#16: implement DKIM key provisioning, Ed25519 & RSA dual signatures and rotation

services/protocols/internal/infrastructure/dkim/signer.go

@@ -0,0 +1,87 @@
+package dkim
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"log"
+
+	msgauth "github.com/emersion/go-msgauth/dkim"
+
+	"lambdamail/protocols/internal/application/port"
+)
+
+// signedHeaders are the headers covered by the signature. Signing a header
+// that is absent is harmless, but signing one that a forwarder may add later
+// would break the signature, so the list stays close to RFC 6376 appendix D.
+var signedHeaders = []string{
+	"From", "To", "Cc", "Subject", "Date", "Message-ID",
+	"MIME-Version", "Content-Type", "Content-Transfer-Encoding",
+	"In-Reply-To", "References",
+}
+
+// Signer applies every active key of a domain to an outbound message.
+type Signer struct {
+	keys port.DkimKeyRepository
+}
+
+func NewSigner(keys port.DkimKeyRepository) *Signer {
+	return &Signer{keys: keys}
+}
+
+// Sign adds one DKIM-Signature header per active key. PLAN.md section 5 signs
+// with Ed25519 and RSA at the same time so that receivers which do not yet
+// validate Ed25519 still see a passing RSA signature.
+//
+// A key that fails to sign is logged and skipped rather than failing the
+// send: delivering with one signature, or none, beats not delivering.
+func (s *Signer) Sign(ctx context.Context, domainName string, message []byte) ([]byte, error) {
+	if s.keys == nil {
+		return message, nil
+	}
+
+	keys, err := s.keys.FindActiveKeys(ctx, domainName)
+	if err != nil {
+		return nil, fmt.Errorf("load dkim keys for %s: %w", domainName, err)
+	}
+
+	signed := message
+	for _, key := range keys {
+		next, err := signOnce(signed, key)
+		if err != nil {
+			log.Printf("dkim: skipping %s signature for %s: %v", key.Algorithm, domainName, err)
+			continue
+		}
+		signed = next
+	}
+
+	return signed, nil
+}
+
+func signOnce(message []byte, key port.DkimSigningKey) ([]byte, error) {
+	signer, err := ParsePrivateKey(key.PrivateKeyPEM)
+	if err != nil {
+		return nil, err
+	}
+
+	options := &msgauth.SignOptions{
+		Domain:                 key.DomainName,
+		Selector:               key.Selector,
+		Signer:                 signer,
+		HeaderKeys:             signedHeaders,
+		HeaderCanonicalization: msgauth.CanonicalizationRelaxed,
+		BodyCanonicalization:   msgauth.CanonicalizationRelaxed,
+	}
+
+	// Hash is left at its zero value on purpose: SHA-256 is the only hash
+	// either algorithm admits (RFC 6376 for RSA, RFC 8463 for Ed25519), and
+	// the library selects it per key type.
+	var out bytes.Buffer
+	if err := msgauth.Sign(&out, bytes.NewReader(message), options); err != nil {
+		return nil, err
+	}
+	return out.Bytes(), nil
+}
+
+// Ensure port interface compliance
+var _ port.DkimSigner = (*Signer)(nil)