Make decryptReader fulfill flate.Reader

benchmark               old ns/op     new ns/op     delta
    BenchmarkLoadJSONID     50232171      48596972      -3.26%

    benchmark               old allocs     new allocs     delta
    BenchmarkLoadJSONID     43643          42884          -1.74%

    benchmark               old bytes     new bytes     delta
    BenchmarkLoadJSONID     5773048       3785517       -34.43%

key.go

@@ -543,6 +543,24 @@ func (d *decryptReader) Read(dst []byte) (int, error) {
 	return n, nil
 }
 
+func (d *decryptReader) ReadByte() (c byte, err error) {
+	if d.buf == nil {
+		return 0, io.EOF
+	}
+
+	remaining := len(d.buf) - d.pos
+	if remaining == 1 {
+		c = d.buf[d.pos]
+		d.Close()
+		return c, io.EOF
+	}
+
+	c = d.buf[d.pos]
+	d.pos++
+
+	return
+}
+
 func (d *decryptReader) Close() error {
 	if d.buf == nil {
 		return nil

server.go

@@ -2,6 +2,7 @@ package restic
 
 import (
 	"bytes"
+	"compress/flate"
 	"compress/zlib"
 	"crypto/sha256"
 	"encoding/json"
@@ -146,8 +147,10 @@ func (s Server) LoadJSONID(t backend.Type, storageID backend.ID, item interface{
 	}
 
 	// unzip
+	br := decryptRd.(flate.Reader)
+
 	unzipRd := zReaderPool.Get().(zReader)
-	err = unzipRd.Reset(decryptRd, nil)
+	err = unzipRd.Reset(br, nil)
 	defer func() {
 		unzipRd.Close()
 		zReaderPool.Put(unzipRd)