Add API for fetching Fulcio configuration (#608)

This API provides the following:
* All OIDC issuers, including the meta/wildcard issuers
* The expected audience of the token
* The claim that must be signed for a proof of possession
* The SPIFFE trust domain, when the issuer is of type SPIFFE

Also fix the workflow's regex.

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

.github/workflows/main.yml

@@ -45,4 +45,4 @@ jobs:
       - name: Upload Coverage Report
         uses: codecov/codecov-action@81cd2dc8148241f03f5839d295e000b8f761e378 # v3.0.0
       - name: Ensure no files were modified as a result of the build
-        run: git update-index --refresh && git diff-index --quiet -I"^\/\/\s+protoc(-gen-go)?\s+v[0-9]+\.[0-9]+\.[0-9]+$" HEAD -- || git diff -I"^\/\/\s+protoc(-gen-go)?\s+v[0-9]+\.[0-9]+\.[0-9]+$" --exit-code
+        run: git update-index --refresh && git diff-index --quiet -I"^\/\/\s+(-\s+)?protoc(-gen-go)?\s+v[0-9]+\.[0-9]+\.[0-9]+$" HEAD -- || git diff -I"^\/\/\s+(-\s+)?protoc(-gen-go)?\s+v[0-9]+\.[0-9]+\.[0-9]+$" --exit-code

fulcio.proto

@@ -48,6 +48,15 @@ service CA {
           get: "/api/v2/trustBundle"
         };
     }
+
+    /**
+     * Returns the configuration of supported OIDC issuers, including the required challenge for each issuer.
+     */
+    rpc GetConfiguration (GetConfigurationRequest) returns (Configuration) {
+        option (google.api.http) = {
+          get: "/api/v2/configuration"
+        };
+    }
 }
 
 message CreateSigningCertificateRequest {
@@ -166,3 +175,30 @@ enum PublicKeyAlgorithm {
     ECDSA                            = 2;
     ED25519                          = 3;
 }
+
+// This is created for forward compatibility in case we want to add fields in the future.
+message GetConfigurationRequest {
+}
+
+// The configuration for the Fulcio instance.
+message Configuration {
+    // The OIDC issuers supported by this Fulcio instance.
+    repeated OIDCIssuer issuers = 1;
+}
+
+// Metadata about an OIDC issuer.
+message OIDCIssuer {
+    oneof issuer {
+        // The URL of the OIDC issuer.
+        string issuer_url = 1;
+        // The URL of wildcard OIDC issuer, e.g. "https://oidc.eks.*.amazonaws.com/id/*".
+        // When comparing the issuer, the wildcards will be replaced by "[-_a-zA-Z0-9]+".
+        string wildcard_issuer_url = 2;
+    }
+    // The expected audience of the OIDC token for the issuer.
+    string audience = 3;
+    // The OIDC claim that must be signed for a proof of possession challenge.
+    string challenge_claim = 4;
+    // The expected SPIFFE trust domain. Only present when the OIDC issuer issues tokens for SPIFFE identities.
+    string spiffe_trust_domain = 5;
+}

pkg/config/config.go

@@ -31,6 +31,7 @@ import (
 
 	"github.com/coreos/go-oidc/v3/oidc"
 	lru "github.com/hashicorp/golang-lru"
+	fulciogrpc "github.com/sigstore/fulcio/pkg/generated/protobuf"
 	"github.com/sigstore/fulcio/pkg/log"
 	"github.com/spiffe/go-spiffe/v2/spiffeid"
 )
@@ -158,6 +159,33 @@ func (fc *FulcioConfig) GetVerifier(issuerURL string) (*oidc.IDTokenVerifier, bo
 	return verifier, true
 }
 
+// ToIssuers returns a proto representation of the OIDC issuer configuration.
+func (fc *FulcioConfig) ToIssuers() []*fulciogrpc.OIDCIssuer {
+	var issuers []*fulciogrpc.OIDCIssuer
+
+	for _, cfgIss := range fc.OIDCIssuers {
+		issuer := &fulciogrpc.OIDCIssuer{
+			Issuer:            &fulciogrpc.OIDCIssuer_IssuerUrl{IssuerUrl: cfgIss.IssuerURL},
+			Audience:          cfgIss.ClientID,
+			SpiffeTrustDomain: cfgIss.SPIFFETrustDomain,
+			ChallengeClaim:    issuerToChallengeClaim(cfgIss.Type),
+		}
+		issuers = append(issuers, issuer)
+	}
+
+	for metaIss, cfgIss := range fc.MetaIssuers {
+		issuer := &fulciogrpc.OIDCIssuer{
+			Issuer:            &fulciogrpc.OIDCIssuer_WildcardIssuerUrl{WildcardIssuerUrl: metaIss},
+			Audience:          cfgIss.ClientID,
+			SpiffeTrustDomain: cfgIss.SPIFFETrustDomain,
+			ChallengeClaim:    issuerToChallengeClaim(cfgIss.Type),
+		}
+		issuers = append(issuers, issuer)
+	}
+
+	return issuers
+}
+
 func (fc *FulcioConfig) prepare() error {
 	fc.verifiers = make(map[string]*oidc.IDTokenVerifier, len(fc.OIDCIssuers))
 	for _, iss := range fc.OIDCIssuers {
@@ -274,6 +302,10 @@ func validateConfig(conf *FulcioConfig) error {
 				return err
 			}
 		}
+
+		if issuerToChallengeClaim(issuer.Type) == "" {
+			return errors.New("issuer missing challenge claim")
+		}
 	}
 
 	for _, metaIssuer := range conf.MetaIssuers {
@@ -282,6 +314,10 @@ func validateConfig(conf *FulcioConfig) error {
 			// to trust domains so we fail early and reject this configuration.
 			return errors.New("SPIFFE meta issuers not supported")
 		}
+
+		if issuerToChallengeClaim(metaIssuer.Type) == "" {
+			return errors.New("issuer missing challenge claim")
+		}
 	}
 
 	return nil
@@ -421,3 +457,22 @@ func validateAllowedDomain(subjectHostname, issuerHostname string) error {
 	}
 	return fmt.Errorf("hostname top-level and second-level domains do not match: %s, %s", subjectHostname, issuerHostname)
 }
+
+func issuerToChallengeClaim(issType IssuerType) string {
+	switch issType {
+	case IssuerTypeEmail:
+		return "email"
+	case IssuerTypeGithubWorkflow:
+		return "sub"
+	case IssuerTypeKubernetes:
+		return "sub"
+	case IssuerTypeSpiffe:
+		return "sub"
+	case IssuerTypeURI:
+		return "sub"
+	case IssuerTypeUsername:
+		return "sub"
+	default:
+		return ""
+	}
+}

pkg/config/config_network_test.go

@@ -34,7 +34,10 @@ func TestLoad(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	cfg, _ := Load(cfgPath)
+	cfg, err := Load(cfgPath)
+	if err != nil {
+		t.Fatal(err)
+	}
 	got, ok := cfg.GetIssuer("https://accounts.google.com")
 	if !ok {
 		t.Error("expected true, got false")

pkg/config/config_test.go

@@ -18,20 +18,25 @@ package config
 import (
 	"fmt"
 	"net/url"
+	"reflect"
 	"testing"
+
+	"github.com/sigstore/fulcio/pkg/generated/protobuf"
 )
 
 var validCfg = `
 {
 	"OIDCIssuers": {
 		"https://accounts.google.com": {
 			"IssuerURL": "https://accounts.google.com",
-			"ClientID": "foo"
+			"ClientID": "foo",
+			"Type": "email"
 		}
 	},
 	"MetaIssuers": {
 		"https://oidc.eks.*.amazonaws.com/id/*": {
-			"ClientID": "bar"
+			"ClientID": "bar",
+			"Type": "kubernetes"
 		}
 	}
 }
@@ -299,6 +304,18 @@ func TestValidateConfig(t *testing.T) {
 			},
 			WantError: true,
 		},
+		"type without challenge claim is invalid": {
+			Config: &FulcioConfig{
+				OIDCIssuers: map[string]OIDCIssuer{
+					"https://issuer.example.com": {
+						IssuerURL: "htts://issuer.example.com",
+						ClientID:  "sigstore",
+						Type:      "invalid",
+					},
+				},
+			},
+			WantError: true,
+		},
 		"nil config isn't valid": {
 			Config:    nil,
 			WantError: true,
@@ -451,3 +468,72 @@ func Test_validateAllowedDomain(t *testing.T) {
 		})
 	}
 }
+
+func Test_issuerToChallengeClaim(t *testing.T) {
+	if claim := issuerToChallengeClaim(IssuerTypeEmail); claim != "email" {
+		t.Fatalf("expected email subject claim for email issuer, got %s", claim)
+	}
+	if claim := issuerToChallengeClaim(IssuerTypeSpiffe); claim != "sub" {
+		t.Fatalf("expected sub subject claim for SPIFFE issuer, got %s", claim)
+	}
+	if claim := issuerToChallengeClaim(IssuerTypeUsername); claim != "sub" {
+		t.Fatalf("expected sub subject claim for username issuer, got %s", claim)
+	}
+	if claim := issuerToChallengeClaim(IssuerTypeURI); claim != "sub" {
+		t.Fatalf("expected sub subject claim for URI issuer, got %s", claim)
+	}
+	if claim := issuerToChallengeClaim(IssuerTypeGithubWorkflow); claim != "sub" {
+		t.Fatalf("expected sub subject claim for GitHub issuer, got %s", claim)
+	}
+	if claim := issuerToChallengeClaim(IssuerTypeKubernetes); claim != "sub" {
+		t.Fatalf("expected sub subject claim for K8S issuer, got %s", claim)
+	}
+	// unexpected issuer has empty claim
+	if claim := issuerToChallengeClaim("invalid"); claim != "" {
+		t.Fatalf("expected no claim for invalid issuer, got %s", claim)
+	}
+}
+
+func TestToIssuers(t *testing.T) {
+	config := &FulcioConfig{
+		OIDCIssuers: map[string]OIDCIssuer{
+			"example.com": {
+				IssuerURL: "example.com",
+				ClientID:  "sigstore",
+				Type:      IssuerTypeEmail,
+			},
+		},
+		MetaIssuers: map[string]OIDCIssuer{
+			"wildcard.*.example.com": {
+				ClientID: "sigstore",
+				Type:     IssuerTypeKubernetes,
+			},
+		},
+	}
+
+	issuers := config.ToIssuers()
+	if len(issuers) != 2 {
+		t.Fatalf("unexpected number of issues, expected 2, got %v", len(issuers))
+	}
+
+	iss := &protobuf.OIDCIssuer{
+		Audience:       "sigstore",
+		ChallengeClaim: "email",
+		Issuer: &protobuf.OIDCIssuer_IssuerUrl{
+			IssuerUrl: "example.com",
+		},
+	}
+	if !reflect.DeepEqual(issuers[0], iss) {
+		t.Fatalf("expected issuer %v, got %v", iss, issuers[0])
+	}
+	iss = &protobuf.OIDCIssuer{
+		Audience:       "sigstore",
+		ChallengeClaim: "sub",
+		Issuer: &protobuf.OIDCIssuer_WildcardIssuerUrl{
+			WildcardIssuerUrl: "wildcard.*.example.com",
+		},
+	}
+	if !reflect.DeepEqual(issuers[1], iss) {
+		t.Fatalf("expected issuer %v, got %v", iss, issuers[1])
+	}
+}