user.rb

# Because we have some old legacy users in the database, we need to
# override #has_secure_passwords' method for checking if a password is valid.
# We first ask if the password is valid, and if it throws an InvalidHash
# exception, we know that we're dealing with a legacy user, so we check the
# password against the SHA1 algorithm that was used to hash the password in
# the old database.  

#SOURCES OF SOLUTION:
# http://stackoverflow.com/questions/6113375/converting-existing-password-hash-to-devise
# https://github.com/binarylogic/authlogic/blob/master/lib/authlogic/crypto_providers/sha512.rb
# https://github.com/plataformatec/devise/blob/master/lib/devise/encryptors/authlogic_sha512.rb
    
# using old authlogic with crypted_password

  has_secure_password

  def authenticate(password)
    begin
      super(password)
    rescue ::BCrypt::Errors::InvalidHash
      stretches = 20
      digest = [password, password_salt].flatten.join('')
      stretches.times {digest = Digest::SHA512.hexdigest(digest)}
      if digest == self.encrypted_password
        self.password = self.password_confirmation = password

        # deletes sha512 once user has logged in and updated to bcrypt
        self.encrypted_password = self.password_salt = nil

        self.save
        return true
      else
        # if bcrypt password and not old authlogic sha512 password doesn't authenticate user
        return false
      end
    end
  end