Fix folding signature algorithm

Closes: #18
Co-authored-by: Ludovico Russo <[email protected]>

dkim/header.go

@@ -92,35 +92,48 @@ func parseHeaderParams(s string) (map[string]string, error) {
 	return params, nil
 }
 
-func formatHeaderParams(params map[string]string) string {
-	keys := make([]string, 0, len(params))
-	found := false
-	for k := range params {
-		if k == "b" {
-			found = true
-		} else {
-			keys = append(keys, k)
+func formatHeaderParams(headerFieldName string, params map[string]string) string {
+	keys, bvalue, bfound := sortParams(params)
+
+	s := headerFieldName + ":"
+	var line string
+
+	for _, k := range keys {
+		v := params[k]
+		nextLength := 3 + len(line) + len(v) + len(k)
+		if nextLength > 75 {
+			s += line + crlf
+			line = ""
 		}
+		line = fmt.Sprintf("%v %v=%v;", line, k, v)
 	}
-	sort.Strings(keys)
-	if found {
-		keys = append(keys, "b")
+
+	if line != "" {
+		s += line
 	}
 
-	var s string
-	first := true
-	for _, k := range keys {
-		v := params[k]
+	if bfound {
+		bfiled := foldHeaderField(" b=" + bvalue)
+		s += crlf + bfiled
+	}
 
-		if first {
-			first = false
+	return s
+}
+
+func sortParams(params map[string]string) ([]string, string, bool) {
+	keys := make([]string, 0, len(params))
+	bfound := false
+	var bvalue string
+	for k := range params {
+		if k == "b" {
+			bvalue = params["b"]
+			bfound = true
 		} else {
-			s += " "
+			keys = append(keys, k)
 		}
-
-		s += k + "=" + v + ";"
 	}
-	return s
+	sort.Strings(keys)
+	return keys, bvalue, bfound
 }
 
 type headerPicker struct {

dkim/header_test.go

@@ -55,14 +55,48 @@ func TestFormatHeaderParams(t *testing.T) {
 		"d": "example.org",
 	}
 
-	expected := "a=rsa-sha256; d=example.org; v=1;"
+	expected := "DKIM-Signature: a=rsa-sha256; d=example.org; v=1;"
 
-	s := formatHeaderParams(params)
+	s := formatHeaderParams("DKIM-Signature", params)
 	if s != expected {
 		t.Errorf("Expected formatted params to be %q, but got %q", expected, s)
 	}
 }
 
+func TestLongHeaderFolding(t *testing.T) {
+	// see #29 and #27
+
+	params := map[string]string{
+		"v": "1",
+		"a": "rsa-sha256",
+		"d": "example.org",
+		"h": "From:To:Subject:Date:Message-ID:Long-Header-Name",
+	}
+
+	expected := "DKIM-Signature: a=rsa-sha256; d=example.org;\r\n h=From:To:Subject:Date:Message-ID:Long-Header-Name; v=1;"
+
+	s := formatHeaderParams("DKIM-Signature", params)
+	if s != expected {
+		t.Errorf("Expected formatted params to be\n\n %q\n\n, but got\n\n %q", expected, s)
+	}
+}
+
+func TestSignedHeaderFolding(t *testing.T) {
+	hValue := "From:To:Subject:Date:Message-ID:Long-Header-Name:Another-Long-Header-Name"
+
+	params := map[string]string{
+		"v": "1",
+		"a": "rsa-sha256",
+		"d": "example.org",
+		"h": hValue,
+	}
+
+	s := formatHeaderParams("DKIM-Signature", params)
+	if !strings.Contains(s, hValue) {
+		t.Errorf("Signed Headers names (%v) are not well folded in the signed header %q", hValue, s)
+	}
+}
+
 func TestParseHeaderParams_malformed(t *testing.T) {
 	_, err := parseHeaderParams("abc; def")
 	if err == nil {

dkim/sign.go

@@ -333,8 +333,8 @@ func Sign(w io.Writer, r io.Reader, options *SignOptions) error {
 }
 
 func formatSignature(params map[string]string) string {
-	sig := headerFieldName + ": " + formatHeaderParams(params)
-	return foldHeaderField(sig)
+	sig := formatHeaderParams(headerFieldName, params)
+	return sig
 }
 
 func formatTagList(l []string) string {

dkim/sign_ed25519_test.go

@@ -7,10 +7,11 @@ import (
 	"testing"
 )
 
-const signedEd25519MailString = "DKIM-Signature: a=ed25519-sha256; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJ" + "\r\n" +
-	" " + "VOzv8=; c=simple/simple; d=football.example.com; h=From:To:Subject:Date:Mes" + "\r\n" +
-	" " + "sage-ID; s=brisbane; t=424242; v=1; b=ZduPZq83AOTqjhScIfHll6W90tMG1nf34a34Q" + "\r\n" +
-	" " + "XKat3iFtP7NQE/3AwnHOrcsR2r5nVNoW+LeZURpT2obCthPCw==;" + "\r\n" +
+const signedEd25519MailString = "DKIM-Signature: a=ed25519-sha256; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;" + "\r\n" +
+	" " + "c=simple/simple; d=football.example.com;" + "\r\n" +
+	" " + "h=From:To:Subject:Date:Message-ID; s=brisbane; t=424242; v=1;" + "\r\n" +
+	" " + "b=k1LzRxs9/DfN/whlMICYKNIJhqUSmup0d5yw8tpi+Cfiqe6I3oSBmJ+HEp+moWy7/XvcUY/t" + "\r\n" +
+	" " + "ERHc3D2m7vw1AA==" + "\r\n" +
 	mailHeaderString +
 	"\r\n" +
 	mailBodyString

dkim/sign_test.go

@@ -22,11 +22,12 @@ const mailBodyString = "Hi.\r\n" +
 
 const mailString = mailHeaderString + "\r\n" + mailBodyString
 
-const signedMailString = "DKIM-Signature: a=rsa-sha256; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv" + "\r\n" +
-	" " + "8=; c=simple/simple; d=example.org; h=From:To:Subject:Date:Message-ID; s=br" + "\r\n" +
-	" " + "isbane; t=424242; v=1; b=bXtqB8uOEvtd1Xv/DHatdjb9onP0+vnzdYBbPMZm1qrRmhSuFH" + "\r\n" +
-	" " + "WsbkETafswNvJ4VqNX0gMoaYvzcmoMkUhW9m4pgZqR5y+62yA+B7WJCd6mz82UVkS1qEJeGjMxX" + "\r\n" +
-	" " + "mmPDkmLDA5HHL5LLTc3DLrxkwWMLzwrhQL48WhNFD1d6L4=;" + "\r\n" +
+const signedMailString = "DKIM-Signature: a=rsa-sha256; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;" + "\r\n" +
+	" " + "c=simple/simple; d=example.org; h=From:To:Subject:Date:Message-ID;" + "\r\n" +
+	" " + "s=brisbane; t=424242; v=1;" + "\r\n" +
+	" " + "b=MobyyDTeHhMhNJCEI6ATNK63ZQ7deSXK9umyzAvYwFqE6oGGvlQBQwqr1aC11hWpktjMLP1/" + "\r\n" +
+	" " + "m0PBi9v7cRLKMXXBIv2O0B1mIWdZPqd9jveRJqKzCb7SpqH2u5kK6i2vZI639ENTQzRQdxSAGXc" + "\r\n" +
+	" " + "PcPYjrgkqj7xklnrNBs0aIUA=" + "\r\n" +
 	mailHeaderString +
 	"\r\n" +
 	mailBodyString