🔒 Add SASL ANONYMOUS mechanism

See https://tools.ietf.org/html/rfc4505 for the specification.

lib/net/imap.rb

@@ -1002,6 +1002,12 @@ def starttls(options = {}, verify = true)
     # Each mechanism has different properties and requirements.  Please consult
     # the documentation for the specific mechanisms you are using:
     #
+    # +ANONYMOUS+::
+    #     See AnonymousAuthenticator[Net::IMAP::SASL::AnonymousAuthenticator].
+    #
+    #     Allows the user to gain access to public services or resources without
+    #     authenticating or disclosing an identity.
+    #
     # +OAUTHBEARER+::
     #     See OAuthBearerAuthenticator[rdoc-ref:Net::IMAP::SASL::OAuthBearerAuthenticator].
     #

lib/net/imap/sasl.rb

@@ -27,6 +27,12 @@ class IMAP
     # Each mechanism has different properties and requirements.  Please consult
     # the documentation for the specific mechanisms you are using:
     #
+    # +ANONYMOUS+::
+    #     See AnonymousAuthenticator[Net::IMAP::SASL::AnonymousAuthenticator].
+    #
+    #     Allows the user to gain access to public services or resources without
+    #     authenticating or disclosing an identity.
+    #
     # +OAUTHBEARER+::
     #     See OAuthBearerAuthenticator.
     #
@@ -77,6 +83,8 @@ module SASL
       sasl_dir = File.expand_path("sasl", __dir__)
       autoload :Authenticators,           "#{sasl_dir}/authenticators"
       autoload :GS2Header,                "#{sasl_dir}/gs2_header"
+
+      autoload :AnonymousAuthenticator,   "#{sasl_dir}/anonymous_authenticator"
       autoload :OAuthBearerAuthenticator, "#{sasl_dir}/oauthbearer_authenticator"
       autoload :PlainAuthenticator,       "#{sasl_dir}/plain_authenticator"
       autoload :XOAuth2Authenticator,     "#{sasl_dir}/xoauth2_authenticator"

lib/net/imap/sasl/anonymous_authenticator.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+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
+
+        # :call-seq:
+        #   new(anonymous_message = "", **) -> authenticator
+        #   new(anonymous_message:  "", **) -> 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.
+        #
+        # #anonymous_message is an optional message which is sent to the server.
+        # It may be sent as a positional argument or as a keyword argument.
+        #
+        # Any other keyword parameters are quietly ignored.
+        def initialize(anon_msg = nil, anonymous_message: nil, **)
+          message = (anonymous_message || anon_msg || "").to_str
+          @anonymous_message = StringPrep::Trace.stringprep_trace message
+          if (size = @anonymous_message&.length)&.> 255
+            raise ArgumentError,
+                  "anonymous_message is too long.  (%d codepoints)" % [size]
+          end
+        end
+
+        # A token sent for the +ANONYMOUS+ mechanism.
+        #
+        # If it contains an "@" sign, the message must be a valid email address
+        # (+addr-spec+ from RFC-2822[https://tools.ietf.org/html/rfc2822]).
+        # Email syntax is _not_ validated by AnonymousAuthenticator.
+        #
+        # Otherwise, it can be any UTF8 string which is permitted by the
+        # StringPrep::Trace profile, up to 255 UTF-8 characters in length.
+        attr_reader :anonymous_message
+
+        # :call-seq:
+        #   initial_response? -> true
+        #
+        # +ANONYMOUS+ can send an initial client response.
+        def initial_response?; true end
+
+        # Returns #anonymous_message.
+        def process(_server_challenge_string) anonymous_message end
+
+      end
+    end
+  end
+end

lib/net/imap/sasl/authenticators.rb

@@ -33,6 +33,7 @@ class Authenticators
     def initialize(use_defaults: false)
       @authenticators = {}
       if use_defaults
+        add_authenticator "Anonymous"
         add_authenticator "OAuthBearer"
         add_authenticator "Plain"
         add_authenticator "XOAuth2"

test/net/imap/test_imap_authenticators.rb

@@ -100,6 +100,41 @@ def test_xoauth2_supports_initial_response
     assert Net::IMAP::SASL.initial_response?(xoauth2("foo", "bar"))
   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(ArgumentError) { anonymous("a" * 256).process(nil) }
+  end
+
   # ----------------------
   # LOGIN (obsolete)
   # ----------------------