stupidgcm: use aead_seal for gcm as well

$ benchstat old.txt new.txt
name         old time/op   new time/op   delta
StupidGCM-4   7.87µs ± 1%   6.64µs ± 2%  -15.65%  (p=0.000 n=10+10)

name         old speed     new speed     delta
StupidGCM-4  520MB/s ± 1%  617MB/s ± 2%  +18.56%  (p=0.000 n=10+10)

internal/stupidgcm/stupidgcm.go

@@ -5,6 +5,7 @@
 package stupidgcm
 
 // #include <openssl/evp.h>
+// #include "chacha.h"
 // #cgo pkg-config: libcrypto
 import "C"
 
@@ -76,63 +77,17 @@ func (g *StupidGCM) Seal(dst, iv, in, authData []byte) []byte {
 		buf = make([]byte, outLen)
 	}
 
-	// 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_EncryptInit_ex(ctx, C.EVP_aes_256_gcm(), nil, nil, nil) != 1 {
-		log.Panic("EVP_EncryptInit_ex I failed")
-	}
-
-	// Use 16-byte IV
-	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_IVLEN, ivLen, nil) != 1 {
-		log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_SET_IVLEN failed")
-	}
-
-	// Set key and IV
-	if C.EVP_EncryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
-		log.Panic("EVP_EncryptInit_ex II failed")
-	}
-
-	// Provide authentication data
-	var resultLen C.int
-	if C.EVP_EncryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
-		log.Panic("EVP_EncryptUpdate authData failed")
-	}
-	if int(resultLen) != len(authData) {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Encrypt "in" into "buf"
-	if C.EVP_EncryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&in[0]), C.int(len(in))) != 1 {
-		log.Panic("EVP_EncryptUpdate failed")
-	}
-	if int(resultLen) != len(in) {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Finalise encryption
-	// Because GCM is a stream encryption, this will not write out any data.
-	dummy := make([]byte, 16)
-	if C.EVP_EncryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen) != 1 {
-		log.Panic("EVP_EncryptFinal_ex failed")
-	}
-	if resultLen != 0 {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Get GMAC tag and append it to the ciphertext in "buf"
-	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_GET_TAG, tagLen, (unsafe.Pointer)(&buf[len(in)])) != 1 {
-		log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_GET_TAG failed")
-	}
-
-	// Free scratch space
-	C.EVP_CIPHER_CTX_free(ctx)
+	C.aead_seal(C.aeadTypeGcm,
+		(*C.uchar)(&in[0]),
+		C.int(len(in)),
+		(*C.uchar)(&authData[0]),
+		C.int(len(authData)),
+		(*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 inplace {
 		return dst[:len(dst)+outLen]