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, 35 insertions, 0 deletions

diff --git a/internal/stupidgcm/chacha.go b/internal/stupidgcm/chacha.go
new file mode 100644
index 0000000..37f7e1f
--- /dev/null
+++ b/internal/stupidgcm/chacha.go
@@ -0,0 +1,35 @@
+// +build !without_openssl
+
+package stupidgcm
+
+import (
+	"crypto/cipher"
+	"log"
+
+	"golang.org/x/crypto/chacha20poly1305"
+)
+
+/*
+#include <openssl/evp.h>
+*/
+import "C"
+
+type stupidChacha20poly1305 struct {
+	stupidAEADCommon
+}
+
+// Verify that we satisfy the cipher.AEAD interface
+var _ cipher.AEAD = &stupidChacha20poly1305{}
+
+func newChacha20poly1305(key []byte) *stupidChacha20poly1305 {
+	if len(key) != chacha20poly1305.KeySize {
+		log.Panicf("Only %d-byte keys are supported, you passed %d bytes", chacha20poly1305.KeySize, len(key))
+	}
+	return &stupidChacha20poly1305{
+		stupidAEADCommon{
+			key:              append([]byte{}, key...), // private copy
+			openSSLEVPCipher: C.EVP_chacha20_poly1305(),
+			nonceSize:        chacha20poly1305.NonceSize,
+		},
+	}
+}