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, 29 insertions, 0 deletions

diff --git a/internal/stupidgcm/openssl_aead.h b/internal/stupidgcm/openssl_aead.h
new file mode 100644
index 0000000..6a818b6
--- /dev/null
+++ b/internal/stupidgcm/openssl_aead.h
@@ -0,0 +1,29 @@
+#include <openssl/evp.h>
+
+int openssl_aead_seal(
+    const EVP_CIPHER* evpCipher,
+    const unsigned char* const plaintext,
+    const int plaintextLen,
+    const unsigned char* const authData,
+    const int authDataLen,
+    const unsigned char* const key,
+    const int keyLen,
+    const unsigned char* const iv,
+    const int ivLen,
+    unsigned char* const ciphertext,
+    const int ciphertextBufLen);
+
+int openssl_aead_open(
+    const EVP_CIPHER* evpCipher,
+    const unsigned char* const ciphertext,
+    const int ciphertextLen,
+    const unsigned char* const authData,
+    const int authDataLen,
+    unsigned char* const tag,
+    const int tagLen,
+    const unsigned char* const key,
+    const int keyLen,
+    const unsigned char* const iv,
+    const int ivLen,
+    unsigned char* const plaintext,
+    const int plaintextBufLen);