1 files changed, 48 insertions, 9 deletions
diff --git a/yamn/encoder/encoder.go b/yamn/encoder/encoder.go
index 3b595b1..6c730c9 100644
--- a/yamn/encoder/encoder.go
+++ b/yamn/encoder/encoder.go
@@ -24,13 +24,15 @@ import (
 )
 
 const (
-	maxChainLength = 10
-	headerBytes    = 256
-	encHeadBytes   = 160
-	bodyBytes      = 17920
-	maxPlainBytes  = 17910
-	messageBytes   = maxChainLength*headerBytes + bodyBytes
-	armorVersion   = "0.2.7"
+	maxChainLength     = 10
+	headerBytes        = 256
+	encHeadBytes       = 160
+	bodyBytes          = 17920
+	maxPlainBytes      = 17910
+	maxReferencesBytes = 900
+	maxReferenceIDs    = 20
+	messageBytes       = maxChainLength*headerBytes + bodyBytes
+	armorVersion       = "0.2.7"
 )
 
 var (
@@ -137,6 +139,14 @@ func Validate(r Request) error {
 			return fmt.Errorf("%w: invalid header value", ErrInvalidRequest)
 		}
 	}
+	if strings.TrimSpace(r.ReplyTo) != "" {
+		if _, err := mail.ParseAddress(r.ReplyTo); err != nil {
+			return fmt.Errorf("%w: invalid Reply-To address", ErrInvalidRequest)
+		}
+	}
+	if !validReferences(r.References) {
+		return fmt.Errorf("%w: invalid References message ID", ErrInvalidRequest)
+	}
 	return nil
 }
 
@@ -158,8 +168,9 @@ func composeMessage(r Request) ([]byte, error) {
 	if r.Newsgroup != "" {
 		b.WriteString("Newsgroups: " + r.Newsgroup + "\n")
 	}
-	if r.References != "" {
-		b.WriteString("References: " + r.References + "\n")
+	if references := strings.Fields(r.References); len(references) > 0 {
+		b.WriteString("References: " + strings.Join(references, " ") + "\n")
+		b.WriteString("In-Reply-To: " + references[len(references)-1] + "\n")
 	}
 	b.WriteString("\n")
 	b.WriteString(r.Body)
@@ -167,6 +178,34 @@ func composeMessage(r Request) ([]byte, error) {
 }
 
 func validHeaderValue(value string) bool { return !strings.ContainsAny(value, "\r\n\x00") }
+func validReferences(value string) bool {
+	if value == "" {
+		return true
+	}
+	if len(value) > maxReferencesBytes {
+		return false
+	}
+	references := strings.Fields(value)
+	if len(references) == 0 || len(references) > maxReferenceIDs {
+		return false
+	}
+	for _, reference := range references {
+		if len(reference) < 5 || reference[0] != '<' || reference[len(reference)-1] != '>' {
+			return false
+		}
+		messageID := reference[1 : len(reference)-1]
+		if strings.Count(messageID, "@") != 1 || strings.HasPrefix(messageID, "@") || strings.HasSuffix(messageID, "@") {
+			return false
+		}
+		for _, character := range messageID {
+			if character < 33 || character > 126 || character == '<' || character == '>' {
+				return false
+			}
+		}
+	}
+	return true
+}
+
 func isRemailerName(s string) bool {
 	if s == "" {
 		return false