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

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Date: Tue, 29 Sep 2015 03:05:59 +0300
From: Alexander Cherepanov <ch3root@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Specification of a modular crypt format (2)

On 2015-09-28 14:12, Thomas Pornin wrote:
> /*
>   * Decode an Argon2i hash string into the provided structure 'pp'.
>   * Returned value is 1 on success, 0 on error.
>   */
> int
> argon2i_decode_string(argon2i_params *pp, const char *str)
> {
> #define CC(prefix)   do { \
> 		size_t cc_len = strlen(prefix); \
> 		if (strncmp(str, prefix, cc_len) != 0) { \
> 			return 0; \
> 		} \
> 		str += cc_len; \
> 	} while (0)
>
> #define CC_opt(prefix, code)   do { \
> 		size_t cc_len = strlen(prefix); \
> 		if (strncmp(str, prefix, cc_len) == 0) { \
> 			str += cc_len; \
> 			{ code; } \
> 		} \
> 	} while (0)
>
> #define DECIMAL(x)   do { \
> 		char *dec_ep; \
> 		(x) = strtoul(str, &dec_ep, 10); \
 > 		if (dec_ep == str) { \

AIUI the decoder tries to be strict. Then perhaps it's worth to check 
that 'str' doesn't start with ' ' or '-' here and try to catch overflows.

Right now decoder accepts the following strings:

"$argon2i$m= 120,t=5000,p=2"
"$argon2i$m=-120,t=5000,p=2"
"$argon2i$m=120000000000000000000000000000000000000,t=5000,p=2"

(the last two only in 32-bit build due to the bounds checks later in the 
code).

> 			return 0; \
> 		} \
> 		str = (const char *)dec_ep; \
> 	} while (0)
>
> #define BIN(buf, max_len, len)   do { \
> 		size_t bin_len = (max_len); \
> 		str = from_base64(buf, &bin_len, str); \
> 		if (str == NULL) { \
> 			return 0; \
> 		} \
> 		(len) = bin_len; \
> 	} while (0)
>
> 	pp->key_id_len = 0;
> 	pp->associated_data_len = 0;
> 	pp->salt_len = 0;
> 	pp->output_len = 0;
> 	CC("$argon2i");
> 	CC("$m=");
> 	DECIMAL(pp->m);
> 	CC(",t=");
> 	DECIMAL(pp->t);
> 	CC(",p=");
> 	DECIMAL(pp->p);
> 	if (pp->m < 1 || (pp->m >> 30) > 3) {

Why not just compare with a constant here?

> 		return 0;
> 	}
> 	if (pp->t < 1 || (pp->t >> 30) > 3) {
> 		return 0;
> 	}
> 	if (pp->p < 1 || pp->p > 64) {

255 here?

> 		return 0;
> 	}
> 	if (pp->m < (pp->p << 3)) {
> 		return 0;
> 	}
> 	CC_opt(",keyid=", BIN(pp->key_id, sizeof pp->key_id, pp->key_id_len));
> 	CC_opt(",data=", BIN(pp->associated_data, sizeof pp->associated_data,
> 		pp->associated_data_len));
> 	if (*str == 0) {
> 		return 1;
> 	}
> 	CC("$");
> 	BIN(pp->salt, sizeof pp->salt, pp->salt_len);
> 	if (pp->salt_len < 8) {
> 		return 0;
> 	}
> 	if (*str == 0) {
> 		return 1;
> 	}
> 	CC("$");
> 	BIN(pp->output, sizeof pp->output, pp->output_len);
> 	if (pp->output_len < 12) {
> 		return 0;
> 	}
> 	return *str == 0;
>
> #undef CC
> #undef CC_opt
> #undef DECIMAL
> #undef BIN
> }

-- 
Alexander Cherepanov