✨ SASL ANONYMOUS: Add new mechanism

lib/net/imap/sasl.rb

@@ -38,6 +38,11 @@ class IMAP
     #                 Non-standard and obsoleted by +OAUTHBEARER+, but widely
     #                 supported.
     #
+    # +ANONYMOUS+::   See AnonymousAuthenticator.
+    #                 Allow the user to gain access to public services or
+    #                 resources without authenticating or disclosing an
+    #                 identity.
+    #
     # === Deprecated mechanisms
     #
     # <em>Obsolete mechanisms are available for backwards compatibility.  Using
@@ -62,6 +67,7 @@ module SASL
       sasl_dir = File.expand_path("sasl", __dir__)
       autoload :Authenticator,            "#{sasl_dir}/authenticator"
       autoload :Authenticators,           "#{sasl_dir}/authenticators"
+      autoload :AnonymousAuthenticator,   "#{sasl_dir}/anonymous_authenticator"
       autoload :PlainAuthenticator,       "#{sasl_dir}/plain_authenticator"
       autoload :XOAuth2Authenticator,     "#{sasl_dir}/xoauth2_authenticator"
 
@@ -72,6 +78,7 @@ module SASL
       # Authenticators are all lazy loaded
       def self.authenticators
         @authenticators ||= SASL::Authenticators.new.tap do |registry|
+          registry.add_authenticator "Anonymous"
           registry.add_authenticator "Plain"
           registry.add_authenticator "XOAuth2"
           registry.add_authenticator "Login"      # deprecated

lib/net/imap/sasl/anonymous_authenticator.rb

@@ -0,0 +1,127 @@
+# frozen_string_literal: true
+
+require_relative "authenticator"
+
+module Net
+  class IMAP < Protocol
+    module SASL
+
+      # Authenticator for the "+ANONYMOUS+" SASL mechanism, as specified by
+      # RFC-4505[https://tools.ietf.org/html/rfc4505].  See
+      # Net::IMAP#authenticate.
+      class AnonymousAuthenticator < Authenticator
+
+        # :call-seq:
+        #   initial_response? -> true
+        #
+        # +ANONYMOUS+ can send an initial client response.
+        def initial_response?; true end
+
+        ##
+        # :call-seq:
+        #   new                                 -> authenticator
+        #   new(anonymous_message,  **)         -> authenticator
+        #   new(anonymous_message:, **)         -> authenticator
+        #   new(message:,           **)         -> authenticator
+        #   new {|propname, auth_ctx| propval } -> authenticator
+        #
+        # Creates an Authenticator for the "+ANONYMOUS+" SASL mechanism, as
+        # specified in RFC-4505[https://tools.ietf.org/html/rfc4505].  To use
+        # this, see Net::IMAP#authenticate or your client's authentication
+        # method.
+        #
+        # ==== Properties
+        # Only one optional property:
+        #
+        # * #anonymous_message --- an optional message sent to the server which
+        #   doesn't contain an <tt>"@"</tt> character, or if it does have an
+        #   <tt>"@"</tt> it must be a valid email address.
+        #
+        #   May be sent as positional argument or as a keyword argument.
+        #   Aliased as #message.
+        #
+        # See Net::IMAP::SASL::Authenticator@Properties for a detailed
+        # description of property assignment, lazy loading, and callbacks.
+        def initialize(message_arg = nil, anonymous_message: nil, message: nil)
+          super
+          propinit :anonymous_message, message_arg, anonymous_message, message
+        end
+
+        ##
+        # method: anonymous_message
+        # :call-seq:
+        #   anonymous_message -> string or nil
+        #
+        # A token sent for the +ANONYMOUS+ mechanism.
+        #
+        # Restricted to 255 UTF8 encoded characters, which will be validated by
+        # #process.
+        #
+        # If an "@" sign is included, the message must be a valid email address
+        # (+addr-spec+ from RFC-2822[https://tools.ietf.org/html/rfc2822]).
+        # Email syntax will _not_ be validated by AnonymousAuthenticator.
+        #
+        # Otherwise, it can be any UTF8 string which is permitted by the
+        # StringPrep "+trace+" profile.  This is validated by #process.
+        # See AnonymousAuthenticator.stringprep_trace.
+        property :anonymous_message
+        alias message anonymous_message
+
+        # From RFC-4505[https://tools.ietf.org/html/rfc4505] §3, The "trace"
+        # Profile of "Stringprep":
+        # >>>
+        #   Characters from the following tables of [StringPrep] are prohibited:
+        #
+        #   - C.2.1 (ASCII control characters)
+        #   - C.2.2 (Non-ASCII control characters)
+        #   - C.3 (Private use characters)
+        #   - C.4 (Non-character code points)
+        #   - C.5 (Surrogate codes)
+        #   - C.6 (Inappropriate for plain text)
+        #   - C.8 (Change display properties are deprecated)
+        #   - C.9 (Tagging characters)
+        #
+        #   No additional characters are prohibited.
+        SASLPREP_TRACE_TABLES = %w[C.2.1 C.2.2 C.3 C.4 C.5 C.6 C.8 C.9].freeze
+
+        # From RFC-4505[https://tools.ietf.org/html/rfc4505] §3, The "trace"
+        # Profile of "Stringprep":
+        # >>>
+        #   The character repertoire of this profile is Unicode 3.2 [Unicode].
+        #
+        #   No mapping is required by this profile.
+        #
+        #   No Unicode normalization is required by this profile.
+        #
+        #   The list of unassigned code points for this profile is that provided
+        #   in Appendix A of [StringPrep].  Unassigned code points are not
+        #   prohibited.
+        #
+        #   Characters from the following tables of [StringPrep] are prohibited:
+        #   (documented on SASLPREP_TRACE_TABLES)
+        #
+        #   This profile requires bidirectional character checking per Section 6
+        #   of [StringPrep].
+        def self.stringprep_trace(string)
+          StringPrep.check_prohibited!(string,
+                                       *SASLPREP_TRACE_TABLES,
+                                       bidi: true,
+                                       profile: "trace")
+          string
+        end
+
+        # Returns the #anonymous_message, after checking it with
+        # rdoc-ref:AnonymousAuthenticator.stringprep_trace.
+        def process(_server_challenge_string)
+          if (size = anonymous_message&.length)&.> 255
+            raise Error, "anonymous_message is too long.  (%d codepoints)" % [
+              size
+            ]
+          end
+          self.class.stringprep_trace(anonymous_message || "")
+        end
+
+      end
+    end
+  end
+end

test/net/imap/test_imap_authenticators.rb

@@ -86,6 +86,43 @@ def test_xoauth2_callbacks
     )
   end
 
+  # ----------------------
+  # ANONYMOUS
+  # ----------------------
+
+  def anonymous(*args, **kwargs, &block)
+    Net::IMAP::SASL.authenticator("ANONYMOUS", *args, **kwargs, &block)
+  end
+
+  def test_anonymous_matches_mechanism
+    assert_kind_of(Net::IMAP::SASL::AnonymousAuthenticator, anonymous)
+  end
+
+  def test_anonymous_response
+    assert_equal("", anonymous.process(nil))
+    assert_equal("hello world", anonymous("hello world").process(nil))
+    assert_equal("kwargs",
+                 anonymous(anonymous_message: "kwargs").process(nil))
+  end
+
+  def test_anonymous_stringprep
+    assert_raise(Net::IMAP::SASL::ProhibitedCodepoint) {
+      anonymous("no\ncontrol\rchars").process(nil)
+    }
+    assert_raise(Net::IMAP::SASL::ProhibitedCodepoint) {
+      anonymous("regional flags use tagging chars: e.g." \
+                "🏴󠁧󠁢󠁥󠁮󠁧󠁿 England, " \
+                "🏴󠁧󠁢󠁳󠁣󠁴󠁿 Scotland, " \
+                "🏴󠁧󠁢󠁷󠁬󠁳󠁿 Wales.").process(nil)
+    }
+  end
+
+  def test_anonymous_length_over_255
+    assert_raise(Net::IMAP::Error) {
+      anonymous("a" * 256).process(nil)
+    }
+  end
+
   # ----------------------
   # LOGIN (obsolete)
   # ----------------------