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char *secret;

/* initialize secret */

size_t secret_size = strlen(secret);
char *temp_buff;
/* ... */
if (secret_size > SIZE_MAX/2) {
   /* handle error condition */
}
/* calloc() initializes memory to zero */
temp_buff = (char *)calloc(secret_size * 2, sizeof(char));
if (temp_buff == NULL) {
 /* Handle Error */
}

memcpy(temp_buff, secret, secret_size);

/* sanitize the buffer */
memset(secret, '\0', secret_size);

free(secret);
secret = temp_buff; /* install the resized buffer */
temp_buff = NULL;