[Cryptography] encrypting bcrypt hashes

[Ray Dillinger]

On 03/08/2017 03:13 PM, Mark Steward wrote:

> For a decent chance at unique salts, a good rule of thumb is the square of
> the expected number of hashes. So if they have a million customers, use a
> 40-bit salt or longer. This won't protect against someone trying 1234
> against all salts in the whole DB and knowing 10% will match, but it
> maximises the work required to do that.

If it's down to chance whether two different customers get the same
salt, this is exactly true. This is the 'birthday paradox' in action,
and a number of widely-used methods for generating salts (hashing the
customer's name and/or account number for example) provoke exactly this
problem.

But making sure that customers have unique salts is in principle no
harder than assigning them unique account numbers.  There's nothing
secret about a salt, it doesn't need to be hard to guess.

				Bear

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