feat: ex a2d

.gitignore

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 *.pdf
+sheet01/a2/Hash.java
+*.class

sheet01/a1/overview.md

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+## java.util.Random
+
+a) Numbers are generated using a linear congruential generator. They are pseudorandom and the algorithm accepts a 48-bit seed.
+
+b) `java.util.Random` produces uniformly distributed pseudorandom numbers.
+
+c) No, in the documentation `SecureRandom` is recommended as a good alternative for cryptographic use-cases.
+
+## java.lang.Math.random()
+
+java.lang.Math.random() is a wrapper for java.util.Random (therefore same as above).
+
+## java.security.SecureRandom
+
+a) A unpredictable source of randomness should be used, but the algorithm in use can vary since `SecureRandom.getInstance("algorithm")` exists. Multiple ones are available.
+
+b) The distribution of numbers is completely random and unpredictable.
+
+c) Yes, hence the name, `SecureRandom`.
+
+## /dev/random
+
+a) The random number generator gathers environmental noise from device drivers and other sources into an entropy pool. It keeps an estimate of the number of bits of noise in the entropy pool. From this entropy pool, random numbers are created.
+
+b) The distribution of numbers is completely random and unpredictable.
+
+c) Yes, all use cases are fine.
+
+## /dev/urandom
+
+Preferred to random except for applications that require randomness at early boot time since it is not available there. The rest is the same as for `dev/random`.

sheet01/a2/b-sha2.txt

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+SHA256 (plaintext.txt) = 95f3afed37754b0aaec0c57bea62389621f6ce2a299f64b849acd43651e3668c

sheet01/a2/b-sha3.txt

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+SHA3-256 (plaintext.txt) = 8cdcfd9d0414adbba1bb146c214e181d7e5d7c8edde90b79dcb27df005fcc298

sheet01/a2/b.txt

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+cksum -a sha2 --length 256 plaintext.txt > b-sha2.txt
+reads the plaintext file and calculates a unique hash string using the SHA2-256 cryptographic algorithm and writes it into b-sha2.txt
+cksum -a sha3 --length 256 plaintext.txt > b-sha3.txt
+reads the plaintext file and calculates a unique hash string using the SHA3-256 cryptographic algorithm and writes it into b-sha3.txt

sheet01/a2/c-hash.txt

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+03087115

sheet01/a2/c.txt

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+javac Hash.java  (Compiles the java class)
+java Hash plaintext.txt  (runs the custom Hash function on the text file)
+    Output: 03087115

sheet01/a2/d.txt

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+The custom hash is adding the unicode values of each character, the amount of rows and the amount of characters each multiplied with a hex value.
+This leads to it giving the same output when switching around rows, or even putting all characters into one line and adding the required amount of empty lines behind that. 
+
+
+"""
+LJVeuiolknitaaornrdia
+
+
+"""
+
+is producing the same hash as
+
+"""
+Leonard
+Viktoria
+Julian
+"""
+
+and the same as
+
+"""
+Julian
+Leonard
+Viktoria
+"""
+
+It has a good enough "first pre-image resistance" since it is not really possible to find the correct characters in the same order from the hash value that give the same original file
+The "second pre-image resistance" is weak since you can easily calculate a different file which gives the same output.
+It has no collision resistance since you can just switch around characters and get the same hash.

sheet01/a2/plaintext.txt

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+Julian
+Leonard
+Viktoria