The scrypt key derivation function is designed to be far more secure against hardware brute-force attacks than alternative functions such as PBKDF2 or bcrypt.

• http://www.tarsnap.com/scrypt.html
• http://github.com/pbhogan/scrypt

The designers of scrypt estimate that on modern (2009) hardware, if 5 seconds are spent computing a derived key, the cost of a hardware brute-force attack against scrypt is roughly 4000 times greater than the cost of a similar attack against bcrypt (to find the same password), and 20000 times greater than a similar attack against PBKDF2.

gem install scrypt

It works pretty similarly to ruby-bcrypt with a few minor differences, especially where the cost factor is concerned.

require "scrypt"

# hash a user's password
@password = SCrypt::Password.create("my grand secret")
# => "400$8$36$78f4ae6983f76119$37ec6ce55a2b928dc56ff9a7d0cdafbd7dbde49d9282c38a40b1434e88f24cf5"

# compare to strings
@password == "my grand secret" # => true
@password == "a paltry guess"  # => false

Password.create takes five options which will determine the key length and salt size, as well as the cost limits of the computation:

• :key_len specifies the length in bytes of the key you want to generate. The default is 32 bytes (256 bits). Minimum is 16 bytes (128 bits). Maximum is 512 bytes (4096 bits).
• :salt_size specifies the size in bytes of the random salt you want to generate. The default and minimum is 8 bytes (64 bits). Maximum is 32 bytes (256 bits).
• :max_time specifies the maximum number of seconds the computation should take.
• :max_mem specifies the maximum number of bytes the computation should take. A value of 0 specifies no upper limit. The minimum is always 1 MB.
• :max_memfrac specifies the maximum memory in a fraction of available resources to use. Any value equal to 0 or greater than 0.5 will result in 0.5 being used.

Default options will result in calculation time of approx. 200 ms with 1 MB memory use.

require "scrypt"

SCrypt::Engine.calibrate
# => "400$8$25$"

salt = SCrypt::Engine.generate_salt
# => "400$8$26$b62e0f787a5fc373"

SCrypt::Engine.hash_secret "my grand secret", salt
# => "400$8$26$b62e0f787a5fc373$0399ccd4fa26642d92741b17c366b7f6bd12ccea5214987af445d2bed97bc6a2"

# store it safely in the user model
@user.update_attribute(:password, @password)

# read it back later
@user.reload!
@password = SCrypt::Password.new(@user.password)
@password == "my grand secret" # => true