ssh: support for marshaling keys using the OpenSSH format

This adds methods to marshal private keys, encrypted and unencrypted
to the OpenSSH format.

Fixes golang/go#37132

Change-Id: I1a95301f789ce04858e6b147748c6e8b7700384b
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/218620
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>

ssh/keys_test.go

@@ -281,6 +281,74 @@ func TestMarshalParsePublicKey(t *testing.T) {
 	}
 }
 
+func TestMarshalPrivateKey(t *testing.T) {
+	tests := []struct {
+		name string
+	}{
+		{"rsa-openssh-format"},
+		{"ed25519"},
+		{"p256-openssh-format"},
+		{"p384-openssh-format"},
+		{"p521-openssh-format"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			expected, ok := testPrivateKeys[tt.name]
+			if !ok {
+				t.Fatalf("cannot find key %s", tt.name)
+			}
+
+			block, err := MarshalPrivateKey(expected, "test@golang.org")
+			if err != nil {
+				t.Fatalf("cannot marshal %s: %v", tt.name, err)
+			}
+
+			key, err := ParseRawPrivateKey(pem.EncodeToMemory(block))
+			if err != nil {
+				t.Fatalf("cannot parse %s: %v", tt.name, err)
+			}
+
+			if !reflect.DeepEqual(expected, key) {
+				t.Errorf("unexpected marshaled key %s", tt.name)
+			}
+		})
+	}
+}
+
+func TestMarshalPrivateKeyWithPassphrase(t *testing.T) {
+	tests := []struct {
+		name string
+	}{
+		{"rsa-openssh-format"},
+		{"ed25519"},
+		{"p256-openssh-format"},
+		{"p384-openssh-format"},
+		{"p521-openssh-format"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			expected, ok := testPrivateKeys[tt.name]
+			if !ok {
+				t.Fatalf("cannot find key %s", tt.name)
+			}
+
+			block, err := MarshalPrivateKeyWithPassphrase(expected, "test@golang.org", []byte("test-passphrase"))
+			if err != nil {
+				t.Fatalf("cannot marshal %s: %v", tt.name, err)
+			}
+
+			key, err := ParseRawPrivateKeyWithPassphrase(pem.EncodeToMemory(block), []byte("test-passphrase"))
+			if err != nil {
+				t.Fatalf("cannot parse %s: %v", tt.name, err)
+			}
+
+			if !reflect.DeepEqual(expected, key) {
+				t.Errorf("unexpected marshaled key %s", tt.name)
+			}
+		})
+	}
+}
+
 type testAuthResult struct {
 	pubKey   PublicKey
 	options  []string