# Source file src/crypto/ecdsa/ecdsa_legacy.go

```     1  // Copyright 2022 The Go Authors. All rights reserved.
2  // Use of this source code is governed by a BSD-style
4
5  package ecdsa
6
7  import (
8  	"crypto/elliptic"
9  	"errors"
10  	"io"
11  	"math/big"
12
13  	"golang.org/x/crypto/cryptobyte"
14  	"golang.org/x/crypto/cryptobyte/asn1"
15  )
16
17  // This file contains a math/big implementation of ECDSA that is only used for
18  // deprecated custom curves.
19
20  func generateLegacy(c elliptic.Curve, rand io.Reader) (*PrivateKey, error) {
21  	k, err := randFieldElement(c, rand)
22  	if err != nil {
23  		return nil, err
24  	}
25
26  	priv := new(PrivateKey)
27  	priv.PublicKey.Curve = c
28  	priv.D = k
29  	priv.PublicKey.X, priv.PublicKey.Y = c.ScalarBaseMult(k.Bytes())
30  	return priv, nil
31  }
32
33  // hashToInt converts a hash value to an integer. Per FIPS 186-4, Section 6.4,
34  // we use the left-most bits of the hash to match the bit-length of the order of
35  // the curve. This also performs Step 5 of SEC 1, Version 2.0, Section 4.1.3.
36  func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
37  	orderBits := c.Params().N.BitLen()
38  	orderBytes := (orderBits + 7) / 8
39  	if len(hash) > orderBytes {
40  		hash = hash[:orderBytes]
41  	}
42
43  	ret := new(big.Int).SetBytes(hash)
44  	excess := len(hash)*8 - orderBits
45  	if excess > 0 {
46  		ret.Rsh(ret, uint(excess))
47  	}
48  	return ret
49  }
50
51  var errZeroParam = errors.New("zero parameter")
52
53  // Sign signs a hash (which should be the result of hashing a larger message)
54  // using the private key, priv. If the hash is longer than the bit-length of the
55  // private key's curve order, the hash will be truncated to that length. It
56  // returns the signature as a pair of integers. Most applications should use
57  // [SignASN1] instead of dealing directly with r, s.
58  func Sign(rand io.Reader, priv *PrivateKey, hash []byte) (r, s *big.Int, err error) {
59  	sig, err := SignASN1(rand, priv, hash)
60  	if err != nil {
61  		return nil, nil, err
62  	}
63
64  	r, s = new(big.Int), new(big.Int)
65  	var inner cryptobyte.String
66  	input := cryptobyte.String(sig)
68  		!input.Empty() ||
71  		!inner.Empty() {
72  		return nil, nil, errors.New("invalid ASN.1 from SignASN1")
73  	}
74  	return r, s, nil
75  }
76
77  func signLegacy(priv *PrivateKey, csprng io.Reader, hash []byte) (sig []byte, err error) {
78  	c := priv.Curve
79
80  	// SEC 1, Version 2.0, Section 4.1.3
81  	N := c.Params().N
82  	if N.Sign() == 0 {
83  		return nil, errZeroParam
84  	}
85  	var k, kInv, r, s *big.Int
86  	for {
87  		for {
88  			k, err = randFieldElement(c, csprng)
89  			if err != nil {
90  				return nil, err
91  			}
92
93  			kInv = new(big.Int).ModInverse(k, N)
94
95  			r, _ = c.ScalarBaseMult(k.Bytes())
96  			r.Mod(r, N)
97  			if r.Sign() != 0 {
98  				break
99  			}
100  		}
101
102  		e := hashToInt(hash, c)
103  		s = new(big.Int).Mul(priv.D, r)
105  		s.Mul(s, kInv)
106  		s.Mod(s, N) // N != 0
107  		if s.Sign() != 0 {
108  			break
109  		}
110  	}
111
112  	return encodeSignature(r.Bytes(), s.Bytes())
113  }
114
115  // Verify verifies the signature in r, s of hash using the public key, pub. Its
116  // return value records whether the signature is valid. Most applications should
117  // use VerifyASN1 instead of dealing directly with r, s.
118  func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
119  	if r.Sign() <= 0 || s.Sign() <= 0 {
120  		return false
121  	}
122  	sig, err := encodeSignature(r.Bytes(), s.Bytes())
123  	if err != nil {
124  		return false
125  	}
126  	return VerifyASN1(pub, hash, sig)
127  }
128
129  func verifyLegacy(pub *PublicKey, hash []byte, sig []byte) bool {
130  	rBytes, sBytes, err := parseSignature(sig)
131  	if err != nil {
132  		return false
133  	}
134  	r, s := new(big.Int).SetBytes(rBytes), new(big.Int).SetBytes(sBytes)
135
136  	c := pub.Curve
137  	N := c.Params().N
138
139  	if r.Sign() <= 0 || s.Sign() <= 0 {
140  		return false
141  	}
142  	if r.Cmp(N) >= 0 || s.Cmp(N) >= 0 {
143  		return false
144  	}
145
146  	// SEC 1, Version 2.0, Section 4.1.4
147  	e := hashToInt(hash, c)
148  	w := new(big.Int).ModInverse(s, N)
149
150  	u1 := e.Mul(e, w)
151  	u1.Mod(u1, N)
152  	u2 := w.Mul(r, w)
153  	u2.Mod(u2, N)
154
155  	x1, y1 := c.ScalarBaseMult(u1.Bytes())
156  	x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
157  	x, y := c.Add(x1, y1, x2, y2)
158
159  	if x.Sign() == 0 && y.Sign() == 0 {
160  		return false
161  	}
162  	x.Mod(x, N)
163  	return x.Cmp(r) == 0
164  }
165
166  var one = new(big.Int).SetInt64(1)
167
168  // randFieldElement returns a random element of the order of the given
169  // curve using the procedure given in FIPS 186-4, Appendix B.5.2.
170  func randFieldElement(c elliptic.Curve, rand io.Reader) (k *big.Int, err error) {
171  	// See randomPoint for notes on the algorithm. This has to match, or s390x
172  	// signatures will come out different from other architectures, which will
173  	// break TLS recorded tests.
174  	for {
175  		N := c.Params().N
176  		b := make([]byte, (N.BitLen()+7)/8)
177  		if _, err = io.ReadFull(rand, b); err != nil {
178  			return
179  		}
180  		if excess := len(b)*8 - N.BitLen(); excess > 0 {
181  			b[0] >>= excess
182  		}
183  		k = new(big.Int).SetBytes(b)
184  		if k.Sign() != 0 && k.Cmp(N) < 0 {
185  			return
186  		}
187  	}
188  }
189
```

View as plain text