Re-introduce backwards compatibility with Go 1.17 and Go 1.18

The following changes make use of methods that are not available in Go 1.17/Go 1.18. To guarantee at least 4 versions of backwards compatibility, versioned copies of those changes have been back-ported:
- 4d8db00641 (diff-4f6f6bdb9891d4dd271f9f31430420a2e44018fe4ee539576faf458bebb3cee4)
- 58158e990f (diff-772fc9f5d0c86f26e35158fb3e7a71a4967d18b4ec23a5dbb60781ab0babf426)

smtp/auth_cram_md5.go

@@ -11,6 +11,9 @@
 //
 // SPDX-License-Identifier: BSD-3-Clause AND MIT
 
+//go:build go1.19
+// +build go1.19
+
 package smtp
 
 import (

smtp/auth_cram_md5_118.go

@@ -0,0 +1,50 @@
+// SPDX-FileCopyrightText: Copyright 2010 The Go Authors. All rights reserved.
+// SPDX-FileCopyrightText: Copyright (c) 2022-2023 The go-mail Authors
+//
+// Original net/smtp code from the Go stdlib by the Go Authors.
+// Use of this source code is governed by a BSD-style
+// LICENSE file that can be found in this directory.
+//
+// go-mail specific modifications by the go-mail Authors.
+// Licensed under the MIT License.
+// See [PROJECT ROOT]/LICENSES directory for more information.
+//
+// SPDX-License-Identifier: BSD-3-Clause AND MIT
+
+//go:build !go1.19
+// +build !go1.19
+
+package smtp
+
+import (
+	"crypto/hmac"
+	"crypto/md5"
+	"fmt"
+)
+
+// cramMD5Auth is the type that satisfies the Auth interface for the "SMTP CRAM_MD5" auth
+type cramMD5Auth struct {
+	username, secret string
+}
+
+// CRAMMD5Auth returns an Auth that implements the CRAM-MD5 authentication
+// mechanism as defined in RFC 2195.
+// The returned Auth uses the given username and secret to authenticate
+// to the server using the challenge-response mechanism.
+func CRAMMD5Auth(username, secret string) Auth {
+	return &cramMD5Auth{username, secret}
+}
+
+func (a *cramMD5Auth) Start(_ *ServerInfo) (string, []byte, error) {
+	return "CRAM-MD5", nil, nil
+}
+
+func (a *cramMD5Auth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		d := hmac.New(md5.New, []byte(a.secret))
+		d.Write(fromServer)
+		s := make([]byte, 0, d.Size())
+		return []byte(fmt.Sprintf("%s %x", a.username, d.Sum(s))), nil
+	}
+	return nil, nil
+}

smtp/smtp.go

@@ -137,29 +137,6 @@ func (c *Client) helo() error {
 	return err
 }
 
-// ehlo sends the EHLO (extended hello) greeting to the server. It
-// should be the preferred greeting for servers that support it.
-func (c *Client) ehlo() error {
-	_, msg, err := c.cmd(250, "EHLO %s", c.localName)
-	if err != nil {
-		return err
-	}
-	ext := make(map[string]string)
-	extList := strings.Split(msg, "\n")
-	if len(extList) > 1 {
-		extList = extList[1:]
-		for _, line := range extList {
-			k, v, _ := strings.Cut(line, " ")
-			ext[k] = v
-		}
-	}
-	if mechs, ok := ext["AUTH"]; ok {
-		c.auth = strings.Split(mechs, " ")
-	}
-	c.ext = ext
-	return err
-}
-
 // StartTLS sends the STARTTLS command and encrypts all further communication.
 // Only servers that advertise the STARTTLS extension support this function.
 func (c *Client) StartTLS(config *tls.Config) error {

smtp/smtp_ehlo.go

@@ -0,0 +1,42 @@
+// SPDX-FileCopyrightText: Copyright 2010 The Go Authors. All rights reserved.
+// SPDX-FileCopyrightText: Copyright (c) 2022-2023 The go-mail Authors
+//
+// Original net/smtp code from the Go stdlib by the Go Authors.
+// Use of this source code is governed by a BSD-style
+// LICENSE file that can be found in this directory.
+//
+// go-mail specific modifications by the go-mail Authors.
+// Licensed under the MIT License.
+// See [PROJECT ROOT]/LICENSES directory for more information.
+//
+// SPDX-License-Identifier: BSD-3-Clause AND MIT
+
+//go:build go1.18
+// +build go1.18
+
+package smtp
+
+import "strings"
+
+// ehlo sends the EHLO (extended hello) greeting to the server. It
+// should be the preferred greeting for servers that support it.
+func (c *Client) ehlo() error {
+	_, msg, err := c.cmd(250, "EHLO %s", c.localName)
+	if err != nil {
+		return err
+	}
+	ext := make(map[string]string)
+	extList := strings.Split(msg, "\n")
+	if len(extList) > 1 {
+		extList = extList[1:]
+		for _, line := range extList {
+			k, v, _ := strings.Cut(line, " ")
+			ext[k] = v
+		}
+	}
+	if mechs, ok := ext["AUTH"]; ok {
+		c.auth = strings.Split(mechs, " ")
+	}
+	c.ext = ext
+	return err
+}

smtp/smtp_ehlo_117.go

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: Copyright 2010 The Go Authors. All rights reserved.
+// SPDX-FileCopyrightText: Copyright (c) 2022-2023 The go-mail Authors
+//
+// Original net/smtp code from the Go stdlib by the Go Authors.
+// Use of this source code is governed by a BSD-style
+// LICENSE file that can be found in this directory.
+//
+// go-mail specific modifications by the go-mail Authors.
+// Licensed under the MIT License.
+// See [PROJECT ROOT]/LICENSES directory for more information.
+//
+// SPDX-License-Identifier: BSD-3-Clause AND MIT
+
+//go:build !go1.18
+// +build !go1.18
+
+package smtp
+
+import "strings"
+
+// ehlo sends the EHLO (extended hello) greeting to the server. It
+// should be the preferred greeting for servers that support it.
+func (c *Client) ehlo() error {
+	_, msg, err := c.cmd(250, "EHLO %s", c.localName)
+	if err != nil {
+		return err
+	}
+	ext := make(map[string]string)
+	extList := strings.Split(msg, "\n")
+	if len(extList) > 1 {
+		extList = extList[1:]
+		for _, line := range extList {
+			args := strings.SplitN(line, " ", 2)
+			if len(args) > 1 {
+				ext[args[0]] = args[1]
+			} else {
+				ext[args[0]] = ""
+			}
+		}
+	}
+	if mechs, ok := ext["AUTH"]; ok {
+		c.auth = strings.Split(mechs, " ")
+	}
+	c.ext = ext
+	return err
+}