fix(tls): validate RSA keys before marshaling (#23295)

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

util/tls/tls.go

@@ -192,6 +192,11 @@ func publicKey(priv any) any {
 func pemBlockForKey(priv any) *pem.Block {
 	switch k := priv.(type) {
 	case *rsa.PrivateKey:
+		// In Go 1.24+, MarshalPKCS1PrivateKey calls Precompute() which can panic
+		// if the key is invalid. Validate the key first.
+		if k == nil || k.Validate() != nil {
+			return nil
+		}
 		return &pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(k)}
 	case *ecdsa.PrivateKey:
 		b, err := x509.MarshalECPrivateKey(k)
@@ -297,7 +302,11 @@ func generatePEM(opts CertOptions) ([]byte, []byte, error) {
 		return nil, nil, err
 	}
 	certpem := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: certBytes})
-	keypem := pem.EncodeToMemory(pemBlockForKey(privateKey))
+	keyBlock := pemBlockForKey(privateKey)
+	if keyBlock == nil {
+		return nil, nil, errors.New("failed to encode private key")
+	}
+	keypem := pem.EncodeToMemory(keyBlock)
 	return certpem, keypem, nil
 }
 
@@ -320,7 +329,11 @@ func EncodeX509KeyPair(cert tls.Certificate) ([]byte, []byte) {
 	for _, certtmp := range cert.Certificate {
 		certpem = append(certpem, pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: certtmp})...)
 	}
-	keypem := pem.EncodeToMemory(pemBlockForKey(cert.PrivateKey))
+	keyBlock := pemBlockForKey(cert.PrivateKey)
+	if keyBlock == nil {
+		return certpem, []byte{}
+	}
+	keypem := pem.EncodeToMemory(keyBlock)
 	return certpem, keypem
 }
 

util/tls/tls_test.go

@@ -1,11 +1,13 @@
 package tls
 
 import (
+	"crypto/rsa"
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/pem"
 	"errors"
 	"fmt"
+	"math/big"
 	"os"
 	"strings"
 	"testing"
@@ -458,3 +460,74 @@ func TestLoadX509CertPool(t *testing.T) {
 		require.Nil(t, p)
 	})
 }
+
+func TestEncodeX509KeyPair_InvalidRSAKey(t *testing.T) {
+	t.Run("Nil RSA private key", func(t *testing.T) {
+		cert := tls.Certificate{
+			Certificate: [][]byte{{0x30, 0x82}}, // minimal DER certificate bytes
+			PrivateKey:  (*rsa.PrivateKey)(nil),
+		}
+		certPEM, keyPEM := EncodeX509KeyPair(cert)
+		assert.NotEmpty(t, certPEM)
+		assert.Empty(t, keyPEM)
+	})
+
+	t.Run("RSA private key that fails validation", func(t *testing.T) {
+		// Create an RSA key with invalid parameters that will fail Validate()
+		invalidKey := &rsa.PrivateKey{
+			PublicKey: rsa.PublicKey{
+				N: big.NewInt(1), // Too small modulus, will fail validation
+				E: 65537,
+			},
+			D: big.NewInt(1), // Invalid private exponent
+		}
+		cert := tls.Certificate{
+			Certificate: [][]byte{{0x30, 0x82}}, // minimal DER certificate bytes
+			PrivateKey:  invalidKey,
+		}
+		certPEM, keyPEM := EncodeX509KeyPair(cert)
+		assert.NotEmpty(t, certPEM)
+		assert.Empty(t, keyPEM)
+	})
+
+	t.Run("RSA private key with inconsistent parameters", func(t *testing.T) {
+		invalidKey := &rsa.PrivateKey{
+			PublicKey: rsa.PublicKey{
+				N: big.NewInt(35),
+				E: 65537,
+			},
+			D: big.NewInt(99999),
+		}
+		cert := tls.Certificate{
+			Certificate: [][]byte{{0x30, 0x82}}, // minimal DER certificate bytes
+			PrivateKey:  invalidKey,
+		}
+		certPEM, keyPEM := EncodeX509KeyPair(cert)
+		assert.NotEmpty(t, certPEM)
+		assert.Empty(t, keyPEM)
+	})
+
+	t.Run("Unsupported private key type", func(t *testing.T) {
+		// Use a type that's not *rsa.PrivateKey or *ecdsa.PrivateKey
+		cert := tls.Certificate{
+			Certificate: [][]byte{{0x30, 0x82}}, // minimal DER certificate bytes
+			PrivateKey:  "not a private key",    // Unsupported type
+		}
+		certPEM, keyPEM := EncodeX509KeyPair(cert)
+		assert.NotEmpty(t, certPEM)
+		assert.Empty(t, keyPEM)
+	})
+
+	t.Run("Valid RSA private key should work", func(t *testing.T) {
+		// Generate a valid RSA key for testing
+		opts := CertOptions{Hosts: []string{"localhost"}, Organization: "Test"}
+		validCert, err := GenerateX509KeyPair(opts)
+		require.NoError(t, err)
+
+		certPEM, keyPEM := EncodeX509KeyPair(*validCert)
+		assert.NotEmpty(t, certPEM)
+		assert.NotEmpty(t, keyPEM)
+		assert.Contains(t, string(keyPEM), "-----BEGIN RSA PRIVATE KEY-----")
+		assert.Contains(t, string(keyPEM), "-----END RSA PRIVATE KEY-----")
+	})
+}