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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package ecdsa implements the Elliptic Curve Digital Signature Algorithm, as
// defined in FIPS 186-3.
package ecdsa
// References:
// [NSA]: Suite B implementer's guide to FIPS 186-3,
// http://www.nsa.gov/ia/_files/ecdsa.pdf
// [SECG]: SECG, SEC1
// http://www.secg.org/download/aid-780/sec1-v2.pdf
import (
"crypto/elliptic"
"io"
"math/big"
)
// PublicKey represents an ECDSA public key.
type PublicKey struct {
elliptic.Curve
X, Y *big.Int
}
// PrivateKey represents a ECDSA private key.
type PrivateKey struct {
PublicKey
D *big.Int
}
var one = new(big.Int).SetInt64(1)
// randFieldElement returns a random element of the field underlying the given
// curve using the procedure given in [NSA] A.2.1.
func randFieldElement(c elliptic.Curve, rand io.Reader) (k *big.Int, err error) {
params := c.Params()
b := make([]byte, params.BitSize/8+8)
_, err = io.ReadFull(rand, b)
if err != nil {
return
}
k = new(big.Int).SetBytes(b)
n := new(big.Int).Sub(params.N, one)
k.Mod(k, n)
k.Add(k, one)
return
}
// GenerateKey generates a public&private key pair.
func GenerateKey(c elliptic.Curve, rand io.Reader) (priv *PrivateKey, err error) {
k, err := randFieldElement(c, rand)
if err != nil {
return
}
priv = new(PrivateKey)
priv.PublicKey.Curve = c
priv.D = k
priv.PublicKey.X, priv.PublicKey.Y = c.ScalarBaseMult(k.Bytes())
return
}
// hashToInt converts a hash value to an integer. There is some disagreement
// about how this is done. [NSA] suggests that this is done in the obvious
// manner, but [SECG] truncates the hash to the bit-length of the curve order
// first. We follow [SECG] because that's what OpenSSL does. Additionally,
// OpenSSL right shifts excess bits from the number if the hash is too large
// and we mirror that too.
func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
orderBits := c.Params().N.BitLen()
orderBytes := (orderBits + 7) / 8
if len(hash) > orderBytes {
hash = hash[:orderBytes]
}
ret := new(big.Int).SetBytes(hash)
excess := len(hash)*8 - orderBits
if excess > 0 {
ret.Rsh(ret, uint(excess))
}
return ret
}
// Sign signs an arbitrary length hash (which should be the result of hashing a
// larger message) using the private key, priv. It returns the signature as a
// pair of integers. The security of the private key depends on the entropy of
// rand.
func Sign(rand io.Reader, priv *PrivateKey, hash []byte) (r, s *big.Int, err error) {
// See [NSA] 3.4.1
c := priv.PublicKey.Curve
N := c.Params().N
var k, kInv *big.Int
for {
for {
k, err = randFieldElement(c, rand)
if err != nil {
r = nil
return
}
kInv = new(big.Int).ModInverse(k, N)
r, _ = priv.Curve.ScalarBaseMult(k.Bytes())
r.Mod(r, N)
if r.Sign() != 0 {
break
}
}
e := hashToInt(hash, c)
s = new(big.Int).Mul(priv.D, r)
s.Add(s, e)
s.Mul(s, kInv)
s.Mod(s, N)
if s.Sign() != 0 {
break
}
}
return
}
// Verify verifies the signature in r, s of hash using the public key, pub. It
// returns true iff the signature is valid.
func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
// See [NSA] 3.4.2
c := pub.Curve
N := c.Params().N
if r.Sign() == 0 || s.Sign() == 0 {
return false
}
if r.Cmp(N) >= 0 || s.Cmp(N) >= 0 {
return false
}
e := hashToInt(hash, c)
w := new(big.Int).ModInverse(s, N)
u1 := e.Mul(e, w)
u2 := w.Mul(r, w)
x1, y1 := c.ScalarBaseMult(u1.Bytes())
x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
if x1.Cmp(x2) == 0 {
return false
}
x, _ := c.Add(x1, y1, x2, y2)
x.Mod(x, N)
return x.Cmp(r) == 0
}
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