phc-discussions - Re: [PHC] Specification of a modular crypt format (2)

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Message-ID: <560DD7D1.6000901@openwall.com>
Date: Fri, 02 Oct 2015 04:03:13 +0300
From: Alexander Cherepanov <ch3root@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Specification of a modular crypt format (2)

On 2015-09-30 04:59, Peter Gutmann wrote:
> Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com> writes:
>
>> @Thomas: let's copy the updated source code on a new shared doc https://docs.google.com/document/d/13bv-3HarFpQOD1PayJ2hMvRtVIs5QPu1FZ5XonEv57c/edit?usp=sharing ?
>
> I've waited awhile after seeing this but I'm still just getting a blank page,
> is there supposed to be content there?  I was curious to compare the final
> version with my code, which is (in part):
 >
> 	/* Process the numeric string */
> 	for( value = 0, i = 0; i < strLen; i++ )
> 		{
> 		const int valTmp = value * 10;

It's hard to guarantee it without seeing a full program but this 
multiplication very much looks like a signed integer overflow which is 
undefined behavior (UB).

> 		const int ch = byteToInt( str[ i ] ) - '0';
>
> 		if( ch < 0 || ch > 9 )
> 			return( CRYPT_ERROR_BADDATA );
> 		if( value >= ( MAX_INTLENGTH / 10 ) || \
> 			valTmp >= MAX_INTLENGTH - ch )
> 			return( CRYPT_ERROR_BADDATA );
> 		value = valTmp + ch;
> 		if( value < 0 || value > MAX_INTLENGTH )
> 			return( CRYPT_ERROR_BADDATA );
> 		}

The last check seems to be superfluous or at least ineffective. If 
"value" is overflown UB is already triggered. To put it another way 
optimizing compilers will probably remove it anyway. Are you compiling 
without optimizations?

-- 
Alexander Cherepanov