ssh: add ParsePrivateKeysWithPassphrase

ssh package doesn't provide way to parse private keys with passphrase.

Fixes golang/go#18692

Change-Id: Ic139f11b6dfe7ef61690d6125e0673d50a48db16
Reviewed-on: https://go-review.googlesource.com/36079
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/keys_test.go

@@ -148,6 +148,25 @@ func TestParseEncryptedPrivateKeysFails(t *testing.T) {
 	}
 }
 
+// Parse encrypted private keys with passphrase
+func TestParseEncryptedPrivateKeysWithPassphrase(t *testing.T) {
+	data := []byte("sign me")
+	for _, tt := range testdata.PEMEncryptedKeys {
+		s, err := ParsePrivateKeyWithPassphrase(tt.PEMBytes, []byte(tt.EncryptionKey))
+		if err != nil {
+			t.Fatalf("ParsePrivateKeyWithPassphrase returned error: %s", err)
+			continue
+		}
+		sig, err := s.Sign(rand.Reader, data)
+		if err != nil {
+			t.Fatalf("dsa.Sign: %v", err)
+		}
+		if err := s.PublicKey().Verify(data, sig); err != nil {
+			t.Errorf("Verify failed: %v", err)
+		}
+	}
+}
+
 func TestParseDSA(t *testing.T) {
 	// We actually exercise the ParsePrivateKey codepath here, as opposed to
 	// using the ParseRawPrivateKey+NewSignerFromKey path that testdata_test.go