mastodon/app/lib/activitypub/linked_data_signature.rb

# frozen_string_literal: true

class ActivityPub::LinkedDataSignature
  include JsonLdHelper

  CONTEXT = 'https://w3id.org/identity/v1'

  def initialize(json)
    @json = json.with_indifferent_access
  end

  def verify_actor!
    return unless @json['signature'].is_a?(Hash)
    return if unsupported_jsonld_features?(@json)

    signature_options = compact(@json['signature'].merge({ '@context' => CONTEXT }), CONTEXT)

    type        = signature_options['type']
    creator_uri = signature_options['creator']
    signature   = signature_options['signatureValue']

    return if signature_options['expires']&.to_datetime&.past?

    return unless type == 'RsaSignature2017'

    keypair = Keypair.from_keyid(creator_uri)
    keypair = ActivityPub::FetchRemoteKeyService.new.call(creator_uri) if keypair&.public_key.blank?
    return if keypair.nil? || !keypair.usable? || keypair.type != 'rsa'

    options_hash   = hash(signature_options.without('type', 'id', 'signatureValue'))
    document_hash  = hash(@json.without('signature'))
    to_be_verified = options_hash + document_hash

    keypair.actor if keypair.keypair.public_key.verify(OpenSSL::Digest.new('SHA256'), Base64.decode64(signature), to_be_verified)
  rescue OpenSSL::PKey::PKeyError
    false
  end

  def sign!(creator, sign_with: nil, expires_in: 2.days)
    keypair = sign_with.presence || creator.keypair(type: :rsa)

    options = {
      'type' => 'RsaSignature2017',
      'creator' => keypair.full_uri,
      'created' => Time.now.utc.iso8601,
      'expires' => expires_in.from_now.utc.iso8601,
    }

    options_hash  = hash(options.without('type', 'id', 'signatureValue').merge('@context' => CONTEXT))
    document_hash = hash(@json.without('signature'))
    to_be_signed  = options_hash + document_hash

    signature = Base64.strict_encode64(keypair.keypair.sign(OpenSSL::Digest.new('SHA256'), to_be_signed))

    # Mastodon's context is either an array or a single URL
    context_with_security = Array(@json['@context'])
    context_with_security << 'https://w3id.org/security/v1'
    context_with_security.uniq!
    context_with_security = context_with_security.first if context_with_security.size == 1

    @json.merge('signature' => options.merge('signatureValue' => signature), '@context' => context_with_security)
  end

  private

  def hash(obj)
    Digest::SHA256.hexdigest(canonicalize(obj))
  end
end