dkim: accept RSA keys in both raw and SPKI form

RFC 6376 is inconsistent about whether RSA public keys should be
formatted as RSAPublicKey or SubjectPublicKeyInfo.  Erratum 3017
(https://www.rfc-editor.org/errata/eid3017) proposes allowing both.

This commit changes the verifier to accept both formats, and changes
dkim-keygen to generate keys in SubjectPublicKeyInfo format for
consistency with other implementations including opendkim, Gmail,
and Fastmail.

Closes: #43

cmd/dkim-keygen/main.go

@@ -73,7 +73,16 @@ func main() {
 	var pubBytes []byte
 	switch pubKey := privKey.Public().(type) {
 	case *rsa.PublicKey:
-		pubBytes = x509.MarshalPKCS1PublicKey(pubKey)
+		// RFC 6376 is inconsistent about whether RSA public keys should
+		// be formatted as RSAPublicKey or SubjectPublicKeyInfo.
+		// Erratum 3017 (https://www.rfc-editor.org/errata/eid3017)
+		// proposes allowing both.  We use SubjectPublicKeyInfo for
+		// consistency with other implementations including opendkim,
+		// Gmail, and Fastmail.
+		pubBytes, err = x509.MarshalPKIXPublicKey(pubKey)
+		if err != nil {
+			log.Fatalf("Failed to marshal public key: %v", err)
+		}
 	case ed25519.PublicKey:
 		pubBytes = pubKey
 	default:

dkim/query.go

@@ -118,7 +118,14 @@ func parsePublicKey(s string) (*queryResult, error) {
 	case "rsa", "":
 		pub, err := x509.ParsePKIXPublicKey(b)
 		if err != nil {
-			return nil, permFailError("key syntax error: " + err.Error())
+			// RFC 6376 is inconsistent about whether RSA public keys should
+			// be formatted as RSAPublicKey or SubjectPublicKeyInfo.
+			// Erratum 3017 (https://www.rfc-editor.org/errata/eid3017) proposes
+			// allowing both.
+			pub, err = x509.ParsePKCS1PublicKey(b)
+			if err != nil {
+				return nil, permFailError("key syntax error: " + err.Error())
+			}
 		}
 		rsaPub, ok := pub.(*rsa.PublicKey)
 		if !ok {

dkim/query_test.go

@@ -4,6 +4,12 @@ import (
 	"fmt"
 )
 
+const dnsRawRSAPublicKey = "v=DKIM1; p=MIGJAoGBALVI635dLK4cJJAH3Lx6upo3X/L" +
+	"m1tQz3mezcWTA3BUBnyIsdnRf57aD5BtNmhPrYYDlWlzw3" +
+	"UgnKisIxktkk5+iMQMlFtAS10JB8L3YadXNJY+JBcbeSi5" +
+	"TgJe4WFzNgW95FWDAuSTRXSWZfA/8xjflbTLDx0euFZOM7" +
+	"C4T0GwLAgMBAAE="
+
 const dnsPublicKey = "v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ" +
 	"KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt" +
 	"IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v" +
@@ -21,6 +27,8 @@ func queryTest(domain, selector string, txtLookup txtLookupFunc) (*queryResult,
 	switch record {
 	case "brisbane._domainkey.example.com", "brisbane._domainkey.example.org", "test._domainkey.football.example.com":
 		return parsePublicKey(dnsPublicKey)
+	case "newengland._domainkey.example.com":
+		return parsePublicKey(dnsRawRSAPublicKey)
 	case "brisbane._domainkey.football.example.com":
 		return parsePublicKey(dnsEd25519PublicKey)
 	}

dkim/verify_test.go

@@ -114,6 +114,52 @@ func TestVerifyWithOption(t *testing.T) {
 	}
 }
 
+const verifiedRawRSAMailString = `DKIM-Signature: a=rsa-sha256; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
+ c=simple/simple; d=example.com;
+ h=Received:From:To:Subject:Date:Message-ID; [email protected];
+ s=newengland; t=1615825284; v=1;
+ b=Xh4Ujb2wv5x54gXtulCiy4C0e+plRm6pZ4owF+kICpYzs/8WkTVIDBrzhJP0DAYCpnL62T0G
+ k+0OH8pi/yqETVjKtKk+peMnNvKkut0GeWZMTze0bfq3/JUK3Ln3jTzzpXxrgVnvBxeY9EZIL4g
+ s4wwFRRKz/1bksZGSjD8uuSU=
+Received: from client1.football.example.com  [192.0.2.1]
+      by submitserver.example.com with SUBMISSION;
+      Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
+From: Joe SixPack <[email protected]>
+To: Suzie Q <[email protected]>
+Subject: Is dinner ready?
+Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
+Message-ID: <[email protected]>
+
+Hi.
+
+We lost the game. Are you hungry yet?
+
+Joe.
+`
+
+var testRawRSAVerification = &Verification{
+	Domain:     "example.com",
+	Identifier: "[email protected]",
+	HeaderKeys: []string{"Received", "From", "To", "Subject", "Date", "Message-ID"},
+	Time:       time.Unix(1615825284, 0),
+}
+
+func TestVerify_rawRSA(t *testing.T) {
+	r := newMailStringReader(verifiedRawRSAMailString)
+
+	verifications, err := Verify(r)
+	if err != nil {
+		t.Fatalf("Expected no error while verifying signature, got: %v", err)
+	} else if len(verifications) != 1 {
+		t.Fatalf("Expected exactly one verification, got %v", len(verifications))
+	}
+
+	v := verifications[0]
+	if !reflect.DeepEqual(testRawRSAVerification, v) {
+		t.Errorf("Expected verification to be \n%+v\n but got \n%+v", testRawRSAVerification, v)
+	}
+}
+
 const verifiedEd25519MailString = `DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed;
  d=football.example.com; [email protected];
  q=dns/txt; s=brisbane; t=1528637909; h=from : to :