enable SSE-KMS pass-through on S3 gateway (#7788)

This commit relaxes the restriction that the MinIO gateway
does not accept SSE-KMS headers. Now, the S3 gateway allows
SSE-KMS headers for PUT and MULTIPART PUT requests and forwards them
to the S3 gateway backend (AWS). This is considered SSE pass-through
mode.

Fixes #7753

cmd/crypto/header.go

@@ -18,6 +18,7 @@ import (
 	"bytes"
 	"crypto/md5"
 	"encoding/base64"
+	"encoding/json"
 	"net/http"
 	"strings"
 )
@@ -125,6 +126,25 @@ func (s3KMS) IsRequested(h http.Header) bool {
 	return false
 }
 
+// ParseHTTP parses the SSE-KMS headers and returns the SSE-KMS key ID
+// and context, if present, on success.
+func (s3KMS) ParseHTTP(h http.Header) (string, interface{}, error) {
+	algorithm := h.Get(SSEHeader)
+	if algorithm != SSEAlgorithmKMS {
+		return "", nil, ErrInvalidEncryptionMethod
+	}
+
+	contextStr, ok := h[SSEKmsContext]
+	if ok {
+		var context map[string]interface{}
+		if err := json.Unmarshal([]byte(contextStr[0]), &context); err != nil {
+			return "", nil, err
+		}
+		return h.Get(SSEKmsID), context, nil
+	}
+	return h.Get(SSEKmsID), nil, nil
+}
+
 var (
 	// SSEC represents AWS SSE-C. It provides functionality to handle
 	// SSE-C requests.