ecdsa_legacy.go

Documentation: crypto/ecdsa

     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  	"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)
    67  	if !input.ReadASN1(&inner, asn1.SEQUENCE) ||
    68  		!input.Empty() ||
    69  		!inner.ReadASN1Integer(r) ||
    70  		!inner.ReadASN1Integer(s) ||
    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)
   104  		s.Add(s, e)
   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  //
   119  // The inputs are not considered confidential, and may leak through timing side
   120  // channels, or if an attacker has control of part of the inputs.
   121  func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
   122  	if r.Sign() <= 0 || s.Sign() <= 0 {
   123  		return false
   124  	}
   125  	sig, err := encodeSignature(r.Bytes(), s.Bytes())
   126  	if err != nil {
   127  		return false
   128  	}
   129  	return VerifyASN1(pub, hash, sig)
   130  }
   131  
   132  func verifyLegacy(pub *PublicKey, hash []byte, sig []byte) bool {
   133  	rBytes, sBytes, err := parseSignature(sig)
   134  	if err != nil {
   135  		return false
   136  	}
   137  	r, s := new(big.Int).SetBytes(rBytes), new(big.Int).SetBytes(sBytes)
   138  
   139  	c := pub.Curve
   140  	N := c.Params().N
   141  
   142  	if r.Sign() <= 0 || s.Sign() <= 0 {
   143  		return false
   144  	}
   145  	if r.Cmp(N) >= 0 || s.Cmp(N) >= 0 {
   146  		return false
   147  	}
   148  
   149  	// SEC 1, Version 2.0, Section 4.1.4
   150  	e := hashToInt(hash, c)
   151  	w := new(big.Int).ModInverse(s, N)
   152  
   153  	u1 := e.Mul(e, w)
   154  	u1.Mod(u1, N)
   155  	u2 := w.Mul(r, w)
   156  	u2.Mod(u2, N)
   157  
   158  	x1, y1 := c.ScalarBaseMult(u1.Bytes())
   159  	x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
   160  	x, y := c.Add(x1, y1, x2, y2)
   161  
   162  	if x.Sign() == 0 && y.Sign() == 0 {
   163  		return false
   164  	}
   165  	x.Mod(x, N)
   166  	return x.Cmp(r) == 0
   167  }
   168  
   169  var one = new(big.Int).SetInt64(1)
   170  
   171  // randFieldElement returns a random element of the order of the given
   172  // curve using the procedure given in FIPS 186-4, Appendix B.5.2.
   173  func randFieldElement(c elliptic.Curve, rand io.Reader) (k *big.Int, err error) {
   174  	// See randomPoint for notes on the algorithm. This has to match, or s390x
   175  	// signatures will come out different from other architectures, which will
   176  	// break TLS recorded tests.
   177  	for {
   178  		N := c.Params().N
   179  		b := make([]byte, (N.BitLen()+7)/8)
   180  		if _, err = io.ReadFull(rand, b); err != nil {
   181  			return
   182  		}
   183  		if excess := len(b)*8 - N.BitLen(); excess > 0 {
   184  			b[0] >>= excess
   185  		}
   186  		k = new(big.Int).SetBytes(b)
   187  		if k.Sign() != 0 && k.Cmp(N) < 0 {
   188  			return
   189  		}
   190  	}
   191  }
   192
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