write base64 message parts with 76 data bytes on a line instead of 78

As required by RFC 2045 (MIME). The 78 byte lines work in practice, except that
SpamAssassin has rules that give messages with 78-byte lines spam points.

Mentioned by kjetilho on irc.

dsn/dsn.go

@@ -340,7 +340,7 @@ func (m *Message) Compose(log mlog.Log, smtputf8 bool) ([]byte, error) {
 			data := base64.StdEncoding.EncodeToString(headers)
 			for len(data) > 0 {
 				line := data
-				n := min(len(line), 78)
+				n := min(len(line), 76) // ../rfc/2045:1372
 				line, data = data[:n], data[n:]
 				if _, err := origp.Write([]byte(line + "\r\n")); err != nil {
 					return nil, err

imapserver/compress_test.go

@@ -64,7 +64,7 @@ func TestCompressBreak(t *testing.T) {
 	tcheck(t, err, "read random")
 	text := base64.StdEncoding.EncodeToString(buf)
 	for len(text) > 0 {
-		n := min(78, len(text))
+		n := min(76, len(text))
 		msg += text[:n] + "\r\n"
 		text = text[n:]
 	}

message/headerwriter.go

@@ -27,7 +27,7 @@ func (w *HeaderWriter) Add(separator string, texts ...string) {
 	}
 	for _, text := range texts {
 		n := len(text)
-		if w.nonfirst && w.lineLen > 1 && w.lineLen+len(separator)+n > 78 {
+		if w.nonfirst && w.lineLen > 1 && w.lineLen+len(separator)+n > 76 {
 			w.b.WriteString("\r\n\t")
 			w.lineLen = 1
 		} else if w.nonfirst && separator != "" {
@@ -45,7 +45,7 @@ func (w *HeaderWriter) Add(separator string, texts ...string) {
 func (w *HeaderWriter) AddWrap(buf []byte, text bool) {
 	for len(buf) > 0 {
 		line := buf
-		n := 78 - w.lineLen
+		n := 76 - w.lineLen
 		if len(buf) > n {
 			if text {
 				if i := bytes.LastIndexAny(buf[:n], " \t"); i > 0 {

message/qp.go

@@ -11,7 +11,9 @@ import (
 func NeedsQuotedPrintable(text string) bool {
 	// ../rfc/2045:1025
 	for _, line := range strings.Split(text, "\r\n") {
-		if len(line) > 78 || strings.Contains(line, "\r") || strings.Contains(line, "\n") {
+		// 78 should be fine too, qp itself has a requirement of 76 bytes on a line, but
+		// using qp for anything longer than 76 is safer.
+		if len(line) > 76 || strings.Contains(line, "\r") || strings.Contains(line, "\n") {
 			return true
 		}
 	}

moxio/base64writer.go

@@ -13,7 +13,7 @@ func (f closerFunc) Close() error {
 }
 
 // Base64Writer turns a writer for data into one that writes base64 content on
-// \r\n separated lines of max 78+2 characters length.
+// \r\n separated lines of max 76+2 characters length.
 func Base64Writer(w io.Writer) io.WriteCloser {
 	lw := &lineWrapper{w: w}
 	bw := base64.NewEncoder(base64.StdEncoding, lw)
@@ -39,7 +39,8 @@ type lineWrapper struct {
 func (lw *lineWrapper) Write(buf []byte) (int, error) {
 	wrote := 0
 	for len(buf) > 0 {
-		n := min(78-lw.n, len(buf))
+		// base64 has max 76 data bytes on per line. ../rfc/2045:1372
+		n := min(76-lw.n, len(buf))
 		nn, err := lw.w.Write(buf[:n])
 		if nn > 0 {
 			wrote += nn
@@ -49,7 +50,7 @@ func (lw *lineWrapper) Write(buf []byte) (int, error) {
 			return wrote, err
 		}
 		lw.n += nn
-		if lw.n == 78 {
+		if lw.n == 76 {
 			_, err := lw.w.Write([]byte("\r\n"))
 			if err != nil {
 				return wrote, err

moxio/base64writer_test.go

@@ -13,7 +13,7 @@ func TestBase64Writer(t *testing.T) {
 	err = bw.Close()
 	tcheckf(t, err, "close")
 	s := sb.String()
-	exp := "MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nz\r\ng5MDEyMzQ1Njc4OQ==\r\n"
+	exp := "MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2\r\nNzg5MDEyMzQ1Njc4OQ==\r\n"
 	if s != exp {
 		t.Fatalf("base64writer, got %q, expected %q", s, exp)
 	}

spf/received_test.go

@@ -26,7 +26,7 @@ func TestReceived(t *testing.T) {
 		Receiver:     "z",
 		Identity:     ReceivedMailFrom,
 		Mechanism:    "+ip4:0.0.0.0/0",
-	}, "Received-SPF: pass (c) client-ip=0.0.0.0; envelope-from=\"x@x\"; helo=y;\r\n\tproblem=\"a b\\\"\\\\\"; mechanism=\"+ip4:0.0.0.0/0\"; receiver=z; identity=mailfrom\r\n")
+	}, "Received-SPF: pass (c) client-ip=0.0.0.0; envelope-from=\"x@x\"; helo=y;\r\n\tproblem=\"a b\\\"\\\\\"; mechanism=\"+ip4:0.0.0.0/0\"; receiver=z;\r\n\tidentity=mailfrom\r\n")
 
 	test(Received{
 		Result:       StatusPass,
@@ -35,5 +35,5 @@ func TestReceived(t *testing.T) {
 		Helo:         dns.IPDomain{IP: net.ParseIP("2001:db8::1")},
 		Receiver:     "z",
 		Identity:     ReceivedMailFrom,
-	}, "Received-SPF: pass client-ip=0.0.0.0; envelope-from=\"x@x\"; helo=\"2001:db8::1\";\r\n\treceiver=z; identity=mailfrom\r\n")
+	}, "Received-SPF: pass client-ip=0.0.0.0; envelope-from=\"x@x\";\r\n\thelo=\"2001:db8::1\"; receiver=z; identity=mailfrom\r\n")
 }

webapisrv/server.go

@@ -875,7 +875,7 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 
 		for len(base64Data) > 0 {
 			line := base64Data
-			n := min(len(line), 78)
+			n := min(len(line), 76) // ../rfc/2045:1372
 			line, base64Data = base64Data[:n], base64Data[n:]
 			_, err := p.Write([]byte(line))
 			xcheckf(err, "writing attachment")

webmail/api.go

@@ -831,7 +831,7 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 
 			for len(base64Data) > 0 {
 				line := base64Data
-				n := min(len(line), 78)
+				n := min(len(line), 76) // ../rfc/2045:1372
 				line, base64Data = base64Data[:n], base64Data[n:]
 				_, err := ap.Write(line)
 				xcheckf(ctx, err, "writing attachment")