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
|
// Copyright © 2025 siddharth ravikumar <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"math/big"
"time"
"ricketyspace.net/cryptopals/lib"
)
func C42() {
var (
m []byte
s []byte
kp *lib.RSAPair
sigm []byte
err error
)
// Set message.
m = []byte("hi mom")
// Generate 1024 bit key pair.
kp, err = lib.RSAGenKey(1024)
if err != nil {
fmt.Printf("gen key: %v\n", err)
return
}
// Verify RSA signing works.
s, err = kp.Private.Sign(m)
if err != nil {
fmt.Printf("sign: %v\n", err)
return
}
if !kp.Public.LazyVerify(m, s) {
fmt.Printf("sig check failed\n")
}
// Make message that will be given as input to create the RSA
// signature.
sigm, err = mkSigMsg(m, kp.Public)
if err != nil {
fmt.Printf("mk sig msg: %v\n", err)
return
}
var (
found = false
tries = 0
rn = byte(0)
rnIdx = len(sigm)
forgeds []byte // Forged signature.
el time.Time
)
el = time.Now()
defer func() {
fmt.Printf(
"time to find forged signature: %v\n",
time.Since(el),
)
}()
sigm = append(sigm, rn)
for tries < 1000000 {
forgeds = lib.BigIntCubeRoot(
new(big.Int).SetBytes(sigm),
).Bytes()
if forgeds != nil {
var ok = kp.Public.LazyVerify(m, forgeds)
if ok {
found = true
break
}
}
tries += 1
if rn == 255 {
rn = 0
rnIdx += 1
sigm = append(sigm, rn)
} else {
rn += 1
}
sigm[rnIdx] = rn
}
if found {
fmt.Printf("found forged signature: %v\n", forgeds)
} else {
fmt.Printf("unable to find forged signature\n")
}
}
func mkSigMsg(msg []byte, pub *lib.RSAPub) (m []byte, err error) {
var (
dgst lib.Md4
d []byte // Data to sign.
eb []byte
)
dgst.Init([]uint32{})
dgst.Message(msg)
d = dgst.Hash()
// Build encryption block.
eb = []byte{0x00, 0x01, 0xFF, 0xFF, 0x00}
eb = append(eb, d...)
m = eb
return
}
|
