1// Copyright 2011 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Package elgamal implements ElGamal encryption, suitable for OpenPGP,
6// as specified in "A Public-Key Cryptosystem and a Signature Scheme Based on
7// Discrete Logarithms," IEEE Transactions on Information Theory, v. IT-31,
8// n. 4, 1985, pp. 469-472.
9//
10// This form of ElGamal embeds PKCS#1 v1.5 padding, which may make it
11// unsuitable for other protocols. RSA should be used in preference in any
12// case.
13package elgamal // import "golang.org/x/crypto/openpgp/elgamal"
14
15import (
16	"crypto/rand"
17	"crypto/subtle"
18	"errors"
19	"io"
20	"math/big"
21)
22
23// PublicKey represents an ElGamal public key.
24type PublicKey struct {
25	G, P, Y *big.Int
26}
27
28// PrivateKey represents an ElGamal private key.
29type PrivateKey struct {
30	PublicKey
31	X *big.Int
32}
33
34// Encrypt encrypts the given message to the given public key. The result is a
35// pair of integers. Errors can result from reading random, or because msg is
36// too large to be encrypted to the public key.
37func Encrypt(random io.Reader, pub *PublicKey, msg []byte) (c1, c2 *big.Int, err error) {
38	pLen := (pub.P.BitLen() + 7) / 8
39	if len(msg) > pLen-11 {
40		err = errors.New("elgamal: message too long")
41		return
42	}
43
44	// EM = 0x02 || PS || 0x00 || M
45	em := make([]byte, pLen-1)
46	em[0] = 2
47	ps, mm := em[1:len(em)-len(msg)-1], em[len(em)-len(msg):]
48	err = nonZeroRandomBytes(ps, random)
49	if err != nil {
50		return
51	}
52	em[len(em)-len(msg)-1] = 0
53	copy(mm, msg)
54
55	m := new(big.Int).SetBytes(em)
56
57	k, err := rand.Int(random, pub.P)
58	if err != nil {
59		return
60	}
61
62	c1 = new(big.Int).Exp(pub.G, k, pub.P)
63	s := new(big.Int).Exp(pub.Y, k, pub.P)
64	c2 = s.Mul(s, m)
65	c2.Mod(c2, pub.P)
66
67	return
68}
69
70// Decrypt takes two integers, resulting from an ElGamal encryption, and
71// returns the plaintext of the message. An error can result only if the
72// ciphertext is invalid. Users should keep in mind that this is a padding
73// oracle and thus, if exposed to an adaptive chosen ciphertext attack, can
74// be used to break the cryptosystem.  See ``Chosen Ciphertext Attacks
75// Against Protocols Based on the RSA Encryption Standard PKCS #1'', Daniel
76// Bleichenbacher, Advances in Cryptology (Crypto '98),
77func Decrypt(priv *PrivateKey, c1, c2 *big.Int) (msg []byte, err error) {
78	s := new(big.Int).Exp(c1, priv.X, priv.P)
79	if s.ModInverse(s, priv.P) == nil {
80		return nil, errors.New("elgamal: invalid private key")
81	}
82	s.Mul(s, c2)
83	s.Mod(s, priv.P)
84	em := s.Bytes()
85
86	firstByteIsTwo := subtle.ConstantTimeByteEq(em[0], 2)
87
88	// The remainder of the plaintext must be a string of non-zero random
89	// octets, followed by a 0, followed by the message.
90	//   lookingForIndex: 1 iff we are still looking for the zero.
91	//   index: the offset of the first zero byte.
92	var lookingForIndex, index int
93	lookingForIndex = 1
94
95	for i := 1; i < len(em); i++ {
96		equals0 := subtle.ConstantTimeByteEq(em[i], 0)
97		index = subtle.ConstantTimeSelect(lookingForIndex&equals0, i, index)
98		lookingForIndex = subtle.ConstantTimeSelect(equals0, 0, lookingForIndex)
99	}
100
101	if firstByteIsTwo != 1 || lookingForIndex != 0 || index < 9 {
102		return nil, errors.New("elgamal: decryption error")
103	}
104	return em[index+1:], nil
105}
106
107// nonZeroRandomBytes fills the given slice with non-zero random octets.
108func nonZeroRandomBytes(s []byte, rand io.Reader) (err error) {
109	_, err = io.ReadFull(rand, s)
110	if err != nil {
111		return
112	}
113
114	for i := 0; i < len(s); i++ {
115		for s[i] == 0 {
116			_, err = io.ReadFull(rand, s[i:i+1])
117			if err != nil {
118				return
119			}
120		}
121	}
122
123	return
124}
125