1 // Copyright 2022 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 ecdsa
6 7 import (
8 "crypto/elliptic"
9 "crypto/internal/fips140only"
10 "errors"
11 "io"
12 "math/big"
13 "math/rand/v2"
14 15 "golang.org/x/crypto/cryptobyte"
16 "golang.org/x/crypto/cryptobyte/asn1"
17 )
18 19 // This file contains a math/big implementation of ECDSA that is only used for
20 // deprecated custom curves.
21 22 func generateLegacy(c elliptic.Curve, rand io.Reader) (*PrivateKey, error) {
23 if fips140only.Enabled {
24 return nil, errors.New("crypto/ecdsa: use of custom curves is not allowed in FIPS 140-only mode")
25 }
26 27 k, err := randFieldElement(c, rand)
28 if err != nil {
29 return nil, err
30 }
31 32 priv := &PrivateKey{}
33 priv.PublicKey.Curve = c
34 priv.D = k
35 priv.PublicKey.X, priv.PublicKey.Y = c.ScalarBaseMult(k.Bytes())
36 return priv, nil
37 }
38 39 // hashToInt converts a hash value to an integer. Per FIPS 186-4, Section 6.4,
40 // we use the left-most bits of the hash to match the bit-length of the order of
41 // the curve. This also performs Step 5 of SEC 1, Version 2.0, Section 4.1.3.
42 func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
43 orderBits := c.Params().N.BitLen()
44 orderBytes := (orderBits + 7) / 8
45 if len(hash) > orderBytes {
46 hash = hash[:orderBytes]
47 }
48 49 ret := (&big.Int{}).SetBytes(hash)
50 excess := len(hash)*8 - orderBits
51 if excess > 0 {
52 ret.Rsh(ret, uint(excess))
53 }
54 return ret
55 }
56 57 var errZeroParam = errors.New("zero parameter")
58 59 // Sign signs a hash (which should be the result of hashing a larger message)
60 // using the private key, priv. If the hash is longer than the bit-length of the
61 // private key's curve order, the hash will be truncated to that length. It
62 // returns the signature as a pair of integers. Most applications should use
63 // [SignASN1] instead of dealing directly with r, s.
64 func Sign(rand io.Reader, priv *PrivateKey, hash []byte) (r, s *big.Int, err error) {
65 sig, err := SignASN1(rand, priv, hash)
66 if err != nil {
67 return nil, nil, err
68 }
69 70 r, s = &big.Int{}, &big.Int{}
71 var inner cryptobyte.String
72 input := cryptobyte.String(sig)
73 if !input.ReadASN1(&inner, asn1.SEQUENCE) ||
74 !input.Empty() ||
75 !inner.ReadASN1Integer(r) ||
76 !inner.ReadASN1Integer(s) ||
77 !inner.Empty() {
78 return nil, nil, errors.New("invalid ASN.1 from SignASN1")
79 }
80 return r, s, nil
81 }
82 83 func signLegacy(priv *PrivateKey, csprng io.Reader, hash []byte) (sig []byte, err error) {
84 if fips140only.Enabled {
85 return nil, errors.New("crypto/ecdsa: use of custom curves is not allowed in FIPS 140-only mode")
86 }
87 88 c := priv.Curve
89 90 // A cheap version of hedged signatures, for the deprecated path.
91 var seed [32]byte
92 if _, err := io.ReadFull(csprng, seed[:]); err != nil {
93 return nil, err
94 }
95 for i, b := range priv.D.Bytes() {
96 seed[i%32] ^= b
97 }
98 for i, b := range hash {
99 seed[i%32] ^= b
100 }
101 csprng = rand.NewChaCha8(seed)
102 103 // SEC 1, Version 2.0, Section 4.1.3
104 N := c.Params().N
105 if N.Sign() == 0 {
106 return nil, errZeroParam
107 }
108 var k, kInv, r, s *big.Int
109 for {
110 for {
111 k, err = randFieldElement(c, csprng)
112 if err != nil {
113 return nil, err
114 }
115 116 kInv = (&big.Int{}).ModInverse(k, N)
117 118 r, _ = c.ScalarBaseMult(k.Bytes())
119 r.Mod(r, N)
120 if r.Sign() != 0 {
121 break
122 }
123 }
124 125 e := hashToInt(hash, c)
126 s = (&big.Int{}).Mul(priv.D, r)
127 s.Add(s, e)
128 s.Mul(s, kInv)
129 s.Mod(s, N) // N != 0
130 if s.Sign() != 0 {
131 break
132 }
133 }
134 135 return encodeSignature(r.Bytes(), s.Bytes())
136 }
137 138 // Verify verifies the signature in r, s of hash using the public key, pub. Its
139 // return value records whether the signature is valid. Most applications should
140 // use VerifyASN1 instead of dealing directly with r, s.
141 //
142 // The inputs are not considered confidential, and may leak through timing side
143 // channels, or if an attacker has control of part of the inputs.
144 func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
145 if r.Sign() <= 0 || s.Sign() <= 0 {
146 return false
147 }
148 sig, err := encodeSignature(r.Bytes(), s.Bytes())
149 if err != nil {
150 return false
151 }
152 return VerifyASN1(pub, hash, sig)
153 }
154 155 func verifyLegacy(pub *PublicKey, hash []byte, sig []byte) bool {
156 if fips140only.Enabled {
157 panic("crypto/ecdsa: use of custom curves is not allowed in FIPS 140-only mode")
158 }
159 160 rBytes, sBytes, err := parseSignature(sig)
161 if err != nil {
162 return false
163 }
164 r, s := (&big.Int{}).SetBytes(rBytes), (&big.Int{}).SetBytes(sBytes)
165 166 c := pub.Curve
167 N := c.Params().N
168 169 if r.Sign() <= 0 || s.Sign() <= 0 {
170 return false
171 }
172 if r.Cmp(N) >= 0 || s.Cmp(N) >= 0 {
173 return false
174 }
175 176 // SEC 1, Version 2.0, Section 4.1.4
177 e := hashToInt(hash, c)
178 w := (&big.Int{}).ModInverse(s, N)
179 180 u1 := e.Mul(e, w)
181 u1.Mod(u1, N)
182 u2 := w.Mul(r, w)
183 u2.Mod(u2, N)
184 185 x1, y1 := c.ScalarBaseMult(u1.Bytes())
186 x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
187 x, y := c.Add(x1, y1, x2, y2)
188 189 if x.Sign() == 0 && y.Sign() == 0 {
190 return false
191 }
192 x.Mod(x, N)
193 return x.Cmp(r) == 0
194 }
195 196 var one = (&big.Int{}).SetInt64(1)
197 198 // randFieldElement returns a random element of the order of the given
199 // curve using the procedure given in FIPS 186-4, Appendix B.5.2.
200 func randFieldElement(c elliptic.Curve, rand io.Reader) (k *big.Int, err error) {
201 for {
202 N := c.Params().N
203 b := []byte{:(N.BitLen()+7)/8}
204 if _, err = io.ReadFull(rand, b); err != nil {
205 return
206 }
207 if excess := len(b)*8 - N.BitLen(); excess > 0 {
208 b[0] >>= excess
209 }
210 k = (&big.Int{}).SetBytes(b)
211 if k.Sign() != 0 && k.Cmp(N) < 0 {
212 return
213 }
214 }
215 }
216