crypto/x509: only disable SHA-1 verification for certificates

Disable SHA-1 signature verification in Certificate.CheckSignatureFrom,
but not in Certificate.CheckSignature. This allows verification of OCSP
responses and CRLs, which still use SHA-1 signatures, but not on
certificates.

Updates golang#41682

Change-Id: Ia705eb5052e6fc2724fed59248b1c4ef8af6c3fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/394294
Trust: Roland Shoemaker <[email protected]>
Run-TryBot: Roland Shoemaker <[email protected]>
Auto-Submit: Roland Shoemaker <[email protected]>
Reviewed-by: Jordan Liggitt <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/crypto/x509/verify.go

@@ -724,6 +724,9 @@ func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *V
 // list. (While this is not specified, it is common practice in order to limit
 // the types of certificates a CA can issue.)
 //
+// Certificates that use SHA1WithRSA and ECDSAWithSHA1 signatures are not supported,
+// and will not be used to build chains.
+//
 // WARNING: this function doesn't do any revocation checking.
 func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
  // Platform-specific verification needs the ASN.1 contents so

src/crypto/x509/x509.go

@@ -184,13 +184,13 @@ const (
 
  MD2WithRSA // Unsupported.
  MD5WithRSA // Only supported for signing, not verification.
- SHA1WithRSA // Only supported for signing, not verification.
+ SHA1WithRSA // Only supported for signing, and verification of CRLs, CSRs, and OCSP responses.
  SHA256WithRSA
  SHA384WithRSA
  SHA512WithRSA
  DSAWithSHA1 // Unsupported.
  DSAWithSHA256 // Unsupported.
- ECDSAWithSHA1 // Only supported for signing, not verification.
+ ECDSAWithSHA1 // Only supported for signing, and verification of CRLs, CSRs, and OCSP responses.
  ECDSAWithSHA256
  ECDSAWithSHA384
  ECDSAWithSHA512
@@ -767,7 +767,7 @@ func (c *Certificate) hasSANExtension() bool {
 }
 
 // CheckSignatureFrom verifies that the signature on c is a valid signature
-// from parent.
+// from parent. SHA1WithRSA and ECDSAWithSHA1 signatures are not supported.
 func (c *Certificate) CheckSignatureFrom(parent *Certificate) error {
  // RFC 5280, 4.2.1.9:
  // "If the basic constraints extension is not present in a version 3
@@ -789,13 +789,13 @@ func (c *Certificate) CheckSignatureFrom(parent *Certificate) error {
 
  // TODO(agl): don't ignore the path length constraint.
 
- return parent.CheckSignature(c.SignatureAlgorithm, c.RawTBSCertificate, c.Signature)
+ return checkSignature(c.SignatureAlgorithm, c.RawTBSCertificate, c.Signature, parent.PublicKey, debugAllowSHA1)
 }
 
 // CheckSignature verifies that signature is a valid signature over signed from
 // c's public key.
 func (c *Certificate) CheckSignature(algo SignatureAlgorithm, signed, signature []byte) error {
- return checkSignature(algo, signed, signature, c.PublicKey)
+ return checkSignature(algo, signed, signature, c.PublicKey, true)
 }
 
 func (c *Certificate) hasNameConstraints() bool {
@@ -815,9 +815,9 @@ func signaturePublicKeyAlgoMismatchError(expectedPubKeyAlgo PublicKeyAlgorithm,
  return fmt.Errorf("x509: signature algorithm specifies an %s public key, but have public key of type %T", expectedPubKeyAlgo.String(), pubKey)
 }
 
-// CheckSignature verifies that signature is a valid signature over signed from
+// checkSignature verifies that signature is a valid signature over signed from
 // a crypto.PublicKey.
-func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey crypto.PublicKey) (err error) {
+func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey crypto.PublicKey, allowSHA1 bool) (err error) {
  var hashType crypto.Hash
  var pubKeyAlgo PublicKeyAlgorithm
 
@@ -836,7 +836,7 @@ func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey
  case crypto.MD5:
  return InsecureAlgorithmError(algo)
  case crypto.SHA1:
- if !debugAllowSHA1 {
+ if !allowSHA1 {
  return InsecureAlgorithmError(algo)
  }
  fallthrough
@@ -1584,11 +1584,11 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
  // Check the signature to ensure the crypto.Signer behaved correctly.
  sigAlg := getSignatureAlgorithmFromAI(signatureAlgorithm)
  switch sigAlg {
- case MD5WithRSA, SHA1WithRSA, ECDSAWithSHA1:
+ case MD5WithRSA:
  // We skip the check if the signature algorithm is only supported for
  // signing, not verification.
  default:
- if err := checkSignature(sigAlg, c.Raw, signature, key.Public()); err != nil {
+ if err := checkSignature(sigAlg, c.Raw, signature, key.Public(), true); err != nil {
  return nil, fmt.Errorf("x509: signature over certificate returned by signer is invalid: %w", err)
  }
  }
@@ -2067,7 +2067,7 @@ func parseCertificateRequest(in *certificateRequest) (*CertificateRequest, error
 
 // CheckSignature reports whether the signature on c is valid.
 func (c *CertificateRequest) CheckSignature() error {
- return checkSignature(c.SignatureAlgorithm, c.RawTBSCertificateRequest, c.Signature, c.PublicKey)
+ return checkSignature(c.SignatureAlgorithm, c.RawTBSCertificateRequest, c.Signature, c.PublicKey, true)
 }
 
 // RevocationList contains the fields used to create an X.509 v2 Certificate

src/crypto/x509/x509_test.go

@@ -2924,30 +2924,15 @@ func TestCreateCertificateBrokenSigner(t *testing.T) {
 }
 
 func TestCreateCertificateLegacy(t *testing.T) {
- ecdsaPriv, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
- if err != nil {
- t.Fatalf("Failed to generate ECDSA key: %s", err)
+ sigAlg := MD5WithRSA
+ template := &Certificate{
+ SerialNumber: big.NewInt(10),
+ DNSNames: []string{"example.com"},
+ SignatureAlgorithm: sigAlg,
  }
-
- for _, sigAlg := range []SignatureAlgorithm{
- MD5WithRSA, SHA1WithRSA, ECDSAWithSHA1,
- } {
- template := &Certificate{
- SerialNumber: big.NewInt(10),
- DNSNames: []string{"example.com"},
- SignatureAlgorithm: sigAlg,
- }
- var k crypto.Signer
- switch sigAlg {
- case MD5WithRSA, SHA1WithRSA:
- k = testPrivateKey
- case ECDSAWithSHA1:
- k = ecdsaPriv
- }
- _, err := CreateCertificate(rand.Reader, template, template, k.Public(), &brokenSigner{k.Public()})
- if err != nil {
- t.Fatalf("CreateCertificate failed when SignatureAlgorithm = %v: %s", sigAlg, err)
- }
+ _, err := CreateCertificate(rand.Reader, template, template, testPrivateKey.Public(), &brokenSigner{testPrivateKey.Public()})
+ if err != nil {
+ t.Fatalf("CreateCertificate failed when SignatureAlgorithm = %v: %s", sigAlg, err)
  }
 }
 
@@ -3396,3 +3381,66 @@ func TestParseUniqueID(t *testing.T) {
  t.Fatalf("unexpected number of extensions (probably because the extension section was not parsed): got %d, want 7", len(cert.Extensions))
  }
 }
+
+func TestDisableSHA1ForCertOnly(t *testing.T) {
+ defer func(old bool) { debugAllowSHA1 = old }(debugAllowSHA1)
+ debugAllowSHA1 = false
+
+ tmpl := &Certificate{
+ SerialNumber: big.NewInt(1),
+ NotBefore: time.Now().Add(-time.Hour),
+ NotAfter: time.Now().Add(time.Hour),
+ SignatureAlgorithm: SHA1WithRSA,
+ BasicConstraintsValid: true,
+ IsCA: true,
+ KeyUsage: KeyUsageCertSign | KeyUsageCRLSign,
+ }
+ certDER, err := CreateCertificate(rand.Reader, tmpl, tmpl, rsaPrivateKey.Public(), rsaPrivateKey)
+ if err != nil {
+ t.Fatalf("failed to generate test cert: %s", err)
+ }
+ cert, err := ParseCertificate(certDER)
+ if err != nil {
+ t.Fatalf("failed to parse test cert: %s", err)
+ }
+
+ err = cert.CheckSignatureFrom(cert)
+ if err == nil {
+ t.Error("expected CheckSignatureFrom to fail")
+ } else if _, ok := err.(InsecureAlgorithmError); !ok {
+ t.Errorf("expected InsecureAlgorithmError error, got %T", err)
+ }
+
+ crlDER, err := CreateRevocationList(rand.Reader, &RevocationList{
+ SignatureAlgorithm: SHA1WithRSA,
+ Number: big.NewInt(1),
+ ThisUpdate: time.Now().Add(-time.Hour),
+ NextUpdate: time.Now().Add(time.Hour),
+ }, cert, rsaPrivateKey)
+ if err != nil {
+ t.Fatalf("failed to generate test CRL: %s", err)
+ }
+ // TODO(rolandshoemaker): this should be ParseRevocationList once it lands
+ crl, err := ParseCRL(crlDER)
+ if err != nil {
+ t.Fatalf("failed to parse test CRL: %s", err)
+ }
+
+ if err = cert.CheckCRLSignature(crl); err != nil {
+ t.Errorf("unexpected error: %s", err)
+ }
+
+ // This is an unrelated OCSP response, which will fail signature verification
+ // but shouldn't return a InsecureAlgorithmError, since SHA1 should be allowed
+ // for OCSP.
+ ocspTBSHex := "30819fa2160414884451ff502a695e2d88f421bad90cf2cecbea7c180f32303133303631383037323434335a30743072304a300906052b0e03021a0500041448b60d38238df8456e4ee5843ea394111802979f0414884451ff502a695e2d88f421bad90cf2cecbea7c021100f78b13b946fc9635d8ab49de9d2148218000180f32303133303631383037323434335aa011180f32303133303632323037323434335a"
+ ocspTBS, err := hex.DecodeString(ocspTBSHex)
+ if err != nil {
+ t.Fatalf("failed to decode OCSP response TBS hex: %s", err)
+ }
+
+ err = cert.CheckSignature(SHA1WithRSA, ocspTBS, nil)
+ if err != rsa.ErrVerification {
+ t.Errorf("unexpected error: %s", err)
+ }
+}