diff --git a/acme/jws.go b/acme/jws.go
index 04f509f..8c3ecce 100644
--- a/acme/jws.go
+++ b/acme/jws.go
@@ -11,27 +11,15 @@
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/sha256"
-	"crypto/sha512"
 	_ "crypto/sha512" // need for EC keys
 	"encoding/asn1"
 	"encoding/base64"
 	"encoding/json"
 	"errors"
 	"fmt"
-	"hash"
 	"math/big"
 )
 
-// MACAlgorithm represents a JWS MAC signature algorithm.
-// See https://tools.ietf.org/html/rfc7518#section-3.1 for more details.
-type MACAlgorithm string
-
-const (
-	MACAlgorithmHS256 = MACAlgorithm("HS256")
-	MACAlgorithmHS384 = MACAlgorithm("HS384")
-	MACAlgorithmHS512 = MACAlgorithm("HS512")
-)
-
 // keyID is the account identity provided by a CA during registration.
 type keyID string
 
@@ -101,24 +89,35 @@
 	return json.Marshal(&enc)
 }
 
-// jwsWithMAC creates and signs a JWS using the given key and algorithm.
-// "rawProtected" and "rawPayload" should not be base64-URL-encoded.
-func jwsWithMAC(key []byte, alg MACAlgorithm, rawProtected, rawPayload []byte) (*jsonWebSignature, error) {
+// jwsWithMAC creates and signs a JWS using the given key and the HS256
+// algorithm. kid and url are included in the protected header. rawPayload
+// should not be base64-URL-encoded.
+func jwsWithMAC(key []byte, kid, url string, rawPayload []byte) (*jsonWebSignature, error) {
 	if len(key) == 0 {
 		return nil, errors.New("acme: cannot sign JWS with an empty MAC key")
 	}
+	header := struct {
+		Algorithm string `json:"alg"`
+		KID       string `json:"kid"`
+		URL       string `json:"url,omitempty"`
+	}{
+		// Only HMAC-SHA256 is supported.
+		Algorithm: "HS256",
+		KID:       kid,
+		URL:       url,
+	}
+	rawProtected, err := json.Marshal(header)
+	if err != nil {
+		return nil, err
+	}
 	protected := base64.RawURLEncoding.EncodeToString(rawProtected)
 	payload := base64.RawURLEncoding.EncodeToString(rawPayload)
 
-	// Only HMACs are currently supported.
-	hmac, err := newHMAC(key, alg)
-	if err != nil {
+	h := hmac.New(sha256.New, key)
+	if _, err := h.Write([]byte(protected + "." + payload)); err != nil {
 		return nil, err
 	}
-	if _, err := hmac.Write([]byte(protected + "." + payload)); err != nil {
-		return nil, err
-	}
-	mac := hmac.Sum(nil)
+	mac := h.Sum(nil)
 
 	return &jsonWebSignature{
 		Protected: protected,
@@ -218,20 +217,6 @@
 	return "", 0
 }
 
-// newHMAC returns an appropriate HMAC for the given MACAlgorithm.
-func newHMAC(key []byte, alg MACAlgorithm) (hash.Hash, error) {
-	switch alg {
-	case MACAlgorithmHS256:
-		return hmac.New(sha256.New, key), nil
-	case MACAlgorithmHS384:
-		return hmac.New(sha512.New384, key), nil
-	case MACAlgorithmHS512:
-		return hmac.New(sha512.New, key), nil
-	default:
-		return nil, fmt.Errorf("acme: unsupported MAC algorithm: %v", alg)
-	}
-}
-
 // JWKThumbprint creates a JWK thumbprint out of pub
 // as specified in https://tools.ietf.org/html/rfc7638.
 func JWKThumbprint(pub crypto.PublicKey) (string, error) {
diff --git a/acme/jws_test.go b/acme/jws_test.go
index c6e3230..c8a1e8b 100644
--- a/acme/jws_test.go
+++ b/acme/jws_test.go
@@ -397,8 +397,6 @@
 	// Example from RFC 7520 Section 4.4.3.
 	// https://tools.ietf.org/html/rfc7520#section-4.4.3
 	b64Key := "hJtXIZ2uSN5kbQfbtTNWbpdmhkV8FJG-Onbc6mxCcYg"
-	alg := MACAlgorithmHS256
-	rawProtected := []byte(`{"alg":"HS256","kid":"018c0ae5-4d9b-471b-bfd6-eef314bc7037"}`)
 	rawPayload := []byte("It\xe2\x80\x99s a dangerous business, Frodo, going out your " +
 		"door. You step onto the road, and if you don't keep your feet, " +
 		"there\xe2\x80\x99s no knowing where you might be swept off " +
@@ -416,7 +414,7 @@
 	if err != nil {
 		t.Fatalf("unable to decode key: %q", b64Key)
 	}
-	got, err := jwsWithMAC(key, alg, rawProtected, rawPayload)
+	got, err := jwsWithMAC(key, "018c0ae5-4d9b-471b-bfd6-eef314bc7037", "", rawPayload)
 	if err != nil {
 		t.Fatalf("jwsWithMAC() = %q", err)
 	}
@@ -432,22 +430,9 @@
 }
 
 func TestJWSWithMACError(t *testing.T) {
-	tt := []struct {
-		desc string
-		alg  MACAlgorithm
-		key  []byte
-	}{
-		{"Unknown Algorithm", MACAlgorithm("UNKNOWN-ALG"), []byte("hmac-key")},
-		{"Empty Key", MACAlgorithmHS256, nil},
-	}
-	for _, tc := range tt {
-		tc := tc
-		t.Run(string(tc.desc), func(t *testing.T) {
-			p := "{}"
-			if _, err := jwsWithMAC(tc.key, tc.alg, []byte(p), []byte(p)); err == nil {
-				t.Errorf("jwsWithMAC(%v, %v, %s, %s) = success; want err", tc.key, tc.alg, p, p)
-			}
-		})
+	p := "{}"
+	if _, err := jwsWithMAC(nil, "", "", []byte(p)); err == nil {
+		t.Errorf("jwsWithMAC(nil, ...) = success; want err")
 	}
 }
 
@@ -525,33 +510,3 @@
 		t.Errorf("err = %q; want %q", err, ErrUnsupportedKey)
 	}
 }
-
-func TestNewHMAC(t *testing.T) {
-	tt := []struct {
-		alg      MACAlgorithm
-		wantSize int
-	}{
-		{MACAlgorithmHS256, 32},
-		{MACAlgorithmHS384, 48},
-		{MACAlgorithmHS512, 64},
-	}
-	for _, tc := range tt {
-		tc := tc
-		t.Run(string(tc.alg), func(t *testing.T) {
-			h, err := newHMAC([]byte("key"), tc.alg)
-			if err != nil {
-				t.Fatalf("newHMAC(%v) = %q", tc.alg, err)
-			}
-			gotSize := len(h.Sum(nil))
-			if gotSize != tc.wantSize {
-				t.Errorf("HMAC produced signature with unexpected length; got %d want %d", gotSize, tc.wantSize)
-			}
-		})
-	}
-}
-
-func TestNewHMACError(t *testing.T) {
-	if h, err := newHMAC([]byte("key"), MACAlgorithm("UNKNOWN-ALG")); err == nil {
-		t.Errorf("newHMAC(UNKNOWN-ALG) = %T, nil; want error", h)
-	}
-}
diff --git a/acme/rfc8555.go b/acme/rfc8555.go
index ceb239d..073cee5 100644
--- a/acme/rfc8555.go
+++ b/acme/rfc8555.go
@@ -5,7 +5,6 @@
 package acme
 
 import (
-	"bytes"
 	"context"
 	"crypto"
 	"encoding/base64"
@@ -93,9 +92,7 @@
 	if err != nil {
 		return nil, err
 	}
-	var rProtected bytes.Buffer
-	fmt.Fprintf(&rProtected, `{"alg":%q,"kid":%q,"url":%q}`, eab.Algorithm, eab.KID, c.dir.RegURL)
-	return jwsWithMAC(eab.Key, eab.Algorithm, rProtected.Bytes(), []byte(jwk))
+	return jwsWithMAC(eab.Key, eab.KID, c.dir.RegURL, []byte(jwk))
 }
 
 // updateRegRFC is equivalent to c.UpdateReg but for CAs implementing RFC 8555.
diff --git a/acme/rfc8555_test.go b/acme/rfc8555_test.go
index b985b3e..c35118e 100644
--- a/acme/rfc8555_test.go
+++ b/acme/rfc8555_test.go
@@ -351,9 +351,8 @@
 
 func TestRFC_RegisterExternalAccountBinding(t *testing.T) {
 	eab := &ExternalAccountBinding{
-		KID:       "kid-1",
-		Key:       []byte("secret"),
-		Algorithm: MACAlgorithmHS256,
+		KID: "kid-1",
+		Key: []byte("secret"),
 	}
 
 	type protected struct {
@@ -403,10 +402,10 @@
 		if prot.KID != eab.KID {
 			t.Errorf("j.ExternalAccountBinding.KID = %s; want %s", prot.KID, eab.KID)
 		}
-		// Ensure same Algorithm.
-		if prot.Algorithm != string(eab.Algorithm) {
+		// Ensure expected Algorithm.
+		if prot.Algorithm != "HS256" {
 			t.Errorf("j.ExternalAccountBinding.Alg = %s; want %s",
-				prot.Algorithm, eab.Algorithm)
+				prot.Algorithm, "HS256")
 		}
 
 		// Ensure same URL as outer JWS.
diff --git a/acme/types.go b/acme/types.go
index 4d89fed..e751bf5 100644
--- a/acme/types.go
+++ b/acme/types.go
@@ -217,13 +217,10 @@
 	// Key is the bytes of the symmetric key that the CA provides to identify
 	// the account. Key must correspond to the KID.
 	Key []byte
-
-	// Algorithm used to sign the JWS.
-	Algorithm MACAlgorithm
 }
 
 func (e *ExternalAccountBinding) String() string {
-	return fmt.Sprintf("&{KID: %q, Key: redacted, Algorithm: %v}", e.KID, e.Algorithm)
+	return fmt.Sprintf("&{KID: %q, Key: redacted}", e.KID)
 }
 
 // Directory is ACME server discovery data.
diff --git a/acme/types_test.go b/acme/types_test.go
index 25fa8b2..9ed7753 100644
--- a/acme/types_test.go
+++ b/acme/types_test.go
@@ -13,12 +13,11 @@
 
 func TestExternalAccountBindingString(t *testing.T) {
 	eab := ExternalAccountBinding{
-		KID:       "kid",
-		Key:       []byte("key"),
-		Algorithm: MACAlgorithmHS256,
+		KID: "kid",
+		Key: []byte("key"),
 	}
 	got := eab.String()
-	want := `&{KID: "kid", Key: redacted, Algorithm: HS256}`
+	want := `&{KID: "kid", Key: redacted}`
 	if got != want {
 		t.Errorf("eab.String() = %q, want: %q", got, want)
 	}