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usersecrets.go
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/
usersecrets.go
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package keysharecore
import (
"crypto/aes"
"crypto/cipher"
"crypto/ecdsa"
"crypto/rand"
"crypto/subtle"
"encoding/binary"
"github.com/fxamacker/cbor"
"github.com/go-errors/errors"
"github.com/golang-jwt/jwt/v4"
"github.com/privacybydesign/gabi/big"
"github.com/privacybydesign/gabi/signed"
)
type (
unencryptedUserSecrets struct {
Pin []byte
KeyshareSecret *big.Int
ID []byte
PublicKey *ecdsa.PublicKey
}
// UserSecrets contains the encrypted data of a keyshare user.
UserSecrets []byte
)
var (
ErrKeyshareSecretTooBig = errors.New("Keyshare secret too big to store")
ErrKeyshareSecretNegative = errors.New("Keyshare secret negative")
ErrNoSuchKey = errors.New("Key identifier unknown")
)
func (s *unencryptedUserSecrets) setPin(pin string) error {
if len(pin) > 64 {
// padBytes also checks the length, but we want to return a specific error in this case
return ErrPinTooLong
}
paddedPin, err := padBytes([]byte(pin), 64)
if err != nil {
return err
}
s.Pin = paddedPin
return nil
}
func (s *unencryptedUserSecrets) setKeyshareSecret(val *big.Int) error {
if val.Sign() == -1 {
return ErrKeyshareSecretNegative
}
// We want the result to fit into 64 bytes
if val.BitLen() > 64*8 {
return ErrKeyshareSecretTooBig
}
s.KeyshareSecret = new(big.Int).Set(val)
return nil
}
func (s *unencryptedUserSecrets) setID(id []byte) error {
paddedID, err := padBytes(id, 32)
if err != nil {
return err
}
s.ID = paddedID
return nil
}
func (s *unencryptedUserSecrets) verifyPin(pin string) error {
paddedPin, err := padBytes([]byte(pin), 64)
if err != nil {
return err
}
if subtle.ConstantTimeCompare(s.Pin, paddedPin) != 1 {
return ErrInvalidPin
}
return nil
}
type marshaledUserSecrets struct {
Pin []byte
KeyshareSecret []byte
ID []byte
PublicKey []byte
}
// MarshalCBOR implements cbor.Marshaler to ensure that all fields have a constant size, to minimize
// differences in the size of the encrypted blobs.
func (s *unencryptedUserSecrets) MarshalCBOR() ([]byte, error) {
secretBts, err := padBytes(s.KeyshareSecret.Bytes(), 64)
if err != nil {
return nil, err
}
var pkBts []byte
if s.PublicKey != nil {
pkBts, err = signed.MarshalPublicKey(s.PublicKey)
if err != nil {
return nil, err
}
}
return cbor.Marshal(marshaledUserSecrets{
s.Pin, secretBts, s.ID, pkBts,
}, cbor.EncOptions{})
}
func (s *unencryptedUserSecrets) UnmarshalCBOR(bytes []byte) error {
raw := marshaledUserSecrets{}
err := cbor.Unmarshal(bytes, &raw)
if err != nil {
return err
}
*s = unencryptedUserSecrets{
Pin: raw.Pin,
KeyshareSecret: new(big.Int).SetBytes(raw.KeyshareSecret),
ID: raw.ID,
}
if len(raw.PublicKey) > 0 {
s.PublicKey, err = signed.UnmarshalPublicKey(raw.PublicKey)
if err != nil {
return err
}
}
return nil
}
// publicKey returns the user's public key. For use in jwt.ParseWithClaims().
func (s *unencryptedUserSecrets) publicKey(_ *jwt.Token) (interface{}, error) {
if s.PublicKey == nil {
return nil, ErrKeyNotFound
}
return s.PublicKey, nil
}
func (c *Core) encryptUserSecrets(secrets unencryptedUserSecrets) (UserSecrets, error) {
encSecrets := make(UserSecrets, 16, 256)
bts, err := cbor.Marshal(secrets, cbor.EncOptions{})
if err != nil {
return nil, err
}
// Store key id
binary.LittleEndian.PutUint32(encSecrets[0:], c.decryptionKeyID)
// Generate and store nonce
_, err = rand.Read(encSecrets[4:16])
if err != nil {
return nil, err
}
// Encrypt secrets
gcm, err := newGCM(c.decryptionKey)
if err != nil {
return nil, err
}
return gcm.Seal(encSecrets[:16], encSecrets[4:16], bts, nil), nil
}
func (c *Core) decryptUserSecrets(secrets UserSecrets) (unencryptedUserSecrets, error) {
// determine key id
id := binary.LittleEndian.Uint32(secrets[0:])
// Fetch key
key, ok := c.decryptionKeys[id]
if !ok {
return unencryptedUserSecrets{}, ErrNoSuchKey
}
// try and decrypt secrets
gcm, err := newGCM(key)
if err != nil {
return unencryptedUserSecrets{}, err
}
bts, err := gcm.Open(nil, secrets[4:16], secrets[16:], nil)
if err != nil {
return unencryptedUserSecrets{}, err
}
var unencSecrets unencryptedUserSecrets
err = cbor.Unmarshal(bts, &unencSecrets)
if err != nil {
return unencryptedUserSecrets{}, err
}
return unencSecrets, nil
}
func (c *Core) decryptUserSecretsIfPinOK(secrets UserSecrets, pin string) (unencryptedUserSecrets, error) {
s, err := c.decryptUserSecrets(secrets)
if err != nil {
return unencryptedUserSecrets{}, err
}
if err = s.verifyPin(pin); err != nil {
return unencryptedUserSecrets{}, err
}
return s, nil
}
func newGCM(key AESKey) (cipher.AEAD, error) {
keyedAes, err := aes.NewCipher(key[:])
if err != nil {
return nil, err
}
gcm, err := cipher.NewGCM(keyedAes)
if err != nil {
return nil, err
}
return gcm, nil
}
// padBytes pads the given byte slice with zeros on the left such that the resulting byte slice
// has the specified length.
func padBytes(src []byte, length int) ([]byte, error) {
if len(src) > length {
return nil, errors.New("padBytes: source slice too long")
}
if len(src) == length {
return src, nil
}
result := make([]byte, length)
copy(result[length-len(src):], src)
return result, nil
}