func defaultTLSChallengeCertTemplate() *x509.Certificate {
	return &x509.Certificate{
		SerialNumber:          big.NewInt(1),
		NotBefore:             time.Now(),
		NotAfter:              time.Now().Add(24 * time.Hour),
		BasicConstraintsValid: true,
		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
	}
}
defaultTLSChallengeCertTemplate is a template used to create challenge certs for TLS challenges.
package acme
import (
	"context"
	"crypto"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/sha256"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/asn1"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"math/big"
	"net/http"
	"strings"
	"sync"
	"time"
)
const (
	LetsEncryptURL = "https://acme-v01.api.letsencrypt.org/directory"
	ALPNProto = "acme-tls/1"
)
var idPeACMEIdentifierV1 = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 30, 1}
const (
	maxChainLen = 5        
	maxCertSize = 1 << 20  
	maxNonces = 100
)
type Client struct {
	Key crypto.Signer
	HTTPClient *http.Client
	DirectoryURL string
	RetryBackoff func(n int, r *http.Request, resp *http.Response) time.Duration
	dirMu sync.Mutex  
	dir   *Directory  
	noncesMu sync.Mutex
	nonces   map[string]struct{}  
}
func (c *Client) Discover(ctx context.Context) (Directory, error) {
	c.dirMu.Lock()
	defer c.dirMu.Unlock()
	if c.dir != nil {
		return *c.dir, nil
	}
	dirURL := c.DirectoryURL
	if dirURL == "" {
		dirURL = LetsEncryptURL
	}
	res, err := c.get(ctx, dirURL, wantStatus(http.StatusOK))
	if err != nil {
		return Directory{}, err
	}
	defer res.Body.Close()
	c.addNonce(res.Header)
	var v struct {
		Reg    string `json:"new-reg"`
		Authz  string `json:"new-authz"`
		Cert   string `json:"new-cert"`
		Revoke string `json:"revoke-cert"`
		Meta   struct {
			Terms   string   `json:"terms-of-service"`
			Website string   `json:"website"`
			CAA     []string `json:"caa-identities"`
		}
	}
	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
		return Directory{}, err
	}
	c.dir = &Directory{
		RegURL:    v.Reg,
		AuthzURL:  v.Authz,
		CertURL:   v.Cert,
		RevokeURL: v.Revoke,
		Terms:     v.Meta.Terms,
		Website:   v.Meta.Website,
		CAA:       v.Meta.CAA,
	}
	return *c.dir, nil
}
func (c *Client) CreateCert(ctx context.Context, csr []byte, exp time.Duration, bundle bool) (der [][]byte, certURL string, err error) {
	if _, err := c.Discover(ctx); err != nil {
		return nil, "", err
	}
	req := struct {
		Resource  string `json:"resource"`
		CSR       string `json:"csr"`
		NotBefore string `json:"notBefore,omitempty"`
		NotAfter  string `json:"notAfter,omitempty"`
	}{
		Resource: "new-cert",
		CSR:      base64.RawURLEncoding.EncodeToString(csr),
	}
	now := timeNow()
	req.NotBefore = now.Format(time.RFC3339)
	if exp > 0 {
		req.NotAfter = now.Add(exp).Format(time.RFC3339)
	}
	res, err := c.post(ctx, c.Key, c.dir.CertURL, req, wantStatus(http.StatusCreated))
	if err != nil {
		return nil, "", err
	}
	defer res.Body.Close()
	curl := res.Header.Get("Location")  
	if res.ContentLength == 0 {
		cert, err := c.FetchCert(ctx, curl, bundle)
		return cert, curl, err
	}
	cert, err := c.responseCert(ctx, res, bundle)
	return cert, curl, err
}
func (c *Client) FetchCert(ctx context.Context, url string, bundle bool) ([][]byte, error) {
	res, err := c.get(ctx, url, wantStatus(http.StatusOK))
	if err != nil {
		return nil, err
	}
	return c.responseCert(ctx, res, bundle)
}
func (c *Client) RevokeCert(ctx context.Context, key crypto.Signer, cert []byte, reason CRLReasonCode) error {
	if _, err := c.Discover(ctx); err != nil {
		return err
	}
	body := &struct {
		Resource string `json:"resource"`
		Cert     string `json:"certificate"`
		Reason   int    `json:"reason"`
	}{
		Resource: "revoke-cert",
		Cert:     base64.RawURLEncoding.EncodeToString(cert),
		Reason:   int(reason),
	}
	if key == nil {
		key = c.Key
	}
	res, err := c.post(ctx, key, c.dir.RevokeURL, body, wantStatus(http.StatusOK))
	if err != nil {
		return err
	}
	defer res.Body.Close()
	return nil
}
func AcceptTOS(tosURL string) bool { return true }
func (c *Client) Register(ctx context.Context, a *Account, prompt func(tosURL string) bool) (*Account, error) {
	if _, err := c.Discover(ctx); err != nil {
		return nil, err
	}
	var err error
	if a, err = c.doReg(ctx, c.dir.RegURL, "new-reg", a); err != nil {
		return nil, err
	}
	var accept bool
	if a.CurrentTerms != "" && a.CurrentTerms != a.AgreedTerms {
		accept = prompt(a.CurrentTerms)
	}
	if accept {
		a.AgreedTerms = a.CurrentTerms
		a, err = c.UpdateReg(ctx, a)
	}
	return a, err
}
func (c *Client) GetReg(ctx context.Context, url string) (*Account, error) {
	a, err := c.doReg(ctx, url, "reg", nil)
	if err != nil {
		return nil, err
	}
	a.URI = url
	return a, nil
}
func (c *Client) UpdateReg(ctx context.Context, a *Account) (*Account, error) {
	uri := a.URI
	a, err := c.doReg(ctx, uri, "reg", a)
	if err != nil {
		return nil, err
	}
	a.URI = uri
	return a, nil
}
func (c *Client) Authorize(ctx context.Context, domain string) (*Authorization, error) {
	if _, err := c.Discover(ctx); err != nil {
		return nil, err
	}
	type authzID struct {
		Type  string `json:"type"`
		Value string `json:"value"`
	}
	req := struct {
		Resource   string  `json:"resource"`
		Identifier authzID `json:"identifier"`
	}{
		Resource:   "new-authz",
		Identifier: authzID{Type: "dns", Value: domain},
	}
	res, err := c.post(ctx, c.Key, c.dir.AuthzURL, req, wantStatus(http.StatusCreated))
	if err != nil {
		return nil, err
	}
	defer res.Body.Close()
	var v wireAuthz
	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
		return nil, fmt.Errorf("acme: invalid response: %v", err)
	}
	if v.Status != StatusPending && v.Status != StatusValid {
		return nil, fmt.Errorf("acme: unexpected status: %s", v.Status)
	}
	return v.authorization(res.Header.Get("Location")), nil
}
func (c *Client) GetAuthorization(ctx context.Context, url string) (*Authorization, error) {
	res, err := c.get(ctx, url, wantStatus(http.StatusOK, http.StatusAccepted))
	if err != nil {
		return nil, err
	}
	defer res.Body.Close()
	var v wireAuthz
	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
		return nil, fmt.Errorf("acme: invalid response: %v", err)
	}
	return v.authorization(url), nil
}
func (c *Client) RevokeAuthorization(ctx context.Context, url string) error {
	req := struct {
		Resource string `json:"resource"`
		Status   string `json:"status"`
		Delete   bool   `json:"delete"`
	}{
		Resource: "authz",
		Status:   "deactivated",
		Delete:   true,
	}
	res, err := c.post(ctx, c.Key, url, req, wantStatus(http.StatusOK))
	if err != nil {
		return err
	}
	defer res.Body.Close()
	return nil
}
func (c *Client) WaitAuthorization(ctx context.Context, url string) (*Authorization, error) {
	for {
		res, err := c.get(ctx, url, wantStatus(http.StatusOK, http.StatusAccepted))
		if err != nil {
			return nil, err
		}
		var raw wireAuthz
		err = json.NewDecoder(res.Body).Decode(&raw)
		res.Body.Close()
		switch {
		case err != nil:
		case raw.Status == StatusValid:
			return raw.authorization(url), nil
		case raw.Status == StatusInvalid:
			return nil, raw.error(url)
		}
		d := retryAfter(res.Header.Get("Retry-After"))
		if d == 0 {
			d = time.Second
		}
		t := time.NewTimer(d)
		select {
		case <-ctx.Done():
			t.Stop()
			return nil, ctx.Err()
		case <-t.C:
		}
	}
}
func (c *Client) GetChallenge(ctx context.Context, url string) (*Challenge, error) {
	res, err := c.get(ctx, url, wantStatus(http.StatusOK, http.StatusAccepted))
	if err != nil {
		return nil, err
	}
	defer res.Body.Close()
	v := wireChallenge{URI: url}
	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
		return nil, fmt.Errorf("acme: invalid response: %v", err)
	}
	return v.challenge(), nil
}
func (c *Client) Accept(ctx context.Context, chal *Challenge) (*Challenge, error) {
	auth, err := keyAuth(c.Key.Public(), chal.Token)
	if err != nil {
		return nil, err
	}
	req := struct {
		Resource string `json:"resource"`
		Type     string `json:"type"`
		Auth     string `json:"keyAuthorization"`
	}{
		Resource: "challenge",
		Type:     chal.Type,
		Auth:     auth,
	}
	res, err := c.post(ctx, c.Key, chal.URI, req, wantStatus(
		http.StatusOK,        
		http.StatusAccepted,  
	))
	if err != nil {
		return nil, err
	}
	defer res.Body.Close()
	var v wireChallenge
	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
		return nil, fmt.Errorf("acme: invalid response: %v", err)
	}
	return v.challenge(), nil
}
func (c *Client) DNS01ChallengeRecord(token string) (string, error) {
	ka, err := keyAuth(c.Key.Public(), token)
	if err != nil {
		return "", err
	}
	b := sha256.Sum256([]byte(ka))
	return base64.RawURLEncoding.EncodeToString(b[:]), nil
}
func (c *Client) HTTP01ChallengeResponse(token string) (string, error) {
	return keyAuth(c.Key.Public(), token)
}
func (c *Client) HTTP01ChallengePath(token string) string {
	return "/.well-known/acme-challenge/" + token
}
func (c *Client) TLSSNI01ChallengeCert(token string, opt ...CertOption) (cert tls.Certificate, name string, err error) {
	ka, err := keyAuth(c.Key.Public(), token)
	if err != nil {
		return tls.Certificate{}, "", err
	}
	b := sha256.Sum256([]byte(ka))
	h := hex.EncodeToString(b[:])
	name = fmt.Sprintf("%s.%s.acme.invalid", h[:32], h[32:])
	cert, err = tlsChallengeCert([]string{name}, opt)
	if err != nil {
		return tls.Certificate{}, "", err
	}
	return cert, name, nil
}
func (c *Client) TLSSNI02ChallengeCert(token string, opt ...CertOption) (cert tls.Certificate, name string, err error) {
	b := sha256.Sum256([]byte(token))
	h := hex.EncodeToString(b[:])
	sanA := fmt.Sprintf("%s.%s.token.acme.invalid", h[:32], h[32:])
	ka, err := keyAuth(c.Key.Public(), token)
	if err != nil {
		return tls.Certificate{}, "", err
	}
	b = sha256.Sum256([]byte(ka))
	h = hex.EncodeToString(b[:])
	sanB := fmt.Sprintf("%s.%s.ka.acme.invalid", h[:32], h[32:])
	cert, err = tlsChallengeCert([]string{sanA, sanB}, opt)
	if err != nil {
		return tls.Certificate{}, "", err
	}
	return cert, sanA, nil
}
func (c *Client) TLSALPN01ChallengeCert(token, domain string, opt ...CertOption) (cert tls.Certificate, err error) {
	ka, err := keyAuth(c.Key.Public(), token)
	if err != nil {
		return tls.Certificate{}, err
	}
	shasum := sha256.Sum256([]byte(ka))
	extValue, err := asn1.Marshal(shasum[:])
	if err != nil {
		return tls.Certificate{}, err
	}
	acmeExtension := pkix.Extension{
		Id:       idPeACMEIdentifierV1,
		Critical: true,
		Value:    extValue,
	}
	tmpl := defaultTLSChallengeCertTemplate()
	var newOpt []CertOption
	for _, o := range opt {
		switch o := o.(type) {
		case *certOptTemplate:
			t := *(*x509.Certificate)(o)  
			tmpl = &t
		default:
			newOpt = append(newOpt, o)
		}
	}
	tmpl.ExtraExtensions = append(tmpl.ExtraExtensions, acmeExtension)
	newOpt = append(newOpt, WithTemplate(tmpl))
	return tlsChallengeCert([]string{domain}, newOpt)
}
func (c *Client) doReg(ctx context.Context, url string, typ string, acct *Account) (*Account, error) {
	req := struct {
		Resource  string   `json:"resource"`
		Contact   []string `json:"contact,omitempty"`
		Agreement string   `json:"agreement,omitempty"`
	}{
		Resource: typ,
	}
	if acct != nil {
		req.Contact = acct.Contact
		req.Agreement = acct.AgreedTerms
	}
	res, err := c.post(ctx, c.Key, url, req, wantStatus(
		http.StatusOK,        
		http.StatusCreated,   
		http.StatusAccepted,  
	))
	if err != nil {
		return nil, err
	}
	defer res.Body.Close()
	var v struct {
		Contact        []string
		Agreement      string
		Authorizations string
		Certificates   string
	}
	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
		return nil, fmt.Errorf("acme: invalid response: %v", err)
	}
	var tos string
	if v := linkHeader(res.Header, "terms-of-service"); len(v) > 0 {
		tos = v[0]
	}
	var authz string
	if v := linkHeader(res.Header, "next"); len(v) > 0 {
		authz = v[0]
	}
	return &Account{
		URI:            res.Header.Get("Location"),
		Contact:        v.Contact,
		AgreedTerms:    v.Agreement,
		CurrentTerms:   tos,
		Authz:          authz,
		Authorizations: v.Authorizations,
		Certificates:   v.Certificates,
	}, nil
}
func (c *Client) popNonce(ctx context.Context, url string) (string, error) {
	c.noncesMu.Lock()
	defer c.noncesMu.Unlock()
	if len(c.nonces) == 0 {
		return c.fetchNonce(ctx, url)
	}
	var nonce string
	for nonce = range c.nonces {
		delete(c.nonces, nonce)
		break
	}
	return nonce, nil
}
func (c *Client) clearNonces() {
	c.noncesMu.Lock()
	defer c.noncesMu.Unlock()
	c.nonces = make(map[string]struct{})
}
func (c *Client) addNonce(h http.Header) {
	v := nonceFromHeader(h)
	if v == "" {
		return
	}
	c.noncesMu.Lock()
	defer c.noncesMu.Unlock()
	if len(c.nonces) >= maxNonces {
		return
	}
	if c.nonces == nil {
		c.nonces = make(map[string]struct{})
	}
	c.nonces[v] = struct{}{}
}
func (c *Client) fetchNonce(ctx context.Context, url string) (string, error) {
	r, err := http.NewRequest("HEAD", url, nil)
	if err != nil {
		return "", err
	}
	resp, err := c.doNoRetry(ctx, r)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()
	nonce := nonceFromHeader(resp.Header)
	if nonce == "" {
		if resp.StatusCode > 299 {
			return "", responseError(resp)
		}
		return "", errors.New("acme: nonce not found")
	}
	return nonce, nil
}
func nonceFromHeader(h http.Header) string {
	return h.Get("Replay-Nonce")
}
func (c *Client) responseCert(ctx context.Context, res *http.Response, bundle bool) ([][]byte, error) {
	b, err := ioutil.ReadAll(io.LimitReader(res.Body, maxCertSize+1))
	if err != nil {
		return nil, fmt.Errorf("acme: response stream: %v", err)
	}
	if len(b) > maxCertSize {
		return nil, errors.New("acme: certificate is too big")
	}
	cert := [][]byte{b}
	if !bundle {
		return cert, nil
	}
	up := linkHeader(res.Header, "up")
	if len(up) == 0 {
		return nil, errors.New("acme: rel=up link not found")
	}
	if len(up) > maxChainLen {
		return nil, errors.New("acme: rel=up link is too large")
	}
	for _, url := range up {
		cc, err := c.chainCert(ctx, url, 0)
		if err != nil {
			return nil, err
		}
		cert = append(cert, cc...)
	}
	return cert, nil
}
func (c *Client) chainCert(ctx context.Context, url string, depth int) ([][]byte, error) {
	if depth >= maxChainLen {
		return nil, errors.New("acme: certificate chain is too deep")
	}
	res, err := c.get(ctx, url, wantStatus(http.StatusOK))
	if err != nil {
		return nil, err
	}
	defer res.Body.Close()
	b, err := ioutil.ReadAll(io.LimitReader(res.Body, maxCertSize+1))
	if err != nil {
		return nil, err
	}
	if len(b) > maxCertSize {
		return nil, errors.New("acme: certificate is too big")
	}
	chain := [][]byte{b}
	uplink := linkHeader(res.Header, "up")
	if len(uplink) > maxChainLen {
		return nil, errors.New("acme: certificate chain is too large")
	}
	for _, up := range uplink {
		cc, err := c.chainCert(ctx, up, depth+1)
		if err != nil {
			return nil, err
		}
		chain = append(chain, cc...)
	}
	return chain, nil
}
func linkHeader(h http.Header, rel string) []string {
	var links []string
	for _, v := range h["Link"] {
		parts := strings.Split(v, ";")
		for _, p := range parts {
			p = strings.TrimSpace(p)
			if !strings.HasPrefix(p, "rel=") {
				continue
			}
			if v := strings.Trim(p[4:], `"`); v == rel {
				links = append(links, strings.Trim(parts[0], "<>"))
			}
		}
	}
	return links
}
// keyAuth generates a key authorization string for a given token.
func keyAuth(pub crypto.PublicKey, token string) (string, error) {
	th, err := JWKThumbprint(pub)
	if err != nil {
		return "", err
	}
	return fmt.Sprintf("%s.%s", token, th), nil
}