Add e2e test that tests IssuerClaim (#605)

We were lacking any test coverage for IssuerClaim. This test shows that
the issuer in the issued certificate will come from the claim specified
in IssuerClaim instead of from the issuer claim.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

pkg/api/grpc_server_test.go

@@ -181,6 +181,7 @@ type oidcTestContainer struct {
 type customClaims struct {
 	Email         string `json:"email"`
 	EmailVerified bool   `json:"email_verified"`
+	OtherIssuer   string `json:"other_issuer"`
 }
 
 // Tests API for email and username subject types
@@ -606,6 +607,84 @@ func TestAPIWithGitHub(t *testing.T) {
 	}
 }
 
+// Tests API with issuer claim in different field in the OIDC token
+func TestAPIWithIssuerClaimConfig(t *testing.T) {
+	emailSigner, emailIssuer := newOIDCIssuer(t)
+
+	// Create a FulcioConfig that supports these issuers.
+	cfg, err := config.Read([]byte(fmt.Sprintf(`{
+		"OIDCIssuers": {
+			%q: {
+				"IssuerURL": %q,
+				"ClientID": "sigstore",
+				"Type": "email",
+				"IssuerClaim": "$.other_issuer"
+			}
+		}
+	}`, emailIssuer, emailIssuer)))
+	if err != nil {
+		t.Fatalf("config.Read() = %v", err)
+	}
+
+	emailSubject := "foo@example.com"
+	otherIssuerVal := "other.issuer.com"
+
+	// Create an OIDC token using this issuer's signer.
+	tok, err := jwt.Signed(emailSigner).Claims(jwt.Claims{
+		Issuer:   emailIssuer,
+		IssuedAt: jwt.NewNumericDate(time.Now()),
+		Expiry:   jwt.NewNumericDate(time.Now().Add(30 * time.Minute)),
+		Subject:  emailSubject,
+		Audience: jwt.Audience{"sigstore"},
+	}).Claims(customClaims{Email: emailSubject, EmailVerified: true, OtherIssuer: otherIssuerVal}).CompactSerialize()
+	if err != nil {
+		t.Fatalf("CompactSerialize() = %v", err)
+	}
+
+	ctClient, eca := createCA(cfg, t)
+	ctx := context.Background()
+	server, conn := setupGRPCForTest(ctx, t, cfg, ctClient, eca)
+	defer func() {
+		server.Stop()
+		conn.Close()
+	}()
+
+	client := protobuf.NewCAClient(conn)
+
+	pubBytes, proof := generateKeyAndProof(emailSubject, t)
+
+	// Hit the API to have it sign our certificate.
+	resp, err := client.CreateSigningCertificate(ctx, &protobuf.CreateSigningCertificateRequest{
+		Credentials: &protobuf.Credentials{
+			Credentials: &protobuf.Credentials_OidcIdentityToken{
+				OidcIdentityToken: tok,
+			},
+		},
+		Key: &protobuf.CreateSigningCertificateRequest_PublicKeyRequest{
+			PublicKeyRequest: &protobuf.PublicKeyRequest{
+				PublicKey: &protobuf.PublicKey{
+					Content: pubBytes,
+				},
+				ProofOfPossession: proof,
+			},
+		},
+	})
+	if err != nil {
+		t.Fatalf("SigningCert() = %v", err)
+	}
+
+	// The issuer should be otherIssuerVal, not emailIssuer
+	leafCert := verifyResponse(resp, eca, otherIssuerVal, t)
+
+	// Expect email subject
+	if len(leafCert.EmailAddresses) != 1 {
+		t.Fatalf("unexpected length of leaf certificate URIs, expected 1, got %d", len(leafCert.URIs))
+	}
+	if leafCert.EmailAddresses[0] != emailSubject {
+		t.Fatalf("subjects do not match: Expected %v, got %v", emailSubject, leafCert.EmailAddresses[0])
+	}
+}
+
 // Tests API with challenge sent as CSR
 func TestAPIWithCSRChallenge(t *testing.T) {
 	emailSigner, emailIssuer := newOIDCIssuer(t)