1 // Copyright 2011 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.
5 // Package ecdsa implements the Elliptic Curve Digital Signature Algorithm, as
6 // defined in FIPS 186-3.
8 // This implementation derives the nonce from an AES-CTR CSPRNG keyed by:
10 // SHA2-512(priv.D || entropy || hash)[:32]
12 // The CSPRNG key is indifferentiable from a random oracle as shown in
13 // [Coron], the AES-CTR stream is indifferentiable from a random oracle
14 // under standard cryptographic assumptions (see [Larsson] for examples).
18 // https://cs.nyu.edu/~dodis/ps/merkle.pdf
20 // https://web.archive.org/web/20040719170906/https://www.nada.kth.se/kurser/kth/2D1441/semteo03/lecturenotes/assump.pdf
23 // Further references:
24 // [NSA]: Suite B implementer's guide to FIPS 186-3
25 // https://apps.nsa.gov/iaarchive/library/ia-guidance/ia-solutions-for-classified/algorithm-guidance/suite-b-implementers-guide-to-fips-186-3-ecdsa.cfm
27 // http://www.secg.org/sec1-v2.pdf
34 "crypto/internal/randutil"
40 "golang.org/x/crypto/cryptobyte"
41 "golang.org/x/crypto/cryptobyte/asn1"
44 // A invertible implements fast inverse mod Curve.Params().N
45 type invertible interface {
46 // Inverse returns the inverse of k in GF(P)
47 Inverse(k *big.Int) *big.Int
50 // combinedMult implements fast multiplication S1*g + S2*p (g - generator, p - arbitrary point)
51 type combinedMult interface {
52 CombinedMult(bigX, bigY *big.Int, baseScalar, scalar []byte) (x, y *big.Int)
56 aesIV = "IV for ECDSA CTR"
59 // PublicKey represents an ECDSA public key.
60 type PublicKey struct {
65 // Any methods implemented on PublicKey might need to also be implemented on
66 // PrivateKey, as the latter embeds the former and will expose its methods.
68 // Equal reports whether pub and x have the same value.
70 // Two keys are only considered to have the same value if they have the same Curve value.
71 // Note that for example elliptic.P256() and elliptic.P256().Params() are different
72 // values, as the latter is a generic not constant time implementation.
73 func (pub *PublicKey) Equal(x crypto.PublicKey) bool {
74 xx, ok := x.(*PublicKey)
78 return pub.X.Cmp(xx.X) == 0 && pub.Y.Cmp(xx.Y) == 0 &&
79 // Standard library Curve implementations are singletons, so this check
80 // will work for those. Other Curves might be equivalent even if not
81 // singletons, but there is no definitive way to check for that, and
82 // better to err on the side of safety.
86 // PrivateKey represents an ECDSA private key.
87 type PrivateKey struct {
92 // Public returns the public key corresponding to priv.
93 func (priv *PrivateKey) Public() crypto.PublicKey {
94 return &priv.PublicKey
97 // Equal reports whether priv and x have the same value.
99 // See PublicKey.Equal for details on how Curve is compared.
100 func (priv *PrivateKey) Equal(x crypto.PrivateKey) bool {
101 xx, ok := x.(*PrivateKey)
105 return priv.PublicKey.Equal(&xx.PublicKey) && priv.D.Cmp(xx.D) == 0
108 // Sign signs digest with priv, reading randomness from rand. The opts argument
109 // is not currently used but, in keeping with the crypto.Signer interface,
110 // should be the hash function used to digest the message.
112 // This method implements crypto.Signer, which is an interface to support keys
113 // where the private part is kept in, for example, a hardware module. Common
114 // uses should use the Sign function in this package directly.
115 func (priv *PrivateKey) Sign(rand io.Reader, digest []byte, opts crypto.SignerOpts) ([]byte, error) {
116 r, s, err := Sign(rand, priv, digest)
121 var b cryptobyte.Builder
122 b.AddASN1(asn1.SEQUENCE, func(b *cryptobyte.Builder) {
129 var one = new(big.Int).SetInt64(1)
131 // randFieldElement returns a random element of the field underlying the given
132 // curve using the procedure given in [NSA] A.2.1.
133 func randFieldElement(c elliptic.Curve, rand io.Reader) (k *big.Int, err error) {
135 b := make([]byte, params.BitSize/8+8)
136 _, err = io.ReadFull(rand, b)
141 k = new(big.Int).SetBytes(b)
142 n := new(big.Int).Sub(params.N, one)
148 // GenerateKey generates a public and private key pair.
149 func GenerateKey(c elliptic.Curve, rand io.Reader) (*PrivateKey, error) {
150 k, err := randFieldElement(c, rand)
155 priv := new(PrivateKey)
156 priv.PublicKey.Curve = c
158 priv.PublicKey.X, priv.PublicKey.Y = c.ScalarBaseMult(k.Bytes())
162 // hashToInt converts a hash value to an integer. There is some disagreement
163 // about how this is done. [NSA] suggests that this is done in the obvious
164 // manner, but [SECG] truncates the hash to the bit-length of the curve order
165 // first. We follow [SECG] because that's what OpenSSL does. Additionally,
166 // OpenSSL right shifts excess bits from the number if the hash is too large
167 // and we mirror that too.
168 func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
169 orderBits := c.Params().N.BitLen()
170 orderBytes := (orderBits + 7) / 8
171 if len(hash) > orderBytes {
172 hash = hash[:orderBytes]
175 ret := new(big.Int).SetBytes(hash)
176 excess := len(hash)*8 - orderBits
178 ret.Rsh(ret, uint(excess))
183 // fermatInverse calculates the inverse of k in GF(P) using Fermat's method.
184 // This has better constant-time properties than Euclid's method (implemented
185 // in math/big.Int.ModInverse) although math/big itself isn't strictly
186 // constant-time so it's not perfect.
187 func fermatInverse(k, N *big.Int) *big.Int {
189 nMinus2 := new(big.Int).Sub(N, two)
190 return new(big.Int).Exp(k, nMinus2, N)
193 var errZeroParam = errors.New("zero parameter")
195 // Sign signs a hash (which should be the result of hashing a larger message)
196 // using the private key, priv. If the hash is longer than the bit-length of the
197 // private key's curve order, the hash will be truncated to that length. It
198 // returns the signature as a pair of integers. The security of the private key
199 // depends on the entropy of rand.
200 func Sign(rand io.Reader, priv *PrivateKey, hash []byte) (r, s *big.Int, err error) {
201 randutil.MaybeReadByte(rand)
203 // Get 256 bits of entropy from rand.
204 entropy := make([]byte, 32)
205 _, err = io.ReadFull(rand, entropy)
210 // Initialize an SHA-512 hash context; digest ...
212 md.Write(priv.D.Bytes()) // the private key,
213 md.Write(entropy) // the entropy,
214 md.Write(hash) // and the input hash;
215 key := md.Sum(nil)[:32] // and compute ChopMD-256(SHA-512),
216 // which is an indifferentiable MAC.
218 // Create an AES-CTR instance to use as a CSPRNG.
219 block, err := aes.NewCipher(key)
224 // Create a CSPRNG that xors a stream of zeros with
225 // the output of the AES-CTR instance.
226 csprng := cipher.StreamReader{
228 S: cipher.NewCTR(block, []byte(aesIV)),
232 c := priv.PublicKey.Curve
233 return sign(priv, &csprng, c, hash)
236 func signGeneric(priv *PrivateKey, csprng *cipher.StreamReader, c elliptic.Curve, hash []byte) (r, s *big.Int, err error) {
239 return nil, nil, errZeroParam
244 k, err = randFieldElement(c, *csprng)
250 if in, ok := priv.Curve.(invertible); ok {
253 kInv = fermatInverse(k, N) // N != 0
256 r, _ = priv.Curve.ScalarBaseMult(k.Bytes())
263 e := hashToInt(hash, c)
264 s = new(big.Int).Mul(priv.D, r)
267 s.Mod(s, N) // N != 0
276 // SignASN1 signs a hash (which should be the result of hashing a larger message)
277 // using the private key, priv. If the hash is longer than the bit-length of the
278 // private key's curve order, the hash will be truncated to that length. It
279 // returns the ASN.1 encoded signature. The security of the private key
280 // depends on the entropy of rand.
281 func SignASN1(rand io.Reader, priv *PrivateKey, hash []byte) ([]byte, error) {
282 return priv.Sign(rand, hash, nil)
285 // Verify verifies the signature in r, s of hash using the public key, pub. Its
286 // return value records whether the signature is valid.
287 func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
292 if r.Sign() <= 0 || s.Sign() <= 0 {
295 if r.Cmp(N) >= 0 || s.Cmp(N) >= 0 {
298 return verify(pub, c, hash, r, s)
301 func verifyGeneric(pub *PublicKey, c elliptic.Curve, hash []byte, r, s *big.Int) bool {
302 e := hashToInt(hash, c)
305 if in, ok := c.(invertible); ok {
308 w = new(big.Int).ModInverse(s, N)
316 // Check if implements S1*g + S2*p
318 if opt, ok := c.(combinedMult); ok {
319 x, y = opt.CombinedMult(pub.X, pub.Y, u1.Bytes(), u2.Bytes())
321 x1, y1 := c.ScalarBaseMult(u1.Bytes())
322 x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
323 x, y = c.Add(x1, y1, x2, y2)
326 if x.Sign() == 0 && y.Sign() == 0 {
333 // VerifyASN1 verifies the ASN.1 encoded signature, sig, of hash using the
334 // public key, pub. Its return value records whether the signature is valid.
335 func VerifyASN1(pub *PublicKey, hash, sig []byte) bool {
337 r, s = &big.Int{}, &big.Int{}
338 inner cryptobyte.String
340 input := cryptobyte.String(sig)
341 if !input.ReadASN1(&inner, asn1.SEQUENCE) ||
343 !inner.ReadASN1Integer(r) ||
344 !inner.ReadASN1Integer(s) ||
348 return Verify(pub, hash, r, s)
355 // Read replaces the contents of dst with zeros.
356 func (z *zr) Read(dst []byte) (n int, err error) {
363 var zeroReader = &zr{}