Basic FASP support (#34031)

app/controllers/api/fasp/base_controller.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+class Api::Fasp::BaseController < ApplicationController
+  class Error < ::StandardError; end
+
+  DIGEST_PATTERN = /sha-256=:(.*?):/
+  KEYID_PATTERN = /keyid="(.*?)"/
+
+  attr_reader :current_provider
+
+  skip_forgery_protection
+
+  before_action :check_fasp_enabled
+  before_action :require_authentication
+  after_action :sign_response
+
+  private
+
+  def require_authentication
+    validate_content_digest!
+    validate_signature!
+  rescue Error, Linzer::Error, ActiveRecord::RecordNotFound => e
+    logger.debug("FASP Authentication error: #{e}")
+    authentication_error
+  end
+
+  def authentication_error
+    respond_to do |format|
+      format.json { head 401 }
+    end
+  end
+
+  def validate_content_digest!
+    content_digest_header = request.headers['content-digest']
+    raise Error, 'content-digest missing' if content_digest_header.blank?
+
+    digest_received = content_digest_header.match(DIGEST_PATTERN)[1]
+
+    digest_computed = OpenSSL::Digest.base64digest('sha256', request.body&.string || '')
+
+    raise Error, 'content-digest does not match' if digest_received != digest_computed
+  end
+
+  def validate_signature!
+    signature_input = request.headers['signature-input']&.encode('UTF-8')
+    raise Error, 'signature-input is missing' if signature_input.blank?
+
+    keyid = signature_input.match(KEYID_PATTERN)[1]
+    provider = Fasp::Provider.find(keyid)
+    linzer_request = Linzer.new_request(
+      request.method,
+      request.original_url,
+      {},
+      {
+        'content-digest' => request.headers['content-digest'],
+        'signature-input' => signature_input,
+        'signature' => request.headers['signature'],
+      }
+    )
+    message = Linzer::Message.new(linzer_request)
+    key = Linzer.new_ed25519_public_key(provider.provider_public_key_pem, keyid)
+    signature = Linzer::Signature.build(message.headers)
+    Linzer.verify(key, message, signature)
+    @current_provider = provider
+  end
+
+  def sign_response
+    response.headers['content-digest'] = "sha-256=:#{OpenSSL::Digest.base64digest('sha256', response.body || '')}:"
+
+    linzer_response = Linzer.new_response(response.body, response.status, { 'content-digest' => response.headers['content-digest'] })
+    message = Linzer::Message.new(linzer_response)
+    key = Linzer.new_ed25519_key(current_provider.server_private_key_pem)
+    signature = Linzer.sign(key, message, %w(@status content-digest))
+
+    response.headers.merge!(signature.to_h)
+  end
+
+  def check_fasp_enabled
+    raise ActionController::RoutingError unless Mastodon::Feature.fasp_enabled?
+  end
+end