crypto/x509: add signature verification to CreateCertificate

This changes checks the signature generated during CreateCertificate
and returns an error if the verification fails. A benchmark is also
added. For RSA keys the delta looks to be insignificant, but for
ECDSA keys it introduces a much larger delta which is not ideal.

name          old time/op  new time/op   delta
RSA_2048-8    1.38ms ± 6%   1.41ms ± 2%      ~     (p=0.182 n=10)
ECDSA_P256-8  42.6µs ± 4%  116.8µs ± 4%  +174.00%  (p=0.000 n=1

Fixes #40458

Change-Id: I22827795bb9bb6868b4fa47391927db1d3bc19a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/259697
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Trust: Roland Shoemaker <roland@golang.org>

doc/go1.16.html

@@ -201,6 +201,13 @@ Do not send CLs removing the interior tags from such phrases.
   contain strings with characters within the ASCII range.
 </p>
 
+<p><!-- CL 259697 -->
+  <a href="/pkg/crypto/x509/#CreateCertificate">CreateCertificate</a> now
+  verifies the generated certificate's signature using the signer's
+  public key. If the signature is invalid, an error is returned, instead
+  of a malformed certificate.
+</p>
+
 <h3 id="net"><a href="/pkg/net/">net</a></h3>
 
 <p><!-- CL 250357 -->

src/crypto/x509/x509.go

@@ -2145,12 +2145,22 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
 		return
 	}
 
-	return asn1.Marshal(certificate{
+	signedCert, err := asn1.Marshal(certificate{
 		nil,
 		c,
 		signatureAlgorithm,
 		asn1.BitString{Bytes: signature, BitLength: len(signature) * 8},
 	})
+	if err != nil {
+		return nil, err
+	}
+
+	// Check the signature to ensure the crypto.Signer behaved correctly.
+	if err := checkSignature(getSignatureAlgorithmFromAI(signatureAlgorithm), c.Raw, signature, key.Public()); err != nil {
+		return nil, fmt.Errorf("x509: signature over certificate returned by signer is invalid: %w", err)
+	}
+
+	return signedCert, nil
 }
 
 // pemCRLPrefix is the magic string that indicates that we have a PEM encoded

src/crypto/x509/x509_test.go

@@ -22,6 +22,7 @@ import (
 	"encoding/pem"
 	"fmt"
 	"internal/testenv"
+	"io"
 	"math/big"
 	"net"
 	"net/url"
@@ -2820,3 +2821,78 @@ func TestIA5SANEnforcement(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkCreateCertificate(b *testing.B) {
+	template := &Certificate{
+		SerialNumber: big.NewInt(10),
+		DNSNames:     []string{"example.com"},
+	}
+	tests := []struct {
+		name string
+		gen  func() crypto.Signer
+	}{
+		{
+			name: "RSA 2048",
+			gen: func() crypto.Signer {
+				k, err := rsa.GenerateKey(rand.Reader, 2048)
+				if err != nil {
+					b.Fatalf("failed to generate test key: %s", err)
+				}
+				return k
+			},
+		},
+		{
+			name: "ECDSA P256",
+			gen: func() crypto.Signer {
+				k, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+				if err != nil {
+					b.Fatalf("failed to generate test key: %s", err)
+				}
+				return k
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		k := tc.gen()
+		b.ResetTimer()
+		b.Run(tc.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_, err := CreateCertificate(rand.Reader, template, template, k.Public(), k)
+				if err != nil {
+					b.Fatalf("failed to create certificate: %s", err)
+				}
+			}
+		})
+	}
+}
+
+type brokenSigner struct {
+	pub crypto.PublicKey
+}
+
+func (bs *brokenSigner) Public() crypto.PublicKey {
+	return bs.pub
+}
+
+func (bs *brokenSigner) Sign(_ io.Reader, _ []byte, _ crypto.SignerOpts) ([]byte, error) {
+	return []byte{1, 2, 3}, nil
+}
+
+func TestCreateCertificateBrokenSigner(t *testing.T) {
+	template := &Certificate{
+		SerialNumber: big.NewInt(10),
+		DNSNames:     []string{"example.com"},
+	}
+	k, err := rsa.GenerateKey(rand.Reader, 1024)
+	if err != nil {
+		t.Fatalf("failed to generate test key: %s", err)
+	}
+	expectedErr := "x509: signature over certificate returned by signer is invalid: crypto/rsa: verification error"
+	_, err = CreateCertificate(rand.Reader, template, template, k.Public(), &brokenSigner{k.Public()})
+	if err == nil {
+		t.Fatal("expected CreateCertificate to fail with a broken signer")
+	} else if err.Error() != expectedErr {
+		t.Fatalf("CreateCertificate returned an unexpected error: got %q, want %q", err, expectedErr)
+	}
+}