Merge 7f7bf80e39 into e1db5bf66a

dummy-chain-cert.pem

@@ -0,0 +1,62 @@
+-----BEGIN CERTIFICATE-----
+MIIFWjCCA0KgAwIBAgIUAi7P4JOR4g8b5DMERUtZQEtw+igwDQYJKoZIhvcNAQEL
+BQAwRTELMAkGA1UEBhMCQVUxEzARBgNVBAgMClNvbWUtU3RhdGUxITAfBgNVBAoM
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+BJGrZEew0lLAWAqqASmGPDaWNBaA0HYqO70g4IyqBwIGNnaHSLVr/vT23BFRMyXf
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+xioBxb1md2AplUQ2fG/KHnu2ZuxHA6MNYcwMJNgv
+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----
+MIIFazCCA1OgAwIBAgIUKU+ta2rnJE/79L2Uxg4vFoF0RxYwDQYJKoZIhvcNAQEL
+BQAwRTELMAkGA1UEBhMCQVUxEzARBgNVBAgMClNvbWUtU3RhdGUxITAfBgNVBAoM
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+c0UrtvLC+K5m61I1iWSUYEK85/bPw2K5dn1e8w3Q2J460Yvc7/ZT7mucZlXQxfl3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=
+-----END CERTIFICATE-----

dummy-child-key.pem

@@ -0,0 +1,52 @@
+-----BEGIN PRIVATE KEY-----
+MIIJQAIBADANBgkqhkiG9w0BAQEFAASCCSowggkmAgEAAoICAQCiLh0JJTRRBhmU
+yiMKALHtTOK7T20Bwy+fG0SO6RlBc+hSuuX/n6znXcNgOBlQ2Gg3+p1on/bmcKnG
+N/SCiVBLpROiwxg3blQbZ7B7Jors/MopGk0LIBOXHPtAuYbF4J6ND5Ol6sgeGjMn
+omwRjZlfeuBlHY345MqvcwH/lPhOlKme+tWD+bsFh08NGS+3NdQGP6dA2bRVrPXh
+LXStHEmqfKO9EMVLWv+77tYhZESD6XgnA8pWjbdr9jajCsrQWrCG3jqHtzHNxtwf
+7xfRwwgoUhLEvue6SBVokZGVmDhvWdRt2sjtLcWWJCI3p7M+NXRt5qf6iu24wLBd
+zIDuWfgooWu5vBzNZjSTh2if6R1as9BdwASwy1n2HMvqpzgA+f/rXDFbvVc7WIKi
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+X+teQq1AqERUmLWx4WlTwxSo5X+d1jY9I8f61CRKVfIRgMvAZUhm2h6RnoVIgq7G
+7W3HdC3RT/f758njI7oIv5bhiyqpgyKr3cYhmn0enjP+YtjY85m51q005qzRLfaT
+YiwMR4qyJW4ZOEPntPs20CD+e+Pi2JLONpRdcsSrkqZusVjm5PFy2e5RyNFXTupU
+H2KVrgTRHL3GG2KWF5PmBdkhQfGG1QIDAQABAoICAAL3IruL6/zP5DPZ9RkiL1m/
+lHPhP7/sWKTfWMPf4ChX6XAHWeYraqNPvx8/bESdstLAvx3piyvcapRupN9DsVTu
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+HuIKzi/J+T2o5hZFOo6+33m5rYgwqZE0tRi+zLa1U6juBF/GiVbdsqupm88KN6y6
+9P+vBWUJihN0D06yZBpnk82riiKIprEqe/URkpLy3b0UmCBsTqUOoCbBUabNEocy
+v7bXMtIXUOo0gm7ZqfYXKHQR3oQbF0wqAxfI0RG0hl2syfqi5WQagMZ+PsW35cMC
+ggEBAM6kIM/zUnNBFla+oTiPCyoII7nGmGz8dT9IhH0+6T5nNzG4O6D28A3MJcs1
+C3pfukCCeWOnDNCIQ7C9Hx5XcCDoI4eD6zCRy+7zXxn0FxYS+O6lczB2mXGE69Yp
+n3qb7P4XqZYex3dT72czJUlY6nFB2e0FmyvSoez8fsH9Ws78c4JGO953klam/emA
+Hc8nB3CyM8rb2JlM3WeQbmo+Sbi0Yvj+MWM2AnTXx0xyaFXKP4WGD8hAxTvf9tSX
+3NAPVBXku4zRpvoXyyrcBVd9vwE0qBr3eWCuci8aDD6RAAJgb8HHX9RjRF/phFpY
+S++xGnGHEUSEl7cWnO8k0RyJzzc=
+-----END PRIVATE KEY-----

encoding.go

@@ -171,17 +171,20 @@ const (
 
 	// TypeTextPlain represents the MIME type for plain text content.
 	TypeTextPlain ContentType = "text/plain"
+  
+  // typeSMimeSigned represents the MIME type for S/MIME singed messages.
+  typeSMimeSigned ContentType = `application/pkcs7-signature; name="smime.p7s"`
 )
 
 const (
 	// MIMEAlternative MIMEType represents a MIME multipart/alternative type, used for emails with multiple versions.
 	MIMEAlternative MIMEType = "alternative"
-
   // MIMEMixed MIMEType represents a MIME multipart/mixed type used for emails containing different types of content.
 	MIMEMixed       MIMEType = "mixed"
-
   // MIMERelated MIMEType represents a MIME multipart/related type, used for emails with related content entities.
 	MIMERelated     MIMEType = "related"
+  // MIMESMime MIMEType represents a MIME multipart/signed type, used for siging emails with S/MIME.
+	MIMESMime       MIMEType = `signed; protocol="application/pkcs7-signature"; micalg=sha-256`
 )
 
 // String satisfies the fmt.Stringer interface for the Charset type.
@@ -219,3 +222,8 @@ func (c ContentType) String() string {
 func (e Encoding) String() string {
 	return string(e)
 }
+
+// String is a standard method to convert an MIMEType into a printable format
+func (e MIMEType) String() string {
+	return string(e)
+}

encoding_test.go

@@ -61,6 +61,11 @@ func TestContentType_String(t *testing.T) {
 			"ContentType: application/pgp-encrypted", TypePGPEncrypted,
 			"application/pgp-encrypted",
 		},
+
+		{
+			"ContentType: pkcs7-signature", typeSMimeSigned,
+			`application/pkcs7-signature; name="smime.p7s"`,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -121,3 +126,24 @@ func TestCharset_String(t *testing.T) {
 		})
 	}
 }
+
+// TestContentType_String tests the mime type method of the MIMEType object
+func TestMimeType_String(t *testing.T) {
+	tests := []struct {
+		mt   MIMEType
+		want string
+	}{
+		{MIMEAlternative, "alternative"},
+		{MIMEMixed, "mixed"},
+		{MIMERelated, "related"},
+		{MIMESMime, `signed; protocol="application/pkcs7-signature"; micalg=sha-256`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.mt.String(), func(t *testing.T) {
+			if tt.mt.String() != tt.want {
+				t.Errorf("wrong string for mime type returned. Expected: %s, got: %s",
+					tt.want, tt.mt.String())
+			}
+		})
+	}
+}

go.mod

@@ -7,6 +7,7 @@ module github.com/wneessen/go-mail
 go 1.16
 
 require (
+	go.mozilla.org/pkcs7 v0.9.0
 	golang.org/x/crypto v0.28.0
 	golang.org/x/text v0.19.0
 )

go.sum

@@ -20,6 +20,8 @@ golang.org/x/net v0.10.0/go.mod h1:0qNGK6F8kojg2nk9dLZ2mShWaEBan6FAoqfSigmmuDg=
 golang.org/x/net v0.15.0/go.mod h1:idbUs1IY1+zTqbi8yxTbhexhEEk5ur9LInksu6HrEpk=
 golang.org/x/net v0.21.0/go.mod h1:bIjVDfnllIU7BJ2DNgfnXvpSvtn8VRwhlsaeUTyUS44=
 golang.org/x/net v0.25.0/go.mod h1:JkAGAh7GEvH74S6FOH42FLoXpXbE/aqXSrIQjXgsiwM=
+go.mozilla.org/pkcs7 v0.9.0 h1:yM4/HS9dYv7ri2biPtxt8ikvB37a980dg69/pKmS+eI=
+go.mozilla.org/pkcs7 v0.9.0/go.mod h1:SNgMg+EgDFwmvSmLRTNKC5fegJjB7v23qTQ0XLGUNHk=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=

msg.go

@@ -7,6 +7,7 @@ package mail
 import (
 	"bytes"
 	"context"
+	"crypto/tls"
 	"embed"
 	"errors"
 	"fmt"
@@ -24,7 +25,7 @@ import (
 )
 
 var (
-	// ErrNoFromAddress indicates that the FROM address is not set, which is required.
+	// ErrNoFromAddress should be used when a FROM address is requested but not set
 	ErrNoFromAddress = errors.New("no FROM address set")
 
 	// ErrNoRcptAddresses indicates that no recipient addresses have been set.
@@ -144,6 +145,9 @@ type Msg struct {
 	//
 	// This can be useful in scenarios where headers are conditionally passed based on receipt - i. e. SMTP proxies.
 	noDefaultUserAgent bool
+
+	// SMime represents a middleware used to sign messages with S/MIME
+	sMime *SMime
 }
 
 // SendmailPath is the default system path to the sendmail binary - at least on standard Unix-like OS.
@@ -333,8 +337,16 @@ func WithNoDefaultUserAgent() MsgOption {
 	}
 }
 
-// SetCharset sets or overrides the currently set encoding charset of the Msg.
+// SignWithSMime configures the Msg to be signed with S/MIME
-//
+func (m *Msg) SignWithSMime(keyPair *tls.Certificate) error {
+	sMime, err := newSMime(keyPair)
+	if err != nil {
+		return err
+	}
+	m.sMime = sMime
+	return nil
+}
+
 // This method allows you to specify a character set for the email message. The charset is
 // important for ensuring that the content of the message is correctly interpreted by
 // mail clients. Common charset values include UTF-8, ISO-8859-1, and others. If a charset
@@ -2115,6 +2127,38 @@ func (m *Msg) applyMiddlewares(msg *Msg) *Msg {
 	return msg
 }
 
+// signMessage sign the Msg with S/MIME
+func (m *Msg) signMessage(msg *Msg) (*Msg, error) {
+	signedPart := msg.GetParts()[0]
+	body, err := signedPart.GetContent()
+	if err != nil {
+		return nil, err
+	}
+
+	signaturePart, err := m.createSignaturePart(signedPart.GetEncoding(), signedPart.GetContentType(), signedPart.GetCharset(), body)
+	if err != nil {
+		return nil, err
+	}
+
+	m.parts = append(m.parts, signaturePart)
+
+	return m, err
+}
+
+// createSignaturePart creates an additional part that be used for storing the S/MIME signature of the given body
+func (m *Msg) createSignaturePart(encoding Encoding, contentType ContentType, charSet Charset, body []byte) (*Part, error) {
+	message := m.sMime.prepareMessage(encoding, contentType, charSet, body)
+	signedMessage, err := m.sMime.signMessage(message)
+	if err != nil {
+		return nil, err
+	}
+
+	signaturePart := m.newPart(typeSMimeSigned, WithPartEncoding(EncodingB64), WithSMimeSinging())
+	signaturePart.SetContent(*signedMessage)
+
+	return signaturePart, nil
+}
+
 // WriteTo writes the formatted Msg into the given io.Writer and satisfies the io.WriterTo interface.
 //
 // This method writes the email message, including its headers, body, and attachments, to the provided
@@ -2132,7 +2176,18 @@ func (m *Msg) applyMiddlewares(msg *Msg) *Msg {
 //   - https://datatracker.ietf.org/doc/html/rfc5322
 func (m *Msg) WriteTo(writer io.Writer) (int64, error) {
 	mw := &msgWriter{writer: writer, charset: m.charset, encoder: m.encoder}
-	mw.writeMsg(m.applyMiddlewares(m))
+	msg := m.applyMiddlewares(m)
+
+	if m.hasSMime() {
+		signedMsg, err := m.signMessage(msg)
+		if err != nil {
+			return 0, err
+		}
+		msg = signedMsg
+	}
+
+	mw.writeMsg(msg)
+
 	return mw.bytesWritten, mw.err
 }
 
@@ -2496,7 +2551,7 @@ func (m *Msg) hasAlt() bool {
 			count++
 		}
 	}
-	return count > 1 && m.pgptype == 0
+	return count > 1 && m.pgptype == 0 && !m.hasSMime()
 }
 
 // hasMixed returns true if the Msg has mixed parts.
@@ -2514,6 +2569,11 @@ func (m *Msg) hasMixed() bool {
 	return m.pgptype == 0 && ((len(m.parts) > 0 && len(m.attachments) > 0) || len(m.attachments) > 1)
 }
 
+// hasSMime returns true if the Msg has should be signed with S/MIME
+func (m *Msg) hasSMime() bool {
+	return m.sMime != nil
+}
+
 // hasRelated returns true if the Msg has related parts.
 //
 // This method checks whether the message contains related parts, such as inline embedded files

msg_test.go

@@ -1907,6 +1907,39 @@ func TestMsg_hasAlt(t *testing.T) {
 	}
 }
 
+// TestMsg_hasAlt tests the hasAlt() method of the Msg with active S/MIME
+func TestMsg_hasAltWithSMime(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	m.SetBodyString(TypeTextPlain, "Plain")
+	m.AddAlternativeString(TypeTextHTML, "<b>HTML</b>")
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	if m.hasAlt() {
+		t.Errorf("mail has alternative parts and S/MIME is active, but hasAlt() returned true")
+	}
+}
+
+// TestMsg_hasSMime tests the hasSMime() method of the Msg
+func TestMsg_hasSMime(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	m.SetBodyString(TypeTextPlain, "Plain")
+	if !m.hasSMime() {
+		t.Errorf("mail has smime configured but hasSMime() returned true")
+	}
+}
+
 // TestMsg_hasRelated tests the hasRelated() method of the Msg
 func TestMsg_hasRelated(t *testing.T) {
 	m := NewMsg()
@@ -1974,6 +2007,70 @@ func TestMsg_WriteToSkipMiddleware(t *testing.T) {
 	}
 }
 
+// TestMsg_WriteToWithSMIME tests the WriteTo() method of the Msg
+func TestMsg_WriteToWithSMIME(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+
+	m := NewMsg()
+	m.Subject("This is a test")
+	m.SetBodyString(TypeTextPlain, "Plain")
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+
+	wbuf := bytes.Buffer{}
+	if _, err = m.WriteTo(&wbuf); err != nil {
+		t.Errorf("WriteTo() failed: %s", err)
+	}
+
+	result := wbuf.String()
+	boundary := result[strings.LastIndex(result, "--")-60 : strings.LastIndex(result, "--")]
+	if strings.Count(result, boundary) != 4 {
+		t.Errorf("WriteTo() failed. False number of boundaries found")
+	}
+
+	parts := strings.Split(result, fmt.Sprintf("--%s", boundary))
+	if len(parts) != 4 {
+		t.Errorf("WriteTo() failed. False number of parts found")
+	}
+
+	preamble := parts[0]
+	if !strings.Contains(preamble, "MIME-Version: 1.0") {
+		t.Errorf("WriteTo() failed. Unable to find MIME-Version")
+	}
+	if !strings.Contains(preamble, "Subject: This is a test") {
+		t.Errorf("WriteTo() failed. Unable to find subject")
+	}
+	if !strings.Contains(preamble, fmt.Sprintf("Content-Type: multipart/signed; protocol=\"application/pkcs7-signature\"; micalg=sha-256;\r\n boundary=%s", boundary)) {
+		t.Errorf("WriteTo() failed. Unable to find Content-Type")
+	}
+
+	signedData := parts[1]
+	if !strings.Contains(signedData, "Content-Transfer-Encoding: quoted-printable") {
+		t.Errorf("WriteTo() failed. Unable to find Content-Transfer-Encoding")
+	}
+	if !strings.Contains(signedData, "Content-Type: text/plain; charset=UTF-8") {
+		t.Errorf("WriteTo() failed. Unable to find Content-Type")
+	}
+	if !strings.Contains(signedData, "Plain") {
+		t.Errorf("WriteTo() failed. Unable to find Content")
+	}
+
+	signature := parts[2]
+	if !strings.Contains(signature, "Content-Transfer-Encoding: base64") {
+		t.Errorf("WriteTo() failed. Unable to find Content-Transfer-Encoding")
+	}
+	if !strings.Contains(signature, `application/pkcs7-signature; name="smime.p7s"`) {
+		t.Errorf("WriteTo() failed. Unable to find Content-Type")
+	}
+	if strings.Contains(signature, "Plain") {
+		t.Errorf("WriteTo() failed. Unable to find signature")
+	}
+}
+
 // TestMsg_WriteTo_fails tests the WriteTo() method of the Msg but with a failing body writer function
 func TestMsg_WriteTo_fails(t *testing.T) {
 	m := NewMsg()
@@ -3246,6 +3343,34 @@ func TestNewMsgWithNoDefaultUserAgent(t *testing.T) {
 	}
 }
 
+// TestSignWithSMime_ValidKeyPair tests WithSMimeSinging with given key pair
+func TestSignWithSMime_ValidKeyPair(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("failed to set sMime. Cause: %v", err)
+	}
+	if m.sMime.privateKey == nil {
+		t.Errorf("WithSMimeSinging() - no private key is given")
+	}
+	if m.sMime.certificate == nil {
+		t.Errorf("WithSMimeSinging() - no certificate is given")
+	}
+}
+
+// TestSignWithSMime_InvalidKeyPair tests WithSMimeSinging with given invalid key pair
+func TestSignWithSMime_InvalidKeyPair(t *testing.T) {
+	m := NewMsg()
+
+	err := m.SignWithSMime(nil)
+	if !errors.Is(err, ErrInvalidKeyPair) {
+		t.Errorf("failed to check sMimeAuthConfig values correctly: %s", err)
+	}
+}
+
 // Fuzzing tests
 func FuzzMsg_Subject(f *testing.F) {
 	f.Add("Testsubject")
@@ -3273,3 +3398,84 @@ func FuzzMsg_From(f *testing.F) {
 		m.Reset()
 	})
 }
+
+// TestMsg_createSignaturePart tests the Msg.createSignaturePart method
+func TestMsg_createSignaturePart(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	body := []byte("This is the body")
+	part, err := m.createSignaturePart(EncodingQP, TypeTextPlain, CharsetUTF7, body)
+	if err != nil {
+		t.Errorf("createSignaturePart() method failed.")
+	}
+
+	if part.GetEncoding() != EncodingB64 {
+		t.Errorf("createSignaturePart() method failed. Expected encoding: %s, got: %s", EncodingB64, part.GetEncoding())
+	}
+	if part.GetContentType() != typeSMimeSigned {
+		t.Errorf("createSignaturePart() method failed. Expected content type: %s, got: %s", typeSMimeSigned, part.GetContentType())
+	}
+	if part.GetCharset() != CharsetUTF8 {
+		t.Errorf("createSignaturePart() method failed. Expected charset: %s, got: %s", CharsetUTF8, part.GetCharset())
+	}
+	if content, err := part.GetContent(); err != nil || len(content) == len(body) {
+		t.Errorf("createSignaturePart() method failed. Expected content should not be equal: %s, got: %s", body, part.GetEncoding())
+	}
+}
+
+// TestMsg_signMessage tests the Msg.signMessage method
+func TestMsg_signMessage(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	body := []byte("This is the body")
+	m := NewMsg()
+	m.SetBodyString(TypeTextPlain, string(body))
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	msg, err := m.signMessage(m)
+	if err != nil {
+		t.Errorf("createSignaturePart() method failed.")
+	}
+
+	parts := msg.GetParts()
+	if len(parts) != 2 {
+		t.Errorf("createSignaturePart() method failed. Expected 2 parts, got: %d", len(parts))
+	}
+
+	signedPart := parts[0]
+	if signedPart.GetEncoding() != EncodingQP {
+		t.Errorf("createSignaturePart() method failed. Expected encoding: %s, got: %s", EncodingB64, signedPart.GetEncoding())
+	}
+	if signedPart.GetContentType() != TypeTextPlain {
+		t.Errorf("createSignaturePart() method failed. Expected content type: %s, got: %s", typeSMimeSigned, signedPart.GetContentType())
+	}
+	if signedPart.GetCharset() != CharsetUTF8 {
+		t.Errorf("createSignaturePart() method failed. Expected charset: %s, got: %s", CharsetUTF8, signedPart.GetCharset())
+	}
+	if content, err := signedPart.GetContent(); err != nil || len(content) != len(body) {
+		t.Errorf("createSignaturePart() method failed. Expected content should be equal: %s, got: %s", body, content)
+	}
+
+	signaturePart := parts[1]
+	if signaturePart.GetEncoding() != EncodingB64 {
+		t.Errorf("createSignaturePart() method failed. Expected encoding: %s, got: %s", EncodingB64, signaturePart.GetEncoding())
+	}
+	if signaturePart.GetContentType() != typeSMimeSigned {
+		t.Errorf("createSignaturePart() method failed. Expected content type: %s, got: %s", typeSMimeSigned, signaturePart.GetContentType())
+	}
+	if signaturePart.GetCharset() != CharsetUTF8 {
+		t.Errorf("createSignaturePart() method failed. Expected charset: %s, got: %s", CharsetUTF8, signaturePart.GetCharset())
+	}
+	if content, err := signaturePart.GetContent(); err != nil || len(content) == len(body) {
+		t.Errorf("createSignaturePart() method failed. Expected content should not be equal: %s, got: %s", body, signaturePart.GetEncoding())
+	}
+}

msgwriter.go

@@ -128,6 +128,10 @@ func (mw *msgWriter) writeMsg(msg *Msg) {
 		}
 	}
 
+	if msg.hasSMime() {
+		mw.startMP(MIMESMime, msg.boundary)
+		mw.writeString(DoubleNewLine)
+	}
 	if msg.hasMixed() {
 		mw.startMP(MIMEMixed, msg.boundary)
 		mw.writeString(DoubleNewLine)
@@ -174,6 +178,10 @@ func (mw *msgWriter) writeMsg(msg *Msg) {
 	if msg.hasMixed() {
 		mw.stopMP()
 	}
+
+	if msg.hasSMime() {
+		mw.stopMP()
+	}
 }
 
 // writeGenHeader writes out all generic headers to the msgWriter.
@@ -314,7 +322,7 @@ func (mw *msgWriter) addFiles(files []*File, isAttachment bool) {
 		}
 
 		if mw.err == nil {
-			mw.writeBody(file.Writer, encoding)
+			mw.writeBody(file.Writer, encoding, false)
 		}
 	}
 }
@@ -347,7 +355,12 @@ func (mw *msgWriter) writePart(part *Part, charset Charset) {
 	if partCharset.String() == "" {
 		partCharset = charset
 	}
-	contentType := fmt.Sprintf("%s; charset=%s", part.contentType, partCharset)
+
+	contentType := part.contentType.String()
+	if !part.IsSMimeSigned() {
+		contentType = strings.Join([]string{contentType, "; charset=", partCharset.String()}, "")
+	}
+
 	contentTransferEnc := part.encoding.String()
 	if mw.depth == 0 {
 		mw.writeHeader(HeaderContentType, contentType)
@@ -363,7 +376,7 @@ func (mw *msgWriter) writePart(part *Part, charset Charset) {
 		mimeHeader.Add(string(HeaderContentTransferEnc), contentTransferEnc)
 		mw.newPart(mimeHeader)
 	}
-	mw.writeBody(part.writeFunc, part.encoding)
+	mw.writeBody(part.writeFunc, part.encoding, part.smime)
 }
 
 // writeString writes a string into the msgWriter's io.Writer interface.
@@ -438,7 +451,8 @@ func (mw *msgWriter) writeHeader(key Header, values ...string) {
 // Parameters:
 //   - writeFunc: A function that writes the body content to the given io.Writer.
 //   - encoding: The encoding type to use when writing the content (e.g., base64, quoted-printable).
-func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encoding Encoding) {
+//   - singingWithSMime: Whether the msg should be signed with S/MIME or not.
+func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encoding Encoding, singingWithSMime bool) {
 	var writer io.Writer
 	var encodedWriter io.WriteCloser
 	var n int64
@@ -453,12 +467,11 @@ func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encodin
 	lineBreaker := Base64LineBreaker{}
 	lineBreaker.out = &writeBuffer
 
-	switch encoding {
+	if encoding == EncodingQP {
-	case EncodingQP:
 		encodedWriter = quotedprintable.NewWriter(&writeBuffer)
-	case EncodingB64:
+	} else if encoding == EncodingB64 && !singingWithSMime {
 		encodedWriter = base64.NewEncoder(base64.StdEncoding, &lineBreaker)
-	case NoEncoding:
+	} else if encoding == NoEncoding || singingWithSMime {
 		_, err = writeFunc(&writeBuffer)
 		if err != nil {
 			mw.err = fmt.Errorf("bodyWriter function: %w", err)
@@ -471,7 +484,7 @@ func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encodin
 			mw.bytesWritten += n
 		}
 		return
-	default:
+	} else {
 		encodedWriter = quotedprintable.NewWriter(writer)
 	}
 

msgwriter_test.go

@@ -154,3 +154,42 @@ func TestMsgWriter_writeMsg_PGP(t *testing.T) {
 		t.Errorf("writeMsg failed. Expected PGP encoding header but didn't find it in message output")
 	}
 }
+
+// TestMsgWriter_writeMsg_SMime tests the writeMsg method of the msgWriter with S/MIME types set
+func TestMsgWriter_writeMsg_SMime(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+
+	m := NewMsg()
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	_ = m.From(`"Toni Tester" <test@example.com>`)
+	_ = m.To(`"Toni Receiver" <receiver@example.com>`)
+	m.Subject("This is a subject")
+	m.SetBodyString(TypeTextPlain, "This is the body")
+	buf := bytes.Buffer{}
+	mw := &msgWriter{writer: &buf, charset: CharsetUTF8, encoder: mime.QEncoding}
+	mw.writeMsg(m)
+	ms := buf.String()
+
+	if !strings.Contains(ms, "MIME-Version: 1.0") {
+		t.Errorf("writeMsg failed. Unable to find MIME-Version")
+	}
+	if !strings.Contains(ms, "Subject: This is a subject") {
+		t.Errorf("writeMsg failed. Unable to find subject")
+	}
+	if !strings.Contains(ms, "From: \"Toni Tester\" <test@example.com>") {
+		t.Errorf("writeMsg failed. Unable to find transmitter")
+	}
+	if !strings.Contains(ms, "To: \"Toni Receiver\" <receiver@example.com>") {
+		t.Errorf("writeMsg failed. Unable to find receiver")
+	}
+
+	boundary := ms[strings.LastIndex(ms, "--")-60 : strings.LastIndex(ms, "--")]
+	if !strings.Contains(ms, fmt.Sprintf("Content-Type: multipart/signed; protocol=\"application/pkcs7-signature\"; micalg=sha-256;\r\n boundary=%s", boundary)) {
+		t.Errorf("writeMsg failed. Unable to find Content-Type")
+	}
+}

part.go

@@ -24,6 +24,7 @@ type Part struct {
 	encoding    Encoding
 	isDeleted   bool
 	writeFunc   func(io.Writer) (int64, error)
+	smime       bool
 }
 
 // GetContent executes the WriteFunc of the Part and returns the content as a byte slice.
@@ -94,6 +95,11 @@ func (p *Part) GetDescription() string {
 	return p.description
 }
 
+// IsSMimeSigned returns true if the Part should be singed with S/MIME
+func (p *Part) IsSMimeSigned() bool {
+	return p.smime
+}
+
 // SetContent overrides the content of the Part with the given string.
 //
 // This function sets the content of the Part by creating a new writeFunc that writes the
@@ -146,6 +152,12 @@ func (p *Part) SetDescription(description string) {
 	p.description = description
 }
 
+
+// SetIsSMimeSigned sets the flag for signing the Part with S/MIME
+func (p *Part) SetIsSMimeSigned(smime bool) {
+	p.smime = smime
+}
+
 // SetWriteFunc overrides the WriteFunc of the Part.
 //
 // This function sets a new WriteFunc for the Part, replacing the existing one. The WriteFunc
@@ -213,3 +225,10 @@ func WithPartContentDescription(description string) PartOption {
 		p.description = description
 	}
 }
+
+// WithSMimeSinging overrides the flag for signing the Part with S/MIME
+func WithSMimeSinging() PartOption {
+	return func(p *Part) {
+		p.smime = true
+	}
+}

part_test.go

@@ -102,6 +102,24 @@ func TestPart_WithPartContentDescription(t *testing.T) {
 	}
 }
 
+// TestPart_WithSMimeSinging tests the WithSMimeSinging method
+func TestPart_WithSMimeSinging(t *testing.T) {
+	m := NewMsg()
+	part := m.newPart(TypeTextPlain, WithSMimeSinging())
+	if part == nil {
+		t.Errorf("newPart() WithSMimeSinging() failed: no part returned")
+		return
+	}
+	if part.smime != true {
+		t.Errorf("newPart() WithSMimeSinging() failed: expected: %v, got: %v", true, part.smime)
+	}
+	part.smime = true
+	part.SetIsSMimeSigned(false)
+	if part.smime != false {
+		t.Errorf("newPart() SetIsSMimeSigned() failed: expected: %v, got: %v", false, part.smime)
+	}
+}
+
 // TestPartContentType tests Part.SetContentType
 func TestPart_SetContentType(t *testing.T) {
 	tests := []struct {
@@ -245,6 +263,33 @@ func TestPart_GetContentBroken(t *testing.T) {
 	}
 }
 
+// TestPart_IsSMimeSigned tests Part.IsSMimeSigned
+func TestPart_IsSMimeSigned(t *testing.T) {
+	tests := []struct {
+		name string
+		want bool
+	}{
+		{"smime:", true},
+		{"smime:", false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			m := NewMsg()
+			m.SetBodyString(TypeTextPlain, "This is a body!")
+			pl, err := getPartList(m)
+			if err != nil {
+				t.Errorf("failed: %s", err)
+				return
+			}
+			pl[0].SetIsSMimeSigned(tt.want)
+			smime := pl[0].IsSMimeSigned()
+			if smime != tt.want {
+				t.Errorf("SetContentType failed. Got: %v, expected: %v", smime, tt.want)
+			}
+		})
+	}
+}
+
 // TestPart_SetWriteFunc tests Part.SetWriteFunc
 func TestPart_SetWriteFunc(t *testing.T) {
 	c := "This is a test with ümläutß"

smime.go

@@ -0,0 +1,151 @@
+package mail
+
+import (
+	"bytes"
+	"crypto/rsa"
+	"crypto/tls"
+	"crypto/x509"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"go.mozilla.org/pkcs7"
+	"strings"
+)
+
+var (
+	// ErrInvalidKeyPair should be used if key pair is invalid
+	ErrInvalidKeyPair = errors.New("invalid key pair")
+
+	// ErrCouldNotInitialize should be used if the signed data could not initialize
+	ErrCouldNotInitialize = errors.New("could not initialize signed data")
+
+	// ErrCouldNotAddSigner should be used if the signer could not be added
+	ErrCouldNotAddSigner = errors.New("could not add signer message")
+
+	// ErrCouldNotFinishSigning should be used if the signing could not be finished
+	ErrCouldNotFinishSigning = errors.New("could not finish signing")
+
+	// ErrCouldNoEncodeToPEM should be used if the signature could not be encoded to PEM
+	ErrCouldNoEncodeToPEM = errors.New("could not encode to PEM")
+)
+
+// SMime is used to sign messages with S/MIME
+type SMime struct {
+	privateKey  *rsa.PrivateKey
+	certificate *x509.Certificate
+}
+
+// NewSMime construct a new instance of SMime with a provided *tls.Certificate
+func newSMime(keyPair *tls.Certificate) (*SMime, error) {
+	if keyPair == nil {
+		return nil, ErrInvalidKeyPair
+	}
+
+	return &SMime{
+		privateKey:  keyPair.PrivateKey.(*rsa.PrivateKey),
+		certificate: keyPair.Leaf,
+	}, nil
+}
+
+// signMessage signs the message with S/MIME
+func (sm *SMime) signMessage(message string) (*string, error) {
+	lines := parseLines([]byte(message))
+	toBeSigned := lines.bytesFromLines([]byte("\r\n"))
+
+	signedData, err := pkcs7.NewSignedData(toBeSigned)
+	signedData.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
+	if err != nil {
+		return nil, ErrCouldNotInitialize
+	}
+
+	if err = signedData.AddSigner(sm.certificate, sm.privateKey, pkcs7.SignerInfoConfig{}); err != nil {
+		return nil, ErrCouldNotAddSigner
+	}
+
+	signedData.Detach()
+
+	signatureDER, err := signedData.Finish()
+	if err != nil {
+		return nil, ErrCouldNotFinishSigning
+	}
+
+	pemMsg, err := encodeToPEM(signatureDER)
+	if err != nil {
+		return nil, ErrCouldNoEncodeToPEM
+	}
+
+	return pemMsg, nil
+}
+
+// createMessage prepares the message that will be used for the sign method later
+func (sm *SMime) prepareMessage(encoding Encoding, contentType ContentType, charset Charset, body []byte) string {
+	return fmt.Sprintf("Content-Transfer-Encoding: %v\r\nContent-Type: %v; charset=%v\r\n\r\n%v", encoding, contentType, charset, string(body))
+}
+
+// encodeToPEM uses the method pem.Encode from the standard library but cuts the typical PEM preamble
+func encodeToPEM(msg []byte) (*string, error) {
+	block := &pem.Block{Bytes: msg}
+
+	var arrayBuffer bytes.Buffer
+	if err := pem.Encode(&arrayBuffer, block); err != nil {
+		return nil, err
+	}
+
+	r := arrayBuffer.String()
+	r = strings.TrimPrefix(r, "-----BEGIN -----")
+	r = strings.Trim(r, "\n")
+	r = strings.TrimSuffix(r, "-----END -----")
+	r = strings.Trim(r, "\n")
+
+	return &r, nil
+}
+
+// line is the representation of one line of the message that will be used for signing purposes
+type line struct {
+	line      []byte
+	endOfLine []byte
+}
+
+// lines is the representation of a message that will be used for signing purposes
+type lines []line
+
+// bytesFromLines creates the line representation with the given endOfLine char
+func (ls lines) bytesFromLines(sep []byte) []byte {
+	var raw []byte
+	for i := range ls {
+		raw = append(raw, ls[i].line...)
+		if len(ls[i].endOfLine) != 0 && sep != nil {
+			raw = append(raw, sep...)
+		} else {
+			raw = append(raw, ls[i].endOfLine...)
+		}
+	}
+	return raw
+}
+
+// parseLines constructs the lines representation of a given message
+func parseLines(raw []byte) lines {
+	oneLine := line{raw, nil}
+	lines := lines{oneLine}
+	lines = lines.splitLine([]byte("\r\n"))
+	lines = lines.splitLine([]byte("\r"))
+	lines = lines.splitLine([]byte("\n"))
+	return lines
+}
+
+// splitLine uses the given endOfLine to split the given line
+func (ls lines) splitLine(sep []byte) lines {
+	nl := lines{}
+	for _, l := range ls {
+		split := bytes.Split(l.line, sep)
+		if len(split) > 1 {
+			for i := 0; i < len(split)-1; i++ {
+				nl = append(nl, line{split[i], sep})
+			}
+			nl = append(nl, line{split[len(split)-1], l.endOfLine})
+		} else {
+			nl = append(nl, l)
+		}
+	}
+	return nl
+}

smime_test.go

@@ -0,0 +1,190 @@
+package mail
+
+import (
+	"bytes"
+	"encoding/base64"
+	"fmt"
+	"strings"
+	"testing"
+)
+
+// TestNewSMime tests the newSMime method
+func TestNewSMime(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("Error getting dummy certificate: %s", err)
+	}
+
+	sMime, err := newSMime(keyPair)
+	if err != nil {
+		t.Errorf("Error creating new SMime from keyPair: %s", err)
+	}
+
+	if sMime.privateKey != keyPair.PrivateKey {
+		t.Errorf("NewSMime() did not return the same private key")
+	}
+	if sMime.certificate != keyPair.Leaf {
+		t.Errorf("NewSMime() did not return the same leaf certificate")
+	}
+}
+
+// TestSign tests the sign method
+func TestSign(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("Error getting dummy certificate: %s", err)
+	}
+
+	sMime, err := newSMime(keyPair)
+	if err != nil {
+		t.Errorf("Error creating new SMime from keyPair: %s", err)
+	}
+
+	message := "This is a test message"
+	singedMessage, err := sMime.signMessage(message)
+	if err != nil {
+		t.Errorf("Error creating singed message: %s", err)
+	}
+
+	if *singedMessage == message {
+		t.Errorf("Sign() did not work")
+	}
+}
+
+// TestPrepareMessage tests the createMessage method
+func TestPrepareMessage(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("Error getting dummy certificate: %s", err)
+	}
+
+	sMime, err := newSMime(keyPair)
+	if err != nil {
+		t.Errorf("Error creating new SMime from keyPair: %s", err)
+	}
+
+	encoding := EncodingB64
+	contentType := TypeTextPlain
+	charset := CharsetUTF8
+	body := []byte("This is the body!")
+	result := sMime.prepareMessage(encoding, contentType, charset, body)
+
+	if !strings.Contains(result, encoding.String()) {
+		t.Errorf("createMessage() did not return the correct encoding")
+	}
+	if !strings.Contains(result, contentType.String()) {
+		t.Errorf("createMessage() did not return the correct contentType")
+	}
+	if !strings.Contains(result, string(body)) {
+		t.Errorf("createMessage() did not return the correct body")
+	}
+	if result != fmt.Sprintf("Content-Transfer-Encoding: %s\r\nContent-Type: %s; charset=%s\r\n\r\n%s", encoding, contentType, charset, string(body)) {
+		t.Errorf("createMessage() did not sucessfully create the message")
+	}
+}
+
+// TestEncodeToPEM tests the encodeToPEM method
+func TestEncodeToPEM(t *testing.T) {
+	message := []byte("This is a test message")
+
+	pemMessage, err := encodeToPEM(message)
+	if err != nil {
+		t.Errorf("Error encoding message: %s", err)
+	}
+
+	base64Encoded := base64.StdEncoding.EncodeToString(message)
+	if *pemMessage != base64Encoded {
+		t.Errorf("encodeToPEM() did not work")
+	}
+}
+
+// TestBytesFromLines tests the bytesFromLines method
+func TestBytesFromLines(t *testing.T) {
+	ls := lines{
+		{line: []byte("Hello"), endOfLine: []byte("\n")},
+		{line: []byte("World"), endOfLine: []byte("\n")},
+	}
+	expected := []byte("Hello\nWorld\n")
+
+	result := ls.bytesFromLines([]byte("\n"))
+	if !bytes.Equal(result, expected) {
+		t.Errorf("Expected %s, but got %s", expected, result)
+	}
+}
+
+// FuzzBytesFromLines tests the bytesFromLines method with fuzzing
+func FuzzBytesFromLines(f *testing.F) {
+	f.Add([]byte("Hello"), []byte("\n"))
+	f.Fuzz(func(t *testing.T, lineData, sep []byte) {
+		ls := lines{
+			{line: lineData, endOfLine: sep},
+		}
+		_ = ls.bytesFromLines(sep)
+	})
+}
+
+// TestParseLines tests the parseLines method
+func TestParseLines(t *testing.T) {
+	input := []byte("Hello\r\nWorld\nHello\rWorld")
+	expected := lines{
+		{line: []byte("Hello"), endOfLine: []byte("\r\n")},
+		{line: []byte("World"), endOfLine: []byte("\n")},
+		{line: []byte("Hello"), endOfLine: []byte("\r")},
+		{line: []byte("World"), endOfLine: []byte("")},
+	}
+
+	result := parseLines(input)
+	if len(result) != len(expected) {
+		t.Errorf("Expected %d lines, but got %d", len(expected), len(result))
+	}
+
+	for i := range result {
+		if !bytes.Equal(result[i].line, expected[i].line) || !bytes.Equal(result[i].endOfLine, expected[i].endOfLine) {
+			t.Errorf("Line %d mismatch. Expected line: %s, endOfLine: %s, got line: %s, endOfLine: %s",
+				i, expected[i].line, expected[i].endOfLine, result[i].line, result[i].endOfLine)
+		}
+	}
+}
+
+// FuzzParseLines tests the parseLines method with fuzzing
+func FuzzParseLines(f *testing.F) {
+	f.Add([]byte("Hello\nWorld\r\nAnother\rLine"))
+	f.Fuzz(func(t *testing.T, input []byte) {
+		_ = parseLines(input)
+	})
+}
+
+// TestSplitLine tests the splitLine method
+func TestSplitLine(t *testing.T) {
+	ls := lines{
+		{line: []byte("Hello\r\nWorld\r\nAnotherLine"), endOfLine: []byte("")},
+	}
+	expected := lines{
+		{line: []byte("Hello"), endOfLine: []byte("\r\n")},
+		{line: []byte("World"), endOfLine: []byte("\r\n")},
+		{line: []byte("AnotherLine"), endOfLine: []byte("")},
+	}
+
+	result := ls.splitLine([]byte("\r\n"))
+	if len(result) != len(expected) {
+		t.Errorf("Expected %d lines, but got %d", len(expected), len(result))
+	}
+
+	for i := range result {
+		if !bytes.Equal(result[i].line, expected[i].line) || !bytes.Equal(result[i].endOfLine, expected[i].endOfLine) {
+			t.Errorf("Line %d mismatch. Expected line: %s, endOfLine: %s, got line: %s, endOfLine: %s",
+				i, expected[i].line, expected[i].endOfLine, result[i].line, result[i].endOfLine)
+		}
+	}
+}
+
+// FuzzSplitLine tests the parseLsplitLineines method with fuzzing
+func FuzzSplitLine(f *testing.F) {
+	f.Add([]byte("Hello\r\nWorld"), []byte("\r\n"))
+	f.Fuzz(func(t *testing.T, input, sep []byte) {
+		ls := lines{
+			{line: input, endOfLine: []byte("")},
+		}
+		_ = ls.splitLine(sep)
+	})
+}

util_test.go

@@ -0,0 +1,23 @@
+package mail
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+)
+
+const (
+	certFilePath = "dummy-chain-cert.pem"
+	keyFilePath  = "dummy-child-key.pem"
+)
+
+// getDummyCertificate loads a certificate and a private key form local disk for testing purposes
+func getDummyCertificate() (*tls.Certificate, error) {
+	keyPair, err := tls.LoadX509KeyPair(certFilePath, keyFilePath)
+	if err != nil {
+		return nil, err
+	}
+
+	keyPair.Leaf, err = x509.ParseCertificate(keyPair.Certificate[0])
+
+	return &keyPair, nil
+}