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diff --git a/src/crypto/ecdsa/ecdsa.go b/src/crypto/ecdsa/ecdsa.go
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+++ b/src/crypto/ecdsa/ecdsa.go
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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 186-4 and SEC 1, Version 2.0.
+//
+// Signatures generated by this package are not deterministic, but entropy is
+// mixed with the private key and the message, achieving the same level of
+// security in case of randomness source failure.
+package ecdsa
+
+// [FIPS 186-4] references ANSI X9.62-2005 for the bulk of the ECDSA algorithm.
+// That standard is not freely available, which is a problem in an open source
+// implementation, because not only the implementer, but also any maintainer,
+// contributor, reviewer, auditor, and learner needs access to it. Instead, this
+// package references and follows the equivalent [SEC 1, Version 2.0].
+//
+// [FIPS 186-4]: https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
+// [SEC 1, Version 2.0]: https://www.secg.org/sec1-v2.pdf
+
+import (
+	"crypto"
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/elliptic"
+	"crypto/internal/randutil"
+	"crypto/sha512"
+	"errors"
+	"io"
+	"math/big"
+
+	"golang.org/x/crypto/cryptobyte"
+	"golang.org/x/crypto/cryptobyte/asn1"
+)
+
+// A invertible implements fast inverse in GF(N).
+type invertible interface {
+	// Inverse returns the inverse of k mod Params().N.
+	Inverse(k *big.Int) *big.Int
+}
+
+// A combinedMult implements fast combined multiplication for verification.
+type combinedMult interface {
+	// CombinedMult returns [s1]G + [s2]P where G is the generator.
+	CombinedMult(Px, Py *big.Int, s1, s2 []byte) (x, y *big.Int)
+}
+
+const (
+	aesIV = "IV for ECDSA CTR"
+)
+
+// PublicKey represents an ECDSA public key.
+type PublicKey struct {
+	elliptic.Curve
+	X, Y *big.Int
+}
+
+// Any methods implemented on PublicKey might need to also be implemented on
+// PrivateKey, as the latter embeds the former and will expose its methods.
+
+// Equal reports whether pub and x have the same value.
+//
+// Two keys are only considered to have the same value if they have the same Curve value.
+// Note that for example elliptic.P256() and elliptic.P256().Params() are different
+// values, as the latter is a generic not constant time implementation.
+func (pub *PublicKey) Equal(x crypto.PublicKey) bool {
+	xx, ok := x.(*PublicKey)
+	if !ok {
+		return false
+	}
+	return pub.X.Cmp(xx.X) == 0 && pub.Y.Cmp(xx.Y) == 0 &&
+		// Standard library Curve implementations are singletons, so this check
+		// will work for those. Other Curves might be equivalent even if not
+		// singletons, but there is no definitive way to check for that, and
+		// better to err on the side of safety.
+		pub.Curve == xx.Curve
+}
+
+// PrivateKey represents an ECDSA private key.
+type PrivateKey struct {
+	PublicKey
+	D *big.Int
+}
+
+// Public returns the public key corresponding to priv.
+func (priv *PrivateKey) Public() crypto.PublicKey {
+	return &priv.PublicKey
+}
+
+// Equal reports whether priv and x have the same value.
+//
+// See PublicKey.Equal for details on how Curve is compared.
+func (priv *PrivateKey) Equal(x crypto.PrivateKey) bool {
+	xx, ok := x.(*PrivateKey)
+	if !ok {
+		return false
+	}
+	return priv.PublicKey.Equal(&xx.PublicKey) && priv.D.Cmp(xx.D) == 0
+}
+
+// Sign signs digest with priv, reading randomness from rand. The opts argument
+// is not currently used but, in keeping with the crypto.Signer interface,
+// should be the hash function used to digest the message.
+//
+// This method implements crypto.Signer, which is an interface to support keys
+// where the private part is kept in, for example, a hardware module. Common
+// uses can use the SignASN1 function in this package directly.
+func (priv *PrivateKey) Sign(rand io.Reader, digest []byte, opts crypto.SignerOpts) ([]byte, error) {
+	r, s, err := Sign(rand, priv, digest)
+	if err != nil {
+		return nil, err
+	}
+
+	var b cryptobyte.Builder
+	b.AddASN1(asn1.SEQUENCE, func(b *cryptobyte.Builder) {
+		b.AddASN1BigInt(r)
+		b.AddASN1BigInt(s)
+	})
+	return b.Bytes()
+}
+
+var one = new(big.Int).SetInt64(1)
+
+// randFieldElement returns a random element of the order of the given
+// curve using the procedure given in FIPS 186-4, Appendix B.5.1.
+func randFieldElement(c elliptic.Curve, rand io.Reader) (k *big.Int, err error) {
+	params := c.Params()
+	// Note that for P-521 this will actually be 63 bits more than the order, as
+	// division rounds down, but the extra bit is inconsequential.
+	b := make([]byte, params.BitSize/8+8) // TODO: use params.N.BitLen()
+	_, 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) (*PrivateKey, error) {
+	k, err := randFieldElement(c, rand)
+	if err != nil {
+		return nil, err
+	}
+
+	priv := new(PrivateKey)
+	priv.PublicKey.Curve = c
+	priv.D = k
+	priv.PublicKey.X, priv.PublicKey.Y = c.ScalarBaseMult(k.Bytes())
+	return priv, nil
+}
+
+// hashToInt converts a hash value to an integer. Per FIPS 186-4, Section 6.4,
+// we use the left-most bits of the hash to match the bit-length of the order of
+// the curve. This also performs Step 5 of SEC 1, Version 2.0, Section 4.1.3.
+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
+// (exponentiation modulo P - 2, per Euler's theorem). This has better
+// constant-time properties than Euclid's method (implemented in
+// math/big.Int.ModInverse and FIPS 186-4, Appendix C.1) 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)
+}
+
+var errZeroParam = errors.New("zero parameter")
+
+// Sign signs a hash (which should be the result of hashing a larger message)
+// using the private key, priv. If the hash is longer than the bit-length of the
+// private key's curve order, the hash will be truncated to that length. It
+// returns the signature as a pair of integers. Most applications should use
+// SignASN1 instead of dealing directly with r, s.
+func Sign(rand io.Reader, priv *PrivateKey, hash []byte) (r, s *big.Int, err error) {
+	randutil.MaybeReadByte(rand)
+
+	// This implementation derives the nonce from an AES-CTR CSPRNG keyed by:
+	//
+	//    SHA2-512(priv.D || entropy || hash)[:32]
+	//
+	// The CSPRNG key is indifferentiable from a random oracle as shown in
+	// [Coron], the AES-CTR stream is indifferentiable from a random oracle
+	// under standard cryptographic assumptions (see [Larsson] for examples).
+	//
+	// [Coron]: https://cs.nyu.edu/~dodis/ps/merkle.pdf
+	// [Larsson]: https://web.archive.org/web/20040719170906/https://www.nada.kth.se/kurser/kth/2D1441/semteo03/lecturenotes/assump.pdf
+
+	// Get 256 bits of entropy from rand.
+	entropy := make([]byte, 32)
+	_, 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)),
+	}
+
+	c := priv.PublicKey.Curve
+	return sign(priv, &csprng, c, hash)
+}
+
+func signGeneric(priv *PrivateKey, csprng *cipher.StreamReader, c elliptic.Curve, hash []byte) (r, s *big.Int, err error) {
+	// SEC 1, Version 2.0, Section 4.1.3
+	N := c.Params().N
+	if N.Sign() == 0 {
+		return nil, nil, errZeroParam
+	}
+	var k, kInv *big.Int
+	for {
+		for {
+			k, err = randFieldElement(c, *csprng)
+			if err != nil {
+				r = nil
+				return
+			}
+
+			if in, ok := priv.Curve.(invertible); ok {
+				kInv = in.Inverse(k)
+			} else {
+				kInv = fermatInverse(k, N) // N != 0
+			}
+
+			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) // N != 0
+		if s.Sign() != 0 {
+			break
+		}
+	}
+
+	return
+}
+
+// SignASN1 signs a hash (which should be the result of hashing a larger message)
+// using the private key, priv. If the hash is longer than the bit-length of the
+// private key's curve order, the hash will be truncated to that length. It
+// returns the ASN.1 encoded signature.
+func SignASN1(rand io.Reader, priv *PrivateKey, hash []byte) ([]byte, error) {
+	return priv.Sign(rand, hash, nil)
+}
+
+// Verify verifies the signature in r, s of hash using the public key, pub. Its
+// return value records whether the signature is valid. Most applications should
+// use VerifyASN1 instead of dealing directly with r, s.
+func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
+	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
+	}
+	return verify(pub, c, hash, r, s)
+}
+
+func verifyGeneric(pub *PublicKey, c elliptic.Curve, hash []byte, r, s *big.Int) bool {
+	// SEC 1, Version 2.0, Section 4.1.4
+	e := hashToInt(hash, c)
+	var w *big.Int
+	N := c.Params().N
+	if in, ok := c.(invertible); ok {
+		w = in.Inverse(s)
+	} else {
+		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)
+
+	// Check if implements S1*g + S2*p
+	var x, y *big.Int
+	if opt, ok := c.(combinedMult); ok {
+		x, y = opt.CombinedMult(pub.X, pub.Y, u1.Bytes(), u2.Bytes())
+	} else {
+		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
+}
+
+// VerifyASN1 verifies the ASN.1 encoded signature, sig, of hash using the
+// public key, pub. Its return value records whether the signature is valid.
+func VerifyASN1(pub *PublicKey, hash, sig []byte) bool {
+	var (
+		r, s  = &big.Int{}, &big.Int{}
+		inner cryptobyte.String
+	)
+	input := cryptobyte.String(sig)
+	if !input.ReadASN1(&inner, asn1.SEQUENCE) ||
+		!input.Empty() ||
+		!inner.ReadASN1Integer(r) ||
+		!inner.ReadASN1Integer(s) ||
+		!inner.Empty() {
+		return false
+	}
+	return Verify(pub, hash, r, s)
+}
+
+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{}