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Well it should be faster than storing a cryptographically secure hash. If hashing the data is too fast, an attacker could just brute force all of the passwords.

That's why you should always use salt with your hash.

Unsalted, your password hash =

Hash({{Your Password}})

The attacker can brute force it from a dictionary or stepping through characters (possibly from another breach somewhere, or a silly password like boobies123).

Salted hashes are way more secure:

Hash({{Your Password}} + {{Secret}})

Now the attacker has to guess an extra secret phrase, which is often really long, that was fed into the hash function along with the user's password. And due to the beauty of one way hash functions both salted and unsalted hashes use the same amount of bytes in the DB... It's a no brainer.

It's not enough to just use a global salt.

You need to use a random salt for each user, which is stored in the DB.

You also need to use an algorithm that takes a lot of time - SHA1 and the rest are designed to be fast, on purpose. Use bcrypt or something.

(FWIW, PHP has the best batteries-included password functions i've seen in a language. `password_hash` etc just do the right thing. Copy what they do and you'll be ok)

The reason to use a salt is mostly that an attacker doesn't then have a precomputed library of hash values.

Say, if someone uses the password 'gwpmkq' and a site uses plain MD5, they store cc733aac12981561dfc4944dd34a595f in their database. Now, even a stupid attacker can google for a hash search engine, input the hash and get the password in seconds.

On the other hand, with salting the value to be hashed could be something like 'luser@fail.com:@362#^h6329hgtew:gwpmkq'. That won't be precomputed anywhere.

Of course it's also a good idea to either try to keep the salt secret or use a per-user random salt (which you store in the database). But when someone gets a full database dump, chances are they'll also get the salt.

The point of per-user salts is to avoid common passwords from revealing themselves, so that an attacker can't limit his brute-force attempts to just the users with shared passwords that will be easy to break.

The specific attack that per-user random salts are designed to prevent are pre-computed rainbow tables. Brute-forcing MD5 is nearly as fast as using rainbow tables, so the benefits are possibly dubious.

Who uses MD5 for hashing password anymore?

People one step above those who think storing plain-text passwords is okay.

I don't mean faster computationally I mean faster in terms of actually developing the product. In the scheme of even the simplest MVPs the additional developer time to hash an input twice is effectively zero.

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