问题
Update:
SHA-512 is one-way so I will need to not attempt to crack passwords because it is easier to simply reset a password. If someone is aware of getting the original password from a SHA-512 hashed text, please let me know. Otherwise, I am moving on from this question. Thank you for all of your answers.
Original Question:
I have read a lot of articles that state that SHA-512 hashing cannot be unhashed. However, there is a source code for the various SHA-1 + algorithms here: https://tls.mbed.org/sha-512-source-code.
I would like to know if it is possible to reverse this coding, in a way, to decrypt SHA-512 hashed text. Linux encrypts their passwords with SHA-512 hashing. As a systems administrator, I would prefer to simply decrypt or unhash this information as needed, rather than guessing whether a password is correct or incorrect and see if the hash matches. Creating new passwords can cause a lot of extra time and money. If you do not feel comfortable publishing this information and would like to discuss it privately, feel free to request my contact information.
Thank you!
回答1:
Why do you not believe what you have read?
Cryptographic hash functions can not be reversed.
Thought experiment: You have 200 bytes you pass to SHA512, out come 64 bytes. Something has been lost. How do you regain what is lost?
In a similar manner if you have an integer, say 123, and mod by 10 the result would be 3. Now reverse that–oh it could have been and of 3, 13, 23, 33, 123, 9343453, *3.
回答2:
I have read a lot of articles that state that SHA-512 hashing cannot be unhashed.
Yes. That is the definition of "hash". This has nothing to do with SHA-512. The definition of a hash function is that it cannot be reversed. Period. If it can be reversed, it's not a hash.
I would like to know if it is possible to reverse this coding, in a way, to decrypt SHA-512 hashed text.
No, you can't decrypt it, because it isn't encrypted, it's hashed.
Linux encrypts their passwords with SHA-512 hashing.
No, it doesn't. It hashes them, it doesn't encrypt them.
As a systems administrator, I would prefer to simply decrypt or unhash this information as needed, rather than guessing whether a password is correct or incorrect and see if the hash matches. Creating new passwords can cause a lot of extra time and money. If you do not feel comfortable publishing this information and would like to discuss it privately, feel free to request my contact information.
As a systems administrator, if you don't understand the difference between encryption and hashing, please tell me where you work, so that I never ever accidentally become of customer of yours! The Pigeonhole Principle is so simple and obvious that it can be understood by a child.
回答3:
The articles you have read are correct.
However, if for example, a user uses a dictionary word, and you aren't salting your hashes, then those circumstances are open to dictionary attacks. Which is why no-one worth their salt, pun intended would use a hash algorithm without a salt.
Frankly I find it unlikely a systems administrator would need to get a password, as generally they have impersonation rights.
来源：https://stackoverflow.com/questions/35899323/how-to-decrypt-sha-512-hashed-data