Add double encryption at S3 gateway. (#6423)

This PR adds pass-through, single encryption at gateway and double
encryption support (gateway encryption with pass through of SSE
headers to backend).

If KMS is set up (either with Vault as KMS or using
MINIO_SSE_MASTER_KEY),gateway will automatically perform
single encryption. If MINIO_GATEWAY_SSE is set up in addition to
Vault KMS, double encryption is performed.When neither KMS nor
MINIO_GATEWAY_SSE is set, do a pass through to backend.

When double encryption is specified, MINIO_GATEWAY_SSE can be set to
"C" for SSE-C encryption at gateway and backend, "S3" for SSE-S3
encryption at gateway/backend or both to support more than one option.

Fixes #6323, #6696

cmd/gateway-common.go

@@ -18,7 +18,10 @@ package cmd
 
 import (
 	"net/http"
+	"os"
+	"strings"
 
+	"github.com/minio/minio/cmd/logger"
 	"github.com/minio/minio/pkg/hash"
 
 	minio "github.com/minio/minio-go"
@@ -30,8 +33,27 @@ var (
 
 	// MustGetUUID function alias.
 	MustGetUUID = mustGetUUID
+
+	// IsMinAllowedPartSize function alias.
+	IsMinAllowedPartSize = isMinAllowedPartSize
+
+	// GetCompleteMultipartMD5 functon alias.
+	GetCompleteMultipartMD5 = getCompleteMultipartMD5
+
+	// Contains function alias.
+	Contains = contains
+
+	// ExtractETag provides extractETag function alias.
+	ExtractETag = extractETag
+	// CleanMetadataKeys provides cleanMetadataKeys function alias.
+	CleanMetadataKeys = cleanMetadataKeys
 )
 
+// StatInfo -  alias for statInfo
+type StatInfo struct {
+	statInfo
+}
+
 // AnonErrToObjectErr - converts standard http codes into meaningful object layer errors.
 func AnonErrToObjectErr(statusCode int, params ...string) error {
 	bucket := ""
@@ -321,3 +343,30 @@ func ErrorRespToObjectError(err error, params ...string) error {
 
 	return err
 }
+
+// parse gateway sse env variable
+func parseGatewaySSE(s string) (gatewaySSE, error) {
+	l := strings.Split(s, ";")
+	var gwSlice = make([]string, 0)
+	for _, val := range l {
+		v := strings.ToUpper(val)
+		if v == gatewaySSES3 || v == gatewaySSEC {
+			gwSlice = append(gwSlice, v)
+			continue
+		}
+		return nil, uiErrInvalidGWSSEValue(nil).Msg("gateway SSE cannot be (%s) ", v)
+	}
+	return gatewaySSE(gwSlice), nil
+}
+
+// handle gateway env vars
+func handleGatewayEnvVars() {
+	gwsseVal, ok := os.LookupEnv("MINIO_GATEWAY_SSE")
+	if ok {
+		var err error
+		GlobalGatewaySSE, err = parseGatewaySSE(gwsseVal)
+		if err != nil {
+			logger.Fatal(err, "Unable to parse MINIO_GATEWAY_SSE value (`%s`)", gwsseVal)
+		}
+	}
+}