restic/internal/crypto/crypto.go

372 lines
8.7 KiB
Go

package crypto
import (
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"encoding/json"
"fmt"
"github.com/restic/restic/internal/errors"
"golang.org/x/crypto/poly1305"
)
const (
aesKeySize = 32 // for AES-256
macKeySizeK = 16 // for AES-128
macKeySizeR = 16 // for Poly1305
macKeySize = macKeySizeK + macKeySizeR // for Poly1305-AES128
ivSize = aes.BlockSize
macSize = poly1305.TagSize
// Extension is the number of bytes a plaintext is enlarged by encrypting it.
Extension = ivSize + macSize
)
var (
// ErrUnauthenticated is returned when ciphertext verification has failed.
ErrUnauthenticated = errors.New("ciphertext verification failed")
)
// Key holds encryption and message authentication keys for a repository. It is stored
// encrypted and authenticated as a JSON data structure in the Data field of the Key
// structure.
type Key struct {
MACKey `json:"mac"`
EncryptionKey `json:"encrypt"`
}
// EncryptionKey is key used for encryption
type EncryptionKey [32]byte
// MACKey is used to sign (authenticate) data.
type MACKey struct {
K [16]byte // for AES-128
R [16]byte // for Poly1305
masked bool // remember if the MAC key has already been masked
}
// mask for key, (cf. http://cr.yp.to/mac/poly1305-20050329.pdf)
var poly1305KeyMask = [16]byte{
0xff,
0xff,
0xff,
0x0f, // 3: top four bits zero
0xfc, // 4: bottom two bits zero
0xff,
0xff,
0x0f, // 7: top four bits zero
0xfc, // 8: bottom two bits zero
0xff,
0xff,
0x0f, // 11: top four bits zero
0xfc, // 12: bottom two bits zero
0xff,
0xff,
0x0f, // 15: top four bits zero
}
func poly1305MAC(msg []byte, nonce []byte, key *MACKey) []byte {
k := poly1305PrepareKey(nonce, key)
var out [16]byte
poly1305.Sum(&out, msg, &k)
return out[:]
}
// mask poly1305 key
func maskKey(k *MACKey) {
if k == nil || k.masked {
return
}
for i := 0; i < poly1305.TagSize; i++ {
k.R[i] = k.R[i] & poly1305KeyMask[i]
}
k.masked = true
}
// construct mac key from slice (k||r), with masking
func macKeyFromSlice(mk *MACKey, data []byte) {
copy(mk.K[:], data[:16])
copy(mk.R[:], data[16:32])
maskKey(mk)
}
// prepare key for low-level poly1305.Sum(): r||n
func poly1305PrepareKey(nonce []byte, key *MACKey) [32]byte {
var k [32]byte
maskKey(key)
cipher, err := aes.NewCipher(key.K[:])
if err != nil {
panic(err)
}
cipher.Encrypt(k[16:], nonce[:])
copy(k[:16], key.R[:])
return k
}
func poly1305Verify(msg []byte, nonce []byte, key *MACKey, mac []byte) bool {
k := poly1305PrepareKey(nonce, key)
var m [16]byte
copy(m[:], mac)
return poly1305.Verify(&m, msg, &k)
}
// NewRandomKey returns new encryption and message authentication keys.
func NewRandomKey() *Key {
k := &Key{}
n, err := rand.Read(k.EncryptionKey[:])
if n != aesKeySize || err != nil {
panic("unable to read enough random bytes for encryption key")
}
n, err = rand.Read(k.MACKey.K[:])
if n != macKeySizeK || err != nil {
panic("unable to read enough random bytes for MAC encryption key")
}
n, err = rand.Read(k.MACKey.R[:])
if n != macKeySizeR || err != nil {
panic("unable to read enough random bytes for MAC key")
}
maskKey(&k.MACKey)
return k
}
// NewRandomNonce returns a new random nonce. It panics on error so that the
// program is safely terminated.
func NewRandomNonce() []byte {
iv := make([]byte, ivSize)
n, err := rand.Read(iv)
if n != ivSize || err != nil {
panic("unable to read enough random bytes for iv")
}
return iv
}
type jsonMACKey struct {
K []byte `json:"k"`
R []byte `json:"r"`
}
// MarshalJSON converts the MACKey to JSON.
func (m *MACKey) MarshalJSON() ([]byte, error) {
return json.Marshal(jsonMACKey{K: m.K[:], R: m.R[:]})
}
// UnmarshalJSON fills the key m with data from the JSON representation.
func (m *MACKey) UnmarshalJSON(data []byte) error {
j := jsonMACKey{}
err := json.Unmarshal(data, &j)
if err != nil {
return errors.Wrap(err, "Unmarshal")
}
copy(m.K[:], j.K)
copy(m.R[:], j.R)
return nil
}
// Valid tests whether the key k is valid (i.e. not zero).
func (m *MACKey) Valid() bool {
nonzeroK := false
for i := 0; i < len(m.K); i++ {
if m.K[i] != 0 {
nonzeroK = true
}
}
if !nonzeroK {
return false
}
for i := 0; i < len(m.R); i++ {
if m.R[i] != 0 {
return true
}
}
return false
}
// MarshalJSON converts the EncryptionKey to JSON.
func (k *EncryptionKey) MarshalJSON() ([]byte, error) {
return json.Marshal(k[:])
}
// UnmarshalJSON fills the key k with data from the JSON representation.
func (k *EncryptionKey) UnmarshalJSON(data []byte) error {
d := make([]byte, aesKeySize)
err := json.Unmarshal(data, &d)
if err != nil {
return errors.Wrap(err, "Unmarshal")
}
copy(k[:], d)
return nil
}
// Valid tests whether the key k is valid (i.e. not zero).
func (k *EncryptionKey) Valid() bool {
for i := 0; i < len(k); i++ {
if k[i] != 0 {
return true
}
}
return false
}
// ErrInvalidCiphertext is returned when trying to encrypt into the slice that
// holds the plaintext.
var ErrInvalidCiphertext = errors.New("invalid ciphertext, same slice used for plaintext")
// validNonce checks that nonce is not all zero.
func validNonce(nonce []byte) bool {
var sum byte
for _, b := range nonce {
sum |= b
}
return sum > 0
}
// statically ensure that *Key implements crypto/cipher.AEAD
var _ cipher.AEAD = &Key{}
// NonceSize returns the size of the nonce that must be passed to Seal
// and Open.
func (k *Key) NonceSize() int {
return ivSize
}
// Overhead returns the maximum difference between the lengths of a
// plaintext and its ciphertext.
func (k *Key) Overhead() int {
return macSize
}
// sliceForAppend takes a slice and a requested number of bytes. It returns a
// slice with the contents of the given slice followed by that many bytes and a
// second slice that aliases into it and contains only the extra bytes. If the
// original slice has sufficient capacity then no allocation is performed.
//
// taken from the stdlib, crypto/aes/aes_gcm.go
func sliceForAppend(in []byte, n int) (head, tail []byte) {
if total := len(in) + n; cap(in) >= total {
head = in[:total]
} else {
head = make([]byte, total)
copy(head, in)
}
tail = head[len(in):]
return
}
// Seal encrypts and authenticates plaintext, authenticates the
// additional data and appends the result to dst, returning the updated
// slice. The nonce must be NonceSize() bytes long and unique for all
// time, for a given key.
//
// The plaintext and dst may alias exactly or not at all. To reuse
// plaintext's storage for the encrypted output, use plaintext[:0] as dst.
func (k *Key) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
if !k.Valid() {
panic("key is invalid")
}
if len(additionalData) > 0 {
panic("additional data is not supported")
}
if len(nonce) != ivSize {
panic("incorrect nonce length")
}
if !validNonce(nonce) {
panic("nonce is invalid")
}
ret, out := sliceForAppend(dst, len(plaintext)+k.Overhead())
c, err := aes.NewCipher(k.EncryptionKey[:])
if err != nil {
panic(fmt.Sprintf("unable to create cipher: %v", err))
}
e := cipher.NewCTR(c, nonce)
e.XORKeyStream(out, plaintext)
mac := poly1305MAC(out[:len(plaintext)], nonce, &k.MACKey)
copy(out[len(plaintext):], mac)
return ret
}
// Open decrypts and authenticates ciphertext, authenticates the
// additional data and, if successful, appends the resulting plaintext
// to dst, returning the updated slice. The nonce must be NonceSize()
// bytes long and both it and the additional data must match the
// value passed to Seal.
//
// The ciphertext and dst may alias exactly or not at all. To reuse
// ciphertext's storage for the decrypted output, use ciphertext[:0] as dst.
//
// Even if the function fails, the contents of dst, up to its capacity,
// may be overwritten.
func (k *Key) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
if !k.Valid() {
return nil, errors.New("invalid key")
}
// check parameters
if len(nonce) != ivSize {
panic("incorrect nonce length")
}
if !validNonce(nonce) {
return nil, errors.New("nonce is invalid")
}
// check for plausible length
if len(ciphertext) < k.Overhead() {
return nil, errors.Errorf("trying to decrypt invalid data: ciphertext too small")
}
l := len(ciphertext) - macSize
ct, mac := ciphertext[:l], ciphertext[l:]
// verify mac
if !poly1305Verify(ct, nonce, &k.MACKey, mac) {
return nil, ErrUnauthenticated
}
ret, out := sliceForAppend(dst, len(ct))
c, err := aes.NewCipher(k.EncryptionKey[:])
if err != nil {
panic(fmt.Sprintf("unable to create cipher: %v", err))
}
e := cipher.NewCTR(c, nonce)
e.XORKeyStream(out, ct)
return ret, nil
}
// Valid tests if the key is valid.
func (k *Key) Valid() bool {
return k.EncryptionKey.Valid() && k.MACKey.Valid()
}