question about time attack

[d.a.glynos]

Eukolo! Prose3e ta sxolia parakatw. 8a antistrepsw ta paradeigmata sou
prokeimenou na fanei pio eukola h diafora. Prwta e3etazw thn ekdosh pou
einai vulnerable:

> int password-check( char *inp, char *pwd) {
> if (strlen(inp) != strlen(pwd)) return 0;

se auth th sun8hkh o evil-do-er [1] mporei na ma8ei ton ari8mo twn
grammatwn tou password sou ka8ws se mia kai mono periptwsh (an dld
valei opoiodipote password me to idio string length) h sunarthsh
8a epistrepsei amesa.

> for( i=0; i < strlen(pwd); ++i)
> if ( inp[i] != pwd[i] )
> return 0;
> return 1;
> }

As to kanoume ligo indent na fanei ti grafeis:

	for (i=0; i<strlen(pwd); i++)
		if ( inp[i] != pwd[i] )
			return 0;
	return 1;

Edw ta pragmata einai akomh xeirotera. An vrei estw kai ena diaforetiko
gramma, epistrefei amesa. Skepsou to senario opou o evil-do-er,
gnwrizontas pleon ton ari8mo twn grammatwn tou password, dokimazei
na vrei ta grammata ena-ena (seiriaka apo aristera pros ta deksia)
dhl apo 'xxxxxxxx' se 'xxxxxxxx'
         ^                    ^

O algori8mos 8a ginetai olo kai pio argos ka8e fora pou 8a vriskei to
'epi8umhto' gramma tou password.

As e3etasoume kai thn or8h periptwsh:

int password-check( char *inp, char *pwd)
{
        oklen = 1;
        /* e3etazoume an exoun idio mege8os kai se periptwsh pou den
           exoun auto 8a apotelesei sfalma sto telos tou programmatos */
	if (strlen(inp) != strlen(pwd)) oklen=0;

	/* to for loop 8a leitourghsei e3etazontas OLA ta grammata
           se OLES tis periptwseis (akomh kai otan vrei estw kai ena
           diaforetiko). An htan telika diaforetika, auto 8a syneisferei
           sto 'sfalma' sto telos tou programmatos */
	for( ok=1, i=0; i < strlen(pwd); ++i)
		if ( inp[i] != pwd[i] )
			ok = ok & 0;
		else
			ok = ok & 1;

	/* Mexri edw ftanoume se oles tis periptwseis (idio/diaforetiko
	mege8os, idia/diaforetika grammata) me ton idio ari8mo vhmatwn
	(cpu instructions an 8es). Synepws mazi me to teleutaio vhma
        den epitrepoume ston evil-do-er na sxhmatisei apopsh
	gia to password me vash to xrono pou phre gia na ektelestoun oi
        parapanw instructions */

	return ok & oklen;
}

Elpizw ta sxolia na einai katanohta.

dimitris