restic/key.go

669 lines
15 KiB
Go

package restic
import (
"crypto/aes"
"crypto/cipher"
"crypto/hmac"
"crypto/rand"
"crypto/sha256"
"encoding/json"
"errors"
"fmt"
"hash"
"io"
"io/ioutil"
"os"
"os/user"
"sync"
"time"
"github.com/restic/restic/backend"
"github.com/restic/restic/chunker"
"golang.org/x/crypto/scrypt"
)
// max size is 8MiB, defined in chunker
const ivSize = aes.BlockSize
const hmacSize = sha256.Size
const maxCiphertextSize = ivSize + chunker.MaxSize + hmacSize
const CiphertextExtension = ivSize + hmacSize
var (
// ErrUnauthenticated is returned when ciphertext verification has failed.
ErrUnauthenticated = errors.New("ciphertext verification failed")
// ErrNoKeyFound is returned when no key for the repository could be decrypted.
ErrNoKeyFound = errors.New("no key could be found")
// ErrBufferTooSmall is returned when the destination slice is too small
// for the ciphertext.
ErrBufferTooSmall = errors.New("destination buffer too small")
)
// TODO: figure out scrypt values on the fly depending on the current
// hardware.
const (
scryptN = 65536
scryptR = 8
scryptP = 1
scryptSaltsize = 64
aesKeysize = 32 // for AES256
hmacKeysize = 32 // for HMAC with SHA256
)
// Key represents an encrypted master key for a repository.
type Key struct {
Created time.Time `json:"created"`
Username string `json:"username"`
Hostname string `json:"hostname"`
Comment string `json:"comment,omitempty"`
KDF string `json:"kdf"`
N int `json:"N"`
R int `json:"r"`
P int `json:"p"`
Salt []byte `json:"salt"`
Data []byte `json:"data"`
user *keys
master *keys
id backend.ID
}
// keys is a JSON structure that holds signing and encryption keys.
type keys struct {
Sign []byte
Encrypt []byte
}
// CreateKey initializes a master key in the given backend and encrypts it with
// the password.
func CreateKey(s Server, password string) (*Key, error) {
return AddKey(s, password, nil)
}
// OpenKey tries do decrypt the key specified by id with the given password.
func OpenKey(s Server, id backend.ID, password string) (*Key, error) {
k, err := LoadKey(s, id)
if err != nil {
return nil, err
}
// check KDF
if k.KDF != "scrypt" {
return nil, errors.New("only supported KDF is scrypt()")
}
// derive user key
k.user, err = k.scrypt(password)
if err != nil {
return nil, err
}
// decrypt master keys
buf, err := k.DecryptUser([]byte{}, k.Data)
if err != nil {
return nil, err
}
// restore json
k.master = &keys{}
err = json.Unmarshal(buf, k.master)
if err != nil {
return nil, err
}
k.id = id
return k, nil
}
// SearchKey tries to decrypt all keys in the backend with the given password.
// If none could be found, ErrNoKeyFound is returned.
func SearchKey(s Server, password string) (*Key, error) {
// list all keys
ids, err := s.List(backend.Key)
if err != nil {
panic(err)
}
// try all keys in repo
var key *Key
for _, id := range ids {
key, err = OpenKey(s, id, password)
if err != nil {
continue
}
return key, nil
}
return nil, ErrNoKeyFound
}
// LoadKey loads a key from the backend.
func LoadKey(s Server, id backend.ID) (*Key, error) {
// extract data from repo
data, err := s.Get(backend.Key, id)
if err != nil {
return nil, err
}
// restore json
k := &Key{}
err = json.Unmarshal(data, k)
if err != nil {
return nil, err
}
return k, err
}
// AddKey adds a new key to an already existing repository.
func AddKey(s Server, password string, template *Key) (*Key, error) {
// fill meta data about key
newkey := &Key{
Created: time.Now(),
KDF: "scrypt",
N: scryptN,
R: scryptR,
P: scryptP,
}
hn, err := os.Hostname()
if err == nil {
newkey.Hostname = hn
}
usr, err := user.Current()
if err == nil {
newkey.Username = usr.Username
}
// generate random salt
newkey.Salt = make([]byte, scryptSaltsize)
n, err := rand.Read(newkey.Salt)
if n != scryptSaltsize || err != nil {
panic("unable to read enough random bytes for salt")
}
// call scrypt() to derive user key
newkey.user, err = newkey.scrypt(password)
if err != nil {
return nil, err
}
if template == nil {
// generate new random master keys
newkey.master, err = newkey.newKeys()
if err != nil {
return nil, err
}
} else {
// copy master keys from old key
newkey.master = template.master
}
// encrypt master keys (as json) with user key
buf, err := json.Marshal(newkey.master)
if err != nil {
return nil, err
}
newkey.Data = GetChunkBuf("key")
n, err = newkey.EncryptUser(newkey.Data, buf)
newkey.Data = newkey.Data[:n]
// dump as json
buf, err = json.Marshal(newkey)
if err != nil {
return nil, err
}
// store in repository and return
blob, err := s.Create(backend.Key)
if err != nil {
return nil, err
}
_, err = blob.Write(buf)
if err != nil {
return nil, err
}
err = blob.Close()
if err != nil {
return nil, err
}
id, err := blob.ID()
if err != nil {
return nil, err
}
newkey.id = id
FreeChunkBuf("key", newkey.Data)
return newkey, nil
}
func (k *Key) scrypt(password string) (*keys, error) {
if len(k.Salt) == 0 {
return nil, fmt.Errorf("scrypt() called with empty salt")
}
keybytes := hmacKeysize + aesKeysize
scryptKeys, err := scrypt.Key([]byte(password), k.Salt, k.N, k.R, k.P, keybytes)
if err != nil {
return nil, fmt.Errorf("error deriving keys from password: %v", err)
}
if len(scryptKeys) != keybytes {
return nil, fmt.Errorf("invalid numbers of bytes expanded from scrypt(): %d", len(scryptKeys))
}
ks := &keys{
Encrypt: scryptKeys[:aesKeysize],
Sign: scryptKeys[aesKeysize:],
}
return ks, nil
}
func (k *Key) newKeys() (*keys, error) {
ks := &keys{
Encrypt: make([]byte, aesKeysize),
Sign: make([]byte, hmacKeysize),
}
n, err := rand.Read(ks.Encrypt)
if n != aesKeysize || err != nil {
panic("unable to read enough random bytes for encryption key")
}
n, err = rand.Read(ks.Sign)
if n != hmacKeysize || err != nil {
panic("unable to read enough random bytes for signing key")
}
return ks, nil
}
func (k *Key) newIV(buf []byte) error {
_, err := io.ReadFull(rand.Reader, buf[:ivSize])
buf = buf[:ivSize]
if err != nil {
return err
}
return nil
}
// Encrypt encrypts and signs data. Stored in ciphertext is IV || Ciphertext ||
// HMAC. Encrypt returns the ciphertext's length. For the hash function, SHA256
// is used, so the overhead is 16+32=48 byte.
func (k *Key) encrypt(ks *keys, ciphertext, plaintext []byte) (int, error) {
if cap(ciphertext) < len(plaintext)+ivSize+hmacSize {
return 0, ErrBufferTooSmall
}
_, err := io.ReadFull(rand.Reader, ciphertext[:ivSize])
if err != nil {
panic(fmt.Sprintf("unable to generate new random iv: %v", err))
}
c, err := aes.NewCipher(ks.Encrypt)
if err != nil {
panic(fmt.Sprintf("unable to create cipher: %v", err))
}
e := cipher.NewCTR(c, ciphertext[:ivSize])
e.XORKeyStream(ciphertext[ivSize:cap(ciphertext)], plaintext)
ciphertext = ciphertext[:ivSize+len(plaintext)]
hm := hmac.New(sha256.New, ks.Sign)
n, err := hm.Write(ciphertext)
if err != nil || n != len(ciphertext) {
panic(fmt.Sprintf("unable to calculate hmac of ciphertext: %v", err))
}
ciphertext = hm.Sum(ciphertext)
return len(ciphertext), nil
}
// EncryptUser encrypts and signs data with the user key. Stored in ciphertext
// is IV || Ciphertext || HMAC. Returns the ciphertext length. For the hash
// function, SHA256 is used, so the overhead is 16+32=48 byte.
func (k *Key) EncryptUser(ciphertext, plaintext []byte) (int, error) {
return k.encrypt(k.user, ciphertext, plaintext)
}
// Encrypt encrypts and signs data with the master key. Stored in ciphertext is
// IV || Ciphertext || HMAC. Returns the ciphertext length. For the hash
// function, SHA256 is used, so the overhead is 16+32=48 byte.
func (k *Key) Encrypt(ciphertext, plaintext []byte) (int, error) {
return k.encrypt(k.master, ciphertext, plaintext)
}
type encryptWriter struct {
iv []byte
wroteIV bool
h hash.Hash
s cipher.Stream
w io.Writer
origWr io.Writer
err error // remember error writing iv
}
func (e *encryptWriter) Close() error {
// write hmac
_, err := e.origWr.Write(e.h.Sum(nil))
if err != nil {
return err
}
return nil
}
const encryptWriterChunkSize = 512 * 1024 // 512 KiB
var encryptWriterBufPool = sync.Pool{
New: func() interface{} {
return make([]byte, encryptWriterChunkSize)
},
}
func (e *encryptWriter) Write(p []byte) (int, error) {
// write iv first
if !e.wroteIV {
_, e.err = e.origWr.Write(e.iv)
e.wroteIV = true
}
if e.err != nil {
return 0, e.err
}
buf := encryptWriterBufPool.Get().([]byte)
defer encryptWriterBufPool.Put(buf)
written := 0
for len(p) > 0 {
max := len(p)
if max > encryptWriterChunkSize {
max = encryptWriterChunkSize
}
e.s.XORKeyStream(buf, p[:max])
n, err := e.w.Write(buf[:max])
if n != max {
if err == nil { // should never happen
err = io.ErrShortWrite
}
}
written += n
p = p[n:]
if err != nil {
e.err = err
return written, err
}
}
return written, nil
}
func (k *Key) encryptTo(ks *keys, wr io.Writer) io.WriteCloser {
ew := &encryptWriter{
iv: make([]byte, ivSize),
h: hmac.New(sha256.New, ks.Sign),
origWr: wr,
}
_, err := io.ReadFull(rand.Reader, ew.iv)
if err != nil {
panic(fmt.Sprintf("unable to generate new random iv: %v", err))
}
// write iv to hmac
_, err = ew.h.Write(ew.iv)
if err != nil {
panic(err)
}
c, err := aes.NewCipher(ks.Encrypt)
if err != nil {
panic(fmt.Sprintf("unable to create cipher: %v", err))
}
ew.s = cipher.NewCTR(c, ew.iv)
ew.w = io.MultiWriter(ew.h, wr)
return ew
}
// EncryptTo encrypts and signs data with the master key. The returned
// io.Writer writes IV || Ciphertext || HMAC. For the hash function, SHA256 is
// used.
func (k *Key) EncryptTo(wr io.Writer) io.WriteCloser {
return k.encryptTo(k.master, wr)
}
// EncryptUserTo encrypts and signs data with the user key. The returned
// io.Writer writes IV || Ciphertext || HMAC. For the hash function, SHA256 is
// used.
func (k *Key) EncryptUserTo(wr io.Writer) io.WriteCloser {
return k.encryptTo(k.user, wr)
}
// Decrypt verifes and decrypts the ciphertext. Ciphertext must be in the form
// IV || Ciphertext || HMAC.
func (k *Key) decrypt(ks *keys, plaintext, ciphertext []byte) ([]byte, error) {
// check for plausible length
if len(ciphertext) < ivSize+hmacSize {
panic("trying to decrypt invalid data: ciphertext too small")
}
if cap(plaintext) < len(ciphertext) {
// extend plaintext
plaintext = append(plaintext, make([]byte, len(ciphertext)-cap(plaintext))...)
}
hm := hmac.New(sha256.New, ks.Sign)
// extract hmac
l := len(ciphertext) - hm.Size()
ciphertext, mac := ciphertext[:l], ciphertext[l:]
// calculate new hmac
n, err := hm.Write(ciphertext)
if err != nil || n != len(ciphertext) {
panic(fmt.Sprintf("unable to calculate hmac of ciphertext, err %v", err))
}
// verify hmac
mac2 := hm.Sum(nil)
if !hmac.Equal(mac, mac2) {
return nil, ErrUnauthenticated
}
// extract iv
iv, ciphertext := ciphertext[:aes.BlockSize], ciphertext[aes.BlockSize:]
// decrypt data
c, err := aes.NewCipher(ks.Encrypt)
if err != nil {
panic(fmt.Sprintf("unable to create cipher: %v", err))
}
// decrypt
e := cipher.NewCTR(c, iv)
plaintext = plaintext[:len(ciphertext)]
e.XORKeyStream(plaintext, ciphertext)
return plaintext, nil
}
// Decrypt verifes and decrypts the ciphertext with the master key. Ciphertext
// must be in the form IV || Ciphertext || HMAC.
func (k *Key) Decrypt(plaintext, ciphertext []byte) ([]byte, error) {
return k.decrypt(k.master, plaintext, ciphertext)
}
// DecryptUser verifes and decrypts the ciphertext with the user key. Ciphertext
// must be in the form IV || Ciphertext || HMAC.
func (k *Key) DecryptUser(plaintext, ciphertext []byte) ([]byte, error) {
return k.decrypt(k.user, plaintext, ciphertext)
}
type decryptReader struct {
buf []byte
pos int
}
func (d *decryptReader) Read(dst []byte) (int, error) {
if d.buf == nil {
return 0, io.EOF
}
if len(dst) == 0 {
return 0, nil
}
remaining := len(d.buf) - d.pos
if len(dst) >= remaining {
n := copy(dst, d.buf[d.pos:])
d.Close()
return n, io.EOF
}
n := copy(dst, d.buf[d.pos:d.pos+len(dst)])
d.pos += n
return n, nil
}
func (d *decryptReader) ReadByte() (c byte, err error) {
if d.buf == nil {
return 0, io.EOF
}
remaining := len(d.buf) - d.pos
if remaining == 1 {
c = d.buf[d.pos]
d.Close()
return c, io.EOF
}
c = d.buf[d.pos]
d.pos++
return
}
func (d *decryptReader) Close() error {
if d.buf == nil {
return nil
}
FreeChunkBuf("decryptReader", d.buf)
d.buf = nil
return nil
}
// decryptFrom verifies and decrypts the ciphertext read from rd with ks and
// makes it available on the returned Reader. Ciphertext must be in the form IV
// || Ciphertext || HMAC. In order to correctly verify the ciphertext, rd is
// drained, locally buffered and made available on the returned Reader
// afterwards. If an HMAC verification failure is observed, it is returned
// immediately.
func (k *Key) decryptFrom(ks *keys, rd io.Reader) (io.ReadCloser, error) {
ciphertext := GetChunkBuf("decryptReader")
ciphertext = ciphertext[0:cap(ciphertext)]
n, err := io.ReadFull(rd, ciphertext)
if err != io.ErrUnexpectedEOF {
// read remaining data
buf, e := ioutil.ReadAll(rd)
ciphertext = append(ciphertext, buf...)
n += len(buf)
err = e
} else {
err = nil
}
if err != nil {
return nil, err
}
ciphertext = ciphertext[:n]
// check for plausible length
if len(ciphertext) < ivSize+hmacSize {
panic("trying to decrypt invalid data: ciphertext too small")
}
hm := hmac.New(sha256.New, ks.Sign)
// extract hmac
l := len(ciphertext) - hm.Size()
ciphertext, mac := ciphertext[:l], ciphertext[l:]
// calculate new hmac
n, err = hm.Write(ciphertext)
if err != nil || n != len(ciphertext) {
panic(fmt.Sprintf("unable to calculate hmac of ciphertext, err %v", err))
}
// verify hmac
mac2 := hm.Sum(nil)
if !hmac.Equal(mac, mac2) {
return nil, ErrUnauthenticated
}
// extract iv
iv, ciphertext := ciphertext[:aes.BlockSize], ciphertext[aes.BlockSize:]
// decrypt data
c, err := aes.NewCipher(ks.Encrypt)
if err != nil {
panic(fmt.Sprintf("unable to create cipher: %v", err))
}
stream := cipher.NewCTR(c, iv)
stream.XORKeyStream(ciphertext, ciphertext)
return &decryptReader{buf: ciphertext}, nil
}
// DecryptFrom verifies and decrypts the ciphertext read from rd and makes it
// available on the returned Reader. Ciphertext must be in the form IV ||
// Ciphertext || HMAC. In order to correctly verify the ciphertext, rd is
// drained, locally buffered and made available on the returned Reader
// afterwards. If an HMAC verification failure is observed, it is returned
// immediately.
func (k *Key) DecryptFrom(rd io.Reader) (io.ReadCloser, error) {
return k.decryptFrom(k.master, rd)
}
// DecryptFrom verifies and decrypts the ciphertext read from rd with the user
// key and makes it available on the returned Reader. Ciphertext must be in the
// form IV || Ciphertext || HMAC. In order to correctly verify the ciphertext,
// rd is drained, locally buffered and made available on the returned Reader
// afterwards. If an HMAC verification failure is observed, it is returned
// immediately.
func (k *Key) DecryptUserFrom(rd io.Reader) (io.ReadCloser, error) {
return k.decryptFrom(k.user, rd)
}
func (k *Key) String() string {
if k == nil {
return "<Key nil>"
}
return fmt.Sprintf("<Key of %s@%s, created on %s>", k.Username, k.Hostname, k.Created)
}
func (k Key) ID() backend.ID {
return k.id
}