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

  include DomainControlHelper

  EXPIRATION_WINDOW_LIMIT = 12.hours
  CLOCK_SKEW_MARGIN       = 1.hour

  class SignatureVerificationError < StandardError; end

  class SignatureParamsParser < Parslet::Parser
    rule(:token)         { match("[0-9a-zA-Z!#$%&'*+.^_`|~-]").repeat(1).as(:token) }
    rule(:quoted_string) { str('"') >> (qdtext | quoted_pair).repeat.as(:quoted_string) >> str('"') }
    # qdtext and quoted_pair are not exactly according to spec but meh
    rule(:qdtext)        { match('[^\\\\"]') }
    rule(:quoted_pair)   { str('\\') >> any }
    rule(:bws)           { match('\s').repeat }
    rule(:param)         { (token.as(:key) >> bws >> str('=') >> bws >> (token | quoted_string).as(:value)).as(:param) }
    rule(:comma)         { bws >> str(',') >> bws }
    # Old versions of node-http-signature add an incorrect "Signature " prefix to the header
    rule(:buggy_prefix)  { str('Signature ') }
    rule(:params)        { buggy_prefix.maybe >> (param >> (comma >> param).repeat).as(:params) }
    root(:params)
  end

  class SignatureParamsTransformer < Parslet::Transform
    rule(params: subtree(:p)) do
      (p.is_a?(Array) ? p : [p]).each_with_object({}) { |(key, val), h| h[key] = val }
    end

    rule(param: { key: simple(:key), value: simple(:val) }) do
      [key, val]
    end

    rule(quoted_string: simple(:string)) do
      string.to_s
    end

    rule(token: simple(:string)) do
      string.to_s
    end
  end

  def require_signature!
    render plain: signature_verification_failure_reason, status: signature_verification_failure_code unless signed_request_account
  end

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

  def signature_verification_failure_reason
    @signature_verification_failure_reason
  end

  def signature_verification_failure_code
    @signature_verification_failure_code || 401
  end

  def signature_key_id
    signature_params['keyId']
  rescue SignatureVerificationError
    nil
  end

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

    raise SignatureVerificationError, 'Request not signed' unless signed_request?
    raise SignatureVerificationError, 'Incompatible request signature. keyId and signature are required' if missing_required_signature_parameters?
    raise SignatureVerificationError, 'Unsupported signature algorithm (only rsa-sha256 and hs2019 are supported)' unless %w(rsa-sha256 hs2019).include?(signature_algorithm)
    raise SignatureVerificationError, 'Signed request date outside acceptable time window' unless matches_time_window?

    verify_signature_strength!
    verify_body_digest!

    account = account_from_key_id(signature_params['keyId'])

    raise SignatureVerificationError, "Public key not found for key #{signature_params['keyId']}" if account.nil?

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

    return account unless verify_signature(account, signature, compare_signed_string).nil?

    account = stoplight_wrap_request { account.possibly_stale? ? account.refresh! : account_refresh_key(account) }

    raise SignatureVerificationError, "Public key not found for key #{signature_params['keyId']}" if account.nil?

    return account unless verify_signature(account, signature, compare_signed_string).nil?

    @signature_verification_failure_reason = "Verification failed for #{account.username}@#{account.domain} #{account.uri} using rsa-sha256 (RSASSA-PKCS1-v1_5 with SHA-256)"
    @signed_request_account = nil
  rescue SignatureVerificationError => e
    @signature_verification_failure_reason = e.message
    @signed_request_account = nil
  end

  def request_body
    @request_body ||= request.raw_post
  end

  private

  def signature_params
    @signature_params ||= begin
      raw_signature = request.headers['Signature']
      tree          = SignatureParamsParser.new.parse(raw_signature)
      SignatureParamsTransformer.new.apply(tree)
    end
  rescue Parslet::ParseFailed
    raise SignatureVerificationError, 'Error parsing signature parameters'
  end

  def signature_algorithm
    signature_params.fetch('algorithm', 'hs2019')
  end

  def signed_headers
    signature_params.fetch('headers', signature_algorithm == 'hs2019' ? '(created)' : 'date').downcase.split(' ')
  end

  def verify_signature_strength!
    raise SignatureVerificationError, 'Mastodon requires the Date header or (created) pseudo-header to be signed' unless signed_headers.include?('date') || signed_headers.include?('(created)')
    raise SignatureVerificationError, 'Mastodon requires the Digest header or (request-target) pseudo-header to be signed' unless signed_headers.include?(Request::REQUEST_TARGET) || signed_headers.include?('digest')
    raise SignatureVerificationError, 'Mastodon requires the Host header to be signed when doing a GET request' if request.get? && !signed_headers.include?('host')
    raise SignatureVerificationError, 'Mastodon requires the Digest header to be signed when doing a POST request' if request.post? && !signed_headers.include?('digest')
  end

  def verify_body_digest!
    return unless signed_headers.include?('digest')
    raise SignatureVerificationError, 'Digest header missing' unless request.headers.key?('Digest')

    digests = request.headers['Digest'].split(',').map { |digest| digest.split('=', 2) }.map { |key, value| [key.downcase, value] }
    sha256  = digests.assoc('sha-256')
    raise SignatureVerificationError, "Mastodon only supports SHA-256 in Digest header. Offered algorithms: #{digests.map(&:first).join(', ')}" if sha256.nil?
    raise SignatureVerificationError, "Invalid Digest value. Computed SHA-256 digest: #{body_digest}; given: #{sha256[1]}" if body_digest != sha256[1]
  end

  def verify_signature(account, signature, compare_signed_string)
    if account.keypair.public_key.verify(OpenSSL::Digest.new('SHA256'), signature, compare_signed_string)
      @signed_request_account = account
      @signed_request_account
    end
  rescue OpenSSL::PKey::RSAError
    nil
  end

  def build_signed_string
    signed_headers.map do |signed_header|
      if signed_header == Request::REQUEST_TARGET
        "#{Request::REQUEST_TARGET}: #{request.method.downcase} #{request.path}"
      elsif signed_header == '(created)'
        raise SignatureVerificationError, 'Invalid pseudo-header (created) for rsa-sha256' unless signature_algorithm == 'hs2019'
        raise SignatureVerificationError, 'Pseudo-header (created) used but corresponding argument missing' if signature_params['created'].blank?

        "(created): #{signature_params['created']}"
      elsif signed_header == '(expires)'
        raise SignatureVerificationError, 'Invalid pseudo-header (expires) for rsa-sha256' unless signature_algorithm == 'hs2019'
        raise SignatureVerificationError, 'Pseudo-header (expires) used but corresponding argument missing' if signature_params['expires'].blank?

        "(expires): #{signature_params['expires']}"
      else
        "#{signed_header}: #{request.headers[to_header_name(signed_header)]}"
      end
    end.join("\n")
  end

  def matches_time_window?
    created_time = nil
    expires_time = nil

    begin
      if signature_algorithm == 'hs2019' && signature_params['created'].present?
        created_time = Time.at(signature_params['created'].to_i).utc
      elsif request.headers['Date'].present?
        created_time = Time.httpdate(request.headers['Date']).utc
      end

      expires_time = Time.at(signature_params['expires'].to_i).utc if signature_params['expires'].present?
    rescue ArgumentError
      return false
    end

    expires_time ||= created_time + 5.minutes unless created_time.nil?
    expires_time = [expires_time, created_time + EXPIRATION_WINDOW_LIMIT].min unless created_time.nil?

    return false if created_time.present? && created_time > Time.now.utc + CLOCK_SKEW_MARGIN
    return false if expires_time.present? && Time.now.utc > expires_time + CLOCK_SKEW_MARGIN

    true
  end

  def body_digest
    @body_digest ||= Digest::SHA256.base64digest(request_body)
  end

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

  def missing_required_signature_parameters?
    signature_params['keyId'].blank? || signature_params['signature'].blank?
  end

  def account_from_key_id(key_id)
    domain = key_id.start_with?('acct:') ? key_id.split('@').last : key_id

    if domain_not_allowed?(domain)
      @signature_verification_failure_code = 403
      return
    end

    if key_id.start_with?('acct:')
      stoplight_wrap_request { ResolveAccountService.new.call(key_id.gsub(/\Aacct:/, '')) }
    elsif !ActivityPub::TagManager.instance.local_uri?(key_id)
      account   = ActivityPub::TagManager.instance.uri_to_resource(key_id, Account)
      account ||= stoplight_wrap_request { ActivityPub::FetchRemoteKeyService.new.call(key_id) }
      account
    end
  rescue Mastodon::HostValidationError
    nil
  end

  def stoplight_wrap_request(&block)
    Stoplight("source:#{request.remote_ip}", &block)
      .with_fallback { nil }
      .with_threshold(1)
      .with_cool_off_time(5.minutes.seconds)
      .with_error_handler { |error, handle| error.is_a?(HTTP::Error) || error.is_a?(OpenSSL::SSL::SSLError) ? handle.call(error) : raise(error) }
      .run
  end

  def account_refresh_key(account)
    return if account.local? || !account.activitypub?
    ActivityPub::FetchRemoteAccountService.new.call(account.uri, only_key: true)
  end
end