fix: panic in certcrypto.ParsePEMPrivateKey (#1596)

certcrypto/crypto.go

@@ -85,6 +85,9 @@ func ParsePEMBundle(bundle []byte) ([]*x509.Certificate, error) {
 // https://github.com/golang/go/blob/693748e9fa385f1e2c3b91ca9acbb6c0ad2d133d/src/crypto/tls/tls.go#L238)
 func ParsePEMPrivateKey(key []byte) (crypto.PrivateKey, error) {
 	keyBlockDER, _ := pem.Decode(key)
+	if keyBlockDER == nil {
+		return nil, fmt.Errorf("invalid PEM block")
+	}
 
 	if keyBlockDER.Type != "PRIVATE KEY" && !strings.HasSuffix(keyBlockDER.Type, " PRIVATE KEY") {
 		return nil, fmt.Errorf("unknown PEM header %q", keyBlockDER.Type)

certcrypto/crypto_test.go

@@ -5,6 +5,7 @@ import (
 	"crypto"
 	"crypto/rand"
 	"crypto/rsa"
+	"encoding/pem"
 	"testing"
 	"time"
 
@@ -140,6 +141,30 @@ func TestParsePEMCertificate(t *testing.T) {
 	assert.Equal(t, expiration.UTC(), cert.NotAfter)
 }
 
+func TestParsePEMPrivateKey(t *testing.T) {
+	privateKey, err := GeneratePrivateKey(RSA2048)
+	require.NoError(t, err, "Error generating private key")
+
+	pemPrivateKey := PEMEncode(privateKey)
+
+	// Decoding a key should work and create an identical key to the original
+	decoded, err := ParsePEMPrivateKey(pemPrivateKey)
+	require.NoError(t, err)
+	assert.Equal(t, decoded, privateKey)
+
+	// Decoding a PEM block that doesn't contain a private key should error
+	_, err = ParsePEMPrivateKey(pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE"}))
+	require.Errorf(t, err, "Expected to return an error for non-private key input")
+
+	// Decoding a PEM block that doesn't actually contain a key should error
+	_, err = ParsePEMPrivateKey(pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY"}))
+	require.Errorf(t, err, "Expected to return an error for empty input")
+
+	// Decoding non-PEM input should return an error
+	_, err = ParsePEMPrivateKey([]byte("This is not PEM"))
+	require.Errorf(t, err, "Expected to return an error for non-PEM input")
+}
+
 type MockRandReader struct {
 	b *bytes.Buffer
 }