barometer: update DMA's vendoring packages

[barometer.git] / src / dma / vendor / golang.org / x / crypto / acme / autocert / autocert.go

// Copyright 2016 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 autocert provides automatic access to certificates from Let's Encrypt
// and any other ACME-based CA.
//
// This package is a work in progress and makes no API stability promises.
package autocert

import (
        "bytes"
        "context"
        "crypto"
        "crypto/ecdsa"
        "crypto/elliptic"
        "crypto/rand"
        "crypto/rsa"
        "crypto/tls"
        "crypto/x509"
        "crypto/x509/pkix"
        "encoding/pem"
        "errors"
        "fmt"
        "io"
        mathrand "math/rand"
        "net"
        "net/http"
        "path"
        "strings"
        "sync"
        "time"

        "golang.org/x/crypto/acme"
        "golang.org/x/net/idna"
)

// createCertRetryAfter is how much time to wait before removing a failed state
// entry due to an unsuccessful createCert call.
// This is a variable instead of a const for testing.
// TODO: Consider making it configurable or an exp backoff?
var createCertRetryAfter = time.Minute

// pseudoRand is safe for concurrent use.
var pseudoRand *lockedMathRand

func init() {
        src := mathrand.NewSource(time.Now().UnixNano())
        pseudoRand = &lockedMathRand{rnd: mathrand.New(src)}
}

// AcceptTOS is a Manager.Prompt function that always returns true to
// indicate acceptance of the CA's Terms of Service during account
// registration.
func AcceptTOS(tosURL string) bool { return true }

// HostPolicy specifies which host names the Manager is allowed to respond to.
// It returns a non-nil error if the host should be rejected.
// The returned error is accessible via tls.Conn.Handshake and its callers.
// See Manager's HostPolicy field and GetCertificate method docs for more details.
type HostPolicy func(ctx context.Context, host string) error

// HostWhitelist returns a policy where only the specified host names are allowed.
// Only exact matches are currently supported. Subdomains, regexp or wildcard
// will not match.
//
// Note that all hosts will be converted to Punycode via idna.Lookup.ToASCII so that
// Manager.GetCertificate can handle the Unicode IDN and mixedcase hosts correctly.
// Invalid hosts will be silently ignored.
func HostWhitelist(hosts ...string) HostPolicy {
        whitelist := make(map[string]bool, len(hosts))
        for _, h := range hosts {
                if h, err := idna.Lookup.ToASCII(h); err == nil {
                        whitelist[h] = true
                }
        }
        return func(_ context.Context, host string) error {
                if !whitelist[host] {
                        return fmt.Errorf("acme/autocert: host %q not configured in HostWhitelist", host)
                }
                return nil
        }
}

// defaultHostPolicy is used when Manager.HostPolicy is not set.
func defaultHostPolicy(context.Context, string) error {
        return nil
}

// Manager is a stateful certificate manager built on top of acme.Client.
// It obtains and refreshes certificates automatically using "tls-alpn-01",
// "tls-sni-01", "tls-sni-02" and "http-01" challenge types,
// as well as providing them to a TLS server via tls.Config.
//
// You must specify a cache implementation, such as DirCache,
// to reuse obtained certificates across program restarts.
// Otherwise your server is very likely to exceed the certificate
// issuer's request rate limits.
type Manager struct {
        // Prompt specifies a callback function to conditionally accept a CA's Terms of Service (TOS).
        // The registration may require the caller to agree to the CA's TOS.
        // If so, Manager calls Prompt with a TOS URL provided by the CA. Prompt should report
        // whether the caller agrees to the terms.
        //
        // To always accept the terms, the callers can use AcceptTOS.
        Prompt func(tosURL string) bool

        // Cache optionally stores and retrieves previously-obtained certificates
        // and other state. If nil, certs will only be cached for the lifetime of
        // the Manager. Multiple Managers can share the same Cache.
        //
        // Using a persistent Cache, such as DirCache, is strongly recommended.
        Cache Cache

        // HostPolicy controls which domains the Manager will attempt
        // to retrieve new certificates for. It does not affect cached certs.
        //
        // If non-nil, HostPolicy is called before requesting a new cert.
        // If nil, all hosts are currently allowed. This is not recommended,
        // as it opens a potential attack where clients connect to a server
        // by IP address and pretend to be asking for an incorrect host name.
        // Manager will attempt to obtain a certificate for that host, incorrectly,
        // eventually reaching the CA's rate limit for certificate requests
        // and making it impossible to obtain actual certificates.
        //
        // See GetCertificate for more details.
        HostPolicy HostPolicy

        // RenewBefore optionally specifies how early certificates should
        // be renewed before they expire.
        //
        // If zero, they're renewed 30 days before expiration.
        RenewBefore time.Duration

        // Client is used to perform low-level operations, such as account registration
        // and requesting new certificates.
        //
        // If Client is nil, a zero-value acme.Client is used with acme.LetsEncryptURL
        // as directory endpoint. If the Client.Key is nil, a new ECDSA P-256 key is
        // generated and, if Cache is not nil, stored in cache.
        //
        // Mutating the field after the first call of GetCertificate method will have no effect.
        Client *acme.Client

        // Email optionally specifies a contact email address.
        // This is used by CAs, such as Let's Encrypt, to notify about problems
        // with issued certificates.
        //
        // If the Client's account key is already registered, Email is not used.
        Email string

        // ForceRSA used to make the Manager generate RSA certificates. It is now ignored.
        //
        // Deprecated: the Manager will request the correct type of certificate based
        // on what each client supports.
        ForceRSA bool

        // ExtraExtensions are used when generating a new CSR (Certificate Request),
        // thus allowing customization of the resulting certificate.
        // For instance, TLS Feature Extension (RFC 7633) can be used
        // to prevent an OCSP downgrade attack.
        //
        // The field value is passed to crypto/x509.CreateCertificateRequest
        // in the template's ExtraExtensions field as is.
        ExtraExtensions []pkix.Extension

        clientMu sync.Mutex
        client   *acme.Client // initialized by acmeClient method

        stateMu sync.Mutex
        state   map[certKey]*certState

        // renewal tracks the set of domains currently running renewal timers.
        renewalMu sync.Mutex
        renewal   map[certKey]*domainRenewal

        // tokensMu guards the rest of the fields: tryHTTP01, certTokens and httpTokens.
        tokensMu sync.RWMutex
        // tryHTTP01 indicates whether the Manager should try "http-01" challenge type
        // during the authorization flow.
        tryHTTP01 bool
        // httpTokens contains response body values for http-01 challenges
        // and is keyed by the URL path at which a challenge response is expected
        // to be provisioned.
        // The entries are stored for the duration of the authorization flow.
        httpTokens map[string][]byte
        // certTokens contains temporary certificates for tls-sni and tls-alpn challenges
        // and is keyed by token domain name, which matches server name of ClientHello.
        // Keys always have ".acme.invalid" suffix for tls-sni. Otherwise, they are domain names
        // for tls-alpn.
        // The entries are stored for the duration of the authorization flow.
        certTokens map[string]*tls.Certificate
        // nowFunc, if not nil, returns the current time. This may be set for
        // testing purposes.
        nowFunc func() time.Time
}

// certKey is the key by which certificates are tracked in state, renewal and cache.
type certKey struct {
        domain  string // without trailing dot
        isRSA   bool   // RSA cert for legacy clients (as opposed to default ECDSA)
        isToken bool   // tls-based challenge token cert; key type is undefined regardless of isRSA
}

func (c certKey) String() string {
        if c.isToken {
                return c.domain + "+token"
        }
        if c.isRSA {
                return c.domain + "+rsa"
        }
        return c.domain
}

// TLSConfig creates a new TLS config suitable for net/http.Server servers,
// supporting HTTP/2 and the tls-alpn-01 ACME challenge type.
func (m *Manager) TLSConfig() *tls.Config {
        return &tls.Config{
                GetCertificate: m.GetCertificate,
                NextProtos: []string{
                        "h2", "http/1.1", // enable HTTP/2
                        acme.ALPNProto, // enable tls-alpn ACME challenges
                },
        }
}

// GetCertificate implements the tls.Config.GetCertificate hook.
// It provides a TLS certificate for hello.ServerName host, including answering
// tls-alpn-01 and *.acme.invalid (tls-sni-01 and tls-sni-02) challenges.
// All other fields of hello are ignored.
//
// If m.HostPolicy is non-nil, GetCertificate calls the policy before requesting
// a new cert. A non-nil error returned from m.HostPolicy halts TLS negotiation.
// The error is propagated back to the caller of GetCertificate and is user-visible.
// This does not affect cached certs. See HostPolicy field description for more details.
//
// If GetCertificate is used directly, instead of via Manager.TLSConfig, package users will
// also have to add acme.ALPNProto to NextProtos for tls-alpn-01, or use HTTPHandler
// for http-01. (The tls-sni-* challenges have been deprecated by popular ACME providers
// due to security issues in the ecosystem.)
func (m *Manager) GetCertificate(hello *tls.ClientHelloInfo) (*tls.Certificate, error) {
        if m.Prompt == nil {
                return nil, errors.New("acme/autocert: Manager.Prompt not set")
        }

        name := hello.ServerName
        if name == "" {
                return nil, errors.New("acme/autocert: missing server name")
        }
        if !strings.Contains(strings.Trim(name, "."), ".") {
                return nil, errors.New("acme/autocert: server name component count invalid")
        }

        // Note that this conversion is necessary because some server names in the handshakes
        // started by some clients (such as cURL) are not converted to Punycode, which will
        // prevent us from obtaining certificates for them. In addition, we should also treat
        // example.com and EXAMPLE.COM as equivalent and return the same certificate for them.
        // Fortunately, this conversion also helped us deal with this kind of mixedcase problems.
        //
        // Due to the "σςΣ" problem (see https://unicode.org/faq/idn.html#22), we can't use
        // idna.Punycode.ToASCII (or just idna.ToASCII) here.
        name, err := idna.Lookup.ToASCII(name)
        if err != nil {
                return nil, errors.New("acme/autocert: server name contains invalid character")
        }

        // In the worst-case scenario, the timeout needs to account for caching, host policy,
        // domain ownership verification and certificate issuance.
        ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
        defer cancel()

        // Check whether this is a token cert requested for TLS-SNI or TLS-ALPN challenge.
        if wantsTokenCert(hello) {
                m.tokensMu.RLock()
                defer m.tokensMu.RUnlock()
                // It's ok to use the same token cert key for both tls-sni and tls-alpn
                // because there's always at most 1 token cert per on-going domain authorization.
                // See m.verify for details.
                if cert := m.certTokens[name]; cert != nil {
                        return cert, nil
                }
                if cert, err := m.cacheGet(ctx, certKey{domain: name, isToken: true}); err == nil {
                        return cert, nil
                }
                // TODO: cache error results?
                return nil, fmt.Errorf("acme/autocert: no token cert for %q", name)
        }

        // regular domain
        ck := certKey{
                domain: strings.TrimSuffix(name, "."), // golang.org/issue/18114
                isRSA:  !supportsECDSA(hello),
        }
        cert, err := m.cert(ctx, ck)
        if err == nil {
                return cert, nil
        }
        if err != ErrCacheMiss {
                return nil, err
        }

        // first-time
        if err := m.hostPolicy()(ctx, name); err != nil {
                return nil, err
        }
        cert, err = m.createCert(ctx, ck)
        if err != nil {
                return nil, err
        }
        m.cachePut(ctx, ck, cert)
        return cert, nil
}

// wantsTokenCert reports whether a TLS request with SNI is made by a CA server
// for a challenge verification.
func wantsTokenCert(hello *tls.ClientHelloInfo) bool {
        // tls-alpn-01
        if len(hello.SupportedProtos) == 1 && hello.SupportedProtos[0] == acme.ALPNProto {
                return true
        }
        // tls-sni-xx
        return strings.HasSuffix(hello.ServerName, ".acme.invalid")
}

func supportsECDSA(hello *tls.ClientHelloInfo) bool {
        // The "signature_algorithms" extension, if present, limits the key exchange
        // algorithms allowed by the cipher suites. See RFC 5246, section 7.4.1.4.1.
        if hello.SignatureSchemes != nil {
                ecdsaOK := false
        schemeLoop:
                for _, scheme := range hello.SignatureSchemes {
                        const tlsECDSAWithSHA1 tls.SignatureScheme = 0x0203 // constant added in Go 1.10
                        switch scheme {
                        case tlsECDSAWithSHA1, tls.ECDSAWithP256AndSHA256,
                                tls.ECDSAWithP384AndSHA384, tls.ECDSAWithP521AndSHA512:
                                ecdsaOK = true
                                break schemeLoop
                        }
                }
                if !ecdsaOK {
                        return false
                }
        }
        if hello.SupportedCurves != nil {
                ecdsaOK := false
                for _, curve := range hello.SupportedCurves {
                        if curve == tls.CurveP256 {
                                ecdsaOK = true
                                break
                        }
                }
                if !ecdsaOK {
                        return false
                }
        }
        for _, suite := range hello.CipherSuites {
                switch suite {
                case tls.TLS_ECDHE_ECDSA_WITH_RC4_128_SHA,
                        tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
                        tls.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA,
                        tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256,
                        tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
                        tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
                        tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305:
                        return true
                }
        }
        return false
}

// HTTPHandler configures the Manager to provision ACME "http-01" challenge responses.
// It returns an http.Handler that responds to the challenges and must be
// running on port 80. If it receives a request that is not an ACME challenge,
// it delegates the request to the optional fallback handler.
//
// If fallback is nil, the returned handler redirects all GET and HEAD requests
// to the default TLS port 443 with 302 Found status code, preserving the original
// request path and query. It responds with 400 Bad Request to all other HTTP methods.
// The fallback is not protected by the optional HostPolicy.
//
// Because the fallback handler is run with unencrypted port 80 requests,
// the fallback should not serve TLS-only requests.
//
// If HTTPHandler is never called, the Manager will only use the "tls-alpn-01"
// challenge for domain verification.
func (m *Manager) HTTPHandler(fallback http.Handler) http.Handler {
        m.tokensMu.Lock()
        defer m.tokensMu.Unlock()
        m.tryHTTP01 = true

        if fallback == nil {
                fallback = http.HandlerFunc(handleHTTPRedirect)
        }
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                if !strings.HasPrefix(r.URL.Path, "/.well-known/acme-challenge/") {
                        fallback.ServeHTTP(w, r)
                        return
                }
                // A reasonable context timeout for cache and host policy only,
                // because we don't wait for a new certificate issuance here.
                ctx, cancel := context.WithTimeout(r.Context(), time.Minute)
                defer cancel()
                if err := m.hostPolicy()(ctx, r.Host); err != nil {
                        http.Error(w, err.Error(), http.StatusForbidden)
                        return
                }
                data, err := m.httpToken(ctx, r.URL.Path)
                if err != nil {
                        http.Error(w, err.Error(), http.StatusNotFound)
                        return
                }
                w.Write(data)
        })
}

func handleHTTPRedirect(w http.ResponseWriter, r *http.Request) {
        if r.Method != "GET" && r.Method != "HEAD" {
                http.Error(w, "Use HTTPS", http.StatusBadRequest)
                return
        }
        target := "https://" + stripPort(r.Host) + r.URL.RequestURI()
        http.Redirect(w, r, target, http.StatusFound)
}

func stripPort(hostport string) string {
        host, _, err := net.SplitHostPort(hostport)
        if err != nil {
                return hostport
        }
        return net.JoinHostPort(host, "443")
}

// cert returns an existing certificate either from m.state or cache.
// If a certificate is found in cache but not in m.state, the latter will be filled
// with the cached value.
func (m *Manager) cert(ctx context.Context, ck certKey) (*tls.Certificate, error) {
        m.stateMu.Lock()
        if s, ok := m.state[ck]; ok {
                m.stateMu.Unlock()
                s.RLock()
                defer s.RUnlock()
                return s.tlscert()
        }
        defer m.stateMu.Unlock()
        cert, err := m.cacheGet(ctx, ck)
        if err != nil {
                return nil, err
        }
        signer, ok := cert.PrivateKey.(crypto.Signer)
        if !ok {
                return nil, errors.New("acme/autocert: private key cannot sign")
        }
        if m.state == nil {
                m.state = make(map[certKey]*certState)
        }
        s := &certState{
                key:  signer,
                cert: cert.Certificate,
                leaf: cert.Leaf,
        }
        m.state[ck] = s
        go m.renew(ck, s.key, s.leaf.NotAfter)
        return cert, nil
}

// cacheGet always returns a valid certificate, or an error otherwise.
// If a cached certificate exists but is not valid, ErrCacheMiss is returned.
func (m *Manager) cacheGet(ctx context.Context, ck certKey) (*tls.Certificate, error) {
        if m.Cache == nil {
                return nil, ErrCacheMiss
        }
        data, err := m.Cache.Get(ctx, ck.String())
        if err != nil {
                return nil, err
        }

        // private
        priv, pub := pem.Decode(data)
        if priv == nil || !strings.Contains(priv.Type, "PRIVATE") {
                return nil, ErrCacheMiss
        }
        privKey, err := parsePrivateKey(priv.Bytes)
        if err != nil {
                return nil, err
        }

        // public
        var pubDER [][]byte
        for len(pub) > 0 {
                var b *pem.Block
                b, pub = pem.Decode(pub)
                if b == nil {
                        break
                }
                pubDER = append(pubDER, b.Bytes)
        }
        if len(pub) > 0 {
                // Leftover content not consumed by pem.Decode. Corrupt. Ignore.
                return nil, ErrCacheMiss
        }

        // verify and create TLS cert
        leaf, err := validCert(ck, pubDER, privKey, m.now())
        if err != nil {
                return nil, ErrCacheMiss
        }
        tlscert := &tls.Certificate{
                Certificate: pubDER,
                PrivateKey:  privKey,
                Leaf:        leaf,
        }
        return tlscert, nil
}

func (m *Manager) cachePut(ctx context.Context, ck certKey, tlscert *tls.Certificate) error {
        if m.Cache == nil {
                return nil
        }

        // contains PEM-encoded data
        var buf bytes.Buffer

        // private
        switch key := tlscert.PrivateKey.(type) {
        case *ecdsa.PrivateKey:
                if err := encodeECDSAKey(&buf, key); err != nil {
                        return err
                }
        case *rsa.PrivateKey:
                b := x509.MarshalPKCS1PrivateKey(key)
                pb := &pem.Block{Type: "RSA PRIVATE KEY", Bytes: b}
                if err := pem.Encode(&buf, pb); err != nil {
                        return err
                }
        default:
                return errors.New("acme/autocert: unknown private key type")
        }

        // public
        for _, b := range tlscert.Certificate {
                pb := &pem.Block{Type: "CERTIFICATE", Bytes: b}
                if err := pem.Encode(&buf, pb); err != nil {
                        return err
                }
        }

        return m.Cache.Put(ctx, ck.String(), buf.Bytes())
}

func encodeECDSAKey(w io.Writer, key *ecdsa.PrivateKey) error {
        b, err := x509.MarshalECPrivateKey(key)
        if err != nil {
                return err
        }
        pb := &pem.Block{Type: "EC PRIVATE KEY", Bytes: b}
        return pem.Encode(w, pb)
}

// createCert starts the domain ownership verification and returns a certificate
// for that domain upon success.
//
// If the domain is already being verified, it waits for the existing verification to complete.
// Either way, createCert blocks for the duration of the whole process.
func (m *Manager) createCert(ctx context.Context, ck certKey) (*tls.Certificate, error) {
        // TODO: maybe rewrite this whole piece using sync.Once
        state, err := m.certState(ck)
        if err != nil {
                return nil, err
        }
        // state may exist if another goroutine is already working on it
        // in which case just wait for it to finish
        if !state.locked {
                state.RLock()
                defer state.RUnlock()
                return state.tlscert()
        }

        // We are the first; state is locked.
        // Unblock the readers when domain ownership is verified
        // and we got the cert or the process failed.
        defer state.Unlock()
        state.locked = false

        der, leaf, err := m.authorizedCert(ctx, state.key, ck)
        if err != nil {
                // Remove the failed state after some time,
                // making the manager call createCert again on the following TLS hello.
                time.AfterFunc(createCertRetryAfter, func() {
                        defer testDidRemoveState(ck)
                        m.stateMu.Lock()
                        defer m.stateMu.Unlock()
                        // Verify the state hasn't changed and it's still invalid
                        // before deleting.
                        s, ok := m.state[ck]
                        if !ok {
                                return
                        }
                        if _, err := validCert(ck, s.cert, s.key, m.now()); err == nil {
                                return
                        }
                        delete(m.state, ck)
                })
                return nil, err
        }
        state.cert = der
        state.leaf = leaf
        go m.renew(ck, state.key, state.leaf.NotAfter)
        return state.tlscert()
}

// certState returns a new or existing certState.
// If a new certState is returned, state.exist is false and the state is locked.
// The returned error is non-nil only in the case where a new state could not be created.
func (m *Manager) certState(ck certKey) (*certState, error) {
        m.stateMu.Lock()
        defer m.stateMu.Unlock()
        if m.state == nil {
                m.state = make(map[certKey]*certState)
        }
        // existing state
        if state, ok := m.state[ck]; ok {
                return state, nil
        }

        // new locked state
        var (
                err error
                key crypto.Signer
        )
        if ck.isRSA {
                key, err = rsa.GenerateKey(rand.Reader, 2048)
        } else {
                key, err = ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
        }
        if err != nil {
                return nil, err
        }

        state := &certState{
                key:    key,
                locked: true,
        }
        state.Lock() // will be unlocked by m.certState caller
        m.state[ck] = state
        return state, nil
}

// authorizedCert starts the domain ownership verification process and requests a new cert upon success.
// The key argument is the certificate private key.
func (m *Manager) authorizedCert(ctx context.Context, key crypto.Signer, ck certKey) (der [][]byte, leaf *x509.Certificate, err error) {
        client, err := m.acmeClient(ctx)
        if err != nil {
                return nil, nil, err
        }

        if err := m.verify(ctx, client, ck.domain); err != nil {
                return nil, nil, err
        }
        csr, err := certRequest(key, ck.domain, m.ExtraExtensions)
        if err != nil {
                return nil, nil, err
        }
        der, _, err = client.CreateCert(ctx, csr, 0, true)
        if err != nil {
                return nil, nil, err
        }
        leaf, err = validCert(ck, der, key, m.now())
        if err != nil {
                return nil, nil, err
        }
        return der, leaf, nil
}

// revokePendingAuthz revokes all authorizations idenfied by the elements of uri slice.
// It ignores revocation errors.
func (m *Manager) revokePendingAuthz(ctx context.Context, uri []string) {
        client, err := m.acmeClient(ctx)
        if err != nil {
                return
        }
        for _, u := range uri {
                client.RevokeAuthorization(ctx, u)
        }
}

// verify runs the identifier (domain) authorization flow
// using each applicable ACME challenge type.
func (m *Manager) verify(ctx context.Context, client *acme.Client, domain string) error {
        // The list of challenge types we'll try to fulfill
        // in this specific order.
        challengeTypes := []string{"tls-alpn-01", "tls-sni-02", "tls-sni-01"}
        m.tokensMu.RLock()
        if m.tryHTTP01 {
                challengeTypes = append(challengeTypes, "http-01")
        }
        m.tokensMu.RUnlock()

        // Keep track of pending authzs and revoke the ones that did not validate.
        pendingAuthzs := make(map[string]bool)
        defer func() {
                var uri []string
                for k, pending := range pendingAuthzs {
                        if pending {
                                uri = append(uri, k)
                        }
                }
                if len(uri) > 0 {
                        // Use "detached" background context.
                        // The revocations need not happen in the current verification flow.
                        go m.revokePendingAuthz(context.Background(), uri)
                }
        }()

        // errs accumulates challenge failure errors, printed if all fail
        errs := make(map[*acme.Challenge]error)
        var nextTyp int // challengeType index of the next challenge type to try
        for {
                // Start domain authorization and get the challenge.
                authz, err := client.Authorize(ctx, domain)
                if err != nil {
                        return err
                }
                // No point in accepting challenges if the authorization status
                // is in a final state.
                switch authz.Status {
                case acme.StatusValid:
                        return nil // already authorized
                case acme.StatusInvalid:
                        return fmt.Errorf("acme/autocert: invalid authorization %q", authz.URI)
                }

                pendingAuthzs[authz.URI] = true

                // Pick the next preferred challenge.
                var chal *acme.Challenge
                for chal == nil && nextTyp < len(challengeTypes) {
                        chal = pickChallenge(challengeTypes[nextTyp], authz.Challenges)
                        nextTyp++
                }
                if chal == nil {
                        errorMsg := fmt.Sprintf("acme/autocert: unable to authorize %q", domain)
                        for chal, err := range errs {
                                errorMsg += fmt.Sprintf("; challenge %q failed with error: %v", chal.Type, err)
                        }
                        return errors.New(errorMsg)
                }
                cleanup, err := m.fulfill(ctx, client, chal, domain)
                if err != nil {
                        errs[chal] = err
                        continue
                }
                defer cleanup()
                if _, err := client.Accept(ctx, chal); err != nil {
                        errs[chal] = err
                        continue
                }

                // A challenge is fulfilled and accepted: wait for the CA to validate.
                if _, err := client.WaitAuthorization(ctx, authz.URI); err != nil {
                        errs[chal] = err
                        continue
                }
                delete(pendingAuthzs, authz.URI)
                return nil
        }
}

// fulfill provisions a response to the challenge chal.
// The cleanup is non-nil only if provisioning succeeded.
func (m *Manager) fulfill(ctx context.Context, client *acme.Client, chal *acme.Challenge, domain string) (cleanup func(), err error) {
        switch chal.Type {
        case "tls-alpn-01":
                cert, err := client.TLSALPN01ChallengeCert(chal.Token, domain)
                if err != nil {
                        return nil, err
                }
                m.putCertToken(ctx, domain, &cert)
                return func() { go m.deleteCertToken(domain) }, nil
        case "tls-sni-01":
                cert, name, err := client.TLSSNI01ChallengeCert(chal.Token)
                if err != nil {
                        return nil, err
                }
                m.putCertToken(ctx, name, &cert)
                return func() { go m.deleteCertToken(name) }, nil
        case "tls-sni-02":
                cert, name, err := client.TLSSNI02ChallengeCert(chal.Token)
                if err != nil {
                        return nil, err
                }
                m.putCertToken(ctx, name, &cert)
                return func() { go m.deleteCertToken(name) }, nil
        case "http-01":
                resp, err := client.HTTP01ChallengeResponse(chal.Token)
                if err != nil {
                        return nil, err
                }
                p := client.HTTP01ChallengePath(chal.Token)
                m.putHTTPToken(ctx, p, resp)
                return func() { go m.deleteHTTPToken(p) }, nil
        }
        return nil, fmt.Errorf("acme/autocert: unknown challenge type %q", chal.Type)
}

func pickChallenge(typ string, chal []*acme.Challenge) *acme.Challenge {
        for _, c := range chal {
                if c.Type == typ {
                        return c
                }
        }
        return nil
}

// putCertToken stores the token certificate with the specified name
// in both m.certTokens map and m.Cache.
func (m *Manager) putCertToken(ctx context.Context, name string, cert *tls.Certificate) {
        m.tokensMu.Lock()
        defer m.tokensMu.Unlock()
        if m.certTokens == nil {
                m.certTokens = make(map[string]*tls.Certificate)
        }
        m.certTokens[name] = cert
        m.cachePut(ctx, certKey{domain: name, isToken: true}, cert)
}

// deleteCertToken removes the token certificate with the specified name
// from both m.certTokens map and m.Cache.
func (m *Manager) deleteCertToken(name string) {
        m.tokensMu.Lock()
        defer m.tokensMu.Unlock()
        delete(m.certTokens, name)
        if m.Cache != nil {
                ck := certKey{domain: name, isToken: true}
                m.Cache.Delete(context.Background(), ck.String())
        }
}

// httpToken retrieves an existing http-01 token value from an in-memory map
// or the optional cache.
func (m *Manager) httpToken(ctx context.Context, tokenPath string) ([]byte, error) {
        m.tokensMu.RLock()
        defer m.tokensMu.RUnlock()
        if v, ok := m.httpTokens[tokenPath]; ok {
                return v, nil
        }
        if m.Cache == nil {
                return nil, fmt.Errorf("acme/autocert: no token at %q", tokenPath)
        }
        return m.Cache.Get(ctx, httpTokenCacheKey(tokenPath))
}

// putHTTPToken stores an http-01 token value using tokenPath as key
// in both in-memory map and the optional Cache.
//
// It ignores any error returned from Cache.Put.
func (m *Manager) putHTTPToken(ctx context.Context, tokenPath, val string) {
        m.tokensMu.Lock()
        defer m.tokensMu.Unlock()
        if m.httpTokens == nil {
                m.httpTokens = make(map[string][]byte)
        }
        b := []byte(val)
        m.httpTokens[tokenPath] = b
        if m.Cache != nil {
                m.Cache.Put(ctx, httpTokenCacheKey(tokenPath), b)
        }
}

// deleteHTTPToken removes an http-01 token value from both in-memory map
// and the optional Cache, ignoring any error returned from the latter.
//
// If m.Cache is non-nil, it blocks until Cache.Delete returns without a timeout.
func (m *Manager) deleteHTTPToken(tokenPath string) {
        m.tokensMu.Lock()
        defer m.tokensMu.Unlock()
        delete(m.httpTokens, tokenPath)
        if m.Cache != nil {
                m.Cache.Delete(context.Background(), httpTokenCacheKey(tokenPath))
        }
}

// httpTokenCacheKey returns a key at which an http-01 token value may be stored
// in the Manager's optional Cache.
func httpTokenCacheKey(tokenPath string) string {
        return path.Base(tokenPath) + "+http-01"
}

// renew starts a cert renewal timer loop, one per domain.
//
// The loop is scheduled in two cases:
// - a cert was fetched from cache for the first time (wasn't in m.state)
// - a new cert was created by m.createCert
//
// The key argument is a certificate private key.
// The exp argument is the cert expiration time (NotAfter).
func (m *Manager) renew(ck certKey, key crypto.Signer, exp time.Time) {
        m.renewalMu.Lock()
        defer m.renewalMu.Unlock()
        if m.renewal[ck] != nil {
                // another goroutine is already on it
                return
        }
        if m.renewal == nil {
                m.renewal = make(map[certKey]*domainRenewal)
        }
        dr := &domainRenewal{m: m, ck: ck, key: key}
        m.renewal[ck] = dr
        dr.start(exp)
}

// stopRenew stops all currently running cert renewal timers.
// The timers are not restarted during the lifetime of the Manager.
func (m *Manager) stopRenew() {
        m.renewalMu.Lock()
        defer m.renewalMu.Unlock()
        for name, dr := range m.renewal {
                delete(m.renewal, name)
                dr.stop()
        }
}

func (m *Manager) accountKey(ctx context.Context) (crypto.Signer, error) {
        const keyName = "acme_account+key"

        // Previous versions of autocert stored the value under a different key.
        const legacyKeyName = "acme_account.key"

        genKey := func() (*ecdsa.PrivateKey, error) {
                return ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
        }

        if m.Cache == nil {
                return genKey()
        }

        data, err := m.Cache.Get(ctx, keyName)
        if err == ErrCacheMiss {
                data, err = m.Cache.Get(ctx, legacyKeyName)
        }
        if err == ErrCacheMiss {
                key, err := genKey()
                if err != nil {
                        return nil, err
                }
                var buf bytes.Buffer
                if err := encodeECDSAKey(&buf, key); err != nil {
                        return nil, err
                }
                if err := m.Cache.Put(ctx, keyName, buf.Bytes()); err != nil {
                        return nil, err
                }
                return key, nil
        }
        if err != nil {
                return nil, err
        }

        priv, _ := pem.Decode(data)
        if priv == nil || !strings.Contains(priv.Type, "PRIVATE") {
                return nil, errors.New("acme/autocert: invalid account key found in cache")
        }
        return parsePrivateKey(priv.Bytes)
}

func (m *Manager) acmeClient(ctx context.Context) (*acme.Client, error) {
        m.clientMu.Lock()
        defer m.clientMu.Unlock()
        if m.client != nil {
                return m.client, nil
        }

        client := m.Client
        if client == nil {
                client = &acme.Client{DirectoryURL: acme.LetsEncryptURL}
        }
        if client.Key == nil {
                var err error
                client.Key, err = m.accountKey(ctx)
                if err != nil {
                        return nil, err
                }
        }
        var contact []string
        if m.Email != "" {
                contact = []string{"mailto:" + m.Email}
        }
        a := &acme.Account{Contact: contact}
        _, err := client.Register(ctx, a, m.Prompt)
        if ae, ok := err.(*acme.Error); err == nil || ok && ae.StatusCode == http.StatusConflict {
                // conflict indicates the key is already registered
                m.client = client
                err = nil
        }
        return m.client, err
}

func (m *Manager) hostPolicy() HostPolicy {
        if m.HostPolicy != nil {
                return m.HostPolicy
        }
        return defaultHostPolicy
}

func (m *Manager) renewBefore() time.Duration {
        if m.RenewBefore > renewJitter {
                return m.RenewBefore
        }
        return 720 * time.Hour // 30 days
}

func (m *Manager) now() time.Time {
        if m.nowFunc != nil {
                return m.nowFunc()
        }
        return time.Now()
}

// certState is ready when its mutex is unlocked for reading.
type certState struct {
        sync.RWMutex
        locked bool              // locked for read/write
        key    crypto.Signer     // private key for cert
        cert   [][]byte          // DER encoding
        leaf   *x509.Certificate // parsed cert[0]; always non-nil if cert != nil
}

// tlscert creates a tls.Certificate from s.key and s.cert.
// Callers should wrap it in s.RLock() and s.RUnlock().
func (s *certState) tlscert() (*tls.Certificate, error) {
        if s.key == nil {
                return nil, errors.New("acme/autocert: missing signer")
        }
        if len(s.cert) == 0 {
                return nil, errors.New("acme/autocert: missing certificate")
        }
        return &tls.Certificate{
                PrivateKey:  s.key,
                Certificate: s.cert,
                Leaf:        s.leaf,
        }, nil
}

// certRequest generates a CSR for the given common name cn and optional SANs.
func certRequest(key crypto.Signer, cn string, ext []pkix.Extension, san ...string) ([]byte, error) {
        req := &x509.CertificateRequest{
                Subject:         pkix.Name{CommonName: cn},
                DNSNames:        san,
                ExtraExtensions: ext,
        }
        return x509.CreateCertificateRequest(rand.Reader, req, key)
}

// Attempt to parse the given private key DER block. OpenSSL 0.9.8 generates
// PKCS#1 private keys by default, while OpenSSL 1.0.0 generates PKCS#8 keys.
// OpenSSL ecparam generates SEC1 EC private keys for ECDSA. We try all three.
//
// Inspired by parsePrivateKey in crypto/tls/tls.go.
func parsePrivateKey(der []byte) (crypto.Signer, error) {
        if key, err := x509.ParsePKCS1PrivateKey(der); err == nil {
                return key, nil
        }
        if key, err := x509.ParsePKCS8PrivateKey(der); err == nil {
                switch key := key.(type) {
                case *rsa.PrivateKey:
                        return key, nil
                case *ecdsa.PrivateKey:
                        return key, nil
                default:
                        return nil, errors.New("acme/autocert: unknown private key type in PKCS#8 wrapping")
                }
        }
        if key, err := x509.ParseECPrivateKey(der); err == nil {
                return key, nil
        }

        return nil, errors.New("acme/autocert: failed to parse private key")
}

// validCert parses a cert chain provided as der argument and verifies the leaf and der[0]
// correspond to the private key, the domain and key type match, and expiration dates
// are valid. It doesn't do any revocation checking.
//
// The returned value is the verified leaf cert.
func validCert(ck certKey, der [][]byte, key crypto.Signer, now time.Time) (leaf *x509.Certificate, err error) {
        // parse public part(s)
        var n int
        for _, b := range der {
                n += len(b)
        }
        pub := make([]byte, n)
        n = 0
        for _, b := range der {
                n += copy(pub[n:], b)
        }
        x509Cert, err := x509.ParseCertificates(pub)
        if err != nil || len(x509Cert) == 0 {
                return nil, errors.New("acme/autocert: no public key found")
        }
        // verify the leaf is not expired and matches the domain name
        leaf = x509Cert[0]
        if now.Before(leaf.NotBefore) {
                return nil, errors.New("acme/autocert: certificate is not valid yet")
        }
        if now.After(leaf.NotAfter) {
                return nil, errors.New("acme/autocert: expired certificate")
        }
        if err := leaf.VerifyHostname(ck.domain); err != nil {
                return nil, err
        }
        // ensure the leaf corresponds to the private key and matches the certKey type
        switch pub := leaf.PublicKey.(type) {
        case *rsa.PublicKey:
                prv, ok := key.(*rsa.PrivateKey)
                if !ok {
                        return nil, errors.New("acme/autocert: private key type does not match public key type")
                }
                if pub.N.Cmp(prv.N) != 0 {
                        return nil, errors.New("acme/autocert: private key does not match public key")
                }
                if !ck.isRSA && !ck.isToken {
                        return nil, errors.New("acme/autocert: key type does not match expected value")
                }
        case *ecdsa.PublicKey:
                prv, ok := key.(*ecdsa.PrivateKey)
                if !ok {
                        return nil, errors.New("acme/autocert: private key type does not match public key type")
                }
                if pub.X.Cmp(prv.X) != 0 || pub.Y.Cmp(prv.Y) != 0 {
                        return nil, errors.New("acme/autocert: private key does not match public key")
                }
                if ck.isRSA && !ck.isToken {
                        return nil, errors.New("acme/autocert: key type does not match expected value")
                }
        default:
                return nil, errors.New("acme/autocert: unknown public key algorithm")
        }
        return leaf, nil
}

type lockedMathRand struct {
        sync.Mutex
        rnd *mathrand.Rand
}

func (r *lockedMathRand) int63n(max int64) int64 {
        r.Lock()
        n := r.rnd.Int63n(max)
        r.Unlock()
        return n
}

// For easier testing.
var (
        // Called when a state is removed.
        testDidRemoveState = func(certKey) {}
)