Rails uses a CSRF token Show archive.org snapshot in forms and AJAX requests to verify a user request. Internally it compares the injected CSRF token of the form data with the CSRF token in the encrypted user session. To prevent SSL BREACH attacks, the CSRF token from the form data is masked Show archive.org snapshot .

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To better debug issues, when these tokens do not match, it is useful to unmask the CSRF token from the form data. Here is a method that works for Rails 5.2.

def decode_csrf_token(csrf_token)
  decoded_masked_token = Base64.strict_decode64(csrf_token)
  one_time_pad = decoded_masked_token[0...32]
  encrypted_csrf_token = decoded_masked_token[32..-1]

  encrypted_csrf_token_bytes = encrypted_csrf_token.bytes
  one_time_pad.each_byte.with_index { |c1, i| encrypted_csrf_token_bytes[i] ^= c1 }
  csrf_token = encrypted_csrf_token_bytes.pack("C*")
  Base64.strict_encode64(csrf_token)
end

Example form:

<input type="hidden" name="authenticity_token" value="af3uqNHd1MDskKeJH7+AiV7o8gozjqCsu78rbndSG/TlE+KPrKe2l3pltzi4O0xtFnUX3cRfsB+MxfP4BXvT6Q==">

Example usage:

decode_csrf_token("af3uqNHd1MDskKeJH7+AiV7o8gozjqCsu78rbndSG/TlE+KPrKe2l3pltzi4O0xtFnUX3cRfsB+MxfP4BXvT6Q==")
=> jO4MJ316YleW9RCxp4TM5Eid5df30RCzN3rYlnIpyB0=

Rails will only accept this request if jO4MJ316YleW9RCxp4TM5Eid5df30RCzN3rYlnIpyB0= is the same as the value in your cookie:

{
  "session_id"=>"8ef662867ab2457717ba74c143c08733",
  "timestamp"=>1572261371,
  "_csrf_token"=>"jO4MJ316YleW9RCxp4TM5Eid5df30RCzN3rYlnIpyB0="
}

Note: The session cookie is encrypted by default. You can decrypt it with Rails: Decrypt a session cookie.

Rails: Verify the CSRF token

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