Using PBKDF2 with HMAC-SHA256 for storing passwords

pbkdf2.rb

p ="password"
#according to PKCS#5, should be at least 8 bytes. Public information, can be stored along with the pwd.
s = OpenSSL::Random.random_bytes(16) 
c = 20000 # varies depending on how fast the system is, tweak until it takes "long enough"
digest = OpenSSL::Digest::SHA256.new
#should be >= the output size of the underlying hash function, but ">" doesn't improve security (says PKCS#5)
dk_len = digest.digest_length

#store the result for new passwords 
value = OpenSSL::PKCS5.pbkdf2_hmac(p, s, c, dk_len, digest)

#Comparison with existing passwords

stored = #...

# We shouldn't do this (http://codahale.com/a-lesson-in-timing-attacks/)
#if value == stored

# So we need a comparison that takes always the same amount of time
def eql_time_compare(a, b)
  unless a.size == b.size
    return false
  end 
  cmp = b.bytes.to_a
  result = 0
  a.bytes.each_with_index {|c, i|
    result |= c ^ cmp[i]
  }
  result == 0
end

return eql_time_compare(stored, value)