[Go] Failure with in-place Seal/Open

[conradoplg]

According to Seal/Open documentations (and the cipher.AEAD interface), they should work when dst and plaintext (resp. ciphertext) overlap entirely.

However, that doesn't work. This test fails:

func TestAESCMACSIVInPlace(t *testing.T) {
	v := loadAESSIVExamples("aes_siv.tjson")[0]

	c, err := NewAESCMACSIV(v.key)
	if err != nil {
		t.Fatalf("NewAESCMACSIV: %s", err)
	}
	pt := make([]byte, len(v.plaintext), len(v.plaintext)+c.Overhead())
	copy(pt, v.plaintext)
	ct, err := c.Seal(pt[:0], pt, v.ad...)
	if err != nil {
		t.Errorf("Seal: %s", err)
	}
	if !bytes.Equal(v.ciphertext, ct) {
		t.Errorf("Seal: expected: %x\ngot: %x", v.ciphertext, ct)
	}

	copy(ct, v.ciphertext)
	pt, err = c.Open(ct[:0], ct, v.ad...)
	if err != nil {
		t.Errorf("Open: %s", err)
	}
	if !bytes.Equal(v.plaintext, pt) {
		t.Errorf("Open: expected: %x\ngot: %x", v.plaintext, pt)
	}
}

I can write a MR fixing this, however, since the tag comes before the ciphertext it seems that will require making a whole copy of the input in this case (since XORKeyStream alse requires its input to overlap entirely or not at all), entirely defeating the purpose of the in-place optimization. However, I don't see a way out without breaking the cipher.AEAD contract, or moving the tag after the ciphertext (#152)

What do you think?

[conradoplg]

There is another way to fix this: use copy which does support overlapped arguments to shuffle things around when doing in-place operations.

The two branches mentioned above implement the two strategies. Which one do you prefer, if you're going down this route?