TRNG I HC BCH KHOA H NI Vin Cng ngh Thng tin v Truyn thng

BI TP LN

Mn hc: AN TON V BO MT THNG TIN

TI: K thut tn cng Padding oracle attack S hiu nhm: Ging vin hng dn: Nguyn Phi L Sinh vin thc hin: H tn Trn Quang Chin Nguyn Bo Chung Saysana Chanthavong Vnh SHSV 20090304 20090333 Lp CNTT1-K54 CNTT1-K54 CNTT1-K54 CNTT1-K54

MC LC

Phn 1 CC KHI NIM C BN

I. M ho i xng 1. Khi nim S dng cng mt kha b mt m ha v gii m. Hm m ha v gii m ging ht nhau. V d: DES , 3DES , AES , RC5 , IDEA v..v.

iu kin an ton: Khi mt ngi no c c mt hay nhiu chui bit c m ha , ngi cng khng c cch no gii m c mu tin ban u , tr khi ngi bit c "secret key" dng cho m ha. Secret key phi c trao i mt cch an ton gia hai party tham gia vo qu trnh m ho 2. u nhc im a. u im: Tn t ti nguyn Tc thc hin nhanh. b. Nhc im: Vn qun l kho (To, lu mt, trao chuyn ) l rt phc tp v ngy cng kh khi s dng trong mi trng trao i gia rt nhiu ngi dng. Vi s lng ngi dng l n th s lng kho

cn to lp l n(n-1)/2. Mi ngi dng phi to v lu n-1 kho b mt lm vic vi n-1 ngi khc trn mng. Nh vy rt kh khn v khng an ton khi n tng ln. Vn th 2 l trn c s m i xng khng th thit lp c khi nim ch k in t (m thc hin c chc nng ca ch k tht) v cng do m khng c dch v non-repudiation (khng th ph nhn c) cho cc giao dch thng mi trn mng. II. M ho bt i xng 1. Khi nim Mt m ha kha cng khai l mt dng mt m ha cho php ngi s dng trao i cc thng tin mt m khng cn phi trao i cc kha chung b mt trc . iu ny c thc hin bng cch s dng mt cp kha c quan h ton hc vi nhau l kha cng khai v kha c nhn (hay kha b mt). Vd: Mt ngi (Alice) c 1 cp kho gm kho cng khai v kho b mt. Bob mun gi mt ti liu cho Alice, Bob s dng kho cng khai ca Alice m ho. Khi Alice nhn c bn m, Alice s dng kho b mt gii m.

iu kin an ton: kho b mt phi c gi b mt.

2. u nhc im a. u im D dng qun l kho (bo mt, trao i). c dng to ch k in t, dng trong cc dch v nonrepudiation. b. Nhc im Tc m ho chm hn so vi m ho i xng. Kch thc ca kho mt ln hn rt nhiu -> tn ng truyn. III. M ho dng Khi nim: L dng m ho dng mt dng kho z = z1z2 . . . v dng n m ho mt xu bn r x = x1x2 . . . theo quy tc: y = y1y2. . . = ez1(x1) ez2(x1). . .

M dng hot ng nh sau. Gi s K K l kho v x = x1x2 . . .l xu bn r. Hm fi c dng to zi (zi l phn t th i ca dng kho) trong fi l mt hm ca kho K v i-1 l k t u tin ca bn r:

zi = fi (K, x1 , . . ., xi -1 ) Phn t zi ca dng kho c dng m xi to ra yi = eiz(xi). Bi vy, m ho xu bn r x1 x2 . . . ta phi tnh lin tip: z1, y1, z2 , y2 ... Vic gii m xu bn m y1y2. . . c th c thc hin bng cch tnh lin tip: z1, x1, z2 , x2 ... IV. M ho khi(Block ciphers) 1. Khi nim: L dng m ho trong tng khi k t c m ho cng mt lc nh mt n v m ho duy nht. Trong dng m ho bock ciphers ny, cc tham s quan trng l di khi v kch thc kho.

iu kin an ton Kch thc khi phi ln chng li cc loi tn cng ph hoi bng phng php thng k Khng gian kho phi ln chng li tm kim vt cn, ngn vic lm kho, phn phi v lu tr hiu qu. V. Ch m mc xch CBC 1. Khi nim

L ch m mi khi tin trc khi c m ho th c XOR vi khi m sinh ra t trc . Khi u tin s c XOR vi vector ngu nhin(IV). X1=X1 XOR IV X2=X2 XOR Y1 Xi =Xi XOR Yi-1

S m ho khi

S gii m 2. u nhc im a. u im Cc khi m ph thuc ln nhau => ngn chn k th sa i, ct xn m truyn tin.

b. Nhc im Nu m truyn i b sai t do nhiu th vn phi pht li hon ton. VI. K thut padding 1. Khi nim Trong m ho khi, hm m ho v gii m thng ly u vo l mt block 1 khi d liu c di nht nh ( 4-bytes, 8-bytes, 16-bytes v.v..). V vy vi 1 on d liu c di ln, ngi ta thng chia n thnh cc on nh (trng vi di input ca thut ton m ho) v m ho ln lt. V on d liu c tch ra trong qu trnh m ho, s sy ra trng hp block cui cng s ngn hn kch thc (size) cn, trong trng hp ny ngi ta s thm vo sau mt s k t, k thut ny gi l PADDING. 2. Cc k thut padding a. Bit padding 1 bit 1 c thm vo sau khi tin, sau l cc bit 0. Vd: m 9 bit vo sau mt on 23 bit c khi tin 32 bit. ... | 1011 1001 1101 0100 0010 0111 0000 0000 |

b. Byte padding a) Theo tiu chun ANSI X.923 Cc byte 0 c thm vo sau, ring byte cui dng xc nh s byte c thm vo. V d: kch thc khi 8 byte, v cn thm 4 byte vo cui (trong h Hexadecimal) ... | DD DD DD DD DD DD DD DD | DD DD DD DD 00 00 00 04 |

b) Theo tiu chun ISO 10126, cc byte ngu nhin c thm vo, byte cui dng xc nh s byte c thm vo.

... | DD DD DD DD DD DD DD DD | DD DD DD DD 81 A6 23 04 |

c) Zero padding Cc byte 0 c thm vo. Vd: kch thc khi l 8 byte, s byte cn thm l 4 byte.(h Hex) c. ... | DD DD DD DD DD DD DD DD | DD DD DD DD 00 00 00 00 |