stupidgcm: introduce stupidAEADCommon and use for both chacha & gcm

Nice deduplication and brings the GCM decrypt speed up to par.

internal/speed$ benchstat old new
name                old time/op   new time/op   delta
StupidGCM-4          4.71µs ± 0%   4.66µs ± 0%   -0.99%  (p=0.008 n=5+5)
StupidGCMDecrypt-4   5.77µs ± 1%   4.51µs ± 0%  -21.80%  (p=0.008 n=5+5)

name                old speed     new speed     delta
StupidGCM-4         870MB/s ± 0%  879MB/s ± 0%   +1.01%  (p=0.008 n=5+5)
StupidGCMDecrypt-4  710MB/s ± 1%  908MB/s ± 0%  +27.87%  (p=0.008 n=5+5)

1 files changed, 20 insertions, 0 deletions

diff --git a/internal/stupidgcm/xchacha_test.go b/internal/stupidgcm/xchacha_test.go
new file mode 100644
index 0000000..fdea8b5
--- /dev/null
+++ b/internal/stupidgcm/xchacha_test.go
@@ -0,0 +1,20 @@
+// +build !without_openssl
+
+package stupidgcm
+
+import (
+	"testing"
+
+	"golang.org/x/crypto/chacha20poly1305"
+)
+
+func TestStupidXchacha20poly1305(t *testing.T) {
+	key := randBytes(32)
+	c := NewXchacha20poly1305(key)
+	ref, err := chacha20poly1305.NewX(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testCiphers(t, c, ref)
+}