问题
I am currently using SHA256 with a salt to hash my passwords. Is it better to continue using SHA256 or should I change to SHA512?
回答1:
Switching to SHA512 will hardly make your website more secure. You should not write your own password hashing function. Instead, use an existing implementation.
SHA256 and SHA512 are message digests, they were never meant to be password-hashing (or key-derivation) functions. (Although a message digest could be used a building block for a KDF, such as in PBKDF2 with HMAC-SHA1.)
A password-hashing function should defend against dictionary attacks and rainbow tables. In order to defend against dictionary attacks, a password hashing scheme must include a work factor to make it as slow as is workable.
Currently, the best choice is probably Argon2. This family of password hashing functions won the Password Hashing Competition in 2015.
If Argon2 is not available, the only other standardized password-hashing or key-derivation function is PBKDF2, which is an oldish NIST standard. Other choices, if using a standard is not required, include bcrypt and scrypt.
Wikipedia has pages for these functions:
- https://en.wikipedia.org/wiki/Argon2
- https://en.wikipedia.org/wiki/Bcrypt
- https://en.wikipedia.org/wiki/Scrypt
- https://en.wikipedia.org/wiki/PBKDF2
回答2:
This has already been answered reasonably well, if you ask me: https://stackoverflow.com/questions/3897434/password-security-sha1-sha256-or-sha512
Jeff had an interesting post on hashing, too: http://www.codinghorror.com/blog/2012/04/speed-hashing.html
Note that SHA512 is a lot slower to compute than SHA256. In the context of secure hashing, this is an asset. Slower to compute hashes mean it takes more compute time to crack, so if you can afford the compute cost SHA512 will be more secure for this reason.
回答3:
SHA512 may be significantly faster when calculated on most 64-bit processors as SHA256ses 32-bit math, an operation that is often slightly slower.
回答4:
SHA256 is still NIST Approved, but it would be good to change to SHA512, or bcrypt, if you can.
The list of NIST approved hash functions, at time of writing, is: SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, SHA-512/256, and SHA3-224, SHA3-256, SHA3-384, and SHA3-512, SHAKE128 and SHAKE256.
See https://csrc.nist.gov/projects/hash-functions
Depending on what operating system you are running, you probably don't have access to the SHA3 or SHAKE hash functions.
Many people prefer bcrypt to SHA512, but bcrypt is also only available on some operating systems.
SHA512 will be available on your system, or if not, you probably have such an old system that choice of hashing algorithm is the least of your problems.
One reason commonly given for preferring bcrypt is that bcrypt is tuneable - you can increase the number of rounds (work factor) to increase the time it takes to crack bcrypt hashes.
But SHA256 and SHA512 are also tuneable. While the default is 5000 rounds, you can specify more if you wish. 500000 takes my current pc about 0.45 seconds to calculate, which feels tolerable.
e.g.:
password required pam_unix.so sha512 shadow rounds=500000 ...
The reason to change from SHA256 to SHA512 is that SHA256 needs a lot more rounds to be as secure as SHA512, so while it's not insecure, it's less secure.
See, for example: https://medium.com/@davidtstrauss/stop-using-sha-256-6adbb55c608
Crypto changes quickly, so any answer you get might be proved wrong tomorrow, but current state of the art is that while bcrypt is possibly better than SHA512, SHA512 is fine.
If SHA512 is what you have available 'out of the box', use it (not SHA256), and don't worry about bcrypt or any of the SHA3 family until they become standard for your distribution.
As an aside, the current top rated answer has a number of claims that are either wrong or misleading.
"Switching to SHA512 will hardly make your website more secure."
This is misleading. Switching to SHA512 will make your site slightly more secure. SHA256 isn't as good as SHA512, but it isn't dreadful either. There's nothing that is clearly better than SHA512 that is likely to be available on your system yet. Bcrypt might be better, but this isn't clear, and bcrypt isn't available on a lot of systems. The SHA3 family is probably better, but it isn't widely available either.
"SHA256 and SHA512 were never meant to be password-hashing"
This is wrong. Both SHA256 and SHA512 are approved NIST hash algorithms.
"to defend against dictionary attacks, a password hashing scheme must include a work factor to make it as slow as is workable."
This is wrong. A high work factor will protect against brute force hash cracking, but not against a dictionary attack. There is no work factor that is low enough to be usable but high enough to protect against a dictionary attack. If your password is a dictionary word, it will fall to a dictionary attack. The protection against a dictionary attack to not use passwords that can be found in dictionaries.
On my current PC, the limit on rounds seems to be 10 million, which produces a delay of 8.74 seconds for each password entered. That's long enough to be extremely painful, longer than you'd want to use. It's long enough to prevent a brute force attack - but a determined adversary with a good cracking rig and a bit of patience could still iterate through a dictionary if they wanted to.
"A password-hashing function should defend against ... rainbow tables"
This is, at best, misleading. The defence against rainbow tables is to make sure that each password has their own 'salt'. That's pretty much standard practice these days, and it happens before the hash function is called. (Salting means adding a random string to the password before hashing it. The salt is stored with the password, so it's not a secret, but it does mean that even if a user picks a well-known password, the attacker can't just recognise that {this hash} belongs to {that password}, they still need to crack the hash.)
"Currently, the best choice is probably Argon2. This family of password hashing functions won the Password Hashing Competition in 2015."
This is unclear. Any 'new' cryptographic function can have unobvious ways of being broken, which is why most people prefer functions that have been widely used. Besides which, Argon2 is probably not available to you.
"Other choices, if using a standard is not required, include bcrypt and scrypt."
This is unclear. At one point, scrypt was seen as a better bcrypt. However, for various reasons, sentiment has moved away from scrypt towards bcrypt. See, for example: https://blog.ircmaxell.com/2014/03/why-i-dont-recommend-scrypt.html
To repeat, at this point in time, SHA512 appears to be a good choice and so does bcrypt.
SHA512 is NIST approved and bcrypt is not.
SHA512 will almost certainly be available on your system. Bcrypt may or may not be.
If both are on your system, I'd probably recommend bcrypt, but it's a close call. Either is fine.
来源：https://stackoverflow.com/questions/11624372/best-practice-for-hashing-passwords-sha256-or-sha512