Entropy is a measure of information content, which can be viewed as the uncertainty in a random selection.
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What is entropy?
We discuss a lot of topics and use measures of entropy to determine how difficult it is for an attacker to be successful. What does entropy mean in the context of cryptography? How is entropy ...
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How should I calculate the entropy of a password?
If part of the password is a whole regular English word, does the entropy of that part depend on the number of English words in existence, the number of English words known by the choosing algorithm, ...
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Tactics available to help prove security of a new system?
I believe that the accepted tactic to "prove" a system as secure is to allow the crypto-community to review it and if no vulnerabilities are found over a long period of time (5 or 6 years), then a new ...
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Should I use the first or last bits from a sha-256 hash?
I have the need for a hexadecimal token that is smaller than the normal length of the hexadecimal representation of a sha-256 hash.
Should I take the first bits or the last bits? Which of them ...
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Measuring entropy for a ciphertext only attack
When bruteforcing a password (e.g. the common attacks on DES), where you have ciphertext only, you need a way to assess whether a decrypted plaintext is the right one. I believe the EFF DES machine ...
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Why is ciphertext from low entropy plaintext not compressible?
This comes following a discussion with a colleague.
My plaintext file plain consists of a about 100,000 lines of "all work and
no play...". It's size is: 2.2 MB.
...
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Are there any authoritative definitions of “key stretching”?
This is mostly a terminology question, but I suppose that it is best asked and answered here. After browsing the Internet I have come across a fair number of completely different definitions of the ...
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Are picture files “random enough” to be usable as a one-time pad?
Say you have a picture with 1 megapixels taken at random and with $2^{24}$ possible colours per pixel (RGB-24). That image would be unique and the possible combinations $(2^{24})^{10^6}$ immense.
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Feedback on rolling my own entropy gatherer
First of all, I don't recommend doing this. This was something I created when I didn't know better and didn't have a solution available to me.
Long ago I created my own entropy gather for a ...
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Brute forcing CRC-32
I'm working on a cryptosystem which uses IDEA. The designer made the mistake of including a CRC-32B hash of the password unencrypted in the header, so that the system can quickly reject bad ...
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Exhausting the entropy of a hash function
In the case of password storage, consider the following:
I have an idea that one can exhaust the entropy of input to the MD5 function by using a 128 bit random value as the password (indeed, any hash ...
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Stretching a random seed to maximize entropy
I'm using a random number generator that requires me to pass it a big (several kilobytes) pool of random data for initialization.
I've gathered entropy from various system metrics (free memory, ...
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Using chi square for distinguishing between compressed and random data
I am trying to find some measurement for identifying and distinguishing between compressed and random data. I tried this first by computing the entropy of such data, the entropy value is extremely ...
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Is just a bit of random, “fully random”?
I understand that hashing a datetime is a pseudo-random number, and not good in a security context.
But what about hashing a datetime plus a truly-random but constant string (just a string that an ...
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Shannon entropy calculation: is $H(A|R·A) = H(A)$?
Suppose I generate a random $m×m$ matrix $R$, where each of its elements belongs to $\mathbb Z_n$. I ensure that $R$ is invertible in $\mathbb Z_n^{m×m}$.
Now I take a non-random $m×m$ ...
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Seed a PRNG with random data and a password
I'd like to combine a random key file with a password to generate a secure seed for a CSPRNG. The key file is assumed to have very high entropy, but the password will be whatever the user provides. ...
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Using an RSA private key simultanously as an AES encryption key to generate random numbers?
Currently I'm implementing a PRNG for an embedded system. Using the RFC 4086 I've decided to use the X9.17 - to be more specific the succesor X9.31- standard to implement my PRNG. X9.17 uses DES, but ...
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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 ...
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What happens to the entropy of a password when you hash it?
For example, if the entropy of a password is 30 bits, what is the entropy of the password when you hash it with MD5?
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How could block encryption in counter mode be secure from viewpoint of entropy?
A good block cipher e.g. AES running in counter mode is commonly considered to be secure. Assuming a perfectly random key, one could in this case from $128$ bits generate up to $2^{128}ยท128$ bits for ...
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Seeking special-use fingerprinting/hashing algorithm
For a project I wonder if there exists some kind of fixed-size checksumming/fingerprinting function in which based on this fingerprint given data block 1, it is easy to generate more data blocks that ...
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Entropy in natural language texts
I have a couple of related questions:
I read about an estimate, ascribed to Turing, of the entropy of English text to be between 0.6 and 1.3 bits per letter. The modern day corpus, including the huge ...
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Measuring Shannon's diffusion
Shannon's idea of diffusion is fundamental to cryptography.
Besides being a descriptive idea, is there any work on measuring or expressing it? Saying something like "System A has more diffusion than ...
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Entropy of system data - use all and hash, or trim least significant bits?
I'm working on a background entropy collector for key generation that monitors hardware and produces an entropy pool.
Here's my list of sources:
Mouse position
Keyboard timings (i.e. time between ...
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Perfect secrecy and change of plaintext probability distributions
How to prove that if a cryptosystem has perfect secrecy for a given plaintext probability distribution then it will have perfect secrecy for all other possible plaintext probability distributions?
Do ...
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password entropy calculation
I have the following password with a prescribed format: it starts with a fixed value of 111, followed by 2 random lower case letters and then lastly a random digit:
"111""2 Random lowercase ...
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Conditional entropy
Suppose I have a file with random binary strings of the same length in each line. If I m computing the conditional entropy H(Y|X) where Y is the variable string of fixed length l which is after X and ...
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Does a playlist of songs or movies mixed together contain enough random enough for OTP key material?
Ok hypothetical scenario. Lets say I want to generate a lot of key material quickly for a one-time pad. This key material will be divided up and used for many different messages in the future.
1) I ...
