Add support for importing PKCS#8 private keys, and add validation (#1300)

This adds support for PKCS#8 encoded private keys, which means
we can also import ED25519 keys. I've added tests for PKCS#8
RSA and ECDSA keys too.

I also added some validation of keys before importing. For RSA,
we will require that the key size be between 2048 and 4096.
For ECDSA keys, we will only disallow NIST P-224, since Cosign
generates P-256 by default. Other curves are not supported
by Go's crypto library.

Signed-off-by: Hayden Blauzvern <[email protected]>

Author: haydentherapper

pkg/cosign/keys.go

@@ -18,6 +18,7 @@ package cosign
 import (
 	"crypto"
 	"crypto/ecdsa"
+	"crypto/ed25519"
 	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
@@ -37,10 +38,14 @@ import (
 )
 
 const (
-	PrivateKeyPemType    = "ENCRYPTED COSIGN PRIVATE KEY"
+	CosignPrivateKeyPemType = "ENCRYPTED COSIGN PRIVATE KEY"
+	// PEM-encoded PKCS #1 RSA private key
 	RSAPrivateKeyPemType = "RSA PRIVATE KEY"
-	ECPrivateKeyPemType  = "EC PRIVATE KEY"
-	BundleKey            = static.BundleAnnotationKey
+	// PEM-encoded ECDSA private key
+	ECPrivateKeyPemType = "EC PRIVATE KEY"
+	// PEM-encoded PKCS #8 RSA, ECDSA or ED25519 private key
+	PrivateKeyPemType = "PRIVATE KEY"
+	BundleKey         = static.BundleAnnotationKey
 )
 
 type PassFunc func(bool) ([]byte, error)
@@ -56,6 +61,35 @@ type KeysBytes struct {
 	password     []byte
 }
 
+// Enforce a minimum and maximum RSA key size.
+func validateRsaKey(pk *rsa.PrivateKey) error {
+	// Key size is the bit length of modulus
+	keySize := pk.N.BitLen()
+	if keySize < 2048 {
+		return fmt.Errorf("rsa key size too small, expected >= 2048")
+	}
+	if keySize > 4096 {
+		return fmt.Errorf("rsa key size too large, expected <= 4096")
+	}
+	return nil
+}
+
+// Enforce that the ECDSA key curve is one of:
+// * NIST P-256 (secp256r1, prime256v1)
+// * NIST P-384
+// * NIST P-521.
+// Other EC curves, like secp256k1, are not supported by Go.
+func validateEcdsaKey(pk *ecdsa.PrivateKey) error {
+	switch pk.Curve {
+	case elliptic.P224():
+		return fmt.Errorf("unsupported ec curve, expected NIST P-256, P-384, or P-521")
+	case elliptic.P256(), elliptic.P384(), elliptic.P521():
+		return nil
+	default:
+		return fmt.Errorf("unexpected ec curve")
+	}
+}
+
 func GeneratePrivateKey() (*ecdsa.PrivateKey, error) {
 	return ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
 }
@@ -75,15 +109,47 @@ func ImportKeyPair(keyPath string, pf PassFunc) (*KeysBytes, error) {
 
 	switch p.Type {
 	case RSAPrivateKeyPemType:
-		pk, err = x509.ParsePKCS1PrivateKey(p.Bytes)
+		rsaPk, err := x509.ParsePKCS1PrivateKey(p.Bytes)
 		if err != nil {
-			return nil, fmt.Errorf("parsing error")
+			return nil, fmt.Errorf("error parsing rsa private key")
 		}
-	default:
-		pk, err = x509.ParseECPrivateKey(p.Bytes)
+		if err = validateRsaKey(rsaPk); err != nil {
+			return nil, errors.Wrap(err, "error validating rsa key")
+		}
+		pk = rsaPk
+	case ECPrivateKeyPemType:
+		ecdsaPk, err := x509.ParseECPrivateKey(p.Bytes)
 		if err != nil {
-			return nil, fmt.Errorf("parsing error")
+			return nil, fmt.Errorf("error parsing ecdsa private key")
 		}
+		if err = validateEcdsaKey(ecdsaPk); err != nil {
+			return nil, errors.Wrap(err, "error validating ecdsa key")
+		}
+		pk = ecdsaPk
+	case PrivateKeyPemType:
+		pkcs8Pk, err := x509.ParsePKCS8PrivateKey(p.Bytes)
+		if err != nil {
+			return nil, fmt.Errorf("error parsing pkcs #8 private key")
+		}
+		switch k := pkcs8Pk.(type) {
+		case *rsa.PrivateKey:
+			if err = validateRsaKey(k); err != nil {
+				return nil, errors.Wrap(err, "error validating rsa key")
+			}
+			pk = k
+		case *ecdsa.PrivateKey:
+			if err = validateEcdsaKey(k); err != nil {
+				return nil, errors.Wrap(err, "error validating ecdsa key")
+			}
+			pk = k
+		case ed25519.PrivateKey:
+			// Nothing to validate, since ED25519 supports only one key size.
+			pk = k
+		default:
+			return nil, fmt.Errorf("unexpected private key")
+		}
+	default:
+		return nil, fmt.Errorf("unsupported private key")
 	}
 	return marshalKeyPair(Keys{pk, pk.Public()}, pf)
 }
@@ -107,7 +173,7 @@ func marshalKeyPair(keypair Keys, pf PassFunc) (*KeysBytes, error) {
 	// store in PEM format
 	privBytes := pem.EncodeToMemory(&pem.Block{
 		Bytes: encBytes,
-		Type:  PrivateKeyPemType,
+		Type:  CosignPrivateKeyPemType,
 	})
 
 	// Now do the public key
@@ -154,7 +220,7 @@ func LoadPrivateKey(key []byte, pass []byte) (signature.SignerVerifier, error) {
 	if p == nil {
 		return nil, errors.New("invalid pem block")
 	}
-	if p.Type != PrivateKeyPemType {
+	if p.Type != CosignPrivateKeyPemType {
 		return nil, fmt.Errorf("unsupported pem type: %s", p.Type)
 	}