Calculate Rails CSRF token stored in session from authenticity token

README.md

On your rails project root

git clone https://gist.github.com/Jwata/4e5122fa43d719400914716955872cc2 authenticity_token_to_csrf_token

rails runner ./authenticity_token_to_csrf_token/calculate.rb 'some_authenticity_token'
=> 2PqIaqHja/+PllheC7mStH2X+y/E69LlyS5AIfd9MVs= # this is the csrf token stored in your session

calculate.rb

#!/usr/bin/env ruby
authenticity_token = ARGV[0]

# decode
masked_token = Base64.strict_decode64(authenticity_token)

# unmask
one_time_pad = masked_token[0...31]
encrypted_csrf_token = masked_token[32..-1]

encrypted_csrf_token_bytes = encrypted_csrf_token.bytes
one_time_pad.each_byte.with_index { |c, i| encrypted_csrf_token_bytes[i] ^= c }
csrf_token = encrypted_csrf_token_bytes.pack("C*")

puts Base64.encode64(csrf_token)