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Make bcrypt cost configurable #3677

will118 opened this issue Jul 17, 2018 · 3 comments


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commented Jul 17, 2018

Would it be possible to add something to ServerConfig to allow configuring the kBcryptCostFactor?

I feel fairly comfortable making the change if it's something you're interested in.

@adamcfraser adamcfraser added this to the 2.5.0 milestone Jul 19, 2018


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commented Jul 19, 2018

I agree - this is something we should add. We should also review whether the default should be adjusted based on typical modern processor speeds.


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commented Jul 19, 2018

@adamcfraser Go will occasionally increment the default cost factor in their releases based on current hardware, although having said that, the value has not changed from 10 in 7 years.

Incrementing this to 11 will double the time it takes, but there are no strong recommendations above 10 as of right now.


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commented Jul 20, 2018

We need to document the fact that password hashes will only utilise a new cost if an existing user changes their password, unless some logic is implemented to add a rolling cost upgrade... For example:

// An example using types/methods from user.go
user := userImpl{...}

ok := user.Authenticate(password)
if !ok {
    panic("login failed")

if bcrypt.Cost(user.PasswordHash_) < kBcryptCostFactor {
    // cost factor has since increased, so re-hash the given password

@adamcfraser adamcfraser added the backlog label Jul 23, 2018

@adamcfraser adamcfraser added ready and removed backlog labels Oct 1, 2018

@pasin pasin added in progress and removed ready labels Oct 9, 2018

@bbrks bbrks added the review label Oct 9, 2018

adamcfraser added a commit that referenced this issue Oct 10, 2018

#3677 configurable bcrypt cost (#3790)
* Make bcrypt cost configurable

* Rolling-upgrade

* Add unit test for disabled user Authenticate

* Make minimum bcrypt cost the current default

* Improve warning for error when rehashing password
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