diff --git a/acme/acme.go b/acme/acme.go
index 9e28b32..31d07e3 100644
--- a/acme/acme.go
+++ b/acme/acme.go
@@ -119,21 +119,49 @@
 	// identifiable by the server, in case they are causing issues.
 	UserAgent string
 
-	dirMu sync.Mutex // guards writes to dir
-	dir   *Directory // cached result of Client's Discover method
+	cacheMu sync.Mutex
+	dir     *Directory // cached result of Client's Discover method
+	kid     keyID      // cached Account.URI obtained from registerRFC or getAccountRFC
 
 	noncesMu sync.Mutex
 	nonces   map[string]struct{} // nonces collected from previous responses
 }
 
+// accountKID returns a key ID associated with c.Key, the account identity
+// provided by the CA during RFC based registration.
+// It assumes c.Discover has already been called.
+//
+// accountKID requires at most one network roundtrip.
+// It caches only successful result.
+//
+// When in pre-RFC mode or when c.getRegRFC responds with an error, accountKID
+// returns noKeyID.
+func (c *Client) accountKID(ctx context.Context) keyID {
+	c.cacheMu.Lock()
+	defer c.cacheMu.Unlock()
+	if c.dir.OrderURL == "" {
+		// Assume legacy CA.
+		return noKeyID
+	}
+	if c.kid != noKeyID {
+		return c.kid
+	}
+	a, err := c.getRegRFC(ctx)
+	if err != nil {
+		return noKeyID
+	}
+	c.kid = keyID(a.URI)
+	return c.kid
+}
+
 // Discover performs ACME server discovery using c.DirectoryURL.
 //
 // It caches successful result. So, subsequent calls will not result in
 // a network round-trip. This also means mutating c.DirectoryURL after successful call
 // of this method will have no effect.
 func (c *Client) Discover(ctx context.Context) (Directory, error) {
-	c.dirMu.Lock()
-	defer c.dirMu.Unlock()
+	c.cacheMu.Lock()
+	defer c.cacheMu.Unlock()
 	if c.dir != nil {
 		return *c.dir, nil
 	}
@@ -235,7 +263,7 @@
 		req.NotAfter = now.Add(exp).Format(time.RFC3339)
 	}
 
-	res, err := c.post(ctx, c.Key, c.dir.CertURL, req, wantStatus(http.StatusCreated))
+	res, err := c.post(ctx, nil, c.dir.CertURL, req, wantStatus(http.StatusCreated))
 	if err != nil {
 		return nil, "", err
 	}
@@ -289,9 +317,6 @@
 		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
@@ -304,20 +329,32 @@
 // during account registration. See Register method of Client for more details.
 func AcceptTOS(tosURL string) bool { return true }
 
-// Register creates a new account registration by following the "new-reg" flow.
-// It returns the registered account. The account is not modified.
+// Register creates a new account with the CA using c.Key.
+// It returns the registered account. The account acct is not modified.
 //
 // The registration may require the caller to agree to the CA's Terms of Service (TOS).
 // If so, and the account has not indicated the acceptance of the terms (see Account for details),
 // Register 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 caller can use AcceptTOS.
-func (c *Client) Register(ctx context.Context, a *Account, prompt func(tosURL string) bool) (*Account, error) {
-	if _, err := c.Discover(ctx); err != nil {
+//
+// When interfacing with RFC compliant CA, non-RFC8555 compliant fields of acct are ignored
+// and prompt is called if Directory's Terms field is non-zero.
+// Also see Error's Instance field for when a CA requires already registered accounts to agree
+// to an updated Terms of Service.
+func (c *Client) Register(ctx context.Context, acct *Account, prompt func(tosURL string) bool) (*Account, error) {
+	dir, err := c.Discover(ctx)
+	if err != nil {
 		return nil, err
 	}
 
-	var err error
-	if a, err = c.doReg(ctx, c.dir.RegURL, "new-reg", a); err != nil {
+	// RFC8555 compliant account registration.
+	if dir.OrderURL != "" {
+		return c.registerRFC(ctx, acct, prompt)
+	}
+
+	// Legacy ACME draft registration flow.
+	a, err := c.doReg(ctx, dir.RegURL, "new-reg", acct)
+	if err != nil {
 		return nil, err
 	}
 	var accept bool
@@ -331,9 +368,22 @@
 	return a, err
 }
 
-// GetReg retrieves an existing registration.
-// The url argument is an Account URI.
+// GetReg retrieves an existing account associated with c.Key.
+//
+// The url argument is an Account URI used with pre-RFC8555 CAs.
+// It is ignored when interfacing with an RFC compliant CA.
 func (c *Client) GetReg(ctx context.Context, url string) (*Account, error) {
+	dir, err := c.Discover(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	// Assume RFC8555 compliant CA.
+	if dir.OrderURL != "" {
+		return c.getRegRFC(ctx)
+	}
+
+	// Legacy CA.
 	a, err := c.doReg(ctx, url, "reg", nil)
 	if err != nil {
 		return nil, err
@@ -344,9 +394,23 @@
 
 // UpdateReg updates an existing registration.
 // It returns an updated account copy. The provided account is not modified.
-func (c *Client) UpdateReg(ctx context.Context, a *Account) (*Account, error) {
-	uri := a.URI
-	a, err := c.doReg(ctx, uri, "reg", a)
+//
+// When interfacing with RFC compliant CAs, a.URI is ignored and the account URL
+// associated with c.Key is used instead.
+func (c *Client) UpdateReg(ctx context.Context, acct *Account) (*Account, error) {
+	dir, err := c.Discover(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	// Assume RFC8555 compliant CA.
+	if dir.OrderURL != "" {
+		return c.updateRegRFC(ctx, acct)
+	}
+
+	// Legacy CA.
+	uri := acct.URI
+	a, err := c.doReg(ctx, uri, "reg", acct)
 	if err != nil {
 		return nil, err
 	}
@@ -391,7 +455,7 @@
 		Resource:   "new-authz",
 		Identifier: authzID{Type: typ, Value: val},
 	}
-	res, err := c.post(ctx, c.Key, c.dir.AuthzURL, req, wantStatus(http.StatusCreated))
+	res, err := c.post(ctx, nil, c.dir.AuthzURL, req, wantStatus(http.StatusCreated))
 	if err != nil {
 		return nil, err
 	}
@@ -434,6 +498,11 @@
 //
 // It does not revoke existing certificates.
 func (c *Client) RevokeAuthorization(ctx context.Context, url string) error {
+	// Required for c.accountKID() when in RFC mode.
+	if _, err := c.Discover(ctx); err != nil {
+		return err
+	}
+
 	req := struct {
 		Resource string `json:"resource"`
 		Status   string `json:"status"`
@@ -443,7 +512,7 @@
 		Status:   "deactivated",
 		Delete:   true,
 	}
-	res, err := c.post(ctx, c.Key, url, req, wantStatus(http.StatusOK))
+	res, err := c.post(ctx, nil, url, req, wantStatus(http.StatusOK))
 	if err != nil {
 		return err
 	}
@@ -520,6 +589,11 @@
 //
 // The server will then perform the validation asynchronously.
 func (c *Client) Accept(ctx context.Context, chal *Challenge) (*Challenge, error) {
+	// Required for c.accountKID() when in RFC mode.
+	if _, err := c.Discover(ctx); err != nil {
+		return nil, err
+	}
+
 	auth, err := keyAuth(c.Key.Public(), chal.Token)
 	if err != nil {
 		return nil, err
@@ -534,7 +608,7 @@
 		Type:     chal.Type,
 		Auth:     auth,
 	}
-	res, err := c.post(ctx, c.Key, chal.URI, req, wantStatus(
+	res, err := c.post(ctx, nil, chal.URI, req, wantStatus(
 		http.StatusOK,       // according to the spec
 		http.StatusAccepted, // Let's Encrypt: see https://goo.gl/WsJ7VT (acme-divergences.md)
 	))
@@ -711,7 +785,7 @@
 		req.Contact = acct.Contact
 		req.Agreement = acct.AgreedTerms
 	}
-	res, err := c.post(ctx, c.Key, url, req, wantStatus(
+	res, err := c.post(ctx, nil, url, req, wantStatus(
 		http.StatusOK,       // updates and deletes
 		http.StatusCreated,  // new account creation
 		http.StatusAccepted, // Let's Encrypt divergent implementation
diff --git a/acme/acme_test.go b/acme/acme_test.go
index 99a4bf8..f171f8d 100644
--- a/acme/acme_test.go
+++ b/acme/acme_test.go
@@ -16,6 +16,7 @@
 	"encoding/hex"
 	"encoding/json"
 	"fmt"
+	"io"
 	"math/big"
 	"net/http"
 	"net/http/httptest"
@@ -28,10 +29,10 @@
 
 // Decodes a JWS-encoded request and unmarshals the decoded JSON into a provided
 // interface.
-func decodeJWSRequest(t *testing.T, v interface{}, r *http.Request) {
+func decodeJWSRequest(t *testing.T, v interface{}, r io.Reader) {
 	// Decode request
 	var req struct{ Payload string }
-	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+	if err := json.NewDecoder(r).Decode(&req); err != nil {
 		t.Fatal(err)
 	}
 	payload, err := base64.RawURLEncoding.DecodeString(req.Payload)
@@ -47,12 +48,14 @@
 type jwsHead struct {
 	Alg   string
 	Nonce string
+	URL   string            `json:"url"`
+	KID   string            `json:"kid"`
 	JWK   map[string]string `json:"jwk"`
 }
 
-func decodeJWSHead(r *http.Request) (*jwsHead, error) {
+func decodeJWSHead(r io.Reader) (*jwsHead, error) {
 	var req struct{ Protected string }
-	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+	if err := json.NewDecoder(r).Decode(&req); err != nil {
 		return nil, err
 	}
 	b, err := base64.RawURLEncoding.DecodeString(req.Protected)
@@ -123,7 +126,7 @@
 			Contact   []string
 			Agreement string
 		}
-		decodeJWSRequest(t, &j, r)
+		decodeJWSRequest(t, &j, r.Body)
 
 		// Test request
 		if j.Resource != "new-reg" {
@@ -193,7 +196,7 @@
 			Contact   []string
 			Agreement string
 		}
-		decodeJWSRequest(t, &j, r)
+		decodeJWSRequest(t, &j, r.Body)
 
 		// Test request
 		if j.Resource != "reg" {
@@ -215,7 +218,11 @@
 	}))
 	defer ts.Close()
 
-	c := Client{Key: testKeyEC}
+	c := Client{
+		Key:          testKeyEC,
+		DirectoryURL: ts.URL,       // don't dial outside of localhost
+		dir:          &Directory{}, // don't do discovery
+	}
 	a := &Account{URI: ts.URL, Contact: contacts, AgreedTerms: terms}
 	var err error
 	if a, err = c.UpdateReg(context.Background(), a); err != nil {
@@ -254,7 +261,7 @@
 			Contact   []string
 			Agreement string
 		}
-		decodeJWSRequest(t, &j, r)
+		decodeJWSRequest(t, &j, r.Body)
 
 		// Test request
 		if j.Resource != "reg" {
@@ -276,7 +283,11 @@
 	}))
 	defer ts.Close()
 
-	c := Client{Key: testKeyEC}
+	c := Client{
+		Key:          testKeyEC,
+		DirectoryURL: ts.URL,       // don't dial outside of localhost
+		dir:          &Directory{}, // don't do discovery
+	}
 	a, err := c.GetReg(context.Background(), ts.URL)
 	if err != nil {
 		t.Fatal(err)
@@ -318,7 +329,7 @@
 						Value string
 					}
 				}
-				decodeJWSRequest(t, &j, r)
+				decodeJWSRequest(t, &j, r.Body)
 
 				// Test request
 				if j.Resource != "new-authz" {
@@ -629,7 +640,7 @@
 				Status   string
 				Delete   bool
 			}
-			decodeJWSRequest(t, &req, r)
+			decodeJWSRequest(t, &req, r.Body)
 			if req.Resource != "authz" {
 				t.Errorf("req.Resource = %q; want authz", req.Resource)
 			}
@@ -644,7 +655,11 @@
 		}
 	}))
 	defer ts.Close()
-	client := &Client{Key: testKey}
+	client := &Client{
+		Key:          testKey,
+		DirectoryURL: ts.URL,       // don't dial outside of localhost
+		dir:          &Directory{}, // don't do discovery
+	}
 	ctx := context.Background()
 	if err := client.RevokeAuthorization(ctx, ts.URL+"/1"); err != nil {
 		t.Errorf("err = %v", err)
@@ -704,7 +719,7 @@
 			Type     string
 			Auth     string `json:"keyAuthorization"`
 		}
-		decodeJWSRequest(t, &j, r)
+		decodeJWSRequest(t, &j, r.Body)
 
 		// Test request
 		if j.Resource != "challenge" {
@@ -730,7 +745,11 @@
 	}))
 	defer ts.Close()
 
-	cl := Client{Key: testKeyEC}
+	cl := Client{
+		Key:          testKeyEC,
+		DirectoryURL: ts.URL,       // don't dial outside of localhost
+		dir:          &Directory{}, // don't do discovery
+	}
 	c, err := cl.Accept(context.Background(), &Challenge{
 		URI:   ts.URL,
 		Token: "token1",
@@ -771,7 +790,7 @@
 			NotBefore string `json:"notBefore,omitempty"`
 			NotAfter  string `json:"notAfter,omitempty"`
 		}
-		decodeJWSRequest(t, &j, r)
+		decodeJWSRequest(t, &j, r.Body)
 
 		// Test request
 		if j.Resource != "new-cert" {
@@ -956,7 +975,7 @@
 			Certificate string
 			Reason      int
 		}
-		decodeJWSRequest(t, &req, r)
+		decodeJWSRequest(t, &req, r.Body)
 		if req.Resource != "revoke-cert" {
 			t.Errorf("req.Resource = %q; want revoke-cert", req.Resource)
 		}
@@ -1117,7 +1136,7 @@
 			w.Write([]byte(`{"status":"valid"}`))
 		}()
 
-		head, err := decodeJWSHead(r)
+		head, err := decodeJWSHead(r.Body)
 		if err != nil {
 			t.Errorf("decodeJWSHead: %v", err)
 			return
diff --git a/acme/http.go b/acme/http.go
index c6535fd..b145292 100644
--- a/acme/http.go
+++ b/acme/http.go
@@ -156,7 +156,7 @@
 }
 
 // post issues a signed POST request in JWS format using the provided key
-// to the specified URL.
+// to the specified URL. If key is nil, c.Key is used instead.
 // It returns a non-error value only when ok reports true.
 //
 // post retries unsuccessful attempts according to c.RetryBackoff
@@ -193,14 +193,28 @@
 }
 
 // postNoRetry signs the body with the given key and POSTs it to the provided url.
-// The body argument must be JSON-serializable.
 // It is used by c.post to retry unsuccessful attempts.
+// The body argument must be JSON-serializable.
+//
+// If key argument is nil, c.Key is used to sign the request.
+// If key argument is nil and c.accountKID returns a non-zero keyID,
+// the request is sent in KID form. Otherwise, JWK form is used.
+//
+// In practice, when interfacing with RFC compliant CAs most requests are sent in KID form
+// and JWK is used only when KID is unavailable: new account endpoint and certificate
+// revocation requests authenticated by a cert key.
+// See jwsEncodeJSON for other details.
 func (c *Client) postNoRetry(ctx context.Context, key crypto.Signer, url string, body interface{}) (*http.Response, *http.Request, error) {
+	kid := noKeyID
+	if key == nil {
+		key = c.Key
+		kid = c.accountKID(ctx)
+	}
 	nonce, err := c.popNonce(ctx, url)
 	if err != nil {
 		return nil, nil, err
 	}
-	b, err := jwsEncodeJSON(body, key, noKeyID, nonce, url)
+	b, err := jwsEncodeJSON(body, key, kid, nonce, url)
 	if err != nil {
 		return nil, nil, err
 	}
diff --git a/acme/http_test.go b/acme/http_test.go
index a5dc35f..79095cc 100644
--- a/acme/http_test.go
+++ b/acme/http_test.go
@@ -87,7 +87,7 @@
 			return
 		}
 
-		head, err := decodeJWSHead(r)
+		head, err := decodeJWSHead(r.Body)
 		switch {
 		case err != nil:
 			t.Errorf("decodeJWSHead: %v", err)
diff --git a/acme/jws.go b/acme/jws.go
index f26bd5b..f8bc2c4 100644
--- a/acme/jws.go
+++ b/acme/jws.go
@@ -30,7 +30,7 @@
 //
 // If kid is non-empty, its quoted value is inserted in the protected head
 // as "kid" field value. Otherwise, JWK is computed using jwkEncode and inserted
-// as "jwk" field value.
+// as "jwk" field value. The "jwk" and "kid" fields are mutually exclusive.
 //
 // See https://tools.ietf.org/html/rfc7515#section-7.
 func jwsEncodeJSON(claimset interface{}, key crypto.Signer, kid keyID, nonce, url string) ([]byte, error) {
diff --git a/acme/rfc8555.go b/acme/rfc8555.go
new file mode 100644
index 0000000..51839a0
--- /dev/null
+++ b/acme/rfc8555.go
@@ -0,0 +1,122 @@
+// Copyright 2019 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 acme
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+)
+
+// DeactivateReg permanently disables an existing account associated with c.Key.
+// A deactivated account can no longer request certificate issuance or access
+// resources related to the account, such as orders or authorizations.
+//
+// It works only with RFC8555 compliant CAs.
+func (c *Client) DeactivateReg(ctx context.Context) error {
+	url := string(c.accountKID(ctx))
+	if url == "" {
+		return ErrNoAccount
+	}
+	req := json.RawMessage(`{"status": "deactivated"}`)
+	res, err := c.post(ctx, nil, url, req, wantStatus(http.StatusOK))
+	if err != nil {
+		return err
+	}
+	res.Body.Close()
+	return nil
+}
+
+// registerRFC is quivalent to c.Register but for RFC-compliant CAs.
+// It expects c.Discover to have already been called.
+// TODO: Implement externalAccountBinding.
+func (c *Client) registerRFC(ctx context.Context, acct *Account, prompt func(tosURL string) bool) (*Account, error) {
+	c.cacheMu.Lock() // guard c.kid access
+	defer c.cacheMu.Unlock()
+
+	req := struct {
+		TermsAgreed bool     `json:"termsOfServiceAgreed,omitempty"`
+		Contact     []string `json:"contact,omitempty"`
+	}{
+		Contact: acct.Contact,
+	}
+	if c.dir.Terms != "" {
+		req.TermsAgreed = prompt(c.dir.Terms)
+	}
+	res, err := c.post(ctx, c.Key, c.dir.RegURL, req, wantStatus(
+		http.StatusOK,      // account with this key already registered
+		http.StatusCreated, // new account created
+	))
+	if err != nil {
+		return nil, err
+	}
+
+	defer res.Body.Close()
+	a, err := responseAccount(res)
+	if err != nil {
+		return nil, err
+	}
+	// Cache Account URL even if we return an error to the caller.
+	// It is by all means a valid and usable "kid" value for future requests.
+	c.kid = keyID(a.URI)
+	if res.StatusCode == http.StatusOK {
+		return nil, ErrAccountAlreadyExists
+	}
+	return a, nil
+}
+
+// updateGegRFC is equivalent to c.UpdateReg but for RFC-compliant CAs.
+// It expects c.Discover to have already been called.
+func (c *Client) updateRegRFC(ctx context.Context, a *Account) (*Account, error) {
+	url := string(c.accountKID(ctx))
+	if url == "" {
+		return nil, ErrNoAccount
+	}
+	req := struct {
+		Contact []string `json:"contact,omitempty"`
+	}{
+		Contact: a.Contact,
+	}
+	res, err := c.post(ctx, nil, url, req, wantStatus(http.StatusOK))
+	if err != nil {
+		return nil, err
+	}
+	defer res.Body.Close()
+	return responseAccount(res)
+}
+
+// getGegRFC is equivalent to c.GetReg but for RFC-compliant CAs.
+// It expects c.Discover to have already been called.
+func (c *Client) getRegRFC(ctx context.Context) (*Account, error) {
+	req := json.RawMessage(`{"onlyReturnExisting": true}`)
+	res, err := c.post(ctx, c.Key, c.dir.RegURL, req, wantStatus(http.StatusOK))
+	if e, ok := err.(*Error); ok && e.ProblemType == "urn:ietf:params:acme:error:accountDoesNotExist" {
+		return nil, ErrNoAccount
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	defer res.Body.Close()
+	return responseAccount(res)
+}
+
+func responseAccount(res *http.Response) (*Account, error) {
+	var v struct {
+		Status  string
+		Contact []string
+		Orders  string
+	}
+	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
+		return nil, fmt.Errorf("acme: invalid response: %v", err)
+	}
+	return &Account{
+		URI:       res.Header.Get("Location"),
+		Status:    v.Status,
+		Contact:   v.Contact,
+		OrdersURL: v.Orders,
+	}, nil
+}
diff --git a/acme/rfc8555_test.go b/acme/rfc8555_test.go
index 4a8d9f5..c366380 100644
--- a/acme/rfc8555_test.go
+++ b/acme/rfc8555_test.go
@@ -5,11 +5,17 @@
 package acme
 
 import (
+	"bytes"
 	"context"
+	"encoding/json"
 	"fmt"
+	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
+	"reflect"
+	"sync"
 	"testing"
+	"time"
 )
 
 // While contents of this file is pertinent only to RFC8555,
@@ -121,3 +127,350 @@
 		t.Error("last cl.popNonce returned nil error")
 	}
 }
+
+func TestRFC_postKID(t *testing.T) {
+	var ts *httptest.Server
+	ts = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/new-nonce":
+			w.Header().Set("Replay-Nonce", "nonce")
+		case "/new-account":
+			w.Header().Set("Location", "/account-1")
+			w.Write([]byte(`{"status":"valid"}`))
+		case "/post":
+			b, _ := ioutil.ReadAll(r.Body) // check err later in decodeJWSxxx
+			head, err := decodeJWSHead(bytes.NewReader(b))
+			if err != nil {
+				t.Errorf("decodeJWSHead: %v", err)
+				return
+			}
+			if head.KID != "/account-1" {
+				t.Errorf("head.KID = %q; want /account-1", head.KID)
+			}
+			if len(head.JWK) != 0 {
+				t.Errorf("head.JWK = %q; want zero map", head.JWK)
+			}
+			if v := ts.URL + "/post"; head.URL != v {
+				t.Errorf("head.URL = %q; want %q", head.URL, v)
+			}
+
+			var payload struct{ Msg string }
+			decodeJWSRequest(t, &payload, bytes.NewReader(b))
+			if payload.Msg != "ping" {
+				t.Errorf("payload.Msg = %q; want ping", payload.Msg)
+			}
+			w.Write([]byte("pong"))
+		default:
+			t.Errorf("unhandled %s %s", r.Method, r.URL)
+			w.WriteHeader(http.StatusBadRequest)
+		}
+	}))
+	defer ts.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	cl := &Client{
+		Key:          testKey,
+		DirectoryURL: ts.URL,
+		dir: &Directory{
+			NonceURL: ts.URL + "/new-nonce",
+			RegURL:   ts.URL + "/new-account",
+			OrderURL: "/force-rfc-mode",
+		},
+	}
+	req := json.RawMessage(`{"msg":"ping"}`)
+	res, err := cl.post(ctx, nil /* use kid */, ts.URL+"/post", req, wantStatus(http.StatusOK))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	b, _ := ioutil.ReadAll(res.Body) // don't care about err - just checking b
+	if string(b) != "pong" {
+		t.Errorf("res.Body = %q; want pong", b)
+	}
+}
+
+// acmeServer simulates a subset of RFC8555 compliant CA.
+//
+// TODO: We also have x/crypto/acme/autocert/acmetest and startACMEServerStub in autocert_test.go.
+// It feels like this acmeServer is a sweet spot between usefulness and added complexity.
+// Also, acmetest and startACMEServerStub were both written for draft-02, no RFC support.
+// The goal is to consolidate all into one ACME test server.
+type acmeServer struct {
+	ts      *httptest.Server
+	handler map[string]http.HandlerFunc // keyed by r.URL.Path
+
+	mu     sync.Mutex
+	nnonce int
+}
+
+func newACMEServer() *acmeServer {
+	return &acmeServer{handler: make(map[string]http.HandlerFunc)}
+}
+
+func (s *acmeServer) handle(path string, f func(http.ResponseWriter, *http.Request)) {
+	s.handler[path] = http.HandlerFunc(f)
+}
+
+func (s *acmeServer) start() {
+	s.ts = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+
+		// Directory request.
+		if r.URL.Path == "/" {
+			fmt.Fprintf(w, `{
+				"newNonce": %q,
+				"newAccount": %q,
+				"newOrder": %q,
+				"newAuthz": %q,
+				"revokeCert": %q,
+				"meta": {"termsOfService": %q}
+				}`,
+				s.url("/acme/new-nonce"),
+				s.url("/acme/new-account"),
+				s.url("/acme/new-order"),
+				s.url("/acme/new-authz"),
+				s.url("/acme/revoke-cert"),
+				s.url("/terms"),
+			)
+			return
+		}
+
+		// All other responses contain a nonce value unconditionally.
+		w.Header().Set("Replay-Nonce", s.nonce())
+		if r.URL.Path == "/acme/new-nonce" {
+			return
+		}
+
+		h := s.handler[r.URL.Path]
+		if h == nil {
+			w.WriteHeader(http.StatusBadRequest)
+			fmt.Fprintf(w, "Unhandled %s", r.URL.Path)
+			return
+		}
+		h.ServeHTTP(w, r)
+	}))
+}
+
+func (s *acmeServer) close() {
+	s.ts.Close()
+}
+
+func (s *acmeServer) url(path string) string {
+	return s.ts.URL + path
+}
+
+func (s *acmeServer) nonce() string {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.nnonce++
+	return fmt.Sprintf("nonce%d", s.nnonce)
+}
+
+func (s *acmeServer) error(w http.ResponseWriter, e *wireError) {
+	w.WriteHeader(e.Status)
+	json.NewEncoder(w).Encode(e)
+}
+
+func TestRFC_Register(t *testing.T) {
+	const email = "mailto:user@example.org"
+
+	s := newACMEServer()
+	s.handle("/acme/new-account", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Location", s.url("/accounts/1"))
+		w.WriteHeader(http.StatusCreated) // 201 means new account created
+		fmt.Fprintf(w, `{
+			"status": "valid",
+			"contact": [%q],
+			"orders": %q
+		}`, email, s.url("/accounts/1/orders"))
+
+		b, _ := ioutil.ReadAll(r.Body) // check err later in decodeJWSxxx
+		head, err := decodeJWSHead(bytes.NewReader(b))
+		if err != nil {
+			t.Errorf("decodeJWSHead: %v", err)
+			return
+		}
+		if len(head.JWK) == 0 {
+			t.Error("head.JWK is empty")
+		}
+
+		var req struct{ Contact []string }
+		decodeJWSRequest(t, &req, bytes.NewReader(b))
+		if len(req.Contact) != 1 || req.Contact[0] != email {
+			t.Errorf("req.Contact = %q; want [%q]", req.Contact, email)
+		}
+	})
+	s.start()
+	defer s.close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	cl := &Client{
+		Key:          testKeyEC,
+		DirectoryURL: s.url("/"),
+	}
+
+	var didPrompt bool
+	a := &Account{Contact: []string{email}}
+	acct, err := cl.Register(ctx, a, func(tos string) bool {
+		didPrompt = true
+		terms := s.url("/terms")
+		if tos != terms {
+			t.Errorf("tos = %q; want %q", tos, terms)
+		}
+		return true
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	okAccount := &Account{
+		URI:       s.url("/accounts/1"),
+		Status:    StatusValid,
+		Contact:   []string{email},
+		OrdersURL: s.url("/accounts/1/orders"),
+	}
+	if !reflect.DeepEqual(acct, okAccount) {
+		t.Errorf("acct = %+v; want %+v", acct, okAccount)
+	}
+	if !didPrompt {
+		t.Error("tos prompt wasn't called")
+	}
+	if v := cl.accountKID(ctx); v != keyID(okAccount.URI) {
+		t.Errorf("account kid = %q; want %q", v, okAccount.URI)
+	}
+}
+
+func TestRFC_RegisterExisting(t *testing.T) {
+	s := newACMEServer()
+	s.handle("/acme/new-account", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Location", s.url("/accounts/1"))
+		w.WriteHeader(http.StatusOK) // 200 means account already exists
+		w.Write([]byte(`{"status": "valid"}`))
+	})
+	s.start()
+	defer s.close()
+
+	cl := &Client{Key: testKeyEC, DirectoryURL: s.url("/")}
+	_, err := cl.Register(context.Background(), &Account{}, AcceptTOS)
+	if err != ErrAccountAlreadyExists {
+		t.Errorf("err = %v; want %v", err, ErrAccountAlreadyExists)
+	}
+	kid := keyID(s.url("/accounts/1"))
+	if v := cl.accountKID(context.Background()); v != kid {
+		t.Errorf("account kid = %q; want %q", v, kid)
+	}
+}
+
+func TestRFC_UpdateReg(t *testing.T) {
+	const email = "mailto:user@example.org"
+
+	s := newACMEServer()
+	s.handle("/acme/new-account", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Location", s.url("/accounts/1"))
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"status": "valid"}`))
+	})
+	var didUpdate bool
+	s.handle("/accounts/1", func(w http.ResponseWriter, r *http.Request) {
+		didUpdate = true
+		w.Header().Set("Location", s.url("/accounts/1"))
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"status": "valid"}`))
+
+		b, _ := ioutil.ReadAll(r.Body) // check err later in decodeJWSxxx
+		head, err := decodeJWSHead(bytes.NewReader(b))
+		if err != nil {
+			t.Errorf("decodeJWSHead: %v", err)
+			return
+		}
+		if len(head.JWK) != 0 {
+			t.Error("head.JWK is non-zero")
+		}
+		kid := s.url("/accounts/1")
+		if head.KID != kid {
+			t.Errorf("head.KID = %q; want %q", head.KID, kid)
+		}
+
+		var req struct{ Contact []string }
+		decodeJWSRequest(t, &req, bytes.NewReader(b))
+		if len(req.Contact) != 1 || req.Contact[0] != email {
+			t.Errorf("req.Contact = %q; want [%q]", req.Contact, email)
+		}
+	})
+	s.start()
+	defer s.close()
+
+	cl := &Client{Key: testKeyEC, DirectoryURL: s.url("/")}
+	_, err := cl.UpdateReg(context.Background(), &Account{Contact: []string{email}})
+	if err != nil {
+		t.Error(err)
+	}
+	if !didUpdate {
+		t.Error("UpdateReg didn't update the account")
+	}
+}
+
+func TestRFC_GetReg(t *testing.T) {
+	s := newACMEServer()
+	s.handle("/acme/new-account", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Location", s.url("/accounts/1"))
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"status": "valid"}`))
+
+		head, err := decodeJWSHead(r.Body)
+		if err != nil {
+			t.Errorf("decodeJWSHead: %v", err)
+			return
+		}
+		if len(head.JWK) == 0 {
+			t.Error("head.JWK is empty")
+		}
+	})
+	s.start()
+	defer s.close()
+
+	cl := &Client{Key: testKeyEC, DirectoryURL: s.url("/")}
+	acct, err := cl.GetReg(context.Background(), "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	okAccount := &Account{
+		URI:    s.url("/accounts/1"),
+		Status: StatusValid,
+	}
+	if !reflect.DeepEqual(acct, okAccount) {
+		t.Errorf("acct = %+v; want %+v", acct, okAccount)
+	}
+}
+
+func TestRFC_GetRegNoAccount(t *testing.T) {
+	s := newACMEServer()
+	s.handle("/acme/new-account", func(w http.ResponseWriter, r *http.Request) {
+		s.error(w, &wireError{
+			Status: http.StatusBadRequest,
+			Type:   "urn:ietf:params:acme:error:accountDoesNotExist",
+		})
+	})
+	s.start()
+	defer s.close()
+
+	cl := &Client{Key: testKeyEC, DirectoryURL: s.url("/")}
+	if _, err := cl.GetReg(context.Background(), ""); err != ErrNoAccount {
+		t.Errorf("err = %v; want %v", err, ErrNoAccount)
+	}
+}
+
+func TestRFC_GetRegOtherError(t *testing.T) {
+	s := newACMEServer()
+	s.handle("/acme/new-account", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+	})
+	s.start()
+	defer s.close()
+
+	cl := &Client{Key: testKeyEC, DirectoryURL: s.url("/")}
+	if _, err := cl.GetReg(context.Background(), ""); err == nil || err == ErrNoAccount {
+		t.Errorf("GetReg: %v; want any other non-nil err", err)
+	}
+}
diff --git a/acme/types.go b/acme/types.go
index a411487..4432afb 100644
--- a/acme/types.go
+++ b/acme/types.go
@@ -16,12 +16,13 @@
 
 // ACME server response statuses used to describe Authorization and Challenge states.
 const (
-	StatusUnknown    = "unknown"
-	StatusPending    = "pending"
-	StatusProcessing = "processing"
-	StatusValid      = "valid"
-	StatusInvalid    = "invalid"
-	StatusRevoked    = "revoked"
+	StatusDeactivated = "deactivated"
+	StatusInvalid     = "invalid"
+	StatusPending     = "pending"
+	StatusProcessing  = "processing"
+	StatusRevoked     = "revoked"
+	StatusUnknown     = "unknown"
+	StatusValid       = "valid"
 )
 
 // CRLReasonCode identifies the reason for a certificate revocation.
@@ -41,8 +42,17 @@
 	CRLReasonAACompromise         CRLReasonCode = 10
 )
 
-// ErrUnsupportedKey is returned when an unsupported key type is encountered.
-var ErrUnsupportedKey = errors.New("acme: unknown key type; only RSA and ECDSA are supported")
+var (
+	// ErrUnsupportedKey is returned when an unsupported key type is encountered.
+	ErrUnsupportedKey = errors.New("acme: unknown key type; only RSA and ECDSA are supported")
+
+	// ErrAccountAlreadyExists indicates that the Client's key has already been registered
+	// with the CA. It is returned by Register method.
+	ErrAccountAlreadyExists = errors.New("acme: account already exists")
+
+	// ErrNoAccount indicates that the Client's key has not been registered with the CA.
+	ErrNoAccount = errors.New("acme: account does not exist")
+)
 
 // Error is an ACME error, defined in Problem Details for HTTP APIs doc
 // http://tools.ietf.org/html/draft-ietf-appsawg-http-problem.
@@ -54,6 +64,12 @@
 	ProblemType string
 	// Detail is a human-readable explanation specific to this occurrence of the problem.
 	Detail string
+	// Instance indicates a URL that the client should direct a human user to visit
+	// in order for instructions on how to agree to the updated Terms of Service.
+	// In such an event CA sets StatusCode to 403, ProblemType to
+	// "urn:ietf:params:acme:error:userActionRequired" and a Link header with relation
+	// "terms-of-service" containing the latest TOS URL.
+	Instance string
 	// Header is the original server error response headers.
 	// It may be nil.
 	Header http.Header
@@ -108,31 +124,57 @@
 }
 
 // Account is a user account. It is associated with a private key.
+// Non-RFC8555 fields are empty when interfacing with a compliant CA.
 type Account struct {
 	// URI is the account unique ID, which is also a URL used to retrieve
 	// account data from the CA.
+	// When interfacing with RFC8555-compliant CAs, URI is the "kid" field
+	// value in JWS signed requests.
 	URI string
 
 	// Contact is a slice of contact info used during registration.
+	// See https://tools.ietf.org/html/rfc8555#section-7.3 for supported
+	// formats.
 	Contact []string
 
+	// Status indicates current account status as returned by the CA.
+	// Possible values are "valid", "deactivated", and "revoked".
+	Status string
+
+	// OrdersURL is a URL from which a list of orders submitted by this account
+	// can be fetched.
+	OrdersURL string
+
 	// The terms user has agreed to.
 	// A value not matching CurrentTerms indicates that the user hasn't agreed
 	// to the actual Terms of Service of the CA.
+	//
+	// It is non-RFC8555 compliant. Package users can store the ToS they agree to
+	// during Client's Register call in the prompt callback function.
 	AgreedTerms string
 
 	// Actual terms of a CA.
+	//
+	// It is non-RFC8555 compliant. Use Directory's Terms field.
+	// When a CA updates their terms and requires an account agreement,
+	// a URL at which instructions to do so is available in Error's Instance field.
 	CurrentTerms string
 
 	// Authz is the authorization URL used to initiate a new authz flow.
+	//
+	// It is non-RFC8555 compliant. Use Directory's AuthzURL or OrderURL.
 	Authz string
 
 	// Authorizations is a URI from which a list of authorizations
 	// granted to this account can be fetched via a GET request.
+	//
+	// It is non-RFC8555 compliant and is obsoleted by OrdersURL.
 	Authorizations string
 
 	// Certificates is a URI from which a list of certificates
 	// issued for this account can be fetched via a GET request.
+	//
+	// It is non-RFC8555 compliant and is obsoleted by OrdersURL.
 	Certificates string
 }
 
@@ -142,8 +184,8 @@
 	// NonceURL indicates an endpoint where to fetch fresh nonce values from.
 	NonceURL string
 
-	// RegURL is an account endpoint URL, allowing for creating new
-	// and modifying existing accounts.
+	// RegURL is an account endpoint URL, allowing for creating new accounts.
+	// Pre-RFC8555 CAs also allow modifying existing accounts at this URL.
 	RegURL string
 
 	// OrderURL is used to initiate the certificate issuance flow
@@ -299,9 +341,10 @@
 // wireError is a subset of fields of the Problem Details object
 // as described in https://tools.ietf.org/html/rfc7807#section-3.1.
 type wireError struct {
-	Status int
-	Type   string
-	Detail string
+	Status   int
+	Type     string
+	Detail   string
+	Instance string
 }
 
 func (e *wireError) error(h http.Header) *Error {
@@ -309,6 +352,7 @@
 		StatusCode:  e.Status,
 		ProblemType: e.Type,
 		Detail:      e.Detail,
+		Instance:    e.Instance,
 		Header:      h,
 	}
 }