ssh: reject unencrypted keys from ParsePrivateKeyWithPassphrase

The behavior of ParsePrivateKeyWithPassphrase when the key is
unencrypted is unspecified. Currently, it just parses them like
ParsePrivateKey, which is unlikely to be what anyone wants: for us to
ignore a passphrase that they explicitly passed. It also makes the
implementation of encrypted OpenSSH keys in the next CL more confused.

Instead, make ParsePrivateKey return a PassphraseNeededError, so the
application logic can be ParsePrivateKey -> detect encrypted key ->
obtain passphrase -> ParsePrivateKeyWithPassphrase. That error will also
let us return the public key for OpenSSH keys.

Change-Id: Ife4fb2499ae538bef36e353adf9bc8e902662386

ssh/keys.go

@@ -837,7 +837,8 @@ func NewPublicKey(key interface{}) (PublicKey, error) {
 }
 
 // ParsePrivateKey returns a Signer from a PEM encoded private key. It supports
-// the same keys as ParseRawPrivateKey.
+// the same keys as ParseRawPrivateKey. If the private key is encrypted, it
+// will return a PassphraseNeededError.
 func ParsePrivateKey(pemBytes []byte) (Signer, error) {
 	key, err := ParseRawPrivateKey(pemBytes)
 	if err != nil {
@@ -850,8 +851,8 @@ func ParsePrivateKey(pemBytes []byte) (Signer, error) {
 // ParsePrivateKeyWithPassphrase returns a Signer from a PEM encoded private
 // key and passphrase. It supports the same keys as
 // ParseRawPrivateKeyWithPassphrase.
-func ParsePrivateKeyWithPassphrase(pemBytes, passPhrase []byte) (Signer, error) {
-	key, err := ParseRawPrivateKeyWithPassphrase(pemBytes, passPhrase)
+func ParsePrivateKeyWithPassphrase(pemBytes, passphrase []byte) (Signer, error) {
+	key, err := ParseRawPrivateKeyWithPassphrase(pemBytes, passphrase)
 	if err != nil {
 		return nil, err
 	}
@@ -867,16 +868,29 @@ func encryptedBlock(block *pem.Block) bool {
 	return strings.Contains(block.Headers["Proc-Type"], "ENCRYPTED")
 }
 
+// A PassphraseNeededError indicates that parsing this private key requires a
+// passphrase. Use ParsePrivateKeyWithPassphrase.
+type PassphraseNeededError struct {
+	// PublicKey will be set if the private key format includes an unencrypted
+	// public key along with the encrypted private key.
+	PublicKey PublicKey
+}
+
+func (*PassphraseNeededError) Error() string {
+	return "ssh: this private key is passphrase protected"
+}
+
 // ParseRawPrivateKey returns a private key from a PEM encoded private key. It
-// supports RSA (PKCS#1), PKCS#8, DSA (OpenSSL), and ECDSA private keys.
+// supports RSA (PKCS#1), PKCS#8, DSA (OpenSSL), and ECDSA private keys. If the
+// private key is encrypted, it will return a PassphraseNeededError.
 func ParseRawPrivateKey(pemBytes []byte) (interface{}, error) {
 	block, _ := pem.Decode(pemBytes)
 	if block == nil {
 		return nil, errors.New("ssh: no key found")
 	}
 
 	if encryptedBlock(block) {
-		return nil, errors.New("ssh: cannot decode encrypted private keys")
+		return nil, &PassphraseNeededError{}
 	}
 
 	switch block.Type {
@@ -899,24 +913,22 @@ func ParseRawPrivateKey(pemBytes []byte) (interface{}, error) {
 // ParseRawPrivateKeyWithPassphrase returns a private key decrypted with
 // passphrase from a PEM encoded private key. If wrong passphrase, return
 // x509.IncorrectPasswordError.
-func ParseRawPrivateKeyWithPassphrase(pemBytes, passPhrase []byte) (interface{}, error) {
+func ParseRawPrivateKeyWithPassphrase(pemBytes, passphrase []byte) (interface{}, error) {
 	block, _ := pem.Decode(pemBytes)
 	if block == nil {
 		return nil, errors.New("ssh: no key found")
 	}
-	buf := block.Bytes
 
-	if encryptedBlock(block) {
-		if x509.IsEncryptedPEMBlock(block) {
-			var err error
-			buf, err = x509.DecryptPEMBlock(block, passPhrase)
-			if err != nil {
-				if err == x509.IncorrectPasswordError {
-					return nil, err
-				}
-				return nil, fmt.Errorf("ssh: cannot decode encrypted private keys: %v", err)
-			}
+	if !encryptedBlock(block) || !x509.IsEncryptedPEMBlock(block) {
+		return nil, errors.New("ssh: not an encrypted key")
+	}
+
+	buf, err := x509.DecryptPEMBlock(block, passphrase)
+	if err != nil {
+		if err == x509.IncorrectPasswordError {
+			return nil, err
 		}
+		return nil, fmt.Errorf("ssh: cannot decode encrypted private keys: %v", err)
 	}
 
 	switch block.Type {
@@ -926,8 +938,6 @@ func ParseRawPrivateKeyWithPassphrase(pemBytes, passPhrase []byte) (interface{},
 		return x509.ParseECPrivateKey(buf)
 	case "DSA PRIVATE KEY":
 		return ParseDSAPrivateKey(buf)
-	case "OPENSSH PRIVATE KEY":
-		return parseOpenSSHPrivateKey(buf)
 	default:
 		return nil, fmt.Errorf("ssh: unsupported key type %q", block.Type)
 	}

ssh/keys_test.go

@@ -180,16 +180,15 @@ func TestParseECPrivateKey(t *testing.T) {
 
 // See Issue https://github.com/golang/go/issues/6650.
 func TestParseEncryptedPrivateKeysFails(t *testing.T) {
-	const wantSubstring = "encrypted"
 	for i, tt := range testdata.PEMEncryptedKeys {
 		_, err := ParsePrivateKey(tt.PEMBytes)
 		if err == nil {
 			t.Errorf("#%d key %s: ParsePrivateKey successfully parsed, expected an error", i, tt.Name)
 			continue
 		}
 
-		if !strings.Contains(err.Error(), wantSubstring) {
-			t.Errorf("#%d key %s: got error %q, want substring %q", i, tt.Name, err, wantSubstring)
+		if _, ok := err.(*PassphraseNeededError); !ok {
+			t.Errorf("#%d key %s: got error %q, want PassphraseNeededError", i, tt.Name, err)
 		}
 	}
 }