2024年4月6日发(作者：栋斯年)
CS255:CryptographyandComputerSecurityWinter2014
Assignment#1
Due:Wednesday,Feb.5,2014,inclass.
’sseeaconcreteexample.
Supposeyouaretoldthattheonetimepadencryptionofthemessage“attackatdawn”is
09e1c5f70a65ac519458e7e55e32(theplaintextlettersareencodedas8-bitASCIIandthegiven
ciphertextiswritteninhex).Whatwouldbetheonetimepadencryptionofthemessage
“attackatdusk”underthesameOTPkey?
Problem1Simplesecretsharing.
eAlicesharesasecretblockcipherkey,K
AB
withBob,andadifferentsecret
blockcipherkey,K
AC
beamethodforAlicetoencryptanm-
blockmessagesuchthatitcanonlybedecryptedwiththecooperationofbothBob
mayassumethatBobandCharliehaveapre-establishedsecretchannelonwhichto
communicate.
,supposeAlicesharesablockcipherkey,K
AB
withBob,ablockcipherkeyK
AC
withCharlie,andablockcipherkeyK
AD
beamethodforAliceto
encryptanm-blockmessagesuchthatanytwoofBob,Charlie,andDavidcandecrypt
(forexample,BobandCharliecandecrypt),butnoneofthemcandecryptthemessage
,theciphertextshouldonlybeaconstantsizegreaterthanmblocks.
Hint:d
threeciphertextblockstotheciphertextheader.
syoursolutionfrompart(b)scaleasweincreasethenumberofrecipients?In
otherwords,supposeAlicehasasecretkeywitheachofnrecipientsandwantstoencrypt
sothatanykoutofnrecipientscandecrypt,butanyk−uldbethe
lengthoftheheaderasafunctionofnandk?
discussafarmoreefficient
solutionlateronintheclass.
Problem2ThemovieindustrywantstoprotectdigitalcontentdistributedonDVD’y
n=2
32
).Weviewthesenplayersastheleavesofabinarytreeofheightlog
2
dev
i
inthisbinarytreecontainsanAESkeyK
i
.Thesekeysarekeptsecretfromconsumersand
arefifacturingtimeeachDVDplayerisassignedaserialnumber
i∈[0,n−1].ConsiderthesetS
i
of1+log
2
nnodesalongthepathfromtheroottoleaf
ufactureroftheDVDplayerembedsinplayernumber
ithe1+log
2
nkeysassociatedwiththenodesinS
i
.InthiswayeachDVDplayershipswith
1+log
2
nkeysembeddedinit(thesekeysaresupposedlyinaccessibletoconsumers).ADVD
1
movieMisencryptedas
2024年4月6日发(作者：栋斯年)
CS255:CryptographyandComputerSecurityWinter2014
Assignment#1
Due:Wednesday,Feb.5,2014,inclass.
’sseeaconcreteexample.
Supposeyouaretoldthattheonetimepadencryptionofthemessage“attackatdawn”is
09e1c5f70a65ac519458e7e55e32(theplaintextlettersareencodedas8-bitASCIIandthegiven
ciphertextiswritteninhex).Whatwouldbetheonetimepadencryptionofthemessage
“attackatdusk”underthesameOTPkey?
Problem1Simplesecretsharing.
eAlicesharesasecretblockcipherkey,K
AB
withBob,andadifferentsecret
blockcipherkey,K
AC
beamethodforAlicetoencryptanm-
blockmessagesuchthatitcanonlybedecryptedwiththecooperationofbothBob
mayassumethatBobandCharliehaveapre-establishedsecretchannelonwhichto
communicate.
,supposeAlicesharesablockcipherkey,K
AB
withBob,ablockcipherkeyK
AC
withCharlie,andablockcipherkeyK
AD
beamethodforAliceto
encryptanm-blockmessagesuchthatanytwoofBob,Charlie,andDavidcandecrypt
(forexample,BobandCharliecandecrypt),butnoneofthemcandecryptthemessage
,theciphertextshouldonlybeaconstantsizegreaterthanmblocks.
Hint:d
threeciphertextblockstotheciphertextheader.
syoursolutionfrompart(b)scaleasweincreasethenumberofrecipients?In
otherwords,supposeAlicehasasecretkeywitheachofnrecipientsandwantstoencrypt
sothatanykoutofnrecipientscandecrypt,butanyk−uldbethe
lengthoftheheaderasafunctionofnandk?
discussafarmoreefficient
solutionlateronintheclass.
Problem2ThemovieindustrywantstoprotectdigitalcontentdistributedonDVD’y
n=2
32
).Weviewthesenplayersastheleavesofabinarytreeofheightlog
2
dev
i
inthisbinarytreecontainsanAESkeyK
i
.Thesekeysarekeptsecretfromconsumersand
arefifacturingtimeeachDVDplayerisassignedaserialnumber
i∈[0,n−1].ConsiderthesetS
i
of1+log
2
nnodesalongthepathfromtheroottoleaf
ufactureroftheDVDplayerembedsinplayernumber
ithe1+log
2
nkeysassociatedwiththenodesinS
i
.InthiswayeachDVDplayershipswith
1+log
2
nkeysembeddedinit(thesekeysaresupposedlyinaccessibletoconsumers).ADVD
1
movieMisencryptedas