add support for SSE-S3 bulk ETag decryption (#14627)

This commit adds support for bulk ETag
decryption for SSE-S3 encrypted objects.

If KES supports a bulk decryption API, then
MinIO will check whether its policy grants
access to this API. If so, MinIO will use
a bulk API call instead of sending encrypted
ETags serially to KES.

Note that MinIO will not use the KES bulk API
if its client certificate is an admin identity.

MinIO will process object listings in batches.
A batch has a configurable size that can be set
via `MINIO_KMS_KES_BULK_API_BATCH_SIZE=N`.
It defaults to `500`.

This env. variable is experimental and may be
renamed / removed in the future.

Signed-off-by: Andreas Auernhammer <hi@aead.dev>

internal/kms/kms.go

@@ -20,8 +20,10 @@ package kms
 import (
 	"encoding"
 	"encoding/json"
+	"strconv"
 
 	jsoniter "github.com/json-iterator/go"
+	"github.com/minio/pkg/env"
 )
 
 // KMS is the generic interface that abstracts over
@@ -51,6 +53,23 @@ type KMS interface {
 	// by the key ID. The context must match the context value
 	// used to generate the ciphertext.
 	DecryptKey(keyID string, ciphertext []byte, context Context) ([]byte, error)
+
+	// DecryptAll decrypts all ciphertexts with the key referenced
+	// by the key ID. The contexts must match the context value
+	// used to generate the ciphertexts.
+	DecryptAll(keyID string, ciphertext [][]byte, context []Context) ([][]byte, error)
+}
+
+// BatchSize returns the size of the batches that should be used during
+// KES bulk decryption API calls.
+func BatchSize() int {
+	const DefaultBatchSize = 500
+	v := env.Get("MINIO_KMS_KES_BULK_API_BATCH_SIZE", strconv.Itoa(DefaultBatchSize))
+	n, err := strconv.Atoi(v)
+	if err != nil {
+		return DefaultBatchSize
+	}
+	return n
 }
 
 // Status describes the current state of a KMS.