hash_hostname_for_ssh_known_hosts.rb

require 'openssl'
require 'base64'
require 'securerandom'

def hash_hostname(hostname, salt_b64=nil)
  if salt_b64
    salt = Base64.decode64(salt_b64)
  else
    salt = SecureRandom.random_bytes(20)
    salt_b64 = Base64.encode64(salt).strip
  end

  sha1_digest = OpenSSL::Digest::Digest.new('sha1')
  hostname_b64 = Base64.encode64(OpenSSL::HMAC.digest(sha1_digest, salt, hostname)).strip

  "|1|#{salt_b64}|#{hostname_b64}"
end