new package: cmd/crypto (#6062)

This commit introduces a new crypto package providing
AWS S3 related cryptographic building blocks to implement
SSE-S3 (master key or KMS) and SSE-C.

This change only adds some basic functionallity esp.
related to SSE-S3 and documents the general approach
for SSE-S3 and SSE-C.

cmd/crypto/error.go

@@ -0,0 +1,23 @@
+// Minio Cloud Storage, (C) 2015, 2016, 2017, 2018 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 crypto
+
+import "errors"
+
+var (
+	// ErrInvalidEncryptionMethod indicates that the specified SSE encryption method
+	// is not supported.
+	ErrInvalidEncryptionMethod = errors.New("The encryption method is not supported")
+)