add Oauth2 support

fixes #129

auth.go

@@ -19,6 +19,9 @@ const (
 
 	// SMTPAuthCramMD5 is the "CRAM-MD5" SASL authentication mechanism as described in RFC 4954
 	SMTPAuthCramMD5 SMTPAuthType = "CRAM-MD5"
+
+	// SMTPAuthXOAUTH2 is the "XOAUTH2" SASL authentication mechanism
+	SMTPAuthXOAUTH2 SMTPAuthType = "XOAUTH2"
 )
 
 // SMTP Auth related static errors
@@ -31,4 +34,7 @@ var (
 
 	// ErrCramMD5AuthNotSupported should be used if the target server does not support the "CRAM-MD5" schema
 	ErrCramMD5AuthNotSupported = errors.New("server does not support SMTP AUTH type: CRAM-MD5")
+
+	// ErrXOauth2AuthNotSupported should be used if the target server does not support the "XOAUTH2" schema
+	ErrXOauth2AuthNotSupported = errors.New("server does not support SMTP AUTH type: XOAUTH2")
 )

client.go

@@ -32,6 +32,9 @@ const (
 	// DefaultTLSMinVersion is the minimum TLS version required for the connection
 	// Nowadays TLS1.2 should be the sane default
 	DefaultTLSMinVersion = tls.VersionTLS12
+
+	// DefaultXOAuth2Variant is the default XOAuth2 variant
+	DefaultXOAuth2Variant = smtp.XOAuth2VariantGoogle
 )
 
 // DSNMailReturnOption is a type to define which MAIL RET option is used when a DSN
@@ -135,6 +138,9 @@ type Client struct {
 	// user is the SMTP AUTH username
 	user string
 
+	// xoauthVariant sets the client to use the provided XOAuth2Variant variant
+	xoauthVariant smtp.XOAuth2Variant
+
 	// dl enables the debug logging on the SMTP client
 	dl bool
 
@@ -198,6 +204,7 @@ func NewClient(h string, o ...Option) (*Client, error) {
 		port:          DefaultPort,
 		tlsconfig:     &tls.Config{ServerName: h, MinVersion: DefaultTLSMinVersion},
 		tlspolicy:     DefaultTLSPolicy,
+		xoauthVariant: DefaultXOAuth2Variant,
 	}
 
 	// Set default HELO/EHLO hostname
@@ -415,6 +422,14 @@ func WithDialContextFunc(f DialContextFunc) Option {
 	}
 }
 
+// WithXOAuth2Variant tells the client to use the provided XOAuth2 variant
+func WithXOAuth2Variant(v smtp.XOAuth2Variant) Option {
+	return func(c *Client) error {
+		c.xoauthVariant = v
+		return nil
+	}
+}
+
 // TLSPolicy returns the currently set TLSPolicy as string
 func (c *Client) TLSPolicy() string {
 	return c.tlspolicy.String()
@@ -430,6 +445,11 @@ func (c *Client) SetTLSPolicy(p TLSPolicy) {
 	c.tlspolicy = p
 }
 
+// SetXOAuth2Variant overrides the current XOAuth2Variant with the given XOAuth2Variant value
+func (c *Client) SetXOAuth2Variant(v smtp.XOAuth2Variant) {
+	c.xoauthVariant = v
+}
+
 // SetSSL tells the Client wether to use SSL or not
 func (c *Client) SetSSL(s bool) {
 	c.ssl = s
@@ -658,6 +678,11 @@ func (c *Client) auth() error {
 				return ErrCramMD5AuthNotSupported
 			}
 			c.sa = smtp.CRAMMD5Auth(c.user, c.pass)
+		case SMTPAuthXOAUTH2:
+			if !strings.Contains(sat, string(SMTPAuthXOAUTH2)) {
+				return ErrXOauth2AuthNotSupported
+			}
+			c.sa = smtp.XOAuth2Auth(c.user, c.pass, c.xoauthVariant)
 		default:
 			return fmt.Errorf("unsupported SMTP AUTH type %q", c.satype)
 		}

client_test.go

@@ -101,6 +101,7 @@ func TestNewClientWithOptions(t *testing.T) {
 		},
 		{"WithUsername()", WithUsername("test"), false},
 		{"WithPassword()", WithPassword("test"), false},
+		{"WithXOAuth2Variant()", WithXOAuth2Variant(smtp.XOAuth2VariantMicrosoft), false},
 		{"WithDSN()", WithDSN(), false},
 		{"WithDSNMailReturnType()", WithDSNMailReturnType(DSNMailReturnFull), false},
 		{"WithDSNMailReturnType() wrong option", WithDSNMailReturnType("FAIL"), true},
@@ -390,6 +391,57 @@ func TestSetSMTPAuth(t *testing.T) {
 	}
 }
 
+// TestWithXOAuth2Variant tests the WithXOAuth2Variant() option for the NewClient() method
+func TestWithXOAuth2Variant(t *testing.T) {
+	tests := []struct {
+		name  string
+		value smtp.XOAuth2Variant
+		want  string
+		sf    bool
+	}{
+		{"Variant: Google", smtp.XOAuth2VariantGoogle, smtp.XOAuth2VariantGoogle.String(), false},
+		{"Variant: Microsoft", smtp.XOAuth2VariantMicrosoft, smtp.XOAuth2VariantMicrosoft.String(), false},
+		{"Variant: Invalid", -1, "Unknown XOAuth2 variant", true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c, err := NewClient(DefaultHost, WithXOAuth2Variant(tt.value))
+			if err != nil && !tt.sf {
+				t.Errorf("failed to create new client: %s", err)
+				return
+			}
+			if c.xoauthVariant.String() != tt.want {
+				t.Errorf("failed to set XOAuth2 variant. Want: %s, got: %s", tt.want, c.xoauthVariant)
+			}
+		})
+	}
+}
+
+// TestSetXOAuth2Variant tests the SetXOAuth2Variant method for the Client object
+func TestSetXOAuth2Variant(t *testing.T) {
+	tests := []struct {
+		name  string
+		value smtp.XOAuth2Variant
+		want  smtp.XOAuth2Variant
+	}{
+		{"Google", smtp.XOAuth2VariantGoogle, smtp.XOAuth2VariantGoogle},
+		{"Google", smtp.XOAuth2VariantMicrosoft, smtp.XOAuth2VariantMicrosoft},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c, err := NewClient(DefaultHost)
+			if err != nil {
+				t.Errorf("failed to create new client: %s", err)
+				return
+			}
+			c.SetXOAuth2Variant(tt.value)
+			if c.xoauthVariant != tt.want {
+				t.Errorf("failed to set XOAuthVariant. Expected %s, got: %s", tt.want, c.xoauthVariant)
+			}
+		})
+	}
+}
+
 // TestWithDSN tests the WithDSN method for the Client object
 func TestWithDSN(t *testing.T) {
 	c, err := NewClient(DefaultHost, WithDSN())

smtp/auth_xoauth2.go

@@ -0,0 +1,71 @@
+// SPDX-FileCopyrightText: Copyright (c) 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+package smtp
+
+import "fmt"
+
+// XOAuth2Variant describes a int alias for the different XOAuth2 variants we support
+type XOAuth2Variant int
+
+// Supported XOAuth2 variants
+const (
+	// XOAuth2VariantGoogle is the Google variant for "XOAUTH2" SASL authentication mechanism.
+	// https://developers.google.com/gmail/imap/xoauth2-protocol
+	XOAuth2VariantGoogle XOAuth2Variant = iota
+
+	// XOAuth2VariantMicrosoft is the Microsoft variant for "XOAUTH2" SASL authentication mechanism.
+	// https://learn.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth
+	XOAuth2VariantMicrosoft
+)
+
+// String is a standard method to convert a XOAuth2Variant into a printable format
+func (v XOAuth2Variant) String() string {
+	switch v {
+	case XOAuth2VariantGoogle:
+		return "Google"
+	case XOAuth2VariantMicrosoft:
+		return "Microsoft"
+	default:
+		return "Unknown XOAuth2 variant"
+	}
+}
+
+type xoauth2Auth struct {
+	username, token string
+	variant         XOAuth2Variant
+}
+
+func XOAuth2Auth(username, token string, variant XOAuth2Variant) Auth {
+	return &xoauth2Auth{username, token, variant}
+}
+
+func (a *xoauth2Auth) getToken() []byte {
+	return []byte("user=" + a.username + "\x01" + "auth=Bearer " + a.token + "\x01\x01")
+}
+
+func (a *xoauth2Auth) Start(_ *ServerInfo) (string, []byte, error) {
+	switch a.variant {
+	case XOAuth2VariantGoogle:
+		return "XOAUTH2", a.getToken(), nil
+	case XOAuth2VariantMicrosoft:
+		return "XOAUTH2", nil, nil
+	default:
+		return "", nil, fmt.Errorf("unsupported XOAuth2 variant %d", a.variant)
+	}
+}
+
+func (a *xoauth2Auth) Next(_ []byte, more bool) ([]byte, error) {
+	if more {
+		switch a.variant {
+		case XOAuth2VariantGoogle:
+			return []byte(""), nil
+		case XOAuth2VariantMicrosoft:
+			return a.getToken(), nil
+		default:
+			return nil, fmt.Errorf("unsupported XOAuth2 variant %d", a.variant)
+		}
+	}
+	return nil, nil
+}

smtp/smtp_test.go

@@ -69,6 +69,20 @@ var authTests = []authTest{
 		[]string{"", "user 287eb355114cf5c471c26a875f1ca4ae"},
 		[]bool{false, false},
 	},
+	{
+		XOAuth2Auth("username", "token", XOAuth2VariantGoogle),
+		[]string{""},
+		"XOAUTH2",
+		[]string{"user=username\x01auth=Bearer token\x01\x01", ""},
+		[]bool{false},
+	},
+	{
+		XOAuth2Auth("username", "token", XOAuth2VariantMicrosoft),
+		[]string{""},
+		"XOAUTH2",
+		[]string{"", "user=username\x01auth=Bearer token\x01\x01"},
+		[]bool{false},
+	},
 }
 
 func TestAuth(t *testing.T) {
@@ -193,6 +207,37 @@ func TestAuthLogin(t *testing.T) {
 	}
 }
 
+func TestXOAuth2VariantToString(t *testing.T) {
+	v := XOAuth2VariantGoogle
+	if val := v.String(); val != "Google" {
+		t.Errorf("got %q; want Google", val)
+	}
+	v = XOAuth2VariantMicrosoft
+	if val := v.String(); val != "Microsoft" {
+		t.Errorf("got %q; want Microsoft", val)
+	}
+	v = XOAuth2Variant(-1)
+	if val := v.String(); val != "Unknown XOAuth2 variant" {
+		t.Errorf("got %q; want Unknown XOAuth2 variant", val)
+	}
+}
+
+func TestXOAuth2UnsupportendVariant(t *testing.T) {
+	serverInfo := &ServerInfo{
+		Name: "servename",
+		Auth: []string{"XOAUTH2"},
+	}
+	auth := &xoauth2Auth{"username", "token", XOAuth2Variant(-1)}
+	_, _, err := auth.Start(serverInfo)
+	if err == nil {
+		t.Error("expected error for auth.Start() using an unsupported variant")
+	}
+	_, err = auth.Next(nil, true)
+	if err == nil {
+		t.Error("expected error for auth.Next() using an unsupported variant")
+	}
+}
+
 // Issue 17794: don't send a trailing space on AUTH command when there's no password.
 func TestClientAuthTrimSpace(t *testing.T) {
 	server := "220 hello world\r\n" +