Why does BCrypt no longer accept hashes?

Question

Last week I upgrade Fedora to the brand new 28 release, which came with a mongodb upgrade to 3.6. See How to repair mongodb service after an upgrade to Fedora 28? for how I managed to resolve my first problem which was that mongod would no longer start. Now I'm facing an other problem on the Rails application that use this same database.

This most probably is unrelated to the mongodb upgrade, but I thought it might worth providing that context and don't miss a solution for not providing enough of it.

So since the system upgrade any login attempt on this Rails project will fail with a BCrypt::Errors::InvalidHash in Devise::SessionsController#create error, raised at bcrypt (3.1.11) lib/bcrypt/password.rb:60:ininitialize'`. Analyzing further in a Rails console of the project, it seems any call to this method will fail:

> BCrypt::Password.create('TestPassword')
BCrypt::Errors::InvalidHash: invalid hash
from /home/psychoslave/.rbenv/versions/2.4.3/lib/ruby/gems/2.4.0/gems/bcrypt-3.1.11/lib/bcrypt/password.rb:60:in `initialize'

I tried to bundle uninstall/reinstall bcrypt, and even use the github repository version of the bcrypt gem instead, but it didn't change anything.

Looking at /home/psychoslave/.rbenv/versions/2.4.3/lib/ruby/gems/2.4.0/gems/bcrypt-3.1.11/lib/bcrypt/password.rb:60:ininitialize'`, the problem seems that the hash is not valid.

# Initializes a BCrypt::Password instance with the data from a stored hash.
def initialize(raw_hash)
  if valid_hash?(raw_hash)
    self.replace(raw_hash)
    @version, @cost, @salt, @checksum = split_hash(self)
  else
    raise Errors::InvalidHash.new("invalid hash")
  end
end

And the corresponding test is as follow:

  def valid_hash?(h)
    h =~ /^\$[0-9a-z]{2}\$[0-9]{2}\$[A-Za-z0-9\.\/]{53}$/
  end

The hash itself is created through BCrypt::Engine.hash_secret(secret, BCrypt::Engine.generate_salt(cost)), which in the platform I use call __bc_crypt(secret.to_s, salt), which seems to be calling bcrypt-3.1.11/ext/mri/bcrypt_ext.c.

More importantly, adding a binding.pry in the valid_hash? method, it's possible to see what the hash value returned for a call to BCrypt::Password.create('TestPassword'), it's actually a rather long string whose start seems usual, but end up with what is most likely misgenerated sequence:

"$2a$10$Eb1f8DSkGh4G1u5GicyTYujBk6SwFXKYCH.nqxapmBlqJ0eFYdX32\x00\x00\x00\x00\xD1F\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x00\x00\x00\x00T\xBD\x02\x00\x00\x00\x00\x00\xF1V\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x00\x00\x00\x00\xE2\xB0\x02\x00\x00\x00\x
00\x00AW\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x00\x00\x00\x00 \x04\x00\x00\x00\x00\x00\x00\x86\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\xB5\xF8\x0E\x00\x00\x00\x00\x00q\xD8\x01\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00…"

I can provide a dump of a whole hash if it might be of any interest (around 32Ko!).

Answer 1

Here is a circumvent solution which make rspec of bcrypt pass all tests successfully again.

This is really a uggly hack while waitting a proper solution, but does the job until then. Just change ~/.rbenv/versions/2.4.3/lib/ruby/gems/2.4.0/gems/bcrypt-3.1.11/lib/bcrypt/engine.rb (path to adapt, of course), line 51 from:

- __bc_crypt(secret.to_s, salt)
+ __bc_crypt(secret.to_s, salt).gsub(/(\n|\x00).*/, '')

That is, trunk the string starting at the first "\x00" or "\n" occurrence, if any.

Credit note: this version of the hack was proposed by Andrey Sitnik, and I replaced the one I proposed here independently, before discovering it.

After that, BCrypt::Password#create will function again:

> BCrypt::Password.create('TestPassword')
=> "$2a$10$YPRnQF3ZihXHpa9kSx7Mpu.j28PlbdwaNs2umSQvAGkS.JJ.syGye"

Answer 2

I had this issue with a (very) old application and BCrypt 3.1.10. Upgrading to 3.1.12 resolved the issue. :)