to be merged: cache cipher in TransparentEncryptionInfo

aes.go

@@ -46,11 +46,15 @@ func aesCtrStep(offset int64, enc *TransparentEncryptionInfo, b []byte) ([]byte,
 		return nil, err
 	}
 
-	block, err := aes.NewCipher(enc.key)
-	if err != nil {
-		return nil, err
+	if enc.cipher == nil {
+		cipher, err := aes.NewCipher(enc.key)
+		if err != nil {
+			return nil, err
+		}
+		enc.cipher = cipher
 	}
-	stream := cipher.NewCTR(block, iv)
+
+	stream := cipher.NewCTR(enc.cipher, iv)
 
 	padding := offset % aes.BlockSize
 	if padding > 0 {

file_reader.go

@@ -1,6 +1,7 @@
 package hdfs
 
 import (
+	"crypto/cipher"
 	"crypto/md5"
 	"errors"
 	"fmt"
@@ -36,8 +37,9 @@ type FileReader struct {
 
 // A TransparentEncryptionInfo is a key and iv to encrypt or decrypt file data
 type TransparentEncryptionInfo struct {
-	key []byte
-	iv  []byte
+	key    []byte
+	iv     []byte
+	cipher cipher.Block
 }
 
 // Open returns an FileReader which can be used for reading.