// Copyright 2011 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 packet
import (
"crypto/cipher"
"crypto/sha1"
"crypto/subtle"
"golang.org/x/crypto/openpgp/errors"
"hash"
"io"
"strconv"
)
// SymmetricallyEncrypted represents a symmetrically encrypted byte string. The
// encrypted contents will consist of more OpenPGP packets. See RFC 4880,
// sections 5.7 and 5.13.
type SymmetricallyEncrypted struct {
MDC bool // true iff this is a type 18 packet and thus has an embedded MAC.
contents io.Reader
prefix []byte
}
const symmetricallyEncryptedVersion = 1
func (se *SymmetricallyEncrypted) parse(r io.Reader) error {
if se.MDC {
// See RFC 4880, section 5.13.
var buf [1]byte
_, err := readFull(r, buf[:])
if err != nil {
return err
}
if buf[0] != symmetricallyEncryptedVersion {
return errors.UnsupportedError("unknown SymmetricallyEncrypted version")
}
}
se.contents = r
return nil
}
// Decrypt returns a ReadCloser, from which the decrypted contents of the
// packet can be read. An incorrect key can, with high probability, be detected
// immediately and this will result in a KeyIncorrect error being returned.
func (se *SymmetricallyEncrypted) Decrypt(c CipherFunction, key []byte) (io.ReadCloser, error) {
keySize := c.KeySize()
if keySize == 0 {
return nil, errors.UnsupportedError("unknown cipher: " + strconv.Itoa(int(c)))
}
if len(key) != keySize {
return nil, errors.InvalidArgumentError("SymmetricallyEncrypted: incorrect key length")
}
if se.prefix == nil {
se.prefix = make([]byte, c.blockSize()+2)
_, err := readFull(se.contents, se.prefix)
if err != nil {
return nil, err
}
} else if len(se.prefix) != c.blockSize()+2 {
return nil, errors.InvalidArgumentError("can't try ciphers with different block lengths")
}
ocfbResync := OCFBResync
if se.MDC {
// MDC packets use a different form of OCFB mode.
ocfbResync = OCFBNoResync
}
s := NewOCFBDecrypter(c.new(key), se.prefix, ocfbResync)
if s == nil {
return nil, errors.ErrKeyIncorrect
}
plaintext := cipher.StreamReader{S: s, R: se.contents}
if se.MDC {
// MDC packets have an embedded hash that we need to check.
h := sha1.New()
h.Write(se.prefix)
return &seMDCReader{in: plaintext, h: h}, nil
}
// Otherwise, we just need to wrap plaintext so that it's a valid ReadCloser.
return seReader{plaintext}, nil
}
// seReader wraps an io.Reader with a no-op Close method.
type seReader struct {
in io.Reader
}
func (ser seReader) Read(buf []byte) (int, error) {
return ser.in.Read(buf)
}
func (ser seReader) Close() error {
return nil
}
const mdcTrailerSize = 1 /* tag byte */ + 1 /* length byte */ + sha1.Size
// An seMDCReader wraps an io.Reader, maintains a running hash and keeps hold
// of the most recent 22 bytes (mdcTrailerSize). Upon EOF, those bytes form an
// MDC packet containing a hash of the previous contents which is checked
// against the running hash. See RFC 4880, section 5.13.
type seMDCReader struct {
in io.Reader
h hash.Hash
trailer [mdcTrailerSize]byte
scratch [mdcTrailerSize]byte
trailerUsed int
error bool
eof bool
}
func (ser *seMDCReader) Read(buf []byte) (n int, err error) {
if ser.error {
err = io.ErrUnexpectedEOF
return
}
if ser.eof {
err = io.EOF
return
}
// If we haven't yet filled the trailer buffer then we must do that
// first.
for ser.trailerUsed < mdcTrailerSize {
n, err = ser.in.Read(ser.trailer[ser.trailerUsed:])
ser.trailerUsed += n
if err == io.EOF {
if ser.trailerUsed != mdcTrailerSize {
n = 0
err = io.ErrUnexpectedEOF
ser.error = true
return
}
ser.eof = true
n = 0
return
}
if err != nil {
n = 0
return
}
}
// If it's a short read then we read into a temporary buffer and shift
// the data into the caller's buffer.
if len(buf) <= mdcTrailerSize {
n, err = readFull(ser.in, ser.scratch[:len(buf)])
copy(buf, ser.trailer[:n])
ser.h.Write(buf[:n])
copy(ser.trailer[:], ser.trailer[n:])
copy(ser.trailer[mdcTrailerSize-n:], ser.scratch[:])
if n < len(buf) {
ser.eof = true
err = io.EOF
}
return
}
n, err = ser.in.Read(buf[mdcTrailerSize:])
copy(buf, ser.trailer[:])
ser.h.Write(buf[:n])
copy(ser.trailer[:], buf[n:])
if err == io.EOF {
ser.eof = true
}
return
}
// This is a new-format packet tag byte for a type 19 (MDC) packet.
const mdcPacketTagByte = byte(0x80) | 0x40 | 19
func (ser *seMDCReader) Close() error {
if ser.error {
return errors.SignatureError("error during reading")
}
for !ser.eof {
// We haven't seen EOF so we need to read to the end
var buf [1024]byte
_, err := ser.Read(buf[:])
if err == io.EOF {
break
}
if err != nil {
return errors.SignatureError("error during reading")
}
}
if ser.trailer[0] != mdcPacketTagByte || ser.trailer[1] != sha1.Size {
return errors.SignatureError("MDC packet not found")
}
ser.h.Write(ser.trailer[:2])
final := ser.h.Sum(nil)
if subtle.ConstantTimeCompare(final, ser.trailer[2:]) != 1 {
return errors.SignatureError("hash mismatch")
}
return nil
}
// An seMDCWriter writes through to an io.WriteCloser while maintains a running
// hash of the data written. On close, it emits an MDC packet containing the
// running hash.
type seMDCWriter struct {
w io.WriteCloser
h hash.Hash
}
func (w *seMDCWriter) Write(buf []byte) (n int, err error) {
w.h.Write(buf)
return w.w.Write(buf)
}
func (w *seMDCWriter) Close() (err error) {
var buf [mdcTrailerSize]byte
buf[0] = mdcPacketTagByte
buf[1] = sha1.Size
w.h.Write(buf[:2])
digest := w.h.Sum(nil)
copy(buf[2:], digest)
_, err = w.w.Write(buf[:])
if err != nil {
return
}
return w.w.Close()
}
// noOpCloser is like an ioutil.NopCloser, but for an io.Writer.
type noOpCloser struct {
w io.Writer
}
func (c noOpCloser) Write(data []byte) (n int, err error) {
return c.w.Write(data)
}
func (c noOpCloser) Close() error {
return nil
}
// SerializeSymmetricallyEncrypted serializes a symmetrically encrypted packet
// to w and returns a WriteCloser to which the to-be-encrypted packets can be
// written.
// If config is nil, sensible defaults will be used.
func SerializeSymmetricallyEncrypted(w io.Writer, c CipherFunction, key []byte, config *Config) (contents io.WriteCloser, err error) {
if c.KeySize() != len(key) {
return nil, errors.InvalidArgumentError("SymmetricallyEncrypted.Serialize: bad key length")
}
writeCloser := noOpCloser{w}
ciphertext, err := serializeStreamHeader(writeCloser, packetTypeSymmetricallyEncryptedMDC)
if err != nil {
return
}
_, err = ciphertext.Write([]byte{symmetricallyEncryptedVersion})
if err != nil {
return
}
block := c.new(key)
blockSize := block.BlockSize()
iv := make([]byte, blockSize)
_, err = config.Random().Read(iv)
if err != nil {
return
}
s, prefix := NewOCFBEncrypter(block, iv, OCFBNoResync)
_, err = ciphertext.Write(prefix)
if err != nil {
return
}
plaintext := cipher.StreamWriter{S: s, W: ciphertext}
h := sha1.New()
h.Write(iv)
h.Write(iv[blockSize-2:])
contents = &seMDCWriter{w: plaintext, h: h}
return
}
