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crypto.go
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crypto.go
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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package zip
import (
"bytes"
"crypto/aes"
"crypto/cipher"
"crypto/hmac"
"crypto/rand"
"crypto/sha1"
"crypto/subtle"
"errors"
"hash"
"io"
"golang.org/x/crypto/pbkdf2"
)
type EncryptionMethod int
const (
StandardEncryption EncryptionMethod = 1
AES128Encryption EncryptionMethod = 2
AES192Encryption EncryptionMethod = 3
AES256Encryption EncryptionMethod = 4
// AES key lengths
aes128 = 16
aes192 = 24
aes256 = 32
)
func aesKeyLen(strength byte) int {
switch strength {
case 1:
return aes128
case 2:
return aes192
case 3:
return aes256
default:
return 0
}
}
// Encryption/Decryption Errors
var (
ErrDecryption = errors.New("zip: decryption error")
ErrPassword = errors.New("zip: invalid password")
ErrAuthentication = errors.New("zip: authentication failed")
)
// Counter (CTR) mode.
// CTR converts a block cipher into a stream cipher by
// repeatedly encrypting an incrementing counter and
// xoring the resulting stream of data with the input.
// This is a re-implementation of Go's CTR mode to allow
// for a little-endian, left-aligned uint32 counter, which
// is required for WinZip AES encryption. Go's cipher.NewCTR
// follows the NIST Standard SP 800-38A, pp 13-15
// which has a big-endian, right-aligned counter.
type ctr struct {
b cipher.Block
ctr []byte
out []byte
outUsed int
}
const streamBufferSize = 512
// NewWinZipCTR returns a Stream which encrypts/decrypts using the given Block in
// counter mode. The counter is initially set to 1 per WinZip AES.
func newWinZipCTR(block cipher.Block) cipher.Stream {
bufSize := streamBufferSize
if bufSize < block.BlockSize() {
bufSize = block.BlockSize()
}
// Set the IV (counter) to 1
iv := make([]byte, block.BlockSize())
iv[0] = 1
return &ctr{
b: block,
ctr: iv,
out: make([]byte, 0, bufSize),
outUsed: 0,
}
}
func (x *ctr) refill() {
remain := len(x.out) - x.outUsed
if remain > x.outUsed {
return
}
copy(x.out, x.out[x.outUsed:])
x.out = x.out[:cap(x.out)]
bs := x.b.BlockSize()
for remain < len(x.out)-bs {
x.b.Encrypt(x.out[remain:], x.ctr)
remain += bs
// Increment counter
// for i := len(x.ctr) - 1; i >= 0; i-- {
// x.ctr[i]++
// if x.ctr[i] != 0 {
// break
// }
// }
// Change to allow for little-endian,
// left-aligned counter
for i := 0; i < len(x.ctr); i++ {
x.ctr[i]++
if x.ctr[i] != 0 {
break
}
}
}
x.out = x.out[:remain]
x.outUsed = 0
}
func (x *ctr) XORKeyStream(dst, src []byte) {
for len(src) > 0 {
if x.outUsed >= len(x.out)-x.b.BlockSize() {
x.refill()
}
n := xorBytes(dst, src, x.out[x.outUsed:])
dst = dst[n:]
src = src[n:]
x.outUsed += n
}
}
func xorBytes(dst, a, b []byte) int {
n := len(a)
if len(b) < n {
n = len(b)
}
for i := 0; i < n; i++ {
dst[i] = a[i] ^ b[i]
}
return n
}
// newAuthReader returns either a buffered or streaming authentication reader.
// Buffered authentication is recommended. Streaming authentication is only
// recommended if: 1. you buffer the data yourself and wait for authentication
// before streaming to another source such as the network, or 2. you just don't
// care about authenticating unknown ciphertext before use :).
func newAuthReader(akey []byte, data, adata io.Reader, streaming bool) io.Reader {
ar := authReader{
data: data,
adata: adata,
mac: hmac.New(sha1.New, akey),
err: nil,
auth: false,
}
if streaming {
return &ar
}
return &bufferedAuthReader{
ar,
new(bytes.Buffer),
}
}
// Streaming authentication
type authReader struct {
data io.Reader // data to be authenticated
adata io.Reader // the authentication code to read
mac hash.Hash // hmac hash
err error
auth bool
}
func (a *authReader) Read(p []byte) (int, error) {
if a.err != nil {
return 0, a.err
}
end := false
// read underlying data
n, err := a.data.Read(p)
if err != nil && err != io.EOF {
a.err = err
return n, a.err
} else if err == io.EOF {
// if we are at the end, calculate the mac
end = true
a.err = err
}
// write any data to mac
_, err = a.mac.Write(p[:n])
if err != nil {
a.err = err
return n, a.err
}
if end {
ab := new(bytes.Buffer)
_, err = io.Copy(ab, a.adata)
if err != nil || ab.Len() != 10 {
a.err = ErrDecryption
return n, a.err
}
if !a.checkAuthentication(ab.Bytes()) {
a.err = ErrAuthentication
return n, a.err
}
}
return n, a.err
}
// buffered authentication
type bufferedAuthReader struct {
authReader
buf *bytes.Buffer // buffer to store data to authenticate
}
func (a *bufferedAuthReader) Read(b []byte) (int, error) {
// check for sticky error
if a.err != nil {
return 0, a.err
}
// make sure we have auth'ed before we send any data
if !a.auth {
_, err := io.Copy(a.buf, a.data)
if err != nil {
a.err = err
return 0, a.err
}
ab := new(bytes.Buffer)
nn, err := io.Copy(ab, a.adata)
if err != nil {
a.err = err
return 0, a.err
} else if nn != 10 {
a.err = ErrDecryption
return 0, a.err
}
_, err = a.mac.Write(a.buf.Bytes())
if err != nil {
a.err = err
return 0, a.err
}
if !a.checkAuthentication(ab.Bytes()) {
a.err = ErrAuthentication
return 0, a.err
}
}
// so we've authenticated the data, now just pass it on.
n, err := a.buf.Read(b)
if err != nil {
a.err = err
}
return n, a.err
}
func (a *authReader) checkAuthentication(authcode []byte) bool {
expectedAuthCode := a.mac.Sum(nil)
// Truncate at the first 10 bytes
expectedAuthCode = expectedAuthCode[:10]
a.auth = subtle.ConstantTimeCompare(expectedAuthCode, authcode) > 0
return a.auth
}
func checkPasswordVerification(pwvv, pwv []byte) bool {
b := subtle.ConstantTimeCompare(pwvv, pwv) > 0
return b
}
func generateKeys(password, salt []byte, keySize int) (encKey, authKey, pwv []byte) {
totalSize := (keySize * 2) + 2 // enc + auth + pv sizes
key := pbkdf2.Key(password, salt, 1000, totalSize, sha1.New)
encKey = key[:keySize]
authKey = key[keySize : keySize*2]
pwv = key[keySize*2:]
return
}
// newDecryptionReader returns an authenticated, decryption reader
func newDecryptionReader(r *io.SectionReader, f *File) (io.Reader, error) {
keyLen := aesKeyLen(f.aesStrength)
saltLen := keyLen / 2 // salt is half of key len
if saltLen == 0 {
return nil, ErrDecryption
}
// grab the salt and pwvv
saltpwvv := make([]byte, saltLen+2)
if _, err := r.Read(saltpwvv); err != nil {
return nil, err
}
salt := saltpwvv[:saltLen]
pwvv := saltpwvv[saltLen : saltLen+2]
// generate keys only if we have a password
if f.password == nil {
return nil, ErrPassword
}
decKey, authKey, pwv := generateKeys(f.password(), salt, keyLen)
if !checkPasswordVerification(pwv, pwvv) {
return nil, ErrPassword
}
dataOff := int64(saltLen + 2)
dataLen := int64(f.CompressedSize64 - uint64(saltLen) - 2 - 10)
// // TODO(alex): Should the compressed sizes be fixed?
// // Not the ideal place to do this.
// f.CompressedSize64 = uint64(dataLen)
// f.CompressedSize = uint32(dataLen)
data := io.NewSectionReader(r, dataOff, dataLen)
authOff := dataOff + dataLen
authcode := io.NewSectionReader(r, authOff, 10)
ar := newAuthReader(authKey, data, authcode, f.DeferAuth)
dr := decryptStream(decKey, ar)
if dr == nil {
return nil, ErrDecryption
}
return dr, nil
}
func decryptStream(key []byte, ciphertext io.Reader) io.Reader {
block, err := aes.NewCipher(key)
if err != nil {
return nil
}
stream := newWinZipCTR(block)
reader := &cipher.StreamReader{S: stream, R: ciphertext}
return reader
}
// writes encrypted data to hmac as it passes through
type authWriter struct {
hmac hash.Hash // from fw.hmac
w io.Writer // this will be the compCount writer
}
func (aw *authWriter) Write(p []byte) (int, error) {
_, err := aw.hmac.Write(p)
if err != nil {
return 0, err
}
return aw.w.Write(p)
}
// writes out the salt, pwv, and then the encrypted file data
type encryptionWriter struct {
pwv []byte
salt []byte
w io.Writer // where to write the salt + pwv
es io.Writer // where to write plaintext
first bool // first write
err error
}
func (ew *encryptionWriter) Write(p []byte) (int, error) {
if ew.err != nil {
return 0, ew.err
}
if ew.first {
// if our first time writing
// must write out the salt and pwv first unencrypted
_, err1 := ew.w.Write(ew.salt)
_, err2 := ew.w.Write(ew.pwv)
if err1 != nil || err2 != nil {
ew.err = errors.New("zip: error writing salt or pwv")
return 0, ew.err
}
ew.first = false
}
// now just pass on to the encryption stream
return ew.es.Write(p)
}
func encryptStream(key []byte, w io.Writer) (io.Writer, error) {
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
stream := newWinZipCTR(block)
writer := &cipher.StreamWriter{S: stream, W: w}
return writer, nil
}
// newEncryptionWriter returns an io.Writer that when written to, 1. writes
// out the salt, 2. writes out pwv, and 3. writes out authenticated, encrypted
// data. The authcode will be written out in fileWriter.close().
func newEncryptionWriter(w io.Writer, password passwordFn, fw *fileWriter, aesstrength byte) (io.Writer, error) {
keysize := aesKeyLen(aesstrength)
salt := make([]byte, keysize/2)
_, err := rand.Read(salt[:])
if err != nil {
return nil, errors.New("zip: unable to generate random salt")
}
ekey, akey, pwv := generateKeys(password(), salt[:], keysize)
fw.hmac = hmac.New(sha1.New, akey)
aw := &authWriter{
hmac: fw.hmac,
w: w,
}
es, err := encryptStream(ekey, aw)
if err != nil {
return nil, err
}
ew := &encryptionWriter{
pwv: pwv,
salt: salt[:],
w: w,
es: es,
first: true,
}
return ew, nil
}
// IsEncrypted indicates whether this file's data is encrypted.
func (h *FileHeader) IsEncrypted() bool {
return h.Flags&0x1 == 1
}
// WinZip AE-2 specifies that no CRC value is written and
// should be skipped when reading.
func (h *FileHeader) isAE2() bool {
return h.ae == 2
}
func (h *FileHeader) writeWinZipExtra() {
// total size is 11 bytes
var buf [11]byte
eb := writeBuf(buf[:])
eb.uint16(winzipAesExtraId) // 0x9901
eb.uint16(7) // following data size is 7
eb.uint16(2) // ae 2
eb.uint16(0x4541) // "AE"
eb.uint8(h.aesStrength) // aes256
eb.uint16(h.Method) // original compression method
h.Extra = append(h.Extra, buf[:]...)
}
// SetEncryptionMethod sets the encryption method.
func (h *FileHeader) SetEncryptionMethod(enc EncryptionMethod) {
h.encryption = enc
switch enc {
case AES128Encryption:
h.aesStrength = 1
case AES192Encryption:
h.aesStrength = 2
case AES256Encryption:
h.aesStrength = 3
}
}
func (h *FileHeader) setEncryptionBit() {
h.Flags |= 0x1
}
// SetPassword sets the password used for encryption/decryption.
func (h *FileHeader) SetPassword(password string) {
if !h.IsEncrypted() {
h.setEncryptionBit()
}
h.password = func() []byte {
return []byte(password)
}
}
// PasswordFn is a function that returns the password
// as a byte slice
type passwordFn func() []byte
// Encrypt adds a file to the zip file using the provided name.
// It returns a Writer to which the file contents should be written. File
// contents will be encrypted with AES-256 using the given password. The
// file's contents must be written to the io.Writer before the next call
// to Create, CreateHeader, or Close.
func (w *Writer) Encrypt(name string, password string, enc EncryptionMethod) (io.Writer, error) {
fh := &FileHeader{
Name: name,
Method: Deflate,
}
fh.SetPassword(password)
fh.SetEncryptionMethod(enc)
return w.CreateHeader(fh)
}