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symmetric_key_encrypted.go
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symmetric_key_encrypted.go
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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package packet
import (
"bytes"
"crypto/cipher"
"io"
"strconv"
"github.com/ProtonMail/go-crypto/openpgp/errors"
"github.com/ProtonMail/go-crypto/openpgp/s2k"
)
// This is the largest session key that we'll support. Since at most 256-bit cipher
// is supported in OpenPGP, this is large enough to contain also the auth tag.
const maxSessionKeySizeInBytes = 64
// SymmetricKeyEncrypted represents a passphrase protected session key. See RFC
// 4880, section 5.3.
type SymmetricKeyEncrypted struct {
Version int
CipherFunc CipherFunction
Mode AEADMode
s2k func(out, in []byte)
iv []byte
encryptedKey []byte // Contains also the authentication tag for AEAD
}
// parse parses an SymmetricKeyEncrypted packet as specified in
// https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-07.html#name-symmetric-key-encrypted-ses
func (ske *SymmetricKeyEncrypted) parse(r io.Reader) error {
var buf [1]byte
// Version
if _, err := readFull(r, buf[:]); err != nil {
return err
}
ske.Version = int(buf[0])
if ske.Version != 4 && ske.Version != 5 {
return errors.UnsupportedError("unknown SymmetricKeyEncrypted version")
}
// Cipher function
if _, err := readFull(r, buf[:]); err != nil {
return err
}
ske.CipherFunc = CipherFunction(buf[0])
if !ske.CipherFunc.IsSupported() {
return errors.UnsupportedError("unknown cipher: " + strconv.Itoa(int(buf[0])))
}
if ske.Version == 5 {
// AEAD mode
if _, err := readFull(r, buf[:]); err != nil {
return errors.StructuralError("cannot read AEAD octet from packet")
}
ske.Mode = AEADMode(buf[0])
}
var err error
if ske.s2k, err = s2k.Parse(r); err != nil {
if _, ok := err.(errors.ErrDummyPrivateKey); ok {
return errors.UnsupportedError("missing key GNU extension in session key")
}
return err
}
if ske.Version == 5 {
// AEAD IV
iv := make([]byte, ske.Mode.IvLength())
_, err := readFull(r, iv)
if err != nil {
return errors.StructuralError("cannot read AEAD IV")
}
ske.iv = iv
}
encryptedKey := make([]byte, maxSessionKeySizeInBytes)
// The session key may follow. We just have to try and read to find
// out. If it exists then we limit it to maxSessionKeySizeInBytes.
n, err := readFull(r, encryptedKey)
if err != nil && err != io.ErrUnexpectedEOF {
return err
}
if n != 0 {
if n == maxSessionKeySizeInBytes {
return errors.UnsupportedError("oversized encrypted session key")
}
ske.encryptedKey = encryptedKey[:n]
}
return nil
}
// Decrypt attempts to decrypt an encrypted session key and returns the key and
// the cipher to use when decrypting a subsequent Symmetrically Encrypted Data
// packet.
func (ske *SymmetricKeyEncrypted) Decrypt(passphrase []byte) ([]byte, CipherFunction, error) {
key := make([]byte, ske.CipherFunc.KeySize())
ske.s2k(key, passphrase)
if len(ske.encryptedKey) == 0 {
return key, ske.CipherFunc, nil
}
switch ske.Version {
case 4:
plaintextKey, cipherFunc, err := ske.decryptV4(key)
return plaintextKey, cipherFunc, err
case 5:
plaintextKey, err := ske.decryptV5(key)
return plaintextKey, CipherFunction(0), err
}
err := errors.UnsupportedError("unknown SymmetricKeyEncrypted version")
return nil, CipherFunction(0), err
}
func (ske *SymmetricKeyEncrypted) decryptV4(key []byte) ([]byte, CipherFunction, error) {
// the IV is all zeros
iv := make([]byte, ske.CipherFunc.blockSize())
c := cipher.NewCFBDecrypter(ske.CipherFunc.new(key), iv)
plaintextKey := make([]byte, len(ske.encryptedKey))
c.XORKeyStream(plaintextKey, ske.encryptedKey)
cipherFunc := CipherFunction(plaintextKey[0])
if cipherFunc.blockSize() == 0 {
return nil, ske.CipherFunc, errors.UnsupportedError(
"unknown cipher: " + strconv.Itoa(int(cipherFunc)))
}
plaintextKey = plaintextKey[1:]
if len(plaintextKey) != cipherFunc.KeySize() {
return nil, cipherFunc, errors.StructuralError(
"length of decrypted key not equal to cipher keysize")
}
return plaintextKey, cipherFunc, nil
}
func (ske *SymmetricKeyEncrypted) decryptV5(key []byte) ([]byte, error) {
adata := []byte{0xc3, byte(5), byte(ske.CipherFunc), byte(ske.Mode)}
aead := getEncryptedKeyAeadInstance(ske.CipherFunc, ske.Mode, key, adata)
plaintextKey, err := aead.Open(nil, ske.iv, ske.encryptedKey, adata)
if err != nil {
return nil, err
}
return plaintextKey, nil
}
// SerializeSymmetricKeyEncrypted serializes a symmetric key packet to w.
// The packet contains a random session key, encrypted by a key derived from
// the given passphrase. The session key is returned and must be passed to
// SerializeSymmetricallyEncrypted.
// If config is nil, sensible defaults will be used.
func SerializeSymmetricKeyEncrypted(w io.Writer, passphrase []byte, config *Config) (key []byte, err error) {
cipherFunc := config.Cipher()
sessionKey := make([]byte, cipherFunc.KeySize())
_, err = io.ReadFull(config.Random(), sessionKey)
if err != nil {
return
}
err = SerializeSymmetricKeyEncryptedReuseKey(w, sessionKey, passphrase, config)
if err != nil {
return
}
key = sessionKey
return
}
// SerializeSymmetricKeyEncryptedReuseKey serializes a symmetric key packet to w.
// The packet contains the given session key, encrypted by a key derived from
// the given passphrase. The returned session key must be passed to
// SerializeSymmetricallyEncrypted.
// If config is nil, sensible defaults will be used.
func SerializeSymmetricKeyEncryptedReuseKey(w io.Writer, sessionKey []byte, passphrase []byte, config *Config) (err error) {
var version int
if config.AEAD() != nil {
version = 5
} else {
version = 4
}
cipherFunc := config.Cipher()
// cipherFunc must be AES
if !cipherFunc.IsSupported() || cipherFunc < CipherAES128 || cipherFunc > CipherAES256 {
return errors.UnsupportedError("unsupported cipher: " + strconv.Itoa(int(cipherFunc)))
}
keySize := cipherFunc.KeySize()
s2kBuf := new(bytes.Buffer)
keyEncryptingKey := make([]byte, keySize)
// s2k.Serialize salts and stretches the passphrase, and writes the
// resulting key to keyEncryptingKey and the s2k descriptor to s2kBuf.
err = s2k.Serialize(s2kBuf, keyEncryptingKey, config.Random(), passphrase, config.S2K())
if err != nil {
return
}
s2kBytes := s2kBuf.Bytes()
var packetLength int
switch version {
case 4:
packetLength = 2 /* header */ + len(s2kBytes) + 1 /* cipher type */ + keySize
case 5:
ivLen := config.AEAD().Mode().IvLength()
tagLen := config.AEAD().Mode().TagLength()
packetLength = 3 + len(s2kBytes) + ivLen + keySize + tagLen
}
err = serializeHeader(w, packetTypeSymmetricKeyEncrypted, packetLength)
if err != nil {
return
}
// Symmetric Key Encrypted Version
buf := []byte{byte(version)}
// Cipher function
buf = append(buf, byte(cipherFunc))
if version == 5 {
// AEAD mode
buf = append(buf, byte(config.AEAD().Mode()))
}
_, err = w.Write(buf)
if err != nil {
return
}
_, err = w.Write(s2kBytes)
if err != nil {
return
}
switch version {
case 4:
iv := make([]byte, cipherFunc.blockSize())
c := cipher.NewCFBEncrypter(cipherFunc.new(keyEncryptingKey), iv)
encryptedCipherAndKey := make([]byte, keySize+1)
c.XORKeyStream(encryptedCipherAndKey, buf[1:])
c.XORKeyStream(encryptedCipherAndKey[1:], sessionKey)
_, err = w.Write(encryptedCipherAndKey)
if err != nil {
return
}
case 5:
mode := config.AEAD().Mode()
adata := []byte{0xc3, byte(5), byte(cipherFunc), byte(mode)}
aead := getEncryptedKeyAeadInstance(cipherFunc, mode, keyEncryptingKey, adata)
// Sample iv using random reader
iv := make([]byte, config.AEAD().Mode().IvLength())
_, err = io.ReadFull(config.Random(), iv)
if err != nil {
return
}
// Seal and write (encryptedData includes auth. tag)
encryptedData := aead.Seal(nil, iv, sessionKey, adata)
_, err = w.Write(iv)
if err != nil {
return
}
_, err = w.Write(encryptedData)
if err != nil {
return
}
}
return
}
func getEncryptedKeyAeadInstance(c CipherFunction, mode AEADMode, inputKey, associatedData []byte) (aead cipher.AEAD) {
blockCipher := c.new(inputKey)
return mode.new(blockCipher)
}