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.

cmd/config/crypto.go

@@ -0,0 +1,138 @@
+// 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 config
+
+import (
+	"bytes"
+	"crypto/rand"
+	"encoding/binary"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+
+	"github.com/minio/minio/pkg/kms"
+	"github.com/secure-io/sio-go"
+	"github.com/secure-io/sio-go/sioutil"
+)
+
+// Encrypt encrypts the plaintext with a key managed by KMS.
+// The context is bound to the returned ciphertext.
+//
+// The same context must be provided when decrypting the
+// ciphertext.
+func Encrypt(KMS kms.KMS, plaintext io.Reader, context kms.Context) (io.Reader, error) {
+	var algorithm = sio.AES_256_GCM
+	if !sioutil.NativeAES() {
+		algorithm = sio.ChaCha20Poly1305
+	}
+
+	key, err := KMS.GenerateKey("", context)
+	if err != nil {
+		return nil, err
+	}
+	stream, err := algorithm.Stream(key.Plaintext)
+	if err != nil {
+		return nil, err
+	}
+	nonce := make([]byte, stream.NonceSize())
+	if _, err := rand.Read(nonce); err != nil {
+		return nil, err
+	}
+
+	const (
+		MaxMetadataSize = 1 << 20 // max. size of the metadata
+		Version         = 1
+	)
+	var (
+		header [5]byte
+		buffer bytes.Buffer
+	)
+	metadata, err := json.Marshal(encryptedObject{
+		KeyID:     key.KeyID,
+		KMSKey:    key.Ciphertext,
+		Algorithm: algorithm,
+		Nonce:     nonce,
+	})
+	if err != nil {
+		return nil, err
+	}
+	if len(metadata) > MaxMetadataSize {
+		return nil, errors.New("config: encryption metadata is too large")
+	}
+	header[0] = Version
+	binary.LittleEndian.PutUint32(header[1:], uint32(len(metadata)))
+	buffer.Write(header[:])
+	buffer.Write(metadata)
+
+	return io.MultiReader(
+		&buffer,
+		stream.EncryptReader(plaintext, nonce, nil),
+	), nil
+}
+
+// Decrypt decrypts the ciphertext using a key managed by the KMS.
+// The same context that have been used during encryption must be
+// provided.
+func Decrypt(KMS kms.KMS, ciphertext io.Reader, context kms.Context) (io.Reader, error) {
+	const (
+		MaxMetadataSize = 1 << 20 // max. size of the metadata
+		Version         = 1
+	)
+
+	var header [5]byte
+	if _, err := io.ReadFull(ciphertext, header[:]); err != nil {
+		return nil, err
+	}
+	if header[0] != Version {
+		return nil, fmt.Errorf("config: unknown ciphertext version %d", header[0])
+	}
+	size := binary.LittleEndian.Uint32(header[1:])
+	if size > MaxMetadataSize {
+		return nil, errors.New("config: encryption metadata is too large")
+	}
+
+	var (
+		metadataBuffer = make([]byte, size)
+		metadata       encryptedObject
+	)
+	if _, err := io.ReadFull(ciphertext, metadataBuffer); err != nil {
+		return nil, err
+	}
+	if err := json.Unmarshal(metadataBuffer, &metadata); err != nil {
+		return nil, err
+	}
+
+	key, err := KMS.DecryptKey(metadata.KeyID, metadata.KMSKey, context)
+	if err != nil {
+		return nil, err
+	}
+	stream, err := metadata.Algorithm.Stream(key)
+	if err != nil {
+		return nil, err
+	}
+	if stream.NonceSize() != len(metadata.Nonce) {
+		return nil, sio.NotAuthentic
+	}
+	return stream.DecryptReader(ciphertext, metadata.Nonce, nil), nil
+}
+
+type encryptedObject struct {
+	KeyID  string `json:"keyid"`
+	KMSKey []byte `json:"kmskey"`
+
+	Algorithm sio.Algorithm `json:"algorithm"`
+	Nonce     []byte        `json:"nonce"`
+}