stupidgcm: stupidChacha20poly1305.Open: batch C calls in aead_open

Gets the decryption speed to the same level as the
encryption speed.

internal/speed$ benchstat old.txt new.txt
name                    old time/op    new time/op    delta
StupidXchacha-4          732MB/s ± 0%   740MB/s ± 0%   ~     (p=1.000 n=1+1)
StupidXchachaDecrypt-4   602MB/s ± 0%   741MB/s ± 0%   ~     (p=1.000 n=1+1)

1 files changed, 15 insertions, 0 deletions

diff --git a/internal/stupidgcm/chacha.h b/internal/stupidgcm/chacha.h
index 780350a..a5eac04 100644
--- a/internal/stupidgcm/chacha.h
+++ b/internal/stupidgcm/chacha.h
@@ -15,3 +15,18 @@ int aead_seal(
     const int ivLen,
     unsigned char* const ciphertext,
     const int ciphertextBufLen);
+
+int aead_open(
+    const enum aeadType cipherId,
+    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);