speed: add decryption benchmarks

gocryptfs/internal/speed$ go test -bench .
goos: linux
goarch: amd64
pkg: github.com/rfjakob/gocryptfs/v2/internal/speed
cpu: Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz
BenchmarkStupidGCM-4              	  263742	      4523 ns/op	 905.61 MB/s
BenchmarkStupidGCMDecrypt-4       	  204858	      5779 ns/op	 708.76 MB/s
BenchmarkGoGCM-4                  	  291259	      4095 ns/op	1000.25 MB/s
BenchmarkGoGCMDecrypt-4           	  293886	      4061 ns/op	1008.53 MB/s
BenchmarkAESSIV-4                 	   46537	     25538 ns/op	 160.39 MB/s
BenchmarkAESSIVDecrypt-4          	   46770	     25627 ns/op	 159.83 MB/s
BenchmarkXchacha-4                	  243619	      4893 ns/op	 837.03 MB/s
BenchmarkXchachaDecrypt-4         	  248857	      4793 ns/op	 854.51 MB/s
BenchmarkStupidXchacha-4          	  213717	      5558 ns/op	 736.99 MB/s
BenchmarkStupidXchachaDecrypt-4   	  176635	      6782 ns/op	 603.96 MB/s
PASS
ok  	github.com/rfjakob/gocryptfs/v2/internal/speed	12.871s

2 files changed, 62 insertions, 4 deletions

diff --git a/internal/speed/speed.go b/internal/speed/speed.go
index 37b9daf..a696703 100644
--- a/internal/speed/speed.go
+++ b/internal/speed/speed.go
@@ -92,6 +92,25 @@ func bEncrypt(b *testing.B, c cipher.AEAD) {
 
 }
 
+func bDecrypt(b *testing.B, c cipher.AEAD) {
+	authData := randBytes(adLen)
+	iv := randBytes(c.NonceSize())
+	plain := randBytes(blockSize)
+	ciphertext := c.Seal(iv, iv, plain, authData)
+
+	b.SetBytes(int64(len(plain)))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		// Reset plain buffer
+		plain = plain[:0]
+		// Decrypt
+		_, err := c.Open(plain, iv, ciphertext[c.NonceSize():], authData)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
 // bStupidGCM benchmarks stupidgcm's openssl GCM
 func bStupidGCM(b *testing.B) {
 	if stupidgcm.BuiltWithoutOpenssl {
diff --git a/internal/speed/speed_test.go b/internal/speed/speed_test.go
index a6f3f30..4d09148 100644
--- a/internal/speed/speed_test.go
+++ b/internal/speed/speed_test.go
@@ -1,5 +1,16 @@
 package speed
 
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"testing"
+
+	"golang.org/x/crypto/chacha20poly1305"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/siv_aead"
+	"github.com/rfjakob/gocryptfs/v2/internal/stupidgcm"
+)
+
 /*
 Make the "-speed" benchmarks also accessible to the standard test system.
 Example run:
@@ -12,26 +23,54 @@ PASS
 ok  	github.com/rfjakob/gocryptfs/v2/internal/speed	6.022s
 */
 
-import (
-	"testing"
-)
-
 func BenchmarkStupidGCM(b *testing.B) {
 	bStupidGCM(b)
 }
 
+func BenchmarkStupidGCMDecrypt(b *testing.B) {
+	if stupidgcm.BuiltWithoutOpenssl {
+		b.Skip("openssl has been disabled at compile-time")
+	}
+	bDecrypt(b, stupidgcm.New(randBytes(32), false))
+}
+
 func BenchmarkGoGCM(b *testing.B) {
 	bGoGCM(b)
 }
 
+func BenchmarkGoGCMDecrypt(b *testing.B) {
+	gAES, err := aes.NewCipher(randBytes(32))
+	if err != nil {
+		b.Fatal(err)
+	}
+	gGCM, err := cipher.NewGCMWithNonceSize(gAES, 16)
+	if err != nil {
+		b.Fatal(err)
+	}
+	bDecrypt(b, gGCM)
+}
+
 func BenchmarkAESSIV(b *testing.B) {
 	bAESSIV(b)
 }
 
+func BenchmarkAESSIVDecrypt(b *testing.B) {
+	bEncrypt(b, siv_aead.New(randBytes(64)))
+}
+
 func BenchmarkXchacha(b *testing.B) {
 	bXchacha20poly1305(b)
 }
 
+func BenchmarkXchachaDecrypt(b *testing.B) {
+	c, _ := chacha20poly1305.NewX(randBytes(32))
+	bDecrypt(b, c)
+}
+
 func BenchmarkStupidXchacha(b *testing.B) {
 	bStupidXchacha(b)
 }
+
+func BenchmarkStupidXchachaDecrypt(b *testing.B) {
+	bDecrypt(b, stupidgcm.NewXchacha20poly1305(randBytes(32)))
+}