fileca: add support for intermediate certificates (#320)

Signed-off-by: Nathan Smith <nathan@nfsmith.ca>

pkg/ca/fileca/fileca.go

@@ -20,20 +20,20 @@ import (
 	"crypto"
 	"crypto/rand"
 	"crypto/x509"
-	"encoding/pem"
 	"sync"
 
 	"github.com/fsnotify/fsnotify"
 	"github.com/sigstore/fulcio/pkg/ca"
 	"github.com/sigstore/fulcio/pkg/ca/x509ca"
 	"github.com/sigstore/fulcio/pkg/challenges"
+	"github.com/sigstore/sigstore/pkg/cryptoutils"
 )
 
 type fileCA struct {
 	sync.RWMutex
 
-	cert *x509.Certificate
-	key  crypto.Signer
+	certs []*x509.Certificate
+	key   crypto.Signer
 }
 
 // NewFileCA returns a file backed certificate authority. Expects paths to a
@@ -43,7 +43,7 @@ func NewFileCA(certPath, keyPath, keyPass string, watch bool) (ca.CertificateAut
 	var fca fileCA
 
 	var err error
-	fca.cert, fca.key, err = loadKeyPair(certPath, keyPath, keyPass)
+	fca.certs, fca.key, err = loadKeyPair(certPath, keyPath, keyPass)
 	if err != nil {
 		return nil, err
 	}
@@ -68,21 +68,21 @@ func NewFileCA(certPath, keyPath, keyPass string, watch bool) (ca.CertificateAut
 	return &fca, err
 }
 
-func (fca *fileCA) updateX509KeyPair(cert *x509.Certificate, key crypto.Signer) {
+func (fca *fileCA) updateX509KeyPair(certs []*x509.Certificate, key crypto.Signer) {
 	fca.Lock()
 	defer fca.Unlock()
 
 	// NB: We use the RWLock to unsure a reading thread can't get a mismatching
 	// cert / key pair by reading the attributes halfway through the update
 	// below.
-	fca.cert = cert
+	fca.certs = certs
 	fca.key = key
 }
 
 func (fca *fileCA) getX509KeyPair() (*x509.Certificate, crypto.Signer) {
 	fca.RLock()
 	defer fca.RUnlock()
-	return fca.cert, fca.key
+	return fca.certs[0], fca.key
 }
 
 // CreateCertificate issues code signing certificates
@@ -103,8 +103,8 @@ func (fca *fileCA) CreateCertificate(_ context.Context, subject *challenges.Chal
 }
 
 func (fca *fileCA) Root(ctx context.Context) ([]byte, error) {
-	return pem.EncodeToMemory(&pem.Block{
-		Type:  "CERTIFICATE",
-		Bytes: fca.cert.Raw,
-	}), nil
+	fca.RLock()
+	defer fca.RUnlock()
+
+	return cryptoutils.MarshalCertificatesToPEM(fca.certs)
 }

pkg/ca/fileca/load.go

@@ -27,21 +27,64 @@ import (
 	"go.step.sm/crypto/pemutil"
 )
 
-func loadKeyPair(certPath, keyPath, keyPass string) (*x509.Certificate, crypto.Signer, error) {
+func loadKeyPair(certPath, keyPath, keyPass string) ([]*x509.Certificate, crypto.Signer, error) {
 
 	var (
-		cert *x509.Certificate
-		err  error
-		key  crypto.Signer
+		certs []*x509.Certificate
+		err   error
+		key   crypto.Signer
 	)
 
-	// TODO: Load chain of certs (intermediates and root) instead of just one
-	// certificate.
-	cert, err = pemutil.ReadCertificate(certPath)
+	// NB: certs are ordered from leaf at certs[0] to root at
+	// certs[len(certs)-1]
+	certs, err = pemutil.ReadCertificateBundle(certPath)
 	if err != nil {
 		return nil, nil, err
 	}
 
+	// Verify certificate chain
+	{
+		roots := x509.NewCertPool()
+		roots.AddCert(certs[len(certs)-1])
+
+		intermediates := x509.NewCertPool()
+		if len(certs) > 2 {
+			for _, intermediate := range certs[1 : len(certs)-1] {
+				intermediates.AddCert(intermediate)
+			}
+		}
+
+		opts := x509.VerifyOptions{
+			Roots:         roots,
+			Intermediates: intermediates,
+			KeyUsages: []x509.ExtKeyUsage{
+				x509.ExtKeyUsageCodeSigning,
+			},
+		}
+		if _, err := certs[0].Verify(opts); err != nil {
+			return nil, nil, err
+		}
+
+		if !certs[0].IsCA {
+			return nil, nil, errors.New(`fileca: certificate is not a CA`)
+		}
+
+		// If using an intermediate, verify that code signing extended key
+		// usage is set to satify extended key usage chainging
+		if len(certs) > 1 {
+			var hasExtKeyUsageCodeSigning bool
+			for _, extKeyUsage := range certs[0].ExtKeyUsage {
+				if extKeyUsage == x509.ExtKeyUsageCodeSigning {
+					hasExtKeyUsageCodeSigning = true
+					break
+				}
+			}
+			if !hasExtKeyUsageCodeSigning {
+				return nil, nil, errors.New(`fileca: certificate must have extended key usage code signing set to sign code signing certificates`)
+			}
+		}
+	}
+
 	{
 		opaqueKey, err := pemutil.Read(keyPath, pemutil.WithPassword([]byte(keyPass)))
 		if err != nil {
@@ -55,15 +98,11 @@ func loadKeyPair(certPath, keyPath, keyPass string) (*x509.Certificate, crypto.S
 		}
 	}
 
-	if !valid(cert, key) {
+	if !valid(certs[0], key) {
 		return nil, nil, errors.New(`fileca: certificate public key and private key don't match`)
 	}
 
-	if !cert.IsCA {
-		return nil, nil, errors.New(`fileca: certificate is not a CA`)
-	}
-
-	return cert, key, nil
+	return certs, key, nil
 }
 
 func valid(cert *x509.Certificate, key crypto.Signer) bool {

pkg/ca/fileca/load_test.go

@@ -25,6 +25,9 @@ func TestValidLoadKeyPair(t *testing.T) {
 		"ecdsa",
 		"ed25519",
 		"rsa4096",
+		"openssl",
+		"intermediate-2",
+		"intermediate-3",
 	}
 
 	for _, keypair := range keypairs {
@@ -42,6 +45,7 @@ func TestInvalidLoadKeyPair(t *testing.T) {
 	keypairs := []string{
 		"notca",
 		"mismatch",
+		"eku-chaining-violation",
 	}
 
 	for _, keypair := range keypairs {

pkg/ca/fileca/testdata/ecdsa-cert.pem

@@ -1,12 +1,11 @@
 -----BEGIN CERTIFICATE-----
-MIIBxzCCAU6gAwIBAgIUbQqE8rDPWDqJexmvpaeamgZe/HIwCgYIKoZIzj0EAwIw
-EDEOMAwGA1UEAwwFZWNkc2EwIBcNMjExMjIxMTkxMzI2WhgPMjEyMTExMjcxOTEz
-MjZaMBAxDjAMBgNVBAMMBWVjZHNhMHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEMV9i
-0e3Ld1eQy9UXII5MOymw2IFBo288zuOMeH+7w0ejJlY0PFowY4rItKIhqRIWOqFA
-luSaVC59sKsqjsiLdQQW2CV19eYFhVvYQS1S2QaROpFA5Zt8ALOACyp5s+6+o2cw
-ZTAdBgNVHQ4EFgQU3QF9mKDrefmeiE3lqC46PSmhEOkwHwYDVR0jBBgwFoAU3QF9
-mKDrefmeiE3lqC46PSmhEOkwDwYDVR0TAQH/BAUwAwEB/zASBgNVHRMBAf8ECDAG
-AQH/AgEBMAoGCCqGSM49BAMCA2cAMGQCMAQ/g18eRvqITDZEKdzf4bI4qKF/ZbVL
-GTZ+2HHZYwDvsuHeznTl1Uq1stzmySi4owIwV1jCF8f4gikxT0XCF+u1CJlVYiZP
-tyRnLdZaKl/seNUmBO0RRR72tsRd/X1QR3NK
+MIIBojCCASmgAwIBAgIRAPtUhtNn5zcLWx6IFhFNbqowCgYIKoZIzj0EAwMwEDEO
+MAwGA1UEAxMFZWNkc2EwIBcNMjIwMTE4MjAzOTQ4WhgPMjEyMTEyMjUyMDM5NDda
+MBAxDjAMBgNVBAMTBWVjZHNhMHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEdUISPLNy
+nh6rLd9mDRTNrFygLajEOwmeIaZu6/OW8wJAM2r50ZvkpO0X5bOmf1ezSnuiYWpQ
+VjwQqSJ78zLgV2CHLm1td5g/F/lCIofAY+5w56uJgrzsqrAAGODumWpzo0UwQzAO
+BgNVHQ8BAf8EBAMCAQYwEgYDVR0TAQH/BAgwBgEB/wIBATAdBgNVHQ4EFgQUpqtq
++RZD2VWofB75Lsk67/hlFZcwCgYIKoZIzj0EAwMDZwAwZAIwPjiaLvSRX/ju4rGM
+/4Dq+xfwVtlri3zWkGpmNf65ciZipKceKz8wzAYZ6aCoBmSHAjBhnavATf3NxRik
+sMDePua/9rleC/gaCVC9l022c8Ht+i2tjhhwmQ09p+vOf8ugXsM=
 -----END CERTIFICATE-----

pkg/ca/fileca/testdata/ecdsa-key.pem

@@ -1,8 +1,9 @@
------BEGIN ENCRYPTED PRIVATE KEY-----
-MIIBEzBOBgkqhkiG9w0BBQ0wQTApBgkqhkiG9w0BBQwwHAQIGCeFiJ2rs3wCAggA
-MAwGCCqGSIb3DQIJBQAwFAYIKoZIhvcNAwcECGZQSOBsvcjqBIHAGdU9wj136ikM
-QUqJqDt5oSDKJAU2Yrv+pCRLz6VENGUvxFrPsn9fbjSt69fSZMm09mITfEg1eLO4
-9LDl7PW1Bza803IXKQnZ0xnRUkkY1GQfDtQEGCYFaojRpYNLmmSiHpeFrqAPz83K
-+oXsRTwuRkDrABNpwTCEXYVmcHUmk9NqC6E2qjmYOyDx0ktA4HG62H3/cpGZleBs
-l58oyOg658erxF1rnASN15lw9/1g0lWACsXsMgbkjDnY51LU71pR
------END ENCRYPTED PRIVATE KEY-----
+-----BEGIN EC PRIVATE KEY-----
+Proc-Type: 4,ENCRYPTED
+DEK-Info: AES-256-CBC,1ee56fe067d83265fe430391edfa6586
+
+W5NqqRe5rOVe4OvxehYKm6wscR1JFoyRyd8M+Rutp8Q2lxPuKFhR4FZ61b0yy6pr
+LGJGQWOTIZxrNZ8g4JeS9I3huDWGloZRI2fbTg69HK4EiQQWUc1wS1TWAVoaf4fr
+LclBWxp2UzqHDaNJ0/2DoGFZhaeMU84VA1O41lO+p5Cx4bms0yWeEHwOrf2AmnNY
+l5Zm9zoPpXxaDEPSTs5c1loRmmxPHKgb68oZPxEnsCg=
+-----END EC PRIVATE KEY-----

pkg/ca/fileca/testdata/ed25519-cert.pem

@@ -1,10 +1,9 @@
 -----BEGIN CERTIFICATE-----
-MIIBTzCCAQGgAwIBAgIUeObF4LopbObr0zVOX7BAZbvy4MswBQYDK2VwMBIxEDAO
-BgNVBAMMB2VkMjU1MTkwIBcNMjExMjIxMTkxMzI2WhgPMjEyMTExMjcxOTEzMjZa
-MBIxEDAOBgNVBAMMB2VkMjU1MTkwKjAFBgMrZXADIQBNNJP9Ys+Sx0Cx/c5pQNAF
-cuECdESA0vB2IqXVAG5OiaNnMGUwHQYDVR0OBBYEFJEGm0OzRNsdBVLdDBCcx21i
-nEySMB8GA1UdIwQYMBaAFJEGm0OzRNsdBVLdDBCcx21inEySMA8GA1UdEwEB/wQF
-MAMBAf8wEgYDVR0TAQH/BAgwBgEB/wIBATAFBgMrZXADQQD6quk/tnnZpFgabR2Q
-4WCweJfZ4NfrhMOVvAPdECW/P57NH0P2BUSOK+/DktOBFIjLUWG6ptRExHDcRsFm
-WTsA
+MIIBKTCB3KADAgECAhEAtBnovvbxszrVoAQ52cyprDAFBgMrZXAwEjEQMA4GA1UE
+AxMHZWQyNTUxOTAgFw0yMjAxMTgyMDM5NDVaGA8yMTIxMTIyNTIwMzk0NVowEjEQ
+MA4GA1UEAxMHZWQyNTUxOTAqMAUGAytlcAMhAEvHE6tyPrILBpp/trmJFq47084w
+PPeuGwL1NUeOs85po0UwQzAOBgNVHQ8BAf8EBAMCAQYwEgYDVR0TAQH/BAgwBgEB
+/wIBATAdBgNVHQ4EFgQUvGmDwKfk/xiWf+ogGKx6391FPG0wBQYDK2VwA0EARdJM
+8/WM/6UC/fUOc0m597lqAiln+XXi1o6IJIwYf849WAwwZoXjcDpO9PM5KM6jg3dB
+KecnmX3PBCvtMAF6Bg==
 -----END CERTIFICATE-----

pkg/ca/fileca/testdata/ed25519-key.pem

@@ -1,5 +1,6 @@
 -----BEGIN ENCRYPTED PRIVATE KEY-----
-MIGKME4GCSqGSIb3DQEFDTBBMCkGCSqGSIb3DQEFDDAcBAggWEDSFylYswICCAAw
-DAYIKoZIhvcNAgkFADAUBggqhkiG9w0DBwQIaecFy/8IbAYEOHb3xUdAVad3ZcXk
-dkwJjtPNP2t2PA/6ngVgfsx2dgqKBhjg9JXG98Yw2eeYqJsbZ4jrHAJK0l8E
+MIGkMGAGCSqGSIb3DQEFDTBTMDIGCSqGSIb3DQEFDDAlBBAodg9wkHvRNwsiMIKj
+w7HOAgMBhqAwDAYIKoZIhvcNAgkAADAdBglghkgBZQMEASoEEJ/dx5SGu+D6gLpq
+tNb/o1EEQKKnS4CPbd8fqBfCXebZzq2+yzEmp8LNwQjA4/di+MwqO+Alr/w9QqUC
+MwKOP0K/c4Rul3Gdj1TJeB79gk6+8VY=
 -----END ENCRYPTED PRIVATE KEY-----

pkg/ca/fileca/testdata/eku-chaining-violation-cert.pem

@@ -0,0 +1,31 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----
+MIIBdDCCARqgAwIBAgIRAOcT6tYbqPoC/S3mafr+lCIwCgYIKoZIzj0EAwIwDzEN
+MAsGA1UEAxMEcm9vdDAgFw0yMjAxMTgyMDQwMDBaGA8yMTIxMTIyNTIwNDAwMFow
+GDEWMBQGA1UEAxMNaW50ZXJtZWRpYXRlMTAqMAUGAytlcAMhAP8ne9tAJLrAAf/L
+kQnlS+tiYBvKZfu+hLCQisMfWlFKo3sweTAOBgNVHQ8BAf8EBAMCAQYwEwYDVR0l
+BAwwCgYIKwYBBQUHAwMwEgYDVR0TAQH/BAgwBgEB/wIBATAdBgNVHQ4EFgQUo67t
+zwWUMIZRA9je5JDQuaJ/fXYwHwYDVR0jBBgwFoAUqCLjqJtiXiediSJebsQdCV59
+d9MwCgYIKoZIzj0EAwIDSAAwRQIhALLWa6T98PCO/zO/DXzsY5vLEZmiaeJj6ac3
+GQtfXkjFAiBh/GcSFfh6fHn/GPs4N5HZXQs7x4+zTlWD7hSCo2jezg==
+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----
+MIIBYzCCAQmgAwIBAgIQA8PGipZ4WhNTXXeiHs9HkzAKBggqhkjOPQQDAjAPMQ0w
+CwYDVQQDEwRyb290MCAXDTIyMDExODIwNDAwMFoYDzIxMjExMjI1MjA0MDAwWjAP
+MQ0wCwYDVQQDEwRyb290MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE2a1VgMYP
+5Uh/eMs/smjDFjCLEq7UTaj1D36Shp/GrsvXXCKyH/4ZlcVpTaVDC0oDDQ6os4UV
+aoT/4Ix1hU5bnqNFMEMwDgYDVR0PAQH/BAQDAgEGMBIGA1UdEwEB/wQIMAYBAf8C
+AQIwHQYDVR0OBBYEFKgi46ibYl4nnYkiXm7EHQlefXfTMAoGCCqGSM49BAMCA0gA
+MEUCIQDIuZn/YBfndAo7Rv5FUl/9HM9dr9LrDQFfCkn5UnOYnQIgN6ruDqzt+GHG
+/HJ1qe6CfnglgaEJ+/Vloc2PTcm/rVs=
+-----END CERTIFICATE-----

pkg/ca/fileca/testdata/eku-chaining-violation-key.pem

@@ -0,0 +1,8 @@
+-----BEGIN EC PRIVATE KEY-----
+Proc-Type: 4,ENCRYPTED
+DEK-Info: AES-256-CBC,2e9623757041e0d3ca2814238b20a8a0
+
+in6cGXlJQtojVS3Epbt16OoPR+MIZl+yL2+QMwGszsLHwA0r9OgpyK7cnFQKEJST
+m6KVpEWprRU//yljH8XZ0hOB8xzh0XR2Xbp5chMU/MptOjWm4yv4CS2Qvvob/sB2
+V9e+KmyUJLe123h/IYcZ5upHQdQqlf8kKu2vdL9sLHM=
+-----END EC PRIVATE KEY-----