Merge 46cf2ed498 into 627216425f

dummy-chain-cert.pem

@@ -0,0 +1,62 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----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=
+-----END CERTIFICATE-----

dummy-child-key.pem

@@ -0,0 +1,52 @@
+-----BEGIN PRIVATE KEY-----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+-----END PRIVATE KEY-----

encoding.go

@@ -149,6 +149,7 @@ const (
 	TypePGPEncrypted         ContentType = "application/pgp-encrypted"
 	TypeTextHTML             ContentType = "text/html"
 	TypeTextPlain            ContentType = "text/plain"
+	typeSMimeSigned          ContentType = `application/pkcs7-signature; name="smime.p7s"`
 )
 
 // List of MIMETypes
@@ -156,6 +157,7 @@ const (
 	MIMEAlternative MIMEType = "alternative"
 	MIMEMixed       MIMEType = "mixed"
 	MIMERelated     MIMEType = "related"
+	MIMESMime       MIMEType = `signed; protocol="application/pkcs7-signature"; micalg=sha256`
 )
 
 // String is a standard method to convert an Charset into a printable format

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) {

go.mod

@@ -5,3 +5,5 @@
 module github.com/wneessen/go-mail
 
 go 1.16
+
+require go.mozilla.org/pkcs7 v0.9.0

go.sum

@@ -0,0 +1,2 @@
+go.mozilla.org/pkcs7 v0.9.0 h1:yM4/HS9dYv7ri2biPtxt8ikvB37a980dg69/pKmS+eI=
+go.mozilla.org/pkcs7 v0.9.0/go.mod h1:SNgMg+EgDFwmvSmLRTNKC5fegJjB7v23qTQ0XLGUNHk=

msg.go

@@ -7,6 +7,7 @@ package mail
 import (
 	"bytes"
 	"context"
+	"crypto/tls"
 	"embed"
 	"errors"
 	"fmt"
@@ -120,6 +121,9 @@ type Msg struct {
 
 	// noDefaultUserAgent indicates whether the default User Agent will be excluded for the Msg when it's sent.
 	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
@@ -202,6 +206,16 @@ func WithNoDefaultUserAgent() MsgOption {
 	}
 }
 
+// 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
+}
+
 // SetCharset sets the encoding charset of the Msg
 func (m *Msg) SetCharset(c Charset) {
 	m.charset = c
@@ -979,10 +993,50 @@ 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
+}
+
+func (m *Msg) createSignaturePart(encoding Encoding, contentType ContentType, charSet Charset, body []byte) (*Part, error) {
+	message := m.sMime.createMessage(encoding, contentType, charSet, body)
+	signaturePart := m.newPart(typeSMimeSigned, WithPartEncoding(EncodingB64), WithSMimeSinging())
+
+	if err := m.sMime.sign(signaturePart, message); err != nil {
+		return nil, err
+	}
+
+	return signaturePart, nil
+}
+
 // WriteTo writes the formated Msg into a give io.Writer and satisfies the io.WriteTo interface
 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
 }
 
@@ -1169,7 +1223,7 @@ 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
@@ -1177,6 +1231,11 @@ 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
 func (m *Msg) hasRelated() bool {
 	return m.pgptype == 0 && ((len(m.parts) > 0 && len(m.embeds) > 0) || len(m.embeds) > 1)

msg_test.go

@@ -1907,6 +1907,23 @@ 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_hasRelated tests the hasRelated() method of the Msg
 func TestMsg_hasRelated(t *testing.T) {
 	m := NewMsg()
@@ -3246,6 +3263,37 @@ 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")
+	}
+	if len(m.sMime.parentCertificates) != len(keyPair.Certificate[:1]) {
+		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 +3321,84 @@ 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

@@ -88,6 +88,10 @@ 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)
@@ -134,6 +138,10 @@ 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
@@ -237,7 +245,7 @@ func (mw *msgWriter) addFiles(files []*File, isAttachment bool) {
 		}
 
 		if mw.err == nil {
-			mw.writeBody(file.Writer, encoding)
+			mw.writeBody(file.Writer, encoding, false)
 		}
 	}
 }
@@ -269,7 +277,7 @@ 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
@@ -318,7 +326,7 @@ func (mw *msgWriter) writeHeader(key Header, values ...string) {
 }
 
 // writeBody writes an io.Reader into an io.Writer using provided Encoding
-func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encoding Encoding) {
+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
@@ -333,12 +341,11 @@ 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)
@@ -351,7 +358,7 @@ 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 +154,26 @@ 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, `multipart/signed; protocol="application/pkcs7-signature"; micalg=sha256;`) {
+		t.Errorf("writeMsg failed. Expected PGP encoding header but didn't find it in message output")
+	}
+}

part.go

@@ -20,6 +20,7 @@ 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 byte slice
@@ -56,6 +57,11 @@ 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
 func (p *Part) SetContent(content string) {
 	buffer := bytes.NewBufferString(content)
@@ -82,6 +88,11 @@ 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
 func (p *Part) SetWriteFunc(writeFunc func(io.Writer) (int64, error)) {
 	p.writeFunc = writeFunc
@@ -113,3 +124,10 @@ 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"

sime.go

@@ -0,0 +1,164 @@
+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")
+
+	// ErrInvalidCertificate should be used if a certificate is invalid
+	ErrInvalidCertificate = errors.New("invalid certificate")
+
+	// 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
+	parentCertificates []*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
+	}
+
+	parentCertificates := make([]*x509.Certificate, 0)
+	for _, cert := range keyPair.Certificate[1:] {
+		c, err := x509.ParseCertificate(cert)
+		if err != nil {
+			return nil, ErrInvalidCertificate
+		}
+		parentCertificates = append(parentCertificates, c)
+	}
+
+	return &SMime{
+		privateKey:         keyPair.PrivateKey.(*rsa.PrivateKey),
+		certificate:        keyPair.Leaf,
+		parentCertificates: parentCertificates,
+	}, nil
+}
+
+// sign with the S/MIME method the message of the actual *Part
+func (sm *SMime) sign(signaturePart *Part, message string) error {
+	lines := parseLines([]byte(message))
+	toBeSigned := lines.bytesFromLines([]byte("\r\n"))
+
+	tmp, err := pkcs7.NewSignedData(toBeSigned)
+	tmp.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
+	if err != nil {
+		return ErrCouldNotInitialize
+	}
+
+	if err = tmp.AddSignerChain(sm.certificate, sm.privateKey, sm.parentCertificates, pkcs7.SignerInfoConfig{}); err != nil {
+		return ErrCouldNotAddSigner
+	}
+
+	signatureDER, err := tmp.Finish()
+	if err != nil {
+		return ErrCouldNotFinishSigning
+	}
+
+	pemMsg, err := encodeToPEM(signatureDER)
+	if err != nil {
+		return ErrCouldNoEncodeToPEM
+	}
+	signaturePart.SetContent(*pemMsg)
+
+	return nil
+}
+
+// createMessage prepares the message that will be used for the sign method later
+func (sm *SMime) createMessage(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
+}

util_test.go

@@ -0,0 +1,22 @@
+package mail
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+)
+
+const (
+	certFilePath = "dummy-chain-cert.pem"
+	keyFilePath  = "dummy-child-key.pem"
+)
+
+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
+}