I have found out, how I can convert plain text into an SHA (http://hash.online-convert.com/sha512-generator), but how can I convert a SHA key to plain text?
SHA-1, SHA-256, SHA-512 and all the other SHA functions are cryptographic hash functions. One of the defining properties of cryptographic hash functions is preimage resistance: given a cryptographic hash function F and a value h, it is infeasible to find a text m such that F(m) = h. Note that hashing is not encryption: with encryption, you can find the original if you find the decryption key, but with hashing, you can't find the original except by guessing, period.
If you have the hash of a text, the only ways to find the text are:
- Make an exhaustive search. If you take all the computers existing today and devote them to this task, this will take about 100 quintillion times the age of the universe for SHA-1, and much, much longer for SHA-512. Bring a book.
- Make a fundamental breakthrough in cryptography. This is theoretically possible in that nobody has been able to prove that any of the SHA-* family are actually cryptographic hash functions, we just believe they are because professional cryptographers have tried to break them for years and failed. Publish your technique, you'll be famous.
- Guess the text. It's easy to verify each guess. Be prepared to go through a lot of wrong guesses. Depending on the length and complexity (more precisely, on the entropy introduced by the method used to generate the text), this may range anywhere between quick (e.g. if you know it's a dictionary word) and infeasible (e.g. if it's a string of 50 random letters).
- Figure out what input was passed to the function by non-computational means, such as finding the person who submitted the text and hitting them with a wrench until they reveal the password, or digging through the server logs (if the text was logged somewhere).
It's simple. You can not. This is precisely why it's called Secure Hash Algorithm. You need the source that generates the SHA string to know what generates it. This is why it's used to verify data alteration/manipulation. Through there are ways to manipulate the resultant hash, you can not know the source that created that hash.