🔒 Add SASL OAUTHBEARER mechanism

Also, GS2Header was extracted from OAuthBearerAuthenticator.  It's not
much, but it can be re-used in the implementation of other mechanisms,
e.g. `SCRAM-SHA-*`.

lib/net/imap.rb

@@ -1002,6 +1002,13 @@ def starttls(options = {}, verify = true)
     # Each mechanism has different properties and requirements.  Please consult
     # the documentation for the specific mechanisms you are using:
     #
+    # +OAUTHBEARER+::
+    #     See OAuthBearerAuthenticator[rdoc-ref:Net::IMAP::SASL::OAuthBearerAuthenticator].
+    #
+    #     Login using an OAuth2 Bearer token.  This is the standard mechanism
+    #     for using OAuth2 with \SASL, but it is not yet deployed as widely as
+    #     +XOAUTH2+.
+    #
     # +PLAIN+::
     #     See PlainAuthenticator[rdoc-ref:Net::IMAP::SASL::PlainAuthenticator].
     #

lib/net/imap/sasl.rb

@@ -27,6 +27,13 @@ class IMAP
     # Each mechanism has different properties and requirements.  Please consult
     # the documentation for the specific mechanisms you are using:
     #
+    # +OAUTHBEARER+::
+    #     See OAuthBearerAuthenticator.
+    #
+    #     Login using an OAuth2 Bearer token.  This is the standard mechanism
+    #     for using OAuth2 with \SASL, but it is not yet deployed as widely as
+    #     +XOAUTH2+.
+    #
     # +PLAIN+::
     #     See PlainAuthenticator.
     #
@@ -69,7 +76,8 @@ module SASL
 
       sasl_dir = File.expand_path("sasl", __dir__)
       autoload :Authenticators,           "#{sasl_dir}/authenticators"
-
+      autoload :GS2Header,                "#{sasl_dir}/gs2_header"
+      autoload :OAuthBearerAuthenticator, "#{sasl_dir}/oauthbearer_authenticator"
       autoload :PlainAuthenticator,       "#{sasl_dir}/plain_authenticator"
       autoload :XOAuth2Authenticator,     "#{sasl_dir}/xoauth2_authenticator"
 

lib/net/imap/sasl/authenticators.rb

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

lib/net/imap/sasl/gs2_header.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+module Net
+  class IMAP < Protocol
+    module SASL
+
+      # Originally defined for the GS2 mechanism family in
+      # RFC5801[https://tools.ietf.org/html/rfc5801],
+      # several different mechanisms start with a GS2 header:
+      # * +GS2-*+       --- RFC5801[https://tools.ietf.org/html/rfc5801]
+      # * +SCRAM-*+     --- RFC5802[https://tools.ietf.org/html/rfc5802]
+      # * +SAML20+      --- RFC6595[https://tools.ietf.org/html/rfc6595]
+      # * +OPENID20+    --- RFC6616[https://tools.ietf.org/html/rfc6616]
+      # * +OAUTH10A+    --- RFC7628[https://tools.ietf.org/html/rfc7628]
+      # * +OAUTHBEARER+ --- RFC7628[https://tools.ietf.org/html/rfc7628]
+      #   (OAuthBearerAuthenticator)
+      #
+      # Classes that include this module must implement +#authzid+.
+      module GS2Header
+        NO_NULL_CHARS = /\A[^\x00]+\z/u.freeze # :nodoc:
+
+        ##
+        # Matches {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]
+        # +saslname+.  The output from gs2_saslname_encode matches this Regexp.
+        RFC5801_SASLNAME = /\A(?:[^,=\x00]|=2C|=3D)+\z/u.freeze
+
+        # The {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]
+        # +gs2-header+, which prefixes the #initial_client_response.
+        #
+        # >>>
+        #   <em>Note: the actual GS2 header includes an optional flag to
+        #   indicate that the GSS mechanism is not "standard", but since all of
+        #   the SASL mechanisms using GS2 are "standard", we don't include that
+        #   flag.  A class for a nonstandard GSSAPI mechanism should prefix with
+        #   "+F,+".</em>
+        def gs2_header
+          "#{gs2_cb_flag},#{gs2_authzid},"
+        end
+
+        # The {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]
+        # +gs2-cb-flag+:
+        #
+        # "+n+":: The client doesn't support channel binding.
+        # "+y+":: The client does support channel binding
+        #         but thinks the server does not.
+        # "+p+":: The client requires channel binding.
+        #         The selected channel binding follows "+p=+".
+        #
+        # The default always returns "+n+".  A mechanism that supports channel
+        # binding must override this method.
+        #
+        def gs2_cb_flag; "n" end
+
+        # The {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]
+        # +gs2-authzid+ header, when +#authzid+ is not empty.
+        #
+        # If +#authzid+ is empty or +nil+, an empty string is returned.
+        def gs2_authzid
+          return "" if authzid.nil? || authzid == ""
+          "a=#{gs2_saslname_encode(authzid)}"
+        end
+
+        module_function
+
+        # Encodes +str+ to match RFC5801_SASLNAME.
+        def gs2_saslname_encode(str)
+          str = str.encode("UTF-8")
+          # Regexp#match raises "invalid byte sequence" for invalid UTF-8
+          NO_NULL_CHARS.match str or
+            raise ArgumentError, "invalid saslname: %p" % [str]
+          str
+            .gsub(?=, "=3D")
+            .gsub(?,, "=2C")
+        end
+
+      end
+    end
+  end
+end

lib/net/imap/sasl/oauthbearer_authenticator.rb

@@ -0,0 +1,164 @@
+# frozen_string_literal: true
+
+require_relative "gs2_header"
+
+module Net
+  class IMAP < Protocol
+    module SASL
+
+      # Abstract base class for the SASL mechanisms defined in
+      # RFC7628[https://tools.ietf.org/html/rfc7628]:
+      # * OAUTHBEARER[rdoc-ref:OAuthBearerAuthenticator]
+      #   (OAuthBearerAuthenticator)
+      # * OAUTH10A
+      class OAuthAuthenticator
+        include GS2Header
+
+        # Creates an RFC7628[https://tools.ietf.org/html/rfc7628] OAuth
+        # authenticator.
+        #
+        # === Options
+        #
+        # See child classes for required configuration parameter(s).  The
+        # following parameters are all optional, but protocols or servers may
+        # add requirements for #authzid, #host, #port, or any other parameter.
+        #
+        # * #authzid ― Identity to act as or on behalf of.
+        # * #host — Hostname to which the client connected.
+        # * #port — Service port to which the client connected.
+        # * #mthd — HTTP method
+        # * #path — HTTP path data
+        # * #post — HTTP post data
+        # * #qs   — HTTP query string
+        #
+        def initialize(authzid: nil, host: nil, port: nil,
+                       mthd: nil, path: nil, post: nil, qs: nil, **)
+          @authzid = authzid
+          @host    = host
+          @port    = port
+          @mthd    = mthd
+          @path    = path
+          @post    = post
+          @qs      = qs
+          @done    = false
+        end
+
+        # Authorization identity: an identity to act as or on behalf of.
+        #
+        # If no explicit authorization identity is provided, it is usually
+        # derived from the authentication identity.  For the OAuth-based
+        # mechanisms, the authentication identity is the identity established by
+        # the OAuth credential.
+        attr_reader :authzid
+
+        # Hostname to which the client connected.
+        attr_reader :host
+
+        # Service port to which the client connected.
+        attr_reader :port
+
+        # HTTP method.  (optional)
+        attr_reader :mthd
+
+        # HTTP path data.  (optional)
+        attr_reader :path
+
+        # HTTP post data.  (optional)
+        attr_reader :post
+
+        # The query string.  (optional)
+        attr_reader :qs
+
+        # Stores the most recent server "challenge".  When authentication fails,
+        # this may hold information about the failure reason, as JSON.
+        attr_reader :last_server_response
+
+        # Returns initial_client_response the first time, then "<tt>^A</tt>".
+        def process(data)
+          @last_server_response = data
+          done? ? "\1" : initial_client_response
+        ensure
+          @done = true
+        end
+
+        # Returns true when the initial client response was sent.
+        #
+        # The authentication should not succeed unless this returns true, but it
+        # does *not* indicate success.
+        def done?; @done end
+
+        # The {RFC7628 §3.1}[https://www.rfc-editor.org/rfc/rfc7628#section-3.1]
+        # formatted response.
+        def initial_client_response
+          kv_pairs = {
+            host: host, port: port, mthd: mthd, path: path, post: post, qs: qs,
+            auth: authorization, # authorization is implemented by subclasses
+          }.compact
+          [gs2_header, *kv_pairs.map {|kv| kv.join("=") }, "\1"].join("\1")
+        end
+
+        # Value of the HTTP Authorization header
+        #
+        # <b>Implemented by subclasses.</b>
+        def authorization; raise "must be implemented by subclass" end
+
+      end
+
+      # Authenticator for the "+OAUTHBEARER+" SASL mechanism, specified in
+      # RFC7628[https://tools.ietf.org/html/rfc7628].  Authenticates using OAuth
+      # 2.0 bearer tokens, as described in
+      # RFC6750[https://tools.ietf.org/html/rfc6750].  Use via
+      # Net::IMAP#authenticate.
+      #
+      # RFC6750[https://tools.ietf.org/html/rfc6750] requires Transport Layer
+      # Security (TLS) to secure the protocol interaction between the client and
+      # the resource server.  TLS _MUST_ be used for +OAUTHBEARER+ to protect
+      # the bearer token.
+      class OAuthBearerAuthenticator < OAuthAuthenticator
+
+        # :call-seq:
+        #   new(oauth2_token,  **options) -> authenticator
+        #   new(oauth2_token:, **options) -> authenticator
+        #
+        # Creates an Authenticator for the "+OAUTHBEARER+" SASL mechanism.
+        #
+        # Called by Net::IMAP#authenticate and similar methods on other clients.
+        #
+        # === Options
+        #
+        # Only +oauth2_token+ is required by the mechanism, however protocols
+        # and servers may add requirements for #authzid, #host, #port, or any
+        # other parameter.
+        #
+        # * #oauth2_token — An OAuth2 bearer token or access token. *Required.*
+        #   May be provided as either regular or keyword argument.
+        # * #authzid ― Identity to act as or on behalf of.
+        # * #host — Hostname to which the client connected.
+        # * #port — Service port to which the client connected.
+        # * See OAuthAuthenticator documentation for less common parameters.
+        #
+        def initialize(oauth2_token_arg = nil, oauth2_token: nil, **args, &blk)
+          super(**args, &blk) # handles authzid, host, port, etc
+          oauth2_token && oauth2_token_arg and
+            raise ArgumentError, "conflicting values for oauth2_token"
+          @oauth2_token = oauth2_token || oauth2_token_arg or
+            raise ArgumentError, "missing oauth2_token"
+        end
+
+        # An OAuth2 bearer token, generally the access token.
+        attr_reader :oauth2_token
+
+        # :call-seq:
+        #   initial_response? -> true
+        #
+        # +OAUTHBEARER+ sends an initial client response.
+        def initial_response?; true end
+
+        # Value of the HTTP Authorization header
+        def authorization; "Bearer #{oauth2_token}" end
+
+      end
+    end
+
+  end
+end

test/net/imap/test_imap_authenticators.rb

@@ -56,6 +56,28 @@ def test_plain_no_null_chars
     assert_raise(ArgumentError) { plain("u", "p", authzid: "bad\0authz") }
   end
 
+  # ----------------------
+  # OAUTHBEARER
+  # ----------------------
+
+  def test_oauthbearer_authenticator_matches_mechanism
+    assert_kind_of(Net::IMAP::SASL::OAuthBearerAuthenticator,
+                   Net::IMAP::SASL.authenticator("OAUTHBEARER", "tok"))
+  end
+
+  def oauthbearer(*args, **kwargs, &block)
+    Net::IMAP::SASL.authenticator("OAUTHBEARER", *args, **kwargs, &block)
+  end
+
+  def test_oauthbearer_response
+    assert_equal(
+      "n,[email protected],\1host=server.example.com\1port=587\1" \
+      "auth=Bearer mF_9.B5f-4.1JqM\1\1",
+      oauthbearer("mF_9.B5f-4.1JqM", authzid: "[email protected]",
+                  host: "server.example.com", port: 587).process(nil)
+    )
+  end
+
   # ----------------------
   # XOAUTH2
   # ----------------------