stupidgcm: add GCM-SIV benchmark

On a CPU without AES-NI:

$ go test -bench .
Benchmark4kEncStupidGCM-2   	   50000	     24155 ns/op	 169.57 MB/s
Benchmark4kEncGoGCM-2       	   20000	     93965 ns/op	  43.59 MB/s
Benchmark4kEncGCMSIV-2      	     500	   2576193 ns/op	   1.59 MB/s

internal/stupidgcm/stupidgcm_test.go

@@ -14,6 +14,8 @@ import (
 	"crypto/rand"
 	"encoding/hex"
 	"testing"
+
+	"github.com/rfjakob/gcmsiv"
 )
 
 // Get "n" random bytes from /dev/urandom or panic
@@ -159,3 +161,19 @@ func Benchmark4kEncGoGCM(b *testing.B) {
 		gGCM.Seal(iv, iv, in, authData)
 	}
 }
+
+func Benchmark4kEncGCMSIV(b *testing.B) {
+	key := randBytes(32)
+	authData := randBytes(24)
+	iv := randBytes(16)
+	in := make([]byte, 4096)
+	b.SetBytes(int64(len(in)))
+	gGCM, err := gcmsiv.NewGCMSIV(key)
+	if err != nil {
+		b.Fatal(err)
+	}
+	for i := 0; i < b.N; i++ {
+		// Encrypt and append to nonce
+		gGCM.Seal(iv, iv, in, authData)
+	}
+}