What comes first, the salt or the hash?

Question

Okay, I know this is probably dead simple, but I can't seem to find a straight answer anywhere. Let's say I have the following:

Password: "mypassword"
Salt: 1234567

Is the idea of salting to do something like hash(password + salt) or hash(password) + salt? My guess is that only the former makes any sense at all, but I just want to make sure I'm not missing something.

Please forgive my ignorance.

score 21 · Accepted Answer · answered Apr 22 '11 at 02:49

21

You've got it, it's the former.

If you just concatenated the salt and the hash, then an attacker can simply remove the "salt" and use a rainbow table. By hashing the plaintext + salt, the salt cannot be factored out.

answered Apr 22 '11 at 02:49

John Cromartie

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leonbloy · Answer 2 · 2011-04-28T17:56:11.923

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Actually, it's salt + hash(salt+password) (Salt is part of the hash computation - but you must also keep it in the clear)

edited Apr 28 '11 at 17:56

answered Apr 22 '11 at 02:51

leonbloy

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why do you need the first salt? Wouldn't it be pointless as a hacker would just ignore it? – Diskdrive Apr 29 '11 at 06:19
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You need to store it (in the clear) because without it you won't be able to check the password. – leonbloy Apr 29 '11 at 14:04

score 8 · Answer 3 · answered Apr 22 '11 at 02:50

hash(password + salt). If you concatenate the salt after the hashing, the concatenation is easily reversible and doesn't add any difficulty in reversing the hash on the password (with rainbow tables).

That said, some systems do both, e.g. Django stores salt$hash(salt+password) in database. This is simply so that every piece of data needed to check the password against the hash is available in one place.

score 3 · Answer 4 · edited May 23 '17 at 12:02

I'm adding these links into this question for completeness - this topic of salty hashing requires a broad understanding of the topics feeding into it to avoid costly mistakes.

A key point not quite expressed in the answers here is the necessity of using a unique salt for each password. For details on why, read the linked items.

Why make each salt unique : Salting Your Password: Best Practices?

Broad overview on the topic : Salt Generation and open source software

score 0 · Answer 5 · answered Apr 22 '11 at 02:49

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The salt should be part of the hash calculation

answered Apr 22 '11 at 02:49

Robert

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What comes first, the salt or the hash?

5 Answers5