1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
|
// Package cryptocore wraps OpenSSL and Go GCM crypto and provides
// a nonce generator.
package cryptocore
import (
"crypto/aes"
"crypto/cipher"
"crypto/sha512"
"fmt"
"log"
"runtime"
"github.com/rfjakob/eme"
"github.com/rfjakob/gocryptfs/internal/siv_aead"
"github.com/rfjakob/gocryptfs/internal/stupidgcm"
"github.com/rfjakob/gocryptfs/internal/tlog"
)
const (
// KeyLen is the cipher key length in bytes. 32 for AES-256.
KeyLen = 32
// AuthTagLen is the length of a GCM auth tag in bytes.
AuthTagLen = 16
)
// AEADTypeEnum indicates the type of AEAD backend in use.
type AEADTypeEnum int
const (
// BackendOpenSSL specifies the OpenSSL backend.
BackendOpenSSL AEADTypeEnum = 3
// BackendGoGCM specifies the Go based GCM backend.
BackendGoGCM AEADTypeEnum = 4
// BackendAESSIV specifies an AESSIV backend.
BackendAESSIV AEADTypeEnum = 5
)
func (a AEADTypeEnum) String() string {
switch a {
case BackendOpenSSL:
return "BackendOpenSSL"
case BackendGoGCM:
return "BackendGoGCM"
case BackendAESSIV:
return "BackendAESSIV"
default:
return fmt.Sprintf("%d", a)
}
}
// CryptoCore is the low level crypto implementation.
type CryptoCore struct {
// EME is used for filename encryption.
EMECipher *eme.EMECipher
// GCM or AES-SIV. This is used for content encryption.
AEADCipher cipher.AEAD
// Which backend is behind AEADCipher?
AEADBackend AEADTypeEnum
// GCM needs unique IVs (nonces)
IVGenerator *nonceGenerator
// IVLen in bytes
IVLen int
}
// New returns a new CryptoCore object or panics.
//
// Even though the "GCMIV128" feature flag is now mandatory, we must still
// support 96-bit IVs here because they were used for encrypting the master
// key in gocryptfs.conf up to gocryptfs v1.2. v1.3 switched to 128 bits.
//
// Note: "key" is either the scrypt hash of the password (when decrypting
// a config file) or the masterkey (when finally mounting the filesystem).
func New(key []byte, aeadType AEADTypeEnum, IVBitLen int, useHKDF bool, forceDecode bool) *CryptoCore {
tlog.Debug.Printf("cryptocore.New: key=%d bytes, aeadType=%v, IVBitLen=%d, useHKDF=%v, forceDecode=%v",
len(key), aeadType, IVBitLen, useHKDF, forceDecode)
if len(key) != KeyLen {
log.Panicf("Unsupported key length of %d bytes", len(key))
}
if IVBitLen != 96 && IVBitLen != 128 {
log.Panicf("Unsupported IV length of %d bits", IVBitLen)
}
// Initialize EME for filename encryption.
var emeCipher *eme.EMECipher
var err error
{
var emeBlockCipher cipher.Block
if useHKDF {
emeKey := hkdfDerive(key, hkdfInfoEMENames, KeyLen)
emeBlockCipher, err = aes.NewCipher(emeKey)
for i := range emeKey {
emeKey[i] = 0
}
} else {
emeBlockCipher, err = aes.NewCipher(key)
}
if err != nil {
log.Panic(err)
}
emeCipher = eme.New(emeBlockCipher)
}
// Initialize an AEAD cipher for file content encryption.
var aeadCipher cipher.AEAD
if aeadType == BackendOpenSSL || aeadType == BackendGoGCM {
var gcmKey []byte
if useHKDF {
gcmKey = hkdfDerive(key, hkdfInfoGCMContent, KeyLen)
} else {
// Filesystems created by gocryptfs v0.7 through v1.2 don't use HKDF.
// Example: tests/example_filesystems/v0.9
gcmKey = append([]byte{}, key...)
}
switch aeadType {
case BackendOpenSSL:
if IVBitLen != 128 {
log.Panicf("stupidgcm only supports 128-bit IVs, you wanted %d", IVBitLen)
}
aeadCipher = stupidgcm.New(gcmKey, forceDecode)
case BackendGoGCM:
goGcmBlockCipher, err := aes.NewCipher(gcmKey)
if err != nil {
log.Panic(err)
}
aeadCipher, err = cipher.NewGCMWithNonceSize(goGcmBlockCipher, IVBitLen/8)
if err != nil {
log.Panic(err)
}
}
for i := range gcmKey {
gcmKey[i] = 0
}
} else if aeadType == BackendAESSIV {
if IVBitLen != 128 {
// SIV supports any nonce size, but we only use 128.
log.Panicf("AES-SIV must use 128-bit IVs, you wanted %d", IVBitLen)
}
// AES-SIV uses 1/2 of the key for authentication, 1/2 for
// encryption, so we need a 64-bytes key for AES-256. Derive it from
// the 32-byte master key using HKDF, or, for older filesystems, with
// SHA256.
var key64 []byte
if useHKDF {
key64 = hkdfDerive(key, hkdfInfoSIVContent, siv_aead.KeyLen)
} else {
s := sha512.Sum512(key)
key64 = s[:]
}
aeadCipher = siv_aead.New(key64)
for i := range key64 {
key64[i] = 0
}
} else {
log.Panic("unknown backend cipher")
}
return &CryptoCore{
EMECipher: emeCipher,
AEADCipher: aeadCipher,
AEADBackend: aeadType,
IVGenerator: &nonceGenerator{nonceLen: IVBitLen / 8},
IVLen: IVBitLen / 8,
}
}
type wiper interface {
Wipe()
}
// Wipe tries to wipe secret keys from memory by overwriting them with zeros
// and/or setting references to nil.
//
// This is not bulletproof due to possible GC copies, but
// still raises to bar for extracting the key.
func (c *CryptoCore) Wipe() {
be := c.AEADBackend
if be == BackendOpenSSL || be == BackendAESSIV {
tlog.Debug.Printf("CryptoCore.Wipe: Wiping AEADBackend %d key", be)
// We don't use "x, ok :=" because we *want* to crash loudly if the
// type assertion fails.
w := c.AEADCipher.(wiper)
w.Wipe()
} else {
tlog.Debug.Printf("CryptoCore.Wipe: Only nil'ing stdlib refs")
}
// We have no access to the keys (or key-equivalents) stored inside the
// Go stdlib. Best we can is to nil the references and force a GC.
c.AEADCipher = nil
c.EMECipher = nil
runtime.GC()
}
|
