crypto/x509: add test for asn1.Marshal of rsa.PublicKey

Go 1.10 is adding new API MarshalPKCS1PublicKey and
ParsePKCS1PublicKey for converting rsa.PublicKeys.

Even though we'd prefer that users did not, check that
if users call asn1.Marshal and asn1.Unmarshal directly instead,
they get the same results. We know that code exists in the
wild that depends on this.

Change-Id: Ia385d6954fda2eba7da228dc42f229b6839ef11e
Reviewed-on: https://go-review.googlesource.com/82080
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/x509_test.go

@@ -187,10 +187,28 @@ func TestMarshalRSAPublicKey(t *testing.T) {
 	derBytes := MarshalPKCS1PublicKey(pub)
 	pub2, err := ParsePKCS1PublicKey(derBytes)
 	if err != nil {
-		t.Errorf("error parsing serialized key: %s", err)
+		t.Errorf("ParsePKCS1PublicKey: %s", err)
 	}
 	if pub.N.Cmp(pub2.N) != 0 || pub.E != pub2.E {
-		t.Errorf("got:%+v want:%+v", pub, pub2)
+		t.Errorf("ParsePKCS1PublicKey = %+v, want %+v", pub, pub2)
+	}
+
+	// It's never been documented that asn1.Marshal/Unmarshal on rsa.PublicKey works,
+	// but it does, and we know of code that depends on it.
+	// Lock that in, even though we'd prefer that people use MarshalPKCS1PublicKey and ParsePKCS1PublicKey.
+	derBytes2, err := asn1.Marshal(*pub)
+	if err != nil {
+		t.Errorf("Marshal(rsa.PublicKey): %v", err)
+	} else if !bytes.Equal(derBytes, derBytes2) {
+		t.Errorf("Marshal(rsa.PublicKey) = %x, want %x", derBytes2, derBytes)
+	}
+	pub3 := new(rsa.PublicKey)
+	rest, err := asn1.Unmarshal(derBytes, pub3)
+	if err != nil {
+		t.Errorf("Unmarshal(rsa.PublicKey): %v", err)
+	}
+	if len(rest) != 0 || pub.N.Cmp(pub3.N) != 0 || pub.E != pub3.E {
+		t.Errorf("Unmarshal(rsa.PublicKey) = %+v, %q want %+v, %q", pub, rest, pub2, []byte(nil))
 	}
 
 	publicKeys := []struct {