path: root/swarm/storage/encryption/encryption.go

// Copyright 2018 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"
    "sync"
)

const KeyLength = 32

type Key []byte

type Encryption interface {
    Encrypt(data []byte) ([]byte, error)
    Decrypt(data []byte) ([]byte, error)
}

type encryption struct {
    key      Key              // the encryption key (hashSize bytes long)
    keyLen   int              // length of the key = length of blockcipher block
    padding  int              // encryption will pad the data upto this if > 0
    initCtr  uint32           // initial counter used for counter mode blockcipher
    hashFunc func() hash.Hash // hasher constructor function
}

// New constructs a new encryptor/decryptor
func New(key Key, padding int, initCtr uint32, hashFunc func() hash.Hash) *encryption {
    return &encryption{
        key:      key,
        keyLen:   len(key),
        padding:  padding,
        initCtr:  initCtr,
        hashFunc: hashFunc,
    }
}

// Encrypt encrypts the data and does padding if specified
func (e *encryption) Encrypt(data []byte) ([]byte, error) {
    length := len(data)
    outLength := length
    isFixedPadding := e.padding > 0
    if isFixedPadding {
        if length > e.padding {
            return nil, fmt.Errorf("Data length longer than padding, data length %v padding %v", length, e.padding)
        }
        outLength = e.padding
    }
    out := make([]byte, outLength)
    e.transform(data, out)
    return out, nil
}

// Decrypt decrypts the data, if padding was used caller must know original length and truncate
func (e *encryption) Decrypt(data []byte) ([]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)
    }
    out := make([]byte, length)
    e.transform(data, out)
    return out, nil
}

//
func (e *encryption) transform(in, out []byte) {
    inLength := len(in)
    wg := sync.WaitGroup{}
    wg.Add((inLength-1)/e.keyLen + 1)
    for i := 0; i < inLength; i += e.keyLen {
        l := min(e.keyLen, inLength-i)
        // call transformations per segment (asyncronously)
        go func(i int, x, y []byte) {
            defer wg.Done()
            e.Transcrypt(i, x, y)
        }(i/e.keyLen, in[i:i+l], out[i:i+l])
    }
    // pad the rest if out is longer
    pad(out[inLength:])
    wg.Wait()
}

// used for segmentwise transformation
// if in is shorter than out, padding is used
func (e *encryption) Transcrypt(i int, in []byte, out []byte) {
    // first hash key with counter (initial counter + i)
    hasher := e.hashFunc()
    hasher.Write(e.key)

    ctrBytes := make([]byte, 4)
    binary.LittleEndian.PutUint32(ctrBytes, uint32(i)+e.initCtr)
    hasher.Write(ctrBytes)

    ctrHash := hasher.Sum(nil)
    hasher.Reset()

    // second round of hashing for selective disclosure
    hasher.Write(ctrHash)
    segmentKey := hasher.Sum(nil)
    hasher.Reset()

    // XOR bytes uptil length of in (out must be at least as long)
    inLength := len(in)
    for j := 0; j < inLength; j++ {
        out[j] = in[j] ^ segmentKey[j]
    }
    // insert padding if out is longer
    pad(out[inLength:])
}

func pad(b []byte) {
    l := len(b)
    for total := 0; total < l; {
        read, _ := rand.Read(b[total:])
        total += read
    }
}

// GenerateRandomKey generates a random key of length l
func GenerateRandomKey(l int) Key {
    key := make([]byte, l)
    var total int
    for total < l {
        read, _ := rand.Read(key[total:])
        total += read
    }
    return key
}

func min(x, y int) int {
    if x < y {
        return x
    }
    return y
}