restic/key.go

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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"
"github.com/fd0/khepri/chunker"
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"golang.org/x/crypto/scrypt"
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)
// 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
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var (
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// 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")
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)
// TODO: figure out scrypt values on the fly depending on the current
// hardware.
const (
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scryptN = 65536
scryptR = 8
scryptP = 1
scryptSaltsize = 64
aesKeysize = 32 // for AES256
hmacKeysize = 32 // for HMAC with SHA256
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)
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// Key represents an encrypted master key for a repository.
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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
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id backend.ID
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}
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// keys is a JSON structure that holds signing and encryption keys.
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type keys struct {
Sign []byte
Encrypt []byte
}
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// CreateKey initializes a master key in the given backend and encrypts it with
// the password.
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func CreateKey(be backend.Server, password string) (*Key, error) {
// fill meta data about key
k := &Key{
Created: time.Now(),
KDF: "scrypt",
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N: scryptN,
R: scryptR,
P: scryptP,
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}
hn, err := os.Hostname()
if err == nil {
k.Hostname = hn
}
usr, err := user.Current()
if err == nil {
k.Username = usr.Username
}
// generate random salt
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k.Salt = make([]byte, scryptSaltsize)
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n, err := rand.Read(k.Salt)
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if n != scryptSaltsize || err != nil {
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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 = GetChunkBuf("key")
n, err = k.EncryptUser(k.Data, buf)
k.Data = k.Data[:n]
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// dump as json
buf, err = json.Marshal(k)
if err != nil {
return nil, err
}
// store in repository and return
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id, err := be.Create(backend.Key, buf)
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if err != nil {
return nil, err
}
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k.id = id
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FreeChunkBuf("key", k.Data)
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return k, nil
}
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// OpenKey tries do decrypt the key specified by id with the given password.
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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
}
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k.id = id
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return k, nil
}
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// SearchKey tries to decrypt all keys in the backend with the given password.
// If none could be found, ErrNoKeyFound is returned.
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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
}
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// AddKey adds a new key to an already existing repository.
func (oldkey *Key) AddKey(be backend.Server, password string) (backend.ID, 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
}
// copy master keys from oldkey
newkey.master = oldkey.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
id, err := be.Create(backend.Key, buf)
if err != nil {
return nil, err
}
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newkey.id = id
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FreeChunkBuf("key", newkey.Data)
return id, nil
}
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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
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scryptKeys, err := scrypt.Key([]byte(password), k.Salt, k.N, k.R, k.P, keybytes)
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if err != nil {
return nil, fmt.Errorf("error deriving keys from password: %v", err)
}
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if len(scryptKeys) != keybytes {
return nil, fmt.Errorf("invalid numbers of bytes expanded from scrypt(): %d", len(scryptKeys))
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}
ks := &keys{
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Encrypt: scryptKeys[:aesKeysize],
Sign: scryptKeys[aesKeysize:],
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}
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]
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if err != nil {
return err
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}
return nil
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}
// 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])
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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)]
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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
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}
// 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)
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}
// 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)
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}
// 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) {
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// check for plausible length
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if len(ciphertext) < ivSize+hmacSize {
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panic("trying to decryipt invalid data: ciphertext too small")
}
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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
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e := cipher.NewCTR(c, iv)
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plaintext := make([]byte, len(ciphertext))
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e.XORKeyStream(plaintext, ciphertext)
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return plaintext, nil
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}
// 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)
}
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func (k Key) ID() backend.ID {
return k.id
}