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