diff --git a/authority/provisioner/gcp.go b/authority/provisioner/gcp.go
index c4390f03..dd0fc185 100644
--- a/authority/provisioner/gcp.go
+++ b/authority/provisioner/gcp.go
@@ -16,6 +16,12 @@ import (
 	"github.com/smallstep/cli/jose"
 )
 
+// googleOauth2Certs is the url that servers Google OAuth2 public keys.
+var googleOauth2Certs = "https://www.googleapis.com/oauth2/v3/certs"
+
+// gcpIdentityURL is the base url for the identity document in GCP.
+var gcpIdentityURL = "http://metadata/computeMetadata/v1/instance/service-accounts/default/identity"
+
 // gcpPayload extends jwt.Claims with custom GCP attributes.
 type gcpPayload struct {
 	jose.Claims
@@ -69,8 +75,11 @@ func (p *GCP) GetTokenID(token string) (string, error) {
 	if err = jwt.UnsafeClaimsWithoutVerification(&claims); err != nil {
 		return "", errors.Wrap(err, "error verifying claims")
 	}
-
-	unique := fmt.Sprintf("%s.%d.%d", claims.Google.ComputeEngine.InstanceID, claims.IssuedAt, claims.Expiry)
+	// This string should be mostly unique
+	unique := fmt.Sprintf("%s.%s.%d.%d",
+		p.GetID(), claims.Google.ComputeEngine.InstanceID,
+		*claims.IssuedAt, *claims.Expiry,
+	)
 	sum := sha256.Sum256([]byte(unique))
 	return strings.ToLower(hex.EncodeToString(sum[:])), nil
 }
@@ -90,6 +99,37 @@ func (p *GCP) GetEncryptedKey() (kid string, key string, ok bool) {
 	return "", "", false
 }
 
+// GetIdentityURL returns the url that generates the GCP token.
+func (p *GCP) GetIdentityURL() string {
+	q := url.Values{}
+	q.Add("audience", p.GetID())
+	q.Add("format", "full")
+	q.Add("licenses", "FALSE")
+	return fmt.Sprintf("%s?%s", gcpIdentityURL, q.Encode())
+}
+
+// GetIdentityToken does an HTTP request to the identity url.
+func (p *GCP) GetIdentityToken() (string, error) {
+	req, err := http.NewRequest("GET", p.GetIdentityURL(), http.NoBody)
+	if err != nil {
+		return "", errors.Wrap(err, "error creating identity request")
+	}
+	req.Header.Set("Metadata-Flavor", "Google")
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", errors.Wrap(err, "error doing identity request, are you in a GCP VM?")
+	}
+	defer resp.Body.Close()
+	b, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return "", errors.Wrap(err, "error reading identity request response")
+	}
+	if resp.StatusCode >= 400 {
+		return "", errors.Errorf("error on identity response: status=%d, response=%s", resp.StatusCode, b)
+	}
+	return string(bytes.TrimSpace(b)), nil
+}
+
 // Init validates and initializes the GCP provider.
 func (p *GCP) Init(config Config) error {
 	var err error
@@ -104,7 +144,7 @@ func (p *GCP) Init(config Config) error {
 		return err
 	}
 	// Initialize key store
-	p.keyStore, err = newKeyStore("https://www.googleapis.com/oauth2/v3/certs")
+	p.keyStore, err = newKeyStore(googleOauth2Certs)
 	if err != nil {
 		return err
 	}
@@ -149,31 +189,6 @@ func (p *GCP) AuthorizeRevoke(token string) error {
 	return err
 }
 
-// GetIdentityURL returns the url that generates the GCP token.
-func (p *GCP) GetIdentityURL() string {
-	audience := url.QueryEscape(p.GetID())
-	return fmt.Sprintf("http://metadata/computeMetadata/v1/instance/service-accounts/default/identity?audience=%s&format=full&licenses=FALSE", audience)
-}
-
-// GetIdentityToken does an HTTP request to the identity url.
-func (p *GCP) GetIdentityToken() (string, error) {
-	req, err := http.NewRequest("GET", p.GetIdentityURL(), http.NoBody)
-	if err != nil {
-		return "", errors.Wrap(err, "error creating identity request")
-	}
-	req.Header.Set("Metadata-Flavor", "Google")
-	resp, err := http.DefaultClient.Do(req)
-	if err != nil {
-		return "", errors.Wrap(err, "error doing identity request, are you in a GCP VM?")
-	}
-	defer resp.Body.Close()
-	b, err := ioutil.ReadAll(resp.Body)
-	if err != nil {
-		return "", errors.Wrap(err, "error reading identity request response")
-	}
-	return string(bytes.TrimSpace(b)), nil
-}
-
 // authorizeToken performs common jwt authorization actions and returns the
 // claims for case specific downstream parsing.
 // e.g. a Sign request will auth/validate different fields than a Revoke request.
@@ -191,42 +206,44 @@ func (p *GCP) authorizeToken(token string) (*gcpPayload, error) {
 	kid := jwt.Headers[0].KeyID
 	keys := p.keyStore.Get(kid)
 	for _, key := range keys {
-		if err := jwt.Claims(key, &claims); err == nil {
+		if err := jwt.Claims(key.Public(), &claims); err == nil {
 			found = true
 			break
 		}
 	}
 	if !found {
-		return nil, errors.Errorf("failed to validate payload: cannot find certificate for kid %s", kid)
+		return nil, errors.Errorf("failed to validate payload: cannot find key for kid %s", kid)
 	}
 
 	// According to "rfc7519 JSON Web Token" acceptable skew should be no
 	// more than a few minutes.
 	if err = claims.ValidateWithLeeway(jose.Expected{
 		Issuer:   "https://accounts.google.com",
-		Time:     time.Now().UTC(),
 		Audience: []string{p.GetID()},
+		Time:     time.Now().UTC(),
 	}, time.Minute); err != nil {
 		return nil, errors.Wrapf(err, "invalid token")
 	}
 
-	// validate authorized party
+	// validate subject (service account)
 	if len(p.ServiceAccounts) > 0 {
 		var found bool
 		for _, sa := range p.ServiceAccounts {
-			if sa == claims.AuthorizedParty {
+			if sa == claims.Subject {
 				found = true
 				break
 			}
 		}
 		if !found {
-			return nil, errors.New("invalid token: invalid authorized party claim (azp)")
+			return nil, errors.New("invalid token: invalid subject claim")
 		}
 	}
 
 	switch {
 	case claims.Google.ComputeEngine.InstanceID == "":
 		return nil, errors.New("token google.compute_engine.instance_id cannot be empty")
+	case claims.Google.ComputeEngine.InstanceName == "":
+		return nil, errors.New("token google.compute_engine.instance_name cannot be empty")
 	case claims.Google.ComputeEngine.ProjectID == "":
 		return nil, errors.New("token google.compute_engine.project_id cannot be empty")
 	case claims.Google.ComputeEngine.Zone == "":
diff --git a/authority/provisioner/gcp_test.go b/authority/provisioner/gcp_test.go
new file mode 100644
index 00000000..aa4748a1
--- /dev/null
+++ b/authority/provisioner/gcp_test.go
@@ -0,0 +1,352 @@
+package provisioner
+
+import (
+	"crypto/sha256"
+	"crypto/x509"
+	"encoding/hex"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/smallstep/assert"
+)
+
+func resetGoogleVars() {
+	googleOauth2Certs = "https://www.googleapis.com/oauth2/v3/certs"
+	gcpIdentityURL = "http://metadata/computeMetadata/v1/instance/service-accounts/default/identity"
+}
+
+func TestGCP_Getters(t *testing.T) {
+	p, err := generateGCP()
+	assert.FatalError(t, err)
+	aud := "gcp:" + p.Name
+	if got := p.GetID(); got != aud {
+		t.Errorf("GCP.GetID() = %v, want %v", got, aud)
+	}
+	if got := p.GetName(); got != p.Name {
+		t.Errorf("GCP.GetName() = %v, want %v", got, p.Name)
+	}
+	if got := p.GetType(); got != TypeGCP {
+		t.Errorf("GCP.GetType() = %v, want %v", got, TypeGCP)
+	}
+	kid, key, ok := p.GetEncryptedKey()
+	if kid != "" || key != "" || ok == true {
+		t.Errorf("GCP.GetEncryptedKey() = (%v, %v, %v), want (%v, %v, %v)",
+			kid, key, ok, "", "", false)
+	}
+	expected := fmt.Sprintf("http://metadata/computeMetadata/v1/instance/service-accounts/default/identity?audience=%s&format=full&licenses=FALSE", url.QueryEscape(p.GetID()))
+	if got := p.GetIdentityURL(); got != expected {
+		t.Errorf("GCP.GetIdentityURL() = %v, want %v", got, expected)
+	}
+}
+
+func TestGCP_GetTokenID(t *testing.T) {
+	p1, err := generateGCP()
+	assert.FatalError(t, err)
+	p1.Name = "name"
+
+	now := time.Now()
+	t1, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", "gcp:name",
+		"instance-id", "instance-name", "project-id", "zone",
+		now, &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+
+	unique := fmt.Sprintf("gcp:name.instance-id.%d.%d", now.Unix(), now.Add(5*time.Minute).Unix())
+	sum := sha256.Sum256([]byte(unique))
+	want := strings.ToLower(hex.EncodeToString(sum[:]))
+
+	type args struct {
+		token string
+	}
+	tests := []struct {
+		name    string
+		gcp     *GCP
+		args    args
+		want    string
+		wantErr bool
+	}{
+		{"ok", p1, args{t1}, want, false},
+		{"fail token", p1, args{"token"}, "", true},
+		{"fail claims", p1, args{"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.ey.fooo"}, "", true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := tt.gcp.GetTokenID(tt.args.token)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("GCP.GetTokenID() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if got != tt.want {
+				t.Errorf("GCP.GetTokenID() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestGCP_GetIdentityToken(t *testing.T) {
+	p1, err := generateGCP()
+	assert.FatalError(t, err)
+	defer resetGoogleVars()
+
+	t1, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Println(r.RequestURI)
+		switch r.URL.Path {
+		case "/bad-request":
+			http.Error(w, http.StatusText(http.StatusBadRequest), http.StatusBadRequest)
+		default:
+			w.Write([]byte(t1))
+		}
+	}))
+	defer srv.Close()
+
+	tests := []struct {
+		name        string
+		gcp         *GCP
+		identityURL string
+		want        string
+		wantErr     bool
+	}{
+		{"ok", p1, srv.URL, t1, false},
+		{"bad request", p1, srv.URL + "/bad-request", "", true},
+		{"bad url", p1, "badurl", "", true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gcpIdentityURL = tt.identityURL
+			got, err := tt.gcp.GetIdentityToken()
+			if (err != nil) != tt.wantErr {
+				t.Errorf("GCP.GetIdentityToken() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if got != tt.want {
+				t.Errorf("GCP.GetIdentityToken() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestGCP_Init(t *testing.T) {
+	srv := generateJWKServer(2)
+	defer srv.Close()
+	defer resetGoogleVars()
+
+	config := Config{
+		Claims: globalProvisionerClaims,
+	}
+	badClaims := &Claims{
+		DefaultTLSDur: &Duration{0},
+	}
+
+	type fields struct {
+		Type            string
+		Name            string
+		ServiceAccounts []string
+		Claims          *Claims
+	}
+	type args struct {
+		config   Config
+		certsURL string
+	}
+	tests := []struct {
+		name    string
+		fields  fields
+		args    args
+		wantErr bool
+	}{
+		{"ok", fields{"GCP", "name", nil, nil}, args{config, srv.URL}, false},
+		{"ok", fields{"GCP", "name", []string{"service-account"}, nil}, args{config, srv.URL}, false},
+		{"bad type", fields{"", "name", nil, nil}, args{config, srv.URL}, true},
+		{"bad name", fields{"GCP", "", nil, nil}, args{config, srv.URL}, true},
+		{"bad claims", fields{"GCP", "name", nil, badClaims}, args{config, srv.URL}, true},
+		{"bad certs", fields{"GCP", "name", nil, badClaims}, args{config, srv.URL + "/error"}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			googleOauth2Certs = tt.args.certsURL
+			p := &GCP{
+				Type:            tt.fields.Type,
+				Name:            tt.fields.Name,
+				ServiceAccounts: tt.fields.ServiceAccounts,
+				Claims:          tt.fields.Claims,
+			}
+			if err := p.Init(tt.args.config); (err != nil) != tt.wantErr {
+				t.Errorf("GCP.Init() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestGCP_AuthorizeSign(t *testing.T) {
+	p1, err := generateGCP()
+	assert.FatalError(t, err)
+
+	aKey, err := generateJSONWebKey()
+	assert.FatalError(t, err)
+
+	t1, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+	failKey, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now(), aKey)
+	assert.FatalError(t, err)
+	failIss, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://foo.bar.zar", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	failAud, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", "gcp:foo",
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+	failExp, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now().Add(-360*time.Second), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+	failNbf, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now().Add(360*time.Second), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+	failServiceAccount, err := generateGCPToken("foo",
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+	failInstanceID, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"", "instance-name", "project-id", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+	failInstanceName, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "", "project-id", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+	failProjectID, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+	failZone, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+
+	type args struct {
+		token string
+	}
+	tests := []struct {
+		name    string
+		gcp     *GCP
+		args    args
+		wantErr bool
+	}{
+		{"ok", p1, args{t1}, false},
+		{"fail token", p1, args{"token"}, true},
+		{"fail key", p1, args{failKey}, true},
+		{"fail iss", p1, args{failIss}, true},
+		{"fail aud", p1, args{failAud}, true},
+		{"fail exp", p1, args{failExp}, true},
+		{"fail nbf", p1, args{failNbf}, true},
+		{"fail service account", p1, args{failServiceAccount}, true},
+		{"fail instance id", p1, args{failInstanceID}, true},
+		{"fail instance name", p1, args{failInstanceName}, true},
+		{"fail project id", p1, args{failProjectID}, true},
+		{"fail zone", p1, args{failZone}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := tt.gcp.AuthorizeSign(tt.args.token)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("GCP.AuthorizeSign() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if err != nil {
+				assert.Nil(t, got)
+			} else {
+				assert.Len(t, 5, got)
+			}
+		})
+	}
+}
+
+func TestGCP_AuthorizeRenewal(t *testing.T) {
+	p1, err := generateGCP()
+	assert.FatalError(t, err)
+	p2, err := generateGCP()
+	assert.FatalError(t, err)
+
+	// disable renewal
+	disable := true
+	p2.Claims = &Claims{DisableRenewal: &disable}
+	p2.claimer, err = NewClaimer(p2.Claims, globalProvisionerClaims)
+	assert.FatalError(t, err)
+
+	type args struct {
+		cert *x509.Certificate
+	}
+	tests := []struct {
+		name    string
+		prov    *GCP
+		args    args
+		wantErr bool
+	}{
+		{"ok", p1, args{nil}, false},
+		{"fail", p2, args{nil}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := tt.prov.AuthorizeRenewal(tt.args.cert); (err != nil) != tt.wantErr {
+				t.Errorf("GCP.AuthorizeRenewal() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestGCP_AuthorizeRevoke(t *testing.T) {
+	p1, err := generateGCP()
+	assert.FatalError(t, err)
+
+	t1, err := generateGCPToken(p1.ServiceAccounts[0],
+		"https://accounts.google.com", p1.GetID(),
+		"instance-id", "instance-name", "project-id", "zone",
+		time.Now(), &p1.keyStore.keySet.Keys[0])
+	assert.FatalError(t, err)
+
+	type args struct {
+		token string
+	}
+	tests := []struct {
+		name    string
+		gcp     *GCP
+		args    args
+		wantErr bool
+	}{
+		{"ok", p1, args{t1}, false},
+		{"fail", p1, args{"token"}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := tt.gcp.AuthorizeRevoke(tt.args.token); (err != nil) != tt.wantErr {
+				t.Errorf("GCP.AuthorizeRevoke() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
diff --git a/authority/provisioner/oidc_test.go b/authority/provisioner/oidc_test.go
index 89965cbd..80920f41 100644
--- a/authority/provisioner/oidc_test.go
+++ b/authority/provisioner/oidc_test.go
@@ -278,7 +278,6 @@ func TestOIDC_AuthorizeSign(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			got, err := tt.prov.AuthorizeSign(tt.args.token)
 			if (err != nil) != tt.wantErr {
-				fmt.Println(tt)
 				t.Errorf("OIDC.Authorize() error = %v, wantErr %v", err, tt.wantErr)
 				return
 			}
@@ -386,47 +385,6 @@ func TestOIDC_AuthorizeRenewal(t *testing.T) {
 	}
 }
 
-/*
-func TestOIDC_AuthorizeRevoke(t *testing.T) {
-	srv := generateJWKServer(2)
-	defer srv.Close()
-
-	var keys jose.JSONWebKeySet
-	assert.FatalError(t, getAndDecode(srv.URL+"/private", &keys))
-
-	// Create test provisioners
-	p1, err := generateOIDC()
-	assert.FatalError(t, err)
-
-	// Update configuration endpoints and initialize
-	config := Config{Claims: globalProvisionerClaims}
-	p1.ConfigurationEndpoint = srv.URL + "/.well-known/openid-configuration"
-	assert.FatalError(t, p1.Init(config))
-
-	t1, err := generateSimpleToken("the-issuer", p1.ClientID, &keys.Keys[0])
-	assert.FatalError(t, err)
-
-	type args struct {
-		token string
-	}
-	tests := []struct {
-		name    string
-		prov    *OIDC
-		args    args
-		wantErr bool
-	}{
-		{"disabled", p1, args{t1}, true},
-	}
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			if err := tt.prov.AuthorizeRevoke(tt.args.token); (err != nil) != tt.wantErr {
-				t.Errorf("OIDC.AuthorizeRevoke() error = %v, wantErr %v", err, tt.wantErr)
-			}
-		})
-	}
-}
-*/
-
 func Test_sanitizeEmail(t *testing.T) {
 	tests := []struct {
 		name  string
diff --git a/authority/provisioner/utils_test.go b/authority/provisioner/utils_test.go
index ab68f163..c7022f3c 100644
--- a/authority/provisioner/utils_test.go
+++ b/authority/provisioner/utils_test.go
@@ -163,6 +163,36 @@ func generateOIDC() (*OIDC, error) {
 	}, nil
 }
 
+func generateGCP() (*GCP, error) {
+	name, err := randutil.Alphanumeric(10)
+	if err != nil {
+		return nil, err
+	}
+	serviceAccount, err := randutil.Alphanumeric(10)
+	if err != nil {
+		return nil, err
+	}
+	jwk, err := generateJSONWebKey()
+	if err != nil {
+		return nil, err
+	}
+	claimer, err := NewClaimer(nil, globalProvisionerClaims)
+	if err != nil {
+		return nil, err
+	}
+	return &GCP{
+		Type:            "GCP",
+		Name:            name,
+		ServiceAccounts: []string{serviceAccount},
+		Claims:          &globalProvisionerClaims,
+		claimer:         claimer,
+		keyStore: &keyStore{
+			keySet: jose.JSONWebKeySet{Keys: []jose.JSONWebKey{*jwk}},
+			expiry: time.Now().Add(24 * time.Hour),
+		},
+	}, nil
+}
+
 func generateCollection(nJWK, nOIDC int) (*Collection, error) {
 	col := NewCollection(testAudiences)
 	for i := 0; i < nJWK; i++ {
@@ -220,6 +250,41 @@ func generateToken(sub, iss, aud string, email string, sans []string, iat time.T
 	return jose.Signed(sig).Claims(claims).CompactSerialize()
 }
 
+func generateGCPToken(sub, iss, aud, instanceID, instanceName, projectID, zone string, iat time.Time, jwk *jose.JSONWebKey) (string, error) {
+	sig, err := jose.NewSigner(
+		jose.SigningKey{Algorithm: jose.ES256, Key: jwk.Key},
+		new(jose.SignerOptions).WithType("JWT").WithHeader("kid", jwk.KeyID),
+	)
+	if err != nil {
+		return "", err
+	}
+
+	claims := gcpPayload{
+		Claims: jose.Claims{
+			Subject:   sub,
+			Issuer:    iss,
+			IssuedAt:  jose.NewNumericDate(iat),
+			NotBefore: jose.NewNumericDate(iat),
+			Expiry:    jose.NewNumericDate(iat.Add(5 * time.Minute)),
+			Audience:  []string{aud},
+		},
+		AuthorizedParty: sub,
+		Email:           "foo@developer.gserviceaccount.com",
+		EmailVerified:   true,
+		Google: gcpGooglePayload{
+			ComputeEngine: gcpComputeEnginePayload{
+				InstanceID:                instanceID,
+				InstanceName:              instanceName,
+				InstanceCreationTimestamp: jose.NewNumericDate(iat.Add(-24 * time.Hour)),
+				ProjectID:                 projectID,
+				ProjectNumber:             1234567890,
+				Zone:                      zone,
+			},
+		},
+	}
+	return jose.Signed(sig).Claims(claims).CompactSerialize()
+}
+
 func parseToken(token string) (*jose.JSONWebToken, *jose.Claims, error) {
 	tok, err := jose.ParseSigned(token)
 	if err != nil {