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/header.go

@@ -0,0 +1,49 @@
+// 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 (
+	"net/http"
+)
+
+// SSEHeader is the general AWS SSE HTTP header key.
+const SSEHeader = "X-Amz-Server-Side-Encryption"
+
+// SSEAlgorithmAES256 is the only supported value for the SSE-S3 or SSE-C algorithm header.
+// For SSE-S3 see: https://docs.aws.amazon.com/AmazonS3/latest/dev/SSEUsingRESTAPI.html
+// For SSE-C  see: https://docs.aws.amazon.com/AmazonS3/latest/dev/ServerSideEncryptionCustomerKeys.html
+const SSEAlgorithmAES256 = "AES256"
+
+// S3 represents AWS SSE-S3. It provides functionality to handle
+// SSE-S3 requests.
+var S3 = s3{}
+
+type s3 struct{}
+
+// IsRequested returns true if the HTTP headers indicates that
+// the S3 client requests SSE-S3.
+func (s3) IsRequested(h http.Header) bool {
+	_, ok := h[SSEHeader]
+	return ok
+}
+
+// Parse parses the SSE-S3 related HTTP headers and checks
+// whether they contain valid values.
+func (s3) Parse(h http.Header) (err error) {
+	if h.Get(SSEHeader) != SSEAlgorithmAES256 {
+		err = ErrInvalidEncryptionMethod
+	}
+	return
+}