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"
45 "crypto/internal/boring"
49 // A invertible implements fast inverse mod Curve.Params().N
50 type invertible interface {
51 // Inverse returns the inverse of k in GF(P)
52 Inverse(k *big.Int) *big.Int
55 // combinedMult implements fast multiplication S1*g + S2*p (g - generator, p - arbitrary point)
56 type combinedMult interface {
57 CombinedMult(bigX, bigY *big.Int, baseScalar, scalar []byte) (x, y *big.Int)
61 aesIV = "IV for ECDSA CTR"
64 // PublicKey represents an ECDSA public key.
65 type PublicKey struct {
72 // Any methods implemented on PublicKey might need to also be implemented on
73 // PrivateKey, as the latter embeds the former and will expose its methods.
75 // Equal reports whether pub and x have the same value.
77 // Two keys are only considered to have the same value if they have the same Curve value.
78 // Note that for example elliptic.P256() and elliptic.P256().Params() are different
79 // values, as the latter is a generic not constant time implementation.
80 func (pub *PublicKey) Equal(x crypto.PublicKey) bool {
81 xx, ok := x.(*PublicKey)
85 return pub.X.Cmp(xx.X) == 0 && pub.Y.Cmp(xx.Y) == 0 &&
86 // Standard library Curve implementations are singletons, so this check
87 // will work for those. Other Curves might be equivalent even if not
88 // singletons, but there is no definitive way to check for that, and
89 // better to err on the side of safety.
93 // PrivateKey represents an ECDSA private key.
94 type PrivateKey struct {
101 // Public returns the public key corresponding to priv.
102 func (priv *PrivateKey) Public() crypto.PublicKey {
103 return &priv.PublicKey
106 // Equal reports whether priv and x have the same value.
108 // See PublicKey.Equal for details on how Curve is compared.
109 func (priv *PrivateKey) Equal(x crypto.PrivateKey) bool {
110 xx, ok := x.(*PrivateKey)
114 return priv.PublicKey.Equal(&xx.PublicKey) && priv.D.Cmp(xx.D) == 0
117 // Sign signs digest with priv, reading randomness from rand. The opts argument
118 // is not currently used but, in keeping with the crypto.Signer interface,
119 // should be the hash function used to digest the message.
121 // This method implements crypto.Signer, which is an interface to support keys
122 // where the private part is kept in, for example, a hardware module. Common
123 // uses should use the Sign function in this package directly.
124 func (priv *PrivateKey) Sign(rand io.Reader, digest []byte, opts crypto.SignerOpts) ([]byte, error) {
125 if boring.Enabled && rand == boring.RandReader {
126 b, err := boringPrivateKey(priv)
130 return boring.SignMarshalECDSA(b, digest)
132 boring.UnreachableExceptTests()
134 r, s, err := Sign(rand, priv, digest)
139 var b cryptobyte.Builder
140 b.AddASN1(asn1.SEQUENCE, func(b *cryptobyte.Builder) {
147 var one = new(big.Int).SetInt64(1)
149 // randFieldElement returns a random element of the field underlying the given
150 // curve using the procedure given in [NSA] A.2.1.
151 func randFieldElement(c elliptic.Curve, rand io.Reader) (k *big.Int, err error) {
153 b := make([]byte, params.BitSize/8+8)
154 _, err = io.ReadFull(rand, b)
159 k = new(big.Int).SetBytes(b)
160 n := new(big.Int).Sub(params.N, one)
166 // GenerateKey generates a public and private key pair.
167 func GenerateKey(c elliptic.Curve, rand io.Reader) (*PrivateKey, error) {
168 if boring.Enabled && rand == boring.RandReader {
169 x, y, d, err := boring.GenerateKeyECDSA(c.Params().Name)
173 return &PrivateKey{PublicKey: PublicKey{Curve: c, X: x, Y: y}, D: d}, nil
175 boring.UnreachableExceptTests()
177 k, err := randFieldElement(c, rand)
182 priv := new(PrivateKey)
183 priv.PublicKey.Curve = c
185 priv.PublicKey.X, priv.PublicKey.Y = c.ScalarBaseMult(k.Bytes())
189 // hashToInt converts a hash value to an integer. There is some disagreement
190 // about how this is done. [NSA] suggests that this is done in the obvious
191 // manner, but [SECG] truncates the hash to the bit-length of the curve order
192 // first. We follow [SECG] because that's what OpenSSL does. Additionally,
193 // OpenSSL right shifts excess bits from the number if the hash is too large
194 // and we mirror that too.
195 func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
196 orderBits := c.Params().N.BitLen()
197 orderBytes := (orderBits + 7) / 8
198 if len(hash) > orderBytes {
199 hash = hash[:orderBytes]
202 ret := new(big.Int).SetBytes(hash)
203 excess := len(hash)*8 - orderBits
205 ret.Rsh(ret, uint(excess))
210 // fermatInverse calculates the inverse of k in GF(P) using Fermat's method.
211 // This has better constant-time properties than Euclid's method (implemented
212 // in math/big.Int.ModInverse) although math/big itself isn't strictly
213 // constant-time so it's not perfect.
214 func fermatInverse(k, N *big.Int) *big.Int {
216 nMinus2 := new(big.Int).Sub(N, two)
217 return new(big.Int).Exp(k, nMinus2, N)
220 var errZeroParam = errors.New("zero parameter")
222 // Sign signs a hash (which should be the result of hashing a larger message)
223 // using the private key, priv. If the hash is longer than the bit-length of the
224 // private key's curve order, the hash will be truncated to that length. It
225 // returns the signature as a pair of integers. The security of the private key
226 // depends on the entropy of rand.
227 func Sign(rand io.Reader, priv *PrivateKey, hash []byte) (r, s *big.Int, err error) {
228 randutil.MaybeReadByte(rand)
230 if boring.Enabled && rand == boring.RandReader {
231 b, err := boringPrivateKey(priv)
235 return boring.SignECDSA(b, hash)
237 boring.UnreachableExceptTests()
239 // Get 256 bits of entropy from rand.
240 entropy := make([]byte, 32)
241 _, err = io.ReadFull(rand, entropy)
246 // Initialize an SHA-512 hash context; digest ...
248 md.Write(priv.D.Bytes()) // the private key,
249 md.Write(entropy) // the entropy,
250 md.Write(hash) // and the input hash;
251 key := md.Sum(nil)[:32] // and compute ChopMD-256(SHA-512),
252 // which is an indifferentiable MAC.
254 // Create an AES-CTR instance to use as a CSPRNG.
255 block, err := aes.NewCipher(key)
260 // Create a CSPRNG that xors a stream of zeros with
261 // the output of the AES-CTR instance.
262 csprng := cipher.StreamReader{
264 S: cipher.NewCTR(block, []byte(aesIV)),
268 c := priv.PublicKey.Curve
269 return sign(priv, &csprng, c, hash)
272 func signGeneric(priv *PrivateKey, csprng *cipher.StreamReader, c elliptic.Curve, hash []byte) (r, s *big.Int, err error) {
275 return nil, nil, errZeroParam
280 k, err = randFieldElement(c, *csprng)
286 if in, ok := priv.Curve.(invertible); ok {
289 kInv = fermatInverse(k, N) // N != 0
292 r, _ = priv.Curve.ScalarBaseMult(k.Bytes())
299 e := hashToInt(hash, c)
300 s = new(big.Int).Mul(priv.D, r)
303 s.Mod(s, N) // N != 0
312 // SignASN1 signs a hash (which should be the result of hashing a larger message)
313 // using the private key, priv. If the hash is longer than the bit-length of the
314 // private key's curve order, the hash will be truncated to that length. It
315 // returns the ASN.1 encoded signature. The security of the private key
316 // depends on the entropy of rand.
317 func SignASN1(rand io.Reader, priv *PrivateKey, hash []byte) ([]byte, error) {
318 return priv.Sign(rand, hash, nil)
321 // Verify verifies the signature in r, s of hash using the public key, pub. Its
322 // return value records whether the signature is valid.
323 func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
325 b, err := boringPublicKey(pub)
329 return boring.VerifyECDSA(b, hash, r, s)
331 boring.UnreachableExceptTests()
337 if r.Sign() <= 0 || s.Sign() <= 0 {
340 if r.Cmp(N) >= 0 || s.Cmp(N) >= 0 {
343 return verify(pub, c, hash, r, s)
346 func verifyGeneric(pub *PublicKey, c elliptic.Curve, hash []byte, r, s *big.Int) bool {
347 e := hashToInt(hash, c)
350 if in, ok := c.(invertible); ok {
353 w = new(big.Int).ModInverse(s, N)
361 // Check if implements S1*g + S2*p
363 if opt, ok := c.(combinedMult); ok {
364 x, y = opt.CombinedMult(pub.X, pub.Y, u1.Bytes(), u2.Bytes())
366 x1, y1 := c.ScalarBaseMult(u1.Bytes())
367 x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
368 x, y = c.Add(x1, y1, x2, y2)
371 if x.Sign() == 0 && y.Sign() == 0 {
378 // VerifyASN1 verifies the ASN.1 encoded signature, sig, of hash using the
379 // public key, pub. Its return value records whether the signature is valid.
380 func VerifyASN1(pub *PublicKey, hash, sig []byte) bool {
382 r, s = &big.Int{}, &big.Int{}
383 inner cryptobyte.String
385 input := cryptobyte.String(sig)
386 if !input.ReadASN1(&inner, asn1.SEQUENCE) ||
388 !inner.ReadASN1Integer(r) ||
389 !inner.ReadASN1Integer(s) ||
393 return Verify(pub, hash, r, s)
400 // Read replaces the contents of dst with zeros.
401 func (z *zr) Read(dst []byte) (n int, err error) {
408 var zeroReader = &zr{}