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COMP364:Quiz#1 March15th ,2001 10:30-11:45am

1.Explain,innotmorethan60Englishwords,themeaningsof\authentication"and\non-repudiation"intheareaofcomputerandcommunicationsecurity.
2.Whatmajorassumption(s)mustbesatis.edforacipher-textonlybruteforceattackofa secretkeyalgorithmtobesuccessful.
3.(a) How many di.erent k eys are thereforan a.ne cipherwith N . 3 0 .
(b) In crypt-an alysingan a.ne cipher,wearrive a t twoequations
4 a + b . 8 m od 1 2 6
10a+b .6 mod126

NotethatN.126inthiscase.Findallthepossiblesolutionstothetwoequations. Showyoursteps.
4.(a)Ontheaverage,howmanyDESkeysmapagivenplaintexttoagivenciphertext.
(b)Ontheaverage,howmanyplaintextsaremappedbyagivenDESkeytoagivencipher-text.
5.Asymmetricblockcipher,B,doesitsencryptionasfollows: fori.1to16do REi.LEi;1.Pi. LE.F(REi;1).REi;1.
i
LE17.RE16.P18. RE17.LE16.P17.

whereREiandLEaretherightandlefthalf(each32bits)oftheoutputoftheithround,
i

respectively.F()isafunctionthattakesa32-bitinputandproducesa32-bitoutput..isthe bitwiseexclusiveorfunction.Also,Pi,1.i.18,canbethoughtofastheithperround key(32bits)generatedfromthesecretkey.(RE0andLEarefromtheplaintextdirectly.
0RE17andLE17togetheristheciphertext.) WritedownthedecryptionfunctionintermsofFandPi0 s.UseRDiandRDitodenotethe rightandlefthalfoftheoutputoftheithroundofthedecryptor. Verifythatyourdecryptionfunctionproperlyinversestheencryptionfunction.
6.Useexamplestoshowasecurityweaknessofeachof
(a)theElectronicCodeBook(ECB),and
(b)theCipherBlockChaining(CBC)
modesofoperation.(Oneexampleforeachmode.)Assumetheblockcipherbeingusedis DES.
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