Safe passwords are a very important aspect of application security. Forcing users to change their passwords often results in using the same passwords repeatedly.
In this post, I will not discuss issues related to enforcing password changes. Instead, we will focus on the way you can check if the newly entered password is not the same as the one already stored in the database. At the outset, I would like to reassure all concerned. The passwords in the database should and must be stored in a hashed form!
Comeonin and Bcrypt
In Elixir, perhaps the most popular password management library is Comeonin. It allows us to use many hashing functions such as Pbkdf2, Argon2, and Bcrypt. Argon2 is assumed to be the strongest variant. In this post, I will focus on probably the most popular one: Bcrypt.
We want to compare the new password entered by the user with the one he has already set. Of course, we store the previous password in a hashed form. For example, storing the last five passwords may be a good practice.
Let’s start by understanding how a hashed password is structured.
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iex(1)> Bcrypt.hash_pwd_salt("password")
"$2b$12$wGy5rRdpE/fpInBo41a4iuSohGb0Yv5tAiK2h/.VCgFX5N/GRviuC"
iex(2)> Bcrypt.hash_pwd_salt("password")
"$2b$12$PMpwStb1MzNx4/o8V3n4.eIjB3hX5J67JeScriNKH52a3PTW3WybS"
As you can see, thanks to hash_pwd_salt/1, we get a different result each time.
It is due to the use of randomly generated salt.
However, we can see some patterns:
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|Algorithm Identifier | Cost | Salt | Hash
(Bcrypt)
$2b $12$ wGy5rRdpE/fpInBo41a4iu SohGb0Yv5tAiK2h/.VCgFX5N/GRviuC
The hash starts with the string $2b that identifies Bcrypt.
I am using the default settings, so the computational cost is 12.
Then we have 22 characters for the salt itself and 31 to store the password hash.
After all, we do not know the previous password’s explicit (plain) form. How can password reuse be checked based on the build?
Compare passwords
It is best to read the Elixir code to answer the above question.
We will use Bcrypt.Base.hash_password(password, salt) to check the password.
We can compare whether we get the same hashed password by indicating the salt from the previous password.
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defmodule MyApp.PasswordChecker do
# Length:
# Algorithm Identifier => 3
# Cost => 4
# Salt => 22
# In total = 29 first characters
@password_range 0..28
@doc """
Verify - the password is the same as the given hashed password.
Used Bcrypt internally.
## Example
iex> Bcrypt.hash_pwd_salt("password")
"$2b$12$wGy5rRdpE/fpInBo41a4iuSohGb0Yv5tAiK2h/.VCgFX5N/GRviuC"
iex> old_hashed_password = Bcrypt.hash_pwd_salt("the-old-one")
"$2b$12$LfVwaQwfIVSCfTPPjeuJpen4KS/RqnACq3GlChjuq7YB9aTUmwz8C"
iex> already_used?("the-old-one", old_hashed_password)
true
iex> already_used?("password", old_hashed_password)
false
"""
@spec already_used?(binary(), binary()) :: boolean()
def already_used?(new_password_text, old_hashed_password) do
salt_with_bcrypt_settings = String.slice(old_hashed_password, @password_range)
Bcrypt.Base.hash_password(new_password_text, salt_with_bcrypt_settings) == old_hashed_password
end
end