Add AWS S3 SSE-C example. (#689)

This PR adds an example to showcase how one can use AWS S3
to enable Server-Side Encryption with Customer-Provided
Encryption Keys (SSE-C)

examples/s3/putobject-getobject-sse.go

@@ -0,0 +1,87 @@
+// +build ignore
+
+/*
+ * Minio Go Library for Amazon S3 Compatible Cloud Storage (C) 2017 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 main
+
+import (
+	"bytes"
+	"crypto/md5"
+	"encoding/base64"
+	"io/ioutil"
+	"log"
+	"net/http"
+
+	minio "github.com/minio/minio-go"
+)
+
+func main() {
+	// Note: YOUR-ACCESSKEYID, YOUR-SECRETACCESSKEY, my-testfile, my-bucketname and
+	// my-objectname are dummy values, please replace them with original values.
+
+	// New returns an Amazon S3 compatible client object. API compatibility (v2 or v4) is automatically
+	// determined based on the Endpoint value.
+	minioClient, err := minio.New("s3.amazonaws.com", "YOUR-ACCESSKEYID", "YOUR-SECRETACCESSKEY", true)
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	content := bytes.NewReader([]byte("Hello again"))
+	key := []byte("32byteslongsecretkeymustprovided")
+	h := md5.New()
+	h.Write(key)
+	encryptionKey := base64.StdEncoding.EncodeToString(key)
+	encryptionKeyMD5 := base64.StdEncoding.EncodeToString(h.Sum(nil))
+
+	// Amazon S3 does not store the encryption key you provide.
+	// Instead S3 stores a randomly salted HMAC value of the
+	// encryption key in order to validate future requests.
+	// The salted HMAC value cannot be used to derive the value
+	// of the encryption key or to decrypt the contents of the
+	// encrypted object. That means, if you lose the encryption
+	// key, you lose the object.
+	var metadata = map[string][]string{
+		"x-amz-server-side-encryption-customer-algorithm": []string{"AES256"},
+		"x-amz-server-side-encryption-customer-key":       []string{encryptionKey},
+		"x-amz-server-side-encryption-customer-key-MD5":   []string{encryptionKeyMD5},
+	}
+
+	// minioClient.TraceOn(os.Stderr) // Enable to debug.
+	_, err = minioClient.PutObjectWithMetadata("mybucket", "my-encrypted-object.txt", content, metadata, nil)
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	var reqHeaders = minio.RequestHeaders{Header: http.Header{}}
+	for k, v := range metadata {
+		reqHeaders.Set(k, v[0])
+	}
+	coreClient := minio.Core{minioClient}
+	reader, _, err := coreClient.GetObject("mybucket", "my-encrypted-object.txt", reqHeaders)
+	if err != nil {
+		log.Fatalln(err)
+	}
+	defer reader.Close()
+
+	decBytes, err := ioutil.ReadAll(reader)
+	if err != nil {
+		log.Fatalln(err)
+	}
+	if !bytes.Equal(decBytes, []byte("Hello again")) {
+		log.Fatalln("Expected \"Hello, world\", got %s", string(decBytes))
+	}
+}