crypto/dsa,crypto/x509: deprecate DSA and remove crypto/x509 support

Updates #40337

Change-Id: I5c1218df3ae7e13144a1d9f7d4a4b456e4475c0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/257939
Trust: Filippo Valsorda <filippo@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

doc/go1.16.html

@@ -229,6 +229,25 @@ Do not send CLs removing the interior tags from such phrases.
   TODO
 </p>
 
+<dl id="crypto/dsa"><dt><a href="/pkg/crypto/dsa/">crypto/dsa</a></dt>
+  <dd>
+    <p><!-- CL 257939 -->
+      The <a href="/pkg/crypto/dsa/"><code>crypto/dsa</code></a> package is now deprecated.
+      See <a href="https://golang.org/issue/40337">issue #40337</a>.
+    </p>
+  </dd>
+</dl><!-- crypto/dsa -->
+
+<dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
+  <dd>
+    <p><!-- CL 257939 -->
+      DSA signature verification is no longer supported. Note that DSA signature
+      generation was never supported.
+      See <a href="https://golang.org/issue/40337">issue #40337</a>.
+    </p>
+  </dd>
+</dl><!-- crypto/x509 -->
+
 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
   <dd>
     <p><!-- CL 233637 -->

src/crypto/dsa/dsa.go

@@ -5,6 +5,12 @@
 // Package dsa implements the Digital Signature Algorithm, as defined in FIPS 186-3.
 //
 // The DSA operations in this package are not implemented using constant-time algorithms.
+//
+// Deprecated: DSA is a legacy algorithm, and modern alternatives such as
+// Ed25519 (implemented by package crypto/ed25519) should be used instead. Keys
+// with 1024-bit moduli (L1024N160 parameters) are cryptographically weak, while
+// bigger keys are not widely supported. Note that FIPS 186-5 no longer approves
+// DSA for signature generation.
 package dsa
 
 import (

src/crypto/x509/x509.go

@@ -159,10 +159,6 @@ type dsaAlgorithmParameters struct {
 	P, Q, G *big.Int
 }
 
-type dsaSignature struct {
-	R, S *big.Int
-}
-
 type validity struct {
 	NotBefore, NotAfter time.Time
 }
@@ -182,14 +178,15 @@ type SignatureAlgorithm int
 
 const (
 	UnknownSignatureAlgorithm SignatureAlgorithm = iota
-	MD2WithRSA
+
-	MD5WithRSA
+	MD2WithRSA // Unsupported.
+	MD5WithRSA // Only supported for signing, not verification.
 	SHA1WithRSA
 	SHA256WithRSA
 	SHA384WithRSA
 	SHA512WithRSA
-	DSAWithSHA1
+	DSAWithSHA1   // Unsupported.
-	DSAWithSHA256
+	DSAWithSHA256 // Unsupported.
 	ECDSAWithSHA1
 	ECDSAWithSHA256
 	ECDSAWithSHA384
@@ -223,7 +220,7 @@ type PublicKeyAlgorithm int
 const (
 	UnknownPublicKeyAlgorithm PublicKeyAlgorithm = iota
 	RSA
-	DSA
+	DSA // Unsupported.
 	ECDSA
 	Ed25519
 )
@@ -845,28 +842,6 @@ func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey
 		} else {
 			return rsa.VerifyPKCS1v15(pub, hashType, signed, signature)
 		}
-	case *dsa.PublicKey:
-		if pubKeyAlgo != DSA {
-			return signaturePublicKeyAlgoMismatchError(pubKeyAlgo, pub)
-		}
-		dsaSig := new(dsaSignature)
-		if rest, err := asn1.Unmarshal(signature, dsaSig); err != nil {
-			return err
-		} else if len(rest) != 0 {
-			return errors.New("x509: trailing data after DSA signature")
-		}
-		if dsaSig.R.Sign() <= 0 || dsaSig.S.Sign() <= 0 {
-			return errors.New("x509: DSA signature contained zero or negative values")
-		}
-		// According to FIPS 186-3, section 4.6, the hash must be truncated if it is longer
-		// than the key length, but crypto/dsa doesn't do it automatically.
-		if maxHashLen := pub.Q.BitLen() / 8; maxHashLen < len(signed) {
-			signed = signed[:maxHashLen]
-		}
-		if !dsa.Verify(pub, signed, dsaSig.R, dsaSig.S) {
-			return errors.New("x509: DSA verification failure")
-		}
-		return
 	case *ecdsa.PublicKey:
 		if pubKeyAlgo != ECDSA {
 			return signaturePublicKeyAlgoMismatchError(pubKeyAlgo, pub)

src/crypto/x509/x509_test.go

@@ -988,51 +988,8 @@ func TestVerifyCertificateWithDSASignature(t *testing.T) {
 		t.Fatalf("Failed to parse certificate: %s", err)
 	}
 	// test cert is self-signed
-	if err = cert.CheckSignatureFrom(cert); err != nil {
+	if err = cert.CheckSignatureFrom(cert); err == nil {
-		t.Fatalf("DSA Certificate verification failed: %s", err)
+		t.Fatalf("Expected error verifying DSA certificate")
-	}
-}
-
-const dsaCert1024WithSha256 = `-----BEGIN CERTIFICATE-----
-MIIDKzCCAumgAwIBAgIUOXWPK4gTRZVVY7OSXTU00QEWQU8wCwYJYIZIAWUDBAMC
-MEUxCzAJBgNVBAYTAkFVMRMwEQYDVQQIDApTb21lLVN0YXRlMSEwHwYDVQQKDBhJ
-bnRlcm5ldCBXaWRnaXRzIFB0eSBMdGQwIBcNMTkxMDAxMDYxODUyWhgPMzAxOTAy
-MDEwNjE4NTJaMEUxCzAJBgNVBAYTAkFVMRMwEQYDVQQIDApTb21lLVN0YXRlMSEw
-HwYDVQQKDBhJbnRlcm5ldCBXaWRnaXRzIFB0eSBMdGQwggG4MIIBLAYHKoZIzjgE
-ATCCAR8CgYEAr79m/1ypU1aUbbLX1jikTyX7w2QYP+EkxNtXUiiTuxkC1KBqqxT3
-0Aht2vxFR47ODEK4B79rHO+UevhaqDaAHSH7Z/9umS0h0aS32KLDLb+LI5AneCrn
-eW5YbVhfD03N7uR4kKUCKOnWj5hAk9xiE3y7oFR0bBXzqrrHJF9LMd0CFQCB6lSj
-HSW0rGmNxIZsBl72u7JFLQKBgQCOFd1PGEQmddn0cdFgby5QQfjrqmoD1zNlFZEt
-L0x1EbndFwelLlF1ChNh3NPNUkjwRbla07FDlONs1GMJq6w4vW11ns+pUvAZ2+RM
-EVFjugip8az2ncn3UujGTVdFxnSTLBsRlMP/tFDK3ky//8zn/5ha9SKKw4v1uv6M
-JuoIbwOBhQACgYEAoeKeR90nwrnoPi5MOUPBLQvuzB87slfr+3kL8vFCmgjA6MtB
-7TxQKoBTOo5aVgWDp0lMIMxLd6btzBrm6r3VdRlh/cL8/PtbxkFwBa+Upe4o5NAh
-ISCe2/f2leT1PxtF8xxYjz/fszeUeHsJbVMilE2cuB2SYrR5tMExiqy+QpqjUzBR
-MB0GA1UdDgQWBBQDMIEL8Z3jc1d9wCxWtksUWc8RkjAfBgNVHSMEGDAWgBQDMIEL
-8Z3jc1d9wCxWtksUWc8RkjAPBgNVHRMBAf8EBTADAQH/MAsGCWCGSAFlAwQDAgMv
-ADAsAhQFehZgI4OyKBGpfnXvyJ0Z/0a6nAIUTO265Ane87LfJuQr3FrqvuCI354=
------END CERTIFICATE-----
-`
-
-func TestVerifyCertificateWithDSATooLongHash(t *testing.T) {
-	pemBlock, _ := pem.Decode([]byte(dsaCert1024WithSha256))
-	cert, err := ParseCertificate(pemBlock.Bytes)
-	if err != nil {
-		t.Fatalf("Failed to parse certificate: %s", err)
-	}
-
-	// test cert is self-signed
-	if err = cert.CheckSignatureFrom(cert); err != nil {
-		t.Fatalf("DSA Certificate self-signature verification failed: %s", err)
-	}
-
-	signed := []byte("A wild Gopher appears!\n")
-	signature, _ := hex.DecodeString("302c0214417aca7ff458f5b566e43e7b82f994953da84be50214625901e249e33f4e4838f8b5966020c286dd610e")
-
-	// This signature is using SHA256, but only has 1024 DSA key. The hash has to be truncated
-	// in CheckSignature, otherwise it won't pass.
-	if err = cert.CheckSignature(DSAWithSHA256, signed, signature); err != nil {
-		t.Fatalf("DSA signature verification failed: %s", err)
 	}
 }