src/crypto/ecdsa/ecdsa.go - go - Git at Google

 // 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.
 //
 // This implementation  derives the nonce from an AES-CTR CSPRNG keyed by
 // ChopMD(256, SHA2-512(priv.D || entropy || hash)). The CSPRNG key is IRO by
 // a result of Coron; the AES-CTR stream is IRO under standard assumptions.
 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"
 	"crypto/aes"
 	"crypto/cipher"
 	"crypto/elliptic"
 	"crypto/sha512"
 	"encoding/asn1"
 	"io"
 	"math/big"
 )

 const (
 	aesIV = "IV for ECDSA CTR"
 )

 // 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
 }

 type ecdsaSignature struct {
 	R, S *big.Int
 }

 // Public returns the public key corresponding to priv.
 func (priv *PrivateKey) Public() crypto.PublicKey {
 	return &priv.PublicKey
 }

 // Sign signs msg with priv, reading randomness from rand. This method is
 // intended to support keys where the private part is kept in, for example, a
 // hardware module. Common uses should use the Sign function in this package
 // directly.
 func (priv *PrivateKey) Sign(rand io.Reader, msg []byte, opts crypto.SignerOpts) ([]byte, error) {
 	r, s, err := Sign(rand, priv, msg)
 	if err != nil {
 		return nil, err
 	}

 	return asn1.Marshal(ecdsaSignature{r, s})
 }

 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 and 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
 }

 // fermatInverse calculates the inverse of k in GF(P) using Fermat's method.
 // This has better constant-time properties than Euclid's method (implemented
 // in math/big.Int.ModInverse) although math/big itself isn't strictly
 // constant-time so it's not perfect.
 func fermatInverse(k, N *big.Int) *big.Int {
 	two := big.NewInt(2)
 	nMinus2 := new(big.Int).Sub(N, two)
 	return new(big.Int).Exp(k, nMinus2, N)
 }

 // 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) {
 	// Get max(log2(q) / 2, 256) bits of entropy from rand.
 	entropylen := (priv.Curve.Params().BitSize + 7) / 16
 	if entropylen > 32 {
 		entropylen = 32
 	}
 	entropy := make([]byte, entropylen)
 	_, err = io.ReadFull(rand, entropy)
 	if err != nil {
 		return
 	}

 	// Initialize an SHA-512 hash context; digest ...
 	md := sha512.New()
 	md.Write(priv.D.Bytes()) // the private key,
 	md.Write(entropy)        // the entropy,
 	md.Write(hash)           // and the input hash;
 	key := md.Sum(nil)[:32]  // and compute ChopMD-256(SHA-512),
 	// which is an indifferentiable MAC.

 	// Create an AES-CTR instance to use as a CSPRNG.
 	block, err := aes.NewCipher(key)
 	if err != nil {
 		return nil, nil, err
 	}

 	// Create a CSPRNG that xors a stream of zeros with
 	// the output of the AES-CTR instance.
 	csprng := cipher.StreamReader{
 		R: zeroReader,
 		S: cipher.NewCTR(block, []byte(aesIV)),
 	}

 	// See [NSA] 3.4.1
 	c := priv.PublicKey.Curve
 	N := c.Params().N

 	var k, kInv *big.Int
 	for {
 		for {
 			k, err = randFieldElement(c, csprng)
 			if err != nil {
 				r = nil
 				return
 			}

 			kInv = fermatInverse(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. Its
 // return value records whether 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)
 	u1.Mod(u1, N)
 	u2 := w.Mul(r, w)
 	u2.Mod(u2, N)

 	x1, y1 := c.ScalarBaseMult(u1.Bytes())
 	x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
 	x, y := c.Add(x1, y1, x2, y2)
 	if x.Sign() == 0 && y.Sign() == 0 {
 		return false
 	}
 	x.Mod(x, N)
 	return x.Cmp(r) == 0
 }

 type zr struct {
 	io.Reader
 }

 // Read replaces the contents of dst with zeros.
 func (z *zr) Read(dst []byte) (n int, err error) {
 	for i := range dst {
 		dst[i] = 0
 	}
 	return len(dst), nil
 }

 var zeroReader = &zr{}
	// 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.
	//
	// This implementation derives the nonce from an AES-CTR CSPRNG keyed by
	// ChopMD(256, SHA2-512(priv.D \|\| entropy \|\| hash)). The CSPRNG key is IRO by
	// a result of Coron; the AES-CTR stream is IRO under standard assumptions.
	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"
	"crypto/aes"
	"crypto/cipher"
	"crypto/elliptic"
	"crypto/sha512"
	"encoding/asn1"
	"io"
	"math/big"
	)

	const (
	aesIV = "IV for ECDSA CTR"
	)

	// 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
	}

	type ecdsaSignature struct {
	R, S *big.Int
	}

	// Public returns the public key corresponding to priv.
	func (priv *PrivateKey) Public() crypto.PublicKey {
	return &priv.PublicKey
	}

	// Sign signs msg with priv, reading randomness from rand. This method is
	// intended to support keys where the private part is kept in, for example, a
	// hardware module. Common uses should use the Sign function in this package
	// directly.
	func (priv *PrivateKey) Sign(rand io.Reader, msg []byte, opts crypto.SignerOpts) ([]byte, error) {
	r, s, err := Sign(rand, priv, msg)
	if err != nil {
	return nil, err
	}

	return asn1.Marshal(ecdsaSignature{r, s})
	}

	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 and 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
	}

	// fermatInverse calculates the inverse of k in GF(P) using Fermat's method.
	// This has better constant-time properties than Euclid's method (implemented
	// in math/big.Int.ModInverse) although math/big itself isn't strictly
	// constant-time so it's not perfect.
	func fermatInverse(k, N big.Int) big.Int {
	two := big.NewInt(2)
	nMinus2 := new(big.Int).Sub(N, two)
	return new(big.Int).Exp(k, nMinus2, N)
	}

	// 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) {
	// Get max(log2(q) / 2, 256) bits of entropy from rand.
	entropylen := (priv.Curve.Params().BitSize + 7) / 16
	if entropylen > 32 {
	entropylen = 32
	}
	entropy := make([]byte, entropylen)
	_, err = io.ReadFull(rand, entropy)
	if err != nil {
	return
	}

	// Initialize an SHA-512 hash context; digest ...
	md := sha512.New()
	md.Write(priv.D.Bytes()) // the private key,
	md.Write(entropy) // the entropy,
	md.Write(hash) // and the input hash;
	key := md.Sum(nil)[:32] // and compute ChopMD-256(SHA-512),
	// which is an indifferentiable MAC.

	// Create an AES-CTR instance to use as a CSPRNG.
	block, err := aes.NewCipher(key)
	if err != nil {
	return nil, nil, err
	}

	// Create a CSPRNG that xors a stream of zeros with
	// the output of the AES-CTR instance.
	csprng := cipher.StreamReader{
	R: zeroReader,
	S: cipher.NewCTR(block, []byte(aesIV)),
	}

	// See [NSA] 3.4.1
	c := priv.PublicKey.Curve
	N := c.Params().N

	var k, kInv *big.Int
	for {
	for {
	k, err = randFieldElement(c, csprng)
	if err != nil {
	r = nil
	return
	}

	kInv = fermatInverse(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. Its
	// return value records whether 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)
	u1.Mod(u1, N)
	u2 := w.Mul(r, w)
	u2.Mod(u2, N)

	x1, y1 := c.ScalarBaseMult(u1.Bytes())
	x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
	x, y := c.Add(x1, y1, x2, y2)
	if x.Sign() == 0 && y.Sign() == 0 {
	return false
	}
	x.Mod(x, N)
	return x.Cmp(r) == 0
	}

	type zr struct {
	io.Reader
	}

	// Read replaces the contents of dst with zeros.
	func (z *zr) Read(dst []byte) (n int, err error) {
	for i := range dst {
	dst[i] = 0
	}
	return len(dst), nil
	}

	var zeroReader = &zr{}