I thought sha256 is impossible to crack (so far). But I saw a website, that could decrypt my hash. So is this because they store known hastes in a database or do they really decrypt hashes? Here's an image:
You are correct, the hash functions
SHA512 are (currently) unbroken, so it is not possible to easily reverse them.
SHA1 are considered broken because researchers have found pairs of strings that produce the same hash value, but even these require weeks or months of computation.
(note: I don't like that the site uses "decrypt" because that's a word you use for encryption; you can't "decrypt" a hash)
What they are probably doing is a dictionary attack in which they have pre-computed the hashes for all common dictionary words. This would give them a lookup table which would take in a hash and return the word that produces it.
Just to give an idea of scale, there are 171,476 English words and a
SHA256 hash takes 32 bytes, so storing the
SHA256 hashes of all English words would only take about 5 mb.
Your hash wasn't cracked, it was pre-computed and put into a rainbow table.
Hashing is a one-way cryptographic function designed to be compared to and not decrypted like a value that has been encrypted.
So what this website has done is taken a bunch of strings, hashed them using various algorithms and stored the value of the hash. That way when you enter your hash, it compares that hash to values it's already stored to seemingly "reverse" the hash. This is why using a salt is so important while hashing.