address comments

source/darwin/commoncrypto_aes.c

@@ -511,8 +511,6 @@ static int s_initialize_gcm_cipher_materials(
         }
     }
 
-    aws_byte_buf_clean_up_secure(&cc_cipher->cipher_base.tag);
-
     status = CCCryptorCreateWithMode(
         kCCDecrypt,
         kCCModeGCM,
@@ -553,6 +551,7 @@ static int s_gcm_reset(struct aws_symmetric_cipher *cipher) {
     struct cc_aes_cipher *cc_cipher = cipher->impl;
 
     int ret_val = s_reset(cipher);
+    aws_byte_buf_clean_up_secure(&cc_cipher->cipher_base.tag);
 
     if (ret_val == AWS_OP_SUCCESS) {
         ret_val = s_initialize_gcm_cipher_materials(cc_cipher, NULL, NULL, NULL);

source/unix/openssl_aes.c

@@ -335,7 +335,7 @@ static int s_gcm_decrypt(struct aws_symmetric_cipher *cipher, struct aws_byte_cu
 static int s_finalize_gcm_encryption(struct aws_symmetric_cipher *cipher, struct aws_byte_buf *out) {
     struct openssl_aes_cipher *openssl_cipher = cipher->impl;
 
-    if (!cipher->tag.len) {
+    if (cipher->tag.buffer == NULL) {
         aws_byte_buf_init(&cipher->tag, cipher->allocator, AWS_AES_256_CIPHER_BLOCK_SIZE);
     }
 

source/windows/bcrypt_aes.c

@@ -537,7 +537,7 @@ struct aws_symmetric_cipher *aws_aes_cbc_256_new_impl(
 }
 
 /*
- * The buffer management for this mode is a good deal easier because we don't care about padding.
+ * The buffer management for gcm mode is a good deal easier than ctr and cbc modes because we don't care about padding.
  * In chained mode, BCrypt expects the data to be passed in in multiples of block size,
  * followed by a finalize call that turns off chaining and provides any remaining data.
  * This function takes care of managing this state - you give it data to work and cipher state and
@@ -559,9 +559,10 @@ static struct aws_byte_cursor s_gcm_get_working_slice(
     AWS_ZERO_STRUCT(working_cur);
     /* If there's overflow, prepend it to the working buffer, then append the data */
     if (cipher_impl->overflow.len) {
-        aws_byte_buf_init_copy(scratch, cipher_impl->cipher.allocator, &cipher_impl->overflow);
+        aws_byte_buf_init(scratch, cipher_impl->cipher.allocator, cipher_impl->overflow.len + data.len);        
+        aws_byte_buf_append(scratch, &cipher_impl->overflow);
         aws_byte_buf_reset(&cipher_impl->overflow, true);
-        aws_byte_buf_append_dynamic(scratch, &data);
+        aws_byte_buf_append(scratch, &data);
         working_cur = aws_byte_cursor_from_buf(scratch);
     } else {
         working_cur = data;