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, 68 insertions, 0 deletions

diff --git a/internal/stupidgcm/common.go b/internal/stupidgcm/common.go
new file mode 100644
index 0000000..3788315
--- /dev/null
+++ b/internal/stupidgcm/common.go
@@ -0,0 +1,68 @@
+package stupidgcm
+
+import (
+	"log"
+)
+
+/*
+#include <openssl/evp.h>
+*/
+import "C"
+
+type stupidAEADCommon struct {
+	wiped            bool
+	key              []byte
+	openSSLEVPCipher *C.EVP_CIPHER
+	nonceSize        int
+}
+
+// Overhead returns the number of bytes that are added for authentication.
+//
+// Part of the cipher.AEAD interface.
+func (c *stupidAEADCommon) Overhead() int {
+	return tagLen
+}
+
+// NonceSize returns the required size of the nonce / IV
+//
+// Part of the cipher.AEAD interface.
+func (c *stupidAEADCommon) NonceSize() int {
+	return c.nonceSize
+}
+
+// Seal encrypts "in" using "iv" and "authData" and append the result to "dst"
+//
+// Part of the cipher.AEAD interface.
+func (c *stupidAEADCommon) Seal(dst, iv, in, authData []byte) []byte {
+	return openSSLSeal(c, dst, iv, in, authData)
+}
+
+// Open decrypts "in" using "iv" and "authData" and append the result to "dst"
+//
+// Part of the cipher.AEAD interface.
+func (c *stupidAEADCommon) Open(dst, iv, in, authData []byte) ([]byte, error) {
+	return openSSLOpen(c, dst, iv, in, authData)
+}
+
+// Wipe tries to wipe the key from memory by overwriting it with zeros.
+//
+// This is not bulletproof due to possible GC copies, but
+// still raises the bar for extracting the key.
+func (c *stupidAEADCommon) Wipe() {
+	key := c.key
+	c.wiped = true
+	c.key = nil
+	for i := range key {
+		key[i] = 0
+	}
+}
+
+func (c *stupidAEADCommon) Wiped() bool {
+	if c.wiped {
+		return true
+	}
+	if len(c.key) != keyLen {
+		log.Panicf("wrong key length %d", len(c.key))
+	}
+	return false
+}