fix(openid/client): flatten audience for client assertion

In accordance with OpenID Connect 1.0 Core, draft 36 incorporating
errata set 3:

> aud
>    REQUIRED. Audience. The aud (audience) Claim. [...] The Audience value MUST be the OP's Issuer Identifier passed as a string, and not a single-element array.

pkg/openid/client/client.go

@@ -24,6 +24,10 @@ import (
 	urlpkg "github.com/nais/wonderwall/pkg/url"
 )
 
+func init() {
+	jwt.Settings(jwt.WithFlattenAudience(true))
+}
+
 var (
 	ErrOpenIDClient = errors.New("client error")
 	ErrOpenIDServer = errors.New("server error")
@@ -141,20 +145,16 @@ func (c *Client) MakeAssertion(expiration time.Duration) (string, error) {
 	iat := time.Now().Add(-5 * time.Second).Truncate(time.Second)
 	exp := iat.Add(expiration)
 
-	errs := make([]error, 0)
-
-	tok := jwt.New()
-	errs = append(errs, tok.Set(jwt.IssuerKey, clientCfg.ClientID()))
-	errs = append(errs, tok.Set(jwt.SubjectKey, clientCfg.ClientID()))
-	errs = append(errs, tok.Set(jwt.AudienceKey, providerCfg.Issuer()))
-	errs = append(errs, tok.Set(jwt.IssuedAtKey, iat))
-	errs = append(errs, tok.Set(jwt.ExpirationKey, exp))
-	errs = append(errs, tok.Set(jwt.JwtIDKey, uuid.New().String()))
-
-	for _, err := range errs {
-		if err != nil {
-			return "", fmt.Errorf("setting claim for client assertion: %w", err)
-		}
+	tok, err := jwt.NewBuilder().
+		Issuer(clientCfg.ClientID()).
+		Subject(clientCfg.ClientID()).
+		Audience([]string{providerCfg.Issuer()}). // the aud claim is flattened to a single string value on serialization
+		IssuedAt(iat).
+		Expiration(exp).
+		JwtID(uuid.New().String()).
+		Build()
+	if err != nil {
+		return "", fmt.Errorf("building client assertion: %w", err)
 	}
 
 	encoded, err := jwt.Sign(tok, jwt.WithKey(key.Algorithm(), key))

pkg/openid/client/client_test.go

@@ -1,6 +1,9 @@
 package client_test
 
 import (
+	"encoding/base64"
+	"encoding/json"
+	"strings"
 	"testing"
 	"time"
 
@@ -20,9 +23,11 @@ func TestMakeAssertion(t *testing.T) {
 	c := newTestClientWithConfig(openidConfig)
 
 	expiry := 30 * time.Second
-	assertionString, err := c.MakeAssertion(expiry)
+	jwtAssertion, err := c.MakeAssertion(expiry)
 	assert.NoError(t, err)
 
+	assertFlattenedAudience(t, jwtAssertion)
+
 	key := openidConfig.Client().ClientJWK()
 	publicKey, err := key.PublicKey()
 	assert.NoError(t, err)
@@ -32,8 +37,7 @@ func TestMakeAssertion(t *testing.T) {
 		jwt.WithRequiredClaim(jwt.ExpirationKey),
 		jwt.WithRequiredClaim(jwt.JwtIDKey),
 	}
-
-	assertion, err := jwt.Parse([]byte(assertionString), opts...)
+	assertion, err := jwt.ParseString(jwtAssertion, opts...)
 	assert.NoError(t, err)
 
 	assert.ElementsMatch(t, []string{"some-issuer"}, assertion.Audience())
@@ -45,6 +49,22 @@ func TestMakeAssertion(t *testing.T) {
 	assert.True(t, assertion.Expiration().Before(time.Now().Add(expiry)))
 }
 
+// assertFlattenedAudience asserts that the raw JWT assertion has a flattened audience claim, i.e. aud is a string value.
+// We do this as the jwx library only exposes the audience as a slice of strings for parsed JWTs.
+func assertFlattenedAudience(t *testing.T, jwtAssertion string) {
+	parts := strings.Split(jwtAssertion, ".")
+	assert.Len(t, parts, 3)
+
+	rawClaims, err := base64.RawURLEncoding.DecodeString(parts[1])
+	assert.NoError(t, err)
+
+	claims := make(map[string]any)
+	err = json.Unmarshal(rawClaims, &claims)
+	assert.NoError(t, err)
+
+	assert.Equal(t, "some-issuer", claims["aud"])
+}
+
 func newTestClientWithConfig(config *mock.TestConfiguration) *client.Client {
 	jwksProvider := mock.NewTestJwksProvider()
 	return client.NewClient(config, jwksProvider)