restic/key.go
2014-11-15 17:12:52 +01:00

351 lines
7.7 KiB
Go

package khepri
import (
"crypto/aes"
"crypto/cipher"
"crypto/hmac"
"crypto/rand"
"crypto/sha256"
"encoding/json"
"errors"
"fmt"
"io"
"os"
"os/user"
"time"
"github.com/fd0/khepri/backend"
"code.google.com/p/go.crypto/scrypt"
)
var (
ErrUnauthenticated = errors.New("Ciphertext verification failed")
ErrNoKeyFound = errors.New("No key could be found")
)
// TODO: figure out scrypt values on the fly depending on the current
// hardware.
const (
scrypt_N = 65536
scrypt_r = 8
scrypt_p = 1
scrypt_saltsize = 64
aesKeysize = 32 // for AES256
hmacKeysize = 32 // for HMAC with SHA256
)
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
}
type keys struct {
Sign []byte
Encrypt []byte
}
func CreateKey(be backend.Server, password string) (*Key, error) {
// fill meta data about key
k := &Key{
Created: time.Now(),
KDF: "scrypt",
N: scrypt_N,
R: scrypt_r,
P: scrypt_p,
}
hn, err := os.Hostname()
if err == nil {
k.Hostname = hn
}
usr, err := user.Current()
if err == nil {
k.Username = usr.Username
}
// generate random salt
k.Salt = make([]byte, scrypt_saltsize)
n, err := rand.Read(k.Salt)
if n != scrypt_saltsize || err != nil {
panic("unable to read enough random bytes for salt")
}
// call scrypt() to derive user key
k.user, err = k.scrypt(password)
if err != nil {
return nil, err
}
// generate new random master keys
k.master, err = k.newKeys()
if err != nil {
return nil, err
}
// encrypt master keys (as json) with user key
buf, err := json.Marshal(k.master)
if err != nil {
return nil, err
}
k.Data, err = k.EncryptUser(buf)
// dump as json
buf, err = json.Marshal(k)
if err != nil {
return nil, err
}
// store in repository and return
_, err = be.Create(backend.Key, buf)
if err != nil {
return nil, err
}
return k, nil
}
func OpenKey(be backend.Server, id backend.ID, password string) (*Key, error) {
// extract data from repo
data, err := be.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
}
// 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(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
}
return k, nil
}
func SearchKey(be backend.Server, password string) (*Key, error) {
// list all keys
ids, err := be.List(backend.Key)
if err != nil {
panic(err)
}
// try all keys in repo
var key *Key
for _, id := range ids {
key, err = OpenKey(be, id, password)
if err != nil {
continue
}
return key, nil
}
return nil, ErrNoKeyFound
}
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
scrypt_keys, 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(scrypt_keys) != keybytes {
return nil, fmt.Errorf("invalid numbers of bytes expanded from scrypt(): %d", len(scrypt_keys))
}
ks := &keys{
Encrypt: scrypt_keys[:aesKeysize],
Sign: scrypt_keys[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() ([]byte, error) {
buf := make([]byte, aes.BlockSize)
_, err := io.ReadFull(rand.Reader, buf)
if err != nil {
return nil, err
}
return buf, nil
}
// Encrypt encrypts and signs data. Returned is IV || Ciphertext || HMAC. For
// the hash function, SHA256 is used, so the overhead is 16+32=48 byte.
func (k *Key) encrypt(ks *keys, plaintext []byte) ([]byte, error) {
iv, err := k.newIV()
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, iv)
l := len(iv)
ciphertext := make([]byte, l+len(plaintext))
copy(ciphertext[:l], iv)
e.XORKeyStream(ciphertext[l:], 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))
}
return hm.Sum(ciphertext), nil
}
// EncryptUser encrypts and signs data with the user key. Returned is IV ||
// Ciphertext || HMAC. For the hash function, SHA256 is used, so the overhead
// is 16+32=48 byte.
func (k *Key) EncryptUser(plaintext []byte) ([]byte, error) {
return k.encrypt(k.user, plaintext)
}
// Encrypt encrypts and signs data with the master key. Returned is IV ||
// Ciphertext || HMAC. For the hash function, SHA256 is used, so the overhead
// is 16+32=48 byte.
func (k *Key) Encrypt(plaintext []byte) ([]byte, error) {
return k.encrypt(k.master, plaintext)
}
// Decrypt verifes and decrypts the ciphertext. Ciphertext must be in the form
// IV || Ciphertext || HMAC.
func (k *Key) decrypt(ks *keys, ciphertext []byte) ([]byte, error) {
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 := make([]byte, 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(ciphertext []byte) ([]byte, error) {
return k.decrypt(k.master, ciphertext)
}
// DecryptUser verifes and decrypts the ciphertext with the master key. Ciphertext
// must be in the form IV || Ciphertext || HMAC.
func (k *Key) DecryptUser(ciphertext []byte) ([]byte, error) {
return k.decrypt(k.user, ciphertext)
}
// Each calls backend.Each() with the given parameters, Decrypt() on the
// ciphertext and, on successful decryption, f with the plaintext.
func (k *Key) Each(be backend.Server, t backend.Type, f func(backend.ID, []byte, error)) error {
return backend.Each(be, t, func(id backend.ID, data []byte, e error) {
if e != nil {
f(id, nil, e)
return
}
buf, err := k.Decrypt(data)
if err != nil {
f(id, nil, err)
return
}
f(id, buf, nil)
})
}
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)
}