introduce new package pkg/kms (#12019)

This commit introduces a new package `pkg/kms`.
It contains basic types and functions to interact
with various KMS implementations.

This commit also moves KMS-related code from `cmd/crypto`
to `pkg/kms`. Now, it is possible to implement a KMS-based
config data encryption in the `pkg/config` package.

pkg/kms/dek_test.go

@@ -0,0 +1,69 @@
+// MinIO Cloud Storage, (C) 2021 MinIO, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package kms
+
+import (
+	"bytes"
+	"encoding/base64"
+	"testing"
+)
+
+var dekEncodeDecodeTests = []struct {
+	Key DEK
+}{
+	{
+		Key: DEK{},
+	},
+	{
+		Key: DEK{
+			Plaintext:  nil,
+			Ciphertext: mustDecodeB64("eyJhZWFkIjoiQUVTLTI1Ni1HQ00tSE1BQy1TSEEtMjU2IiwiaXYiOiJ3NmhLUFVNZXVtejZ5UlVZL29pTFVBPT0iLCJub25jZSI6IktMSEU3UE1jRGo2N2UweHkiLCJieXRlcyI6Ik1wUkhjQWJaTzZ1Sm5lUGJGcnpKTkxZOG9pdkxwTmlUcTNLZ0hWdWNGYkR2Y0RlbEh1c1lYT29zblJWVTZoSXIifQ=="),
+		},
+	},
+	{
+		Key: DEK{
+			Plaintext:  mustDecodeB64("GM2UvLXp/X8lzqq0mibFC0LayDCGlmTHQhYLj7qAy7Q="),
+			Ciphertext: mustDecodeB64("eyJhZWFkIjoiQUVTLTI1Ni1HQ00tSE1BQy1TSEEtMjU2IiwiaXYiOiJ3NmhLUFVNZXVtejZ5UlVZL29pTFVBPT0iLCJub25jZSI6IktMSEU3UE1jRGo2N2UweHkiLCJieXRlcyI6Ik1wUkhjQWJaTzZ1Sm5lUGJGcnpKTkxZOG9pdkxwTmlUcTNLZ0hWdWNGYkR2Y0RlbEh1c1lYT29zblJWVTZoSXIifQ=="),
+		},
+	},
+}
+
+func TestEncodeDecodeDEK(t *testing.T) {
+	for i, test := range dekEncodeDecodeTests {
+		text, err := test.Key.MarshalText()
+		if err != nil {
+			t.Fatalf("Test %d: failed to marshal DEK: %v", i, err)
+		}
+
+		var key DEK
+		if err = key.UnmarshalText(text); err != nil {
+			t.Fatalf("Test %d: failed to unmarshal DEK: %v", i, err)
+		}
+		if key.Plaintext != nil {
+			t.Fatalf("Test %d: unmarshaled DEK contains non-nil plaintext", i)
+		}
+		if !bytes.Equal(key.Ciphertext, test.Key.Ciphertext) {
+			t.Fatalf("Test %d: ciphertext mismatch: got %x - want %x", i, key.Ciphertext, test.Key.Ciphertext)
+		}
+	}
+}
+
+func mustDecodeB64(s string) []byte {
+	b, err := base64.StdEncoding.DecodeString(s)
+	if err != nil {
+		panic(err)
+	}
+	return b
+}