ssh: signal incorrect private key passwords with x509.IncorrectPasswordError

Fixes golang/go#20781

Change-Id: Iae42fff3c9b0b9984509e44a92f9bc99a1a12470
Reviewed-on: https://go-review.googlesource.com/46439
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/keys_test.go

@@ -11,6 +11,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/x509"
 	"encoding/base64"
 	"fmt"
 	"reflect"
@@ -165,6 +166,12 @@ func TestParseEncryptedPrivateKeysWithPassphrase(t *testing.T) {
 			t.Errorf("Verify failed: %v", err)
 		}
 	}
+
+	tt := testdata.PEMEncryptedKeys[0]
+	_, err := ParsePrivateKeyWithPassphrase(tt.PEMBytes, []byte("incorrect"))
+	if err != x509.IncorrectPasswordError {
+		t.Fatalf("got %v want IncorrectPasswordError", err)
+	}
 }
 
 func TestParseDSA(t *testing.T) {