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

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

@@ -0,0 +1,119 @@
+// Package dkim generates, stores and applies the DKIM signing keys described
+// in PLAN.md section 5: every outbound message carries an Ed25519 and an
+// RSA-2048 signature, because Ed25519 is smaller and faster but not every
+// receiver validates it yet.
+package dkim
+
+import (
+	"crypto"
+	"crypto/ed25519"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/pem"
+	"fmt"
+)
+
+// Algorithm names match the dkim_keys.algorithm check constraint
+// (PLAN.md section 9).
+const (
+	AlgorithmRSA2048 = "rsa2048"
+	AlgorithmEd25519 = "ed25519"
+)
+
+// GeneratedKey is a fresh key pair: the private half ready to be sealed into
+// the vault, and the public half ready to be published as a DNS TXT record.
+type GeneratedKey struct {
+	Algorithm string
+	// PrivateKeyPEM is PKCS#8, which covers both RSA and Ed25519 with one
+	// encoding and so keeps the storage format uniform.
+	PrivateKeyPEM []byte
+	// PublicKeyBase64 is the bare base64 of the DER SubjectPublicKeyInfo for
+	// RSA, and of the raw 32-byte key for Ed25519, which is what the "p=" tag
+	// carries in each case (RFC 6376 section 3.6.1, RFC 8463 section 3).
+	PublicKeyBase64 string
+}
+
+// Generate produces a key pair for one of the supported algorithms.
+func Generate(algorithm string) (*GeneratedKey, error) {
+	switch algorithm {
+	case AlgorithmRSA2048:
+		return generateRSA(2048)
+	case AlgorithmEd25519:
+		return generateEd25519()
+	default:
+		return nil, fmt.Errorf("dkim: unsupported algorithm %q", algorithm)
+	}
+}
+
+func generateRSA(bits int) (*GeneratedKey, error) {
+	key, err := rsa.GenerateKey(rand.Reader, bits)
+	if err != nil {
+		return nil, fmt.Errorf("generate rsa key: %w", err)
+	}
+
+	privPEM, err := marshalPrivatePEM(key)
+	if err != nil {
+		return nil, err
+	}
+
+	pubDER, err := x509.MarshalPKIXPublicKey(&key.PublicKey)
+	if err != nil {
+		return nil, fmt.Errorf("marshal rsa public key: %w", err)
+	}
+
+	return &GeneratedKey{
+		Algorithm:       AlgorithmRSA2048,
+		PrivateKeyPEM:   privPEM,
+		PublicKeyBase64: base64.StdEncoding.EncodeToString(pubDER),
+	}, nil
+}
+
+func generateEd25519() (*GeneratedKey, error) {
+	pub, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		return nil, fmt.Errorf("generate ed25519 key: %w", err)
+	}
+
+	privPEM, err := marshalPrivatePEM(priv)
+	if err != nil {
+		return nil, err
+	}
+
+	// RFC 8463 publishes the raw public key, not a SubjectPublicKeyInfo.
+	return &GeneratedKey{
+		Algorithm:       AlgorithmEd25519,
+		PrivateKeyPEM:   privPEM,
+		PublicKeyBase64: base64.StdEncoding.EncodeToString(pub),
+	}, nil
+}
+
+func marshalPrivatePEM(key crypto.PrivateKey) ([]byte, error) {
+	der, err := x509.MarshalPKCS8PrivateKey(key)
+	if err != nil {
+		return nil, fmt.Errorf("marshal private key: %w", err)
+	}
+	return pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: der}), nil
+}
+
+// ParsePrivateKey reads back what marshalPrivatePEM wrote and returns it as a
+// crypto.Signer, which is what the signing library consumes for either
+// algorithm.
+func ParsePrivateKey(pemBytes []byte) (crypto.Signer, error) {
+	block, _ := pem.Decode(pemBytes)
+	if block == nil {
+		return nil, fmt.Errorf("dkim: private key is not valid PEM")
+	}
+
+	parsed, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+	if err != nil {
+		return nil, fmt.Errorf("dkim: parse private key: %w", err)
+	}
+
+	signer, ok := parsed.(crypto.Signer)
+	if !ok {
+		return nil, fmt.Errorf("dkim: private key of type %T cannot sign", parsed)
+	}
+	return signer, nil
+}