phc-discussions - Re: [PHC] Argon2 using memset still

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Date: Thu, 27 Aug 2015 17:48:46 +0100
From: Samuel Neves <sneves@....uc.pt>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Argon2 using memset still

On 08/27/2015 05:33 PM, Bill Cox wrote:
> g++ has a habit of optimizing calls to memset away.  I doubt that Argon2
> succeed in it's attempts to clear memory.  I copied this function from
> Blake2:
>
> /* prevents compiler optimizing out memset() */
> static inline void secure_zero_memory( void *v, size_t n )
> {
>   volatile uint8_t *p = ( volatile uint8_t * )v;
>   while( n-- ) *p++ = 0;
> }

Since Argon presumably clears larger amounts of data than BLAKE2 has to, it might be more convenient to use the
alternative implementation for this:

  static void secure_zero_memory( void *v, size_t n ) {
    static void* (* const volatile memset_s)(void*, int, size_t) = &memset;
    memset_s(v, 0, n);
  }

memset_s is, of course, also a C11 function that is guaranteed to have the semantics we want here.  But not available
anywhere interesting yet, as far as I know.