swarm/storage/encryption: Added encryption package

First step towards
https://github.com/ethersphere/swarm/wiki/Symmetric-Encryption-for-Swarm-Content

swarm/storage/encryption/encryption.go

@@ -0,0 +1,107 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package encryption
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"fmt"
+	"hash"
+)
+
+type Key []byte
+
+type Encryption interface {
+	Encrypt(data []byte, key Key) ([]byte, error)
+	Decrypt(data []byte, key Key) ([]byte, error)
+}
+
+type encryption struct {
+	padding  int
+	initCtr  uint32
+	hashFunc func() hash.Hash
+}
+
+func New(padding int, initCtr uint32, hashFunc func() hash.Hash) *encryption {
+	return &encryption{
+		padding:  padding,
+		initCtr:  initCtr,
+		hashFunc: hashFunc,
+	}
+}
+
+func (e *encryption) Encrypt(data []byte, key Key) ([]byte, error) {
+	length := len(data)
+	isFixedPadding := e.padding > 0
+	if isFixedPadding && length > e.padding {
+		return nil, fmt.Errorf("Data length longer than padding, data length %v padding %v", length, e.padding)
+	}
+
+	paddedData := data
+	if isFixedPadding && length < e.padding {
+		paddedData = make([]byte, e.padding)
+		copy(paddedData[:length], data)
+		rand.Read(paddedData[length:])
+	}
+	return e.transform(paddedData, key), nil
+}
+
+func (e *encryption) Decrypt(data []byte, key Key) ([]byte, error) {
+	length := len(data)
+	if e.padding > 0 && length != e.padding {
+		return nil, fmt.Errorf("Data length different than padding, data length %v padding %v", length, e.padding)
+	}
+
+	return e.transform(data, key), nil
+}
+
+func (e *encryption) transform(data []byte, key Key) []byte {
+	dataLength := len(data)
+	transformedData := make([]byte, dataLength)
+	hasher := e.hashFunc()
+	ctr := e.initCtr
+	hashSize := hasher.Size()
+	for i := 0; i < dataLength; i += hashSize {
+		hasher.Write(key)
+
+		ctrBytes := make([]byte, 4)
+		binary.LittleEndian.PutUint32(ctrBytes, ctr)
+
+		hasher.Write(ctrBytes)
+
+		ctrHash := hasher.Sum(nil)
+		hasher.Reset()
+		hasher.Write(ctrHash)
+
+		segmentKey := hasher.Sum(nil)
+
+		hasher.Reset()
+
+		for j := 0; j < min(hashSize, dataLength-i); j++ {
+			transformedData[i+j] = data[i+j] ^ segmentKey[j]
+		}
+		ctr++
+	}
+	return transformedData
+}
+
+func min(x, y int) int {
+	if x < y {
+		return x
+	}
+	return y
+}