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, 17 insertions, 51 deletions

diff --git a/internal/stupidgcm/stupidchacha.go b/internal/stupidgcm/stupidchacha.go
index 18037c6..5073aa3 100644
--- a/internal/stupidgcm/stupidchacha.go
+++ b/internal/stupidgcm/stupidchacha.go
@@ -6,7 +6,6 @@ import (
 	"crypto/cipher"
 	"fmt"
 	"log"
-	"unsafe"
 
 	"golang.org/x/crypto/chacha20poly1305"
 )
@@ -118,59 +117,26 @@ func (g *stupidChacha20poly1305) Open(dst, iv, in, authData []byte) ([]byte, err
 	ciphertext := in[:len(in)-tagLen]
 	tag := in[len(in)-tagLen:]
 
-	// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode
-
-	// Create scratch space "context"
-	ctx := C.EVP_CIPHER_CTX_new()
-	if ctx == nil {
-		log.Panic("EVP_CIPHER_CTX_new failed")
-	}
-
-	// Set cipher to AES-256
-	if C.EVP_DecryptInit_ex(ctx, C.EVP_chacha20_poly1305(), nil, nil, nil) != 1 {
-		log.Panic("EVP_DecryptInit_ex I failed")
-	}
-
-	// Set key and IV
-	if C.EVP_DecryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
-		log.Panic("EVP_DecryptInit_ex II failed")
-	}
-
-	// Set expected MAC tag
-	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_AEAD_SET_TAG, tagLen, (unsafe.Pointer)(&tag[0])) != 1 {
-		log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_AEAD_SET_TAG failed")
-	}
-
-	// Provide authentication data
-	var resultLen C.int
-	if C.EVP_DecryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
-		log.Panic("EVP_DecryptUpdate authData failed")
-	}
-	if int(resultLen) != len(authData) {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Decrypt "ciphertext" into "buf"
-	if C.EVP_DecryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&ciphertext[0]), C.int(len(ciphertext))) != 1 {
-		log.Panic("EVP_DecryptUpdate failed")
-	}
-	if int(resultLen) != len(ciphertext) {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Check MAC
-	dummy := make([]byte, 16)
-	res := C.EVP_DecryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen)
-	if resultLen != 0 {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Free scratch space
-	C.EVP_CIPHER_CTX_free(ctx)
+	res := int(C.aead_open(C.aeadTypeChacha,
+		(*C.uchar)(&ciphertext[0]),
+		C.int(len(ciphertext)),
+		(*C.uchar)(&authData[0]),
+		C.int(len(authData)),
+		(*C.uchar)(&tag[0]),
+		C.int(len(tag)),
+		(*C.uchar)(&g.key[0]),
+		C.int(len(g.key)),
+		(*C.uchar)(&iv[0]),
+		C.int(len(iv)),
+		(*C.uchar)(&buf[0]),
+		C.int(len(buf))))
 
-	if res != 1 {
+	if res < 0 {
 		return nil, ErrAuth
 	}
+	if res != outLen {
+		log.Panicf("unexpected length %d", res)
+	}
 
 	if inplace {
 		return dst[:len(dst)+outLen], nil