Fix bad signature algorithm on EC+RSA PKI

When the root certificate has an EC key and he intermediate has an
RSA key, the signature algorithm of the leafs should be the default
one, SHA256WithRSA, instead of the one that the intermediate has.

Fixes #1033

.golangci.yml

@@ -50,7 +50,6 @@ linters-settings:
 linters:
   disable-all: true
   enable:
-    - deadcode
     - gocritic
     - gofmt
     - gosimple

cas/softcas/softcas.go

@@ -248,8 +248,23 @@ func createCertificate(template, parent *x509.Certificate, pub crypto.PublicKey,
 		if sa, ok := signer.(apiv1.SignatureAlgorithmGetter); ok {
 			template.SignatureAlgorithm = sa.SignatureAlgorithm()
 		} else if _, ok := parent.PublicKey.(*rsa.PublicKey); ok {
+			// For RSA issuers, only overwrite the default algorithm is the
+			// intermediate is signed with an RSA signature scheme.
+			if isRSA(parent.SignatureAlgorithm) {
 				template.SignatureAlgorithm = parent.SignatureAlgorithm
 			}
 		}
+	}
 	return x509util.CreateCertificate(template, parent, pub, signer)
 }
+
+func isRSA(sa x509.SignatureAlgorithm) bool {
+	switch sa {
+	case x509.SHA256WithRSA, x509.SHA384WithRSA, x509.SHA512WithRSA:
+		return true
+	case x509.SHA256WithRSAPSS, x509.SHA384WithRSAPSS, x509.SHA512WithRSAPSS:
+		return true
+	default:
+		return false
+	}
+}

cas/softcas/softcas_test.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"context"
 	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/x509"
@@ -412,6 +414,92 @@ func TestSoftCAS_CreateCertificate_pss(t *testing.T) {
 	}
 }
 
+func TestSoftCAS_CreateCertificate_ec_rsa(t *testing.T) {
+	rootSigner, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	intSigner, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	now := time.Now()
+
+	// Root template
+	template := &x509.Certificate{
+		Subject:               pkix.Name{CommonName: "Test Root CA"},
+		KeyUsage:              x509.KeyUsageCRLSign | x509.KeyUsageCertSign,
+		PublicKey:             rootSigner.Public(),
+		BasicConstraintsValid: true,
+		IsCA:                  true,
+		MaxPathLen:            0,
+		SerialNumber:          big.NewInt(1234),
+		NotBefore:             now,
+		NotAfter:              now.Add(24 * time.Hour),
+	}
+
+	root, err := x509util.CreateCertificate(template, template, rootSigner.Public(), rootSigner)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Intermediate template
+	template = &x509.Certificate{
+		Subject:               pkix.Name{CommonName: "Test Intermediate CA"},
+		KeyUsage:              x509.KeyUsageCRLSign | x509.KeyUsageCertSign,
+		PublicKey:             intSigner.Public(),
+		BasicConstraintsValid: true,
+		IsCA:                  true,
+		MaxPathLen:            0,
+		SerialNumber:          big.NewInt(1234),
+		NotBefore:             now,
+		NotAfter:              now.Add(24 * time.Hour),
+	}
+
+	iss, err := x509util.CreateCertificate(template, root, intSigner.Public(), rootSigner)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if iss.SignatureAlgorithm != x509.ECDSAWithSHA256 {
+		t.Errorf("Certificate.SignatureAlgorithm = %v, want %v", iss.SignatureAlgorithm, x509.ECDSAWithSHA256)
+	}
+
+	c := &SoftCAS{
+		CertificateChain: []*x509.Certificate{iss},
+		Signer:           intSigner,
+	}
+	cert, err := c.CreateCertificate(&apiv1.CreateCertificateRequest{
+		Template: &x509.Certificate{
+			Subject:      pkix.Name{CommonName: "test.smallstep.com"},
+			DNSNames:     []string{"test.smallstep.com"},
+			KeyUsage:     x509.KeyUsageDigitalSignature,
+			ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth, x509.ExtKeyUsageClientAuth},
+			PublicKey:    testSigner.Public(),
+			SerialNumber: big.NewInt(1234),
+		},
+		Lifetime: time.Hour, Backdate: time.Minute,
+	})
+	if err != nil {
+		t.Fatalf("SoftCAS.CreateCertificate() error = %v", err)
+	}
+	if cert.Certificate.SignatureAlgorithm != x509.SHA256WithRSA {
+		t.Errorf("Certificate.SignatureAlgorithm = %v, want %v", iss.SignatureAlgorithm, x509.SHA256WithRSAPSS)
+	}
+
+	pool := x509.NewCertPool()
+	pool.AddCert(iss)
+	if _, err = cert.Certificate.Verify(x509.VerifyOptions{
+		CurrentTime: time.Now(),
+		Roots:       pool,
+		KeyUsages:   []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageServerAuth},
+	}); err != nil {
+		t.Errorf("Certificate.Verify() error = %v", err)
+	}
+}
+
 func TestSoftCAS_RenewCertificate(t *testing.T) {
 	mockNow(t)
 
@@ -772,3 +860,32 @@ func TestSoftCAS_defaultKeyManager(t *testing.T) {
 		})
 	}
 }
+
+func Test_isRSA(t *testing.T) {
+	type args struct {
+		sa x509.SignatureAlgorithm
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{"SHA256WithRSA", args{x509.SHA256WithRSA}, true},
+		{"SHA384WithRSA", args{x509.SHA384WithRSA}, true},
+		{"SHA512WithRSA", args{x509.SHA512WithRSA}, true},
+		{"SHA256WithRSAPSS", args{x509.SHA256WithRSAPSS}, true},
+		{"SHA384WithRSAPSS", args{x509.SHA384WithRSAPSS}, true},
+		{"SHA512WithRSAPSS", args{x509.SHA512WithRSAPSS}, true},
+		{"ECDSAWithSHA256", args{x509.ECDSAWithSHA256}, false},
+		{"ECDSAWithSHA384", args{x509.ECDSAWithSHA384}, false},
+		{"ECDSAWithSHA512", args{x509.ECDSAWithSHA512}, false},
+		{"PureEd25519", args{x509.PureEd25519}, false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isRSA(tt.args.sa); got != tt.want {
+				t.Errorf("isRSA() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}