a form of cryptology?

According to Wikipedia, cryptography, or cryptology, is “the practice and study of techniques for secure communication in the presence of adversarial behavior.” With that definition in mind, this ...
bk2204's user avatar
  • 3,476
11 votes

What are the dangers of using CPU clock drift for generating random data?

A TRNG is never used instead of a CSPRNG. They serve different purposes. A TRNG is used to seed a CSPRNG. A CSPRNG alone isn't enough to generate random data since it's reproducible. A hardware ...
Gilles 'SO- stop being evil''s user avatar
11 votes
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Entropy of a counter

So is it correct that a non-repeating 256-bit counter has 256-bits of entropy, because the numbers are equally distributed in their appearance? Not necessarily. Entropy can be viewed as a ...
poncho's user avatar
  • 148k
10 votes

If a SHA256 hash with high entropy is then hashed with one made from low entropy, is the resulting hash higher/same/lower entropy?

A deterministic function $f(X)$ of a random variable $X$ never has greater entropy than the original random variable $X$. (Side note: If you're not familiar with the technical terms random variable ...
Squeamish Ossifrage's user avatar
10 votes
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Consuming of entropy

Why generating random consumes an entropy? For the same reason that to generate true random bits using coin throws, one needs to throw a coin for each bit generated. Any attempt to use less throws is ...
fgrieu's user avatar
  • 142k
10 votes
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How to analyze the off-by-one error's impact on the entropy of the result?

First of all the first Entropy calculation is not correct. Recall the definition of (Shannon) entropy: $$H(X) = -\sum_{i=1}^n {\mathrm{P}(x_i) \log_{\,b} \mathrm{P}(x_i)}$$ If uniformly chosen a ...
kelalaka's user avatar
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10 votes
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Practical sources of entropy on an Android device?

Just use SecureRandom and let the OS take care of it.
Conrado's user avatar
  • 6,484
9 votes

Password cracking: What if attacker is lucky?

So if at each bit he has a 50% chance, that means that 1 bit is actually only half bit. No, it is not. Because 'bit' is a quantity of information we should know to reduce entropy twice by definition. ...
Enr1g's user avatar
  • 198
9 votes

Tests on /dev/urandom entropy can be considered useful or not?

Bear in mind that entropy is measured against a source using a model. It is unlikely you will find any model or attack against /dev/urandom that worked purely by ...
Neil Slater's user avatar
  • 1,119
9 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Allan Franklin wrote about this in "The Mendel-Fisher Controversy." As practical tests of results against theoretical distributions became better, Sir R. A. Fisher noted that Mendel's ...
ttw's user avatar
  • 191

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