activitypub-academy/app/controllers/concerns/signature_verification.rb

# frozen_string_literal: true

# Implemented according to HTTP signatures (Draft 6)
# <https://tools.ietf.org/html/draft-cavage-http-signatures-06>
module SignatureVerification
  extend ActiveSupport::Concern

  def signed_request?
    request.headers['Signature'].present?
  end

  def signed_request_account
    return @signed_request_account if defined?(@signed_request_account)

    unless signed_request?
      @signed_request_account = nil
      return
    end

    raw_signature    = request.headers['Signature']
    signature_params = {}

    raw_signature.split(',').each do |part|
      parsed_parts = part.match(/([a-z]+)="([^"]+)"/i)
      next if parsed_parts.nil? || parsed_parts.size != 3
      signature_params[parsed_parts[1]] = parsed_parts[2]
    end

    if incompatible_signature?(signature_params)
      @signed_request_account = nil
      return
    end

    account = ResolveRemoteAccountService.new.call(signature_params['keyId'].gsub(/\Aacct:/, ''))

    if account.nil?
      @signed_request_account = nil
      return
    end

    signature             = Base64.decode64(signature_params['signature'])
    compare_signed_string = build_signed_string(signature_params['headers'])

    if account.keypair.public_key.verify(OpenSSL::Digest::SHA256.new, signature, compare_signed_string)
      @signed_request_account = account
      @signed_request_account
    else
      @signed_request_account = nil
    end
  end

  private

  def build_signed_string(signed_headers)
    signed_headers = 'date' if signed_headers.blank?

    signed_headers.split(' ').map do |signed_header|
      if signed_header == Request::REQUEST_TARGET
        "#{Request::REQUEST_TARGET}: #{request.method.downcase} #{request.path}"
      else
        "#{signed_header}: #{request.headers[to_header_name(signed_header)]}"
      end
    end.join("\n")
  end

  def matches_time_window?
    begin
      time_sent = DateTime.httpdate(request.headers['Date'])
    rescue ArgumentError
      return false
    end

    (Time.now.utc - time_sent).abs <= 30
  end

  def to_header_name(name)
    name.split(/-/).map(&:capitalize).join('-')
  end

  def incompatible_signature?(signature_params)
    signature_params['keyId'].blank? ||
      signature_params['signature'].blank? ||
      signature_params['algorithm'].blank? ||
      signature_params['algorithm'] != 'rsa-sha256' ||
      !signature_params['keyId'].start_with?('acct:')
  end
end