feat: implementation of S/MIME signing without tests

msg.go

@@ -7,8 +7,7 @@ package mail
 import (
 	"bytes"
 	"context"
-	"crypto/rsa"
-	"crypto/x509"
+	"crypto/tls"
 	"embed"
 	"errors"
 	"fmt"
@@ -208,8 +207,8 @@ func WithNoDefaultUserAgent() MsgOption {
 }
 
 // SignWithSMime configures the Msg to be signed with S/MIME
-func (m *Msg) SignWithSMime(privateKey *rsa.PrivateKey, certificate *x509.Certificate) error {
-	sMime, err := NewSMime(privateKey, certificate)
+func (m *Msg) SignWithSMime(keyPair *tls.Certificate) error {
+	sMime, err := newSMime(keyPair)
 	if err != nil {
 		return err
 	}
@@ -985,28 +984,31 @@ func (m *Msg) applyMiddlewares(msg *Msg) *Msg {
 
 // signMessage sign the Msg with S/MIME
 func (m *Msg) signMessage(msg *Msg) (*Msg, error) {
-	currentPart := m.GetParts()[0]
-	currentPart.SetEncoding(EncodingUSASCII)
-	currentPart.SetContentType(TypeTextPlain)
-	content, err := currentPart.GetContent()
+	signedPart := msg.GetParts()[0]
+	body, err := signedPart.GetContent()
 	if err != nil {
-		return nil, errors.New("failed to extract content from part")
+		return nil, err
 	}
 
-	signedContent, err := m.sMime.Sign(content)
+	signaturePart, err := m.createSignaturePart(signedPart.GetEncoding(), signedPart.GetContentType(), signedPart.GetCharset(), body)
 	if err != nil {
-		return nil, errors.New("failed to sign message")
+		return nil, err
 	}
 
-	signedPart := msg.newPart(
-		typeSMimeSigned,
-		WithPartEncoding(EncodingB64),
-		WithContentDisposition(DispositionSMime),
-	)
-	signedPart.SetContent(*signedContent)
-	msg.parts = append(msg.parts, signedPart)
+	m.parts = append(m.parts, signaturePart)
 
-	return msg, nil
+	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
@@ -1014,7 +1016,7 @@ func (m *Msg) WriteTo(writer io.Writer) (int64, error) {
 	mw := &msgWriter{writer: writer, charset: m.charset, encoder: m.encoder}
 	msg := m.applyMiddlewares(m)
 
-	if m.sMime != nil {
+	if m.hasSMime() {
 		signedMsg, err := m.signMessage(msg)
 		if err != nil {
 			return 0, err
@@ -1210,7 +1212,7 @@ 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

msgwriter.go

@@ -100,7 +100,7 @@ func (mw *msgWriter) writeMsg(msg *Msg) {
 		mw.startMP(MIMERelated, msg.boundary)
 		mw.writeString(DoubleNewLine)
 	}
-	if msg.hasAlt() && !msg.hasSMime() {
+	if msg.hasAlt() {
 		mw.startMP(MIMEAlternative, msg.boundary)
 		mw.writeString(DoubleNewLine)
 	}
@@ -241,7 +241,7 @@ func (mw *msgWriter) addFiles(files []*File, isAttachment bool) {
 		}
 
 		if mw.err == nil {
-			mw.writeBody(file.Writer, encoding)
+			mw.writeBody(file.Writer, encoding, false)
 		}
 	}
 }
@@ -273,7 +273,7 @@ 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
@@ -322,7 +322,7 @@ 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
@@ -337,12 +337,11 @@ func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encodin
 	lineBreaker := Base64LineBreaker{}
 	lineBreaker.out = &writeBuffer
 
-	switch encoding {
-	case EncodingQP:
+	if encoding == EncodingQP {
 		encodedWriter = quotedprintable.NewWriter(&writeBuffer)
-	case EncodingB64:
+	} else if encoding == EncodingB64 && !singingWithSMime {
 		encodedWriter = base64.NewEncoder(base64.StdEncoding, &lineBreaker)
-	case NoEncoding:
+	} else if encoding == NoEncoding {
 		_, err = writeFunc(&writeBuffer)
 		if err != nil {
 			mw.err = fmt.Errorf("bodyWriter function: %w", err)
@@ -355,7 +354,7 @@ func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encodin
 			mw.bytesWritten += n
 		}
 		return
-	default:
+	} else {
 		encodedWriter = quotedprintable.NewWriter(writer)
 	}
 

sime.go

@@ -1,18 +1,20 @@
 package mail
 
 import (
+	"bytes"
 	"crypto/rsa"
+	"crypto/tls"
 	"crypto/x509"
+	"encoding/pem"
 	"errors"
+	"fmt"
 	"go.mozilla.org/pkcs7"
+	"strings"
 )
 
 var (
-	// ErrInvalidPrivateKey should be used if private key is invalid
-	ErrInvalidPrivateKey = errors.New("invalid private key")
-
-	// ErrInvalidCertificate should be used if certificate is invalid
-	ErrInvalidCertificate = errors.New("invalid certificate")
+	// 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")
@@ -22,49 +24,136 @@ var (
 
 	// 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
+	privateKey         *rsa.PrivateKey
+	certificate        *x509.Certificate
+	parentCertificates []*x509.Certificate
 }
 
-// NewSMime construct a new instance of SMime with a provided *rsa.PrivateKey
-func NewSMime(privateKey *rsa.PrivateKey, certificate *x509.Certificate) (*SMime, error) {
-	if privateKey == nil {
-		return nil, ErrInvalidPrivateKey
+// 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
 	}
 
-	if certificate == nil {
-		return nil, ErrInvalidCertificate
+	parentCertificates := make([]*x509.Certificate, 0)
+	for _, cert := range keyPair.Certificate[1:] {
+		c, err := x509.ParseCertificate(cert)
+		if err != nil {
+			return nil, err
+		}
+		parentCertificates = append(parentCertificates, c)
 	}
 
 	return &SMime{
-		privateKey:  privateKey,
-		certificate: certificate,
+		privateKey:         keyPair.PrivateKey.(*rsa.PrivateKey),
+		certificate:        keyPair.Leaf,
+		parentCertificates: parentCertificates,
 	}, nil
 }
 
-// Sign the content with the given privateKey of the method NewSMime
-func (sm *SMime) Sign(content []byte) (*string, error) {
-	toBeSigned, err := pkcs7.NewSignedData(content)
+// 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"))
 
-	toBeSigned.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
+	tmp, err := pkcs7.NewSignedData(toBeSigned)
+	tmp.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
 	if err != nil {
-		return nil, ErrCouldNotInitialize
+		return ErrCouldNotInitialize
 	}
 
-	if err = toBeSigned.AddSigner(sm.certificate, sm.privateKey, pkcs7.SignerInfoConfig{}); err != nil {
-		return nil, ErrCouldNotAddSigner
+	if err = tmp.AddSignerChain(sm.certificate, sm.privateKey, sm.parentCertificates, pkcs7.SignerInfoConfig{}); err != nil {
+		return ErrCouldNotAddSigner
 	}
 
-	signed, err := toBeSigned.Finish()
+	signatureDER, err := tmp.Finish()
 	if err != nil {
-		return nil, ErrCouldNotFinishSigning
+		return ErrCouldNotFinishSigning
 	}
 
-	signedData := string(signed)
+	pemMsg, err := encodeToPEM(signatureDER)
+	if err != nil {
+		return ErrCouldNoEncodeToPEM
+	}
+	signaturePart.SetContent(*pemMsg)
 
-	return &signedData, nil
+	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.ReplaceAll(r, "-----BEGIN -----\n", "")
+	r = strings.ReplaceAll(r, "-----END -----\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
 }