lecture 02 introduction cryptography

8/6/2019 Lecture 02 Introduction Cryptography

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Introduction toIntroduction toryptograp yryptograp y

Dr. Nguyen Tuan NamDr. Nguyen Tuan Nam

songuku99@yahoo.comsonguku99@yahoo.com


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CryptographyCryptographycomes from the Greek wordscomes from the Greek words

= hidden or secret = hidden or secret = writing = writingThe art of secret writingThe art of secret writing

Basic serviceBasic service The ability to send information between participants in a way thatThe ability to send information between participants in a way that

prevents others from reading itprevents others from reading it e scope o t s c ass: n o cryptograp y w eree scope o t s c ass: n o cryptograp y w ere

Representing information as numbersRepresenting information as numbers Manipulating those numbers mathematicallyManipulating those numbers mathematically

rov es o er serv ces suc asrov es o er serv ces suc as Integrity checkingIntegrity checking AuthenticationAuthentication

2Nguyen Tuan Nam/NetSec/Win2010


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PlaintextPlaintext oror cleartextcleartext

Message in its original formMessage in its original form CiphertextCiphertext

e mang e n orma one mang e n orma on

EncryptionEncryption The rocess for roducin ci hertext from laintext The rocess for roducin ci hertext from laintext

DecryptionDecryption The reverse of encryptionThe reverse of encryption

plaintext cyphertext plaintextencryption decryption



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AnAn algorithmalgorithm

AA secret valuesecret value, known as a, known as a keykey

Why do we need a key?Why do we need a key?

AA ood cr to ra hic schemeood cr to ra hic scheme

Perfectly OK to have everyone (including the bad guys andPerfectly OK to have everyone (including the bad guys andcryptanalysts) know the algorithmcryptanalysts) know the algorithm

Because knowledge of the algorithm without the key does notBecause knowledge of the algorithm without the key does nothelp unhelp un--mangle the information easilymangle the information easily


ow goo s ou t e omputat ona cu ty


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Important for cryptographic algorithms to beImportant for cryptographic algorithms to be reasonably efficientreasonably efficient for thefor the

goo guy o compu egoo guy o compu e

Good guys are the ones with knowledge of the keysGood guys are the ones with knowledge of the keys Cryptographic algorithms are not impossible to break without the keyCryptographic algorithms are not impossible to break without the key. Why?. Why?

the bad guys to break itthe bad guys to break it 10 million years to break using all of the computers in the world10 million years to break using all of the computers in the world consideredconsidered

reasonably securereasonably secure

Combination lock consists of 3 numbersCombination lock consists of 3 numbers Takes 10 seconds to dial in a combinationTakes 10 seconds to dial in a combination reasonably convenient for the goodreasonably convenient for the good

guyguy

How much work is it for the bad guy? (worst case, average)How much work is it for the bad guy? (worst case, average) A scheme can be made more secure byA scheme can be made more secure bymaking the key longermaking the key longer

Combination lock example?Combination lock example?



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Computer can be used to exhaustively try keysComputer can be used to exhaustively try keys

Faster than peopleFaster than people

Dont get tiredDont get tired Thousands or millions of keys can be tried per secondThousands or millions of keys can be tried per second More ke s can be tried in arallel if ou have multi le com utersMore ke s can be tried in arallel if ou have multi le com uters

VariableVariable--length keylength key Can be made more secure by increasing the length of the keyCan be made more secure by increasing the length of the key Increasing the length of the key by 1 bitsIncreasing the length of the key by 1 bits

oo guy s o ust a t ar eroo guy s o ust a t ar er Bad guys job, how much harder?Bad guys job, how much harder?

FixedFixed--length keylength key Similar al orithm with a lon er ke can be devisedSimilar al orithm with a lon er ke can be devised

QuizQuiz If the computers get 1000 times faster, how much longer should the key lengthIf the computers get 1000 times faster, how much longer should the key length

be?be?



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wayway

in the combinationin the combination

Good guys:A kind word is a key to get what you want

,and a gun than you can with a kind word alone



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Keeping a cryptographic algorithm as Publishin the al orithm, so that it is

ewView 2

secret as possible widely known

Common practice today

.the US may be unpublished. Why?


.


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Secret codeSecret code oror ciphercipher

Any method of encrypting dataAny method of encrypting data

Caesar cipherCaesar cipher Substitute for each letter of the message, the letter which is 3 letters laterSubstitute for each letter of the message, the letter which is 3 letters later

in the alphabet (wrap around)in the alphabet (wrap around)

Captain Midnight Secret Decoder ringsCaptain Midnight Secret Decoder rings Pick a number n between 1 and 25Pick a number n between 1 and 25 Su st tute or eac etter o t e message, t e etter w c s n g erSu st tute or eac etter o t e message, t e etter w c s n g er

(wrap around)(wrap around)

MonoMono--alphabetic cipheralphabetic cipher

How many possible parings of letters?How many possible parings of letters? If took 1 microsecond to try each oneIf took 1 microsecond to try each one take about 10 trillion yearstake about 10 trillion years



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,,

erl lqrx qgt iqul!erl lqrx qgt iqul!



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Ciphertext onlyCiphertext only

Chosen plaintextChosen plaintext



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Assumption: Fred, the bad guyAssumption: Fred, the bad guy

Seen some ciphertext (not difficult to obtain)Seen some ciphertext (not difficult to obtain) Can analyze at leisureCan analyze at leisure

HowHow Searches all the keysSearches all the keys Essential for this attack:Essential for this attack:

Recognize when he has succeededRecognize when he has succeededom na on ocom na on oc

Recognizable plaintextRecognizable plaintext attackattack

Enough ciphertextEnough ciphertext XYZXYZ

omet mes, not necessary to searc t roug a ot o eysomet mes, not necessary to searc t roug a ot o eys Statistical analysis (common English words)Statistical analysis (common English words)

A cryptographic algorithmA cryptographic algorithm MUSTMUST be secure against a ciphertextbe secure against a ciphertext


..


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Fred somehow obtained someFred somehow obtained some

pairs. How?pairs. How? With a monoWith a mono--alphabetic cipheralphabetic cipher

A small amount of known plaintext would be bonanza forA small amount of known plaintext would be bonanza forFredFred

Good enough to be secure against ciphertext only attacksGood enough to be secure against ciphertext only attacks

Not good enough against known plaintext attacksNot good enough against known plaintext attacks

Important to design the systems toImportant to design the systems to minimize the possibilityminimize the possibilitythat a bad guy will ever be able to obtain airs



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Can choose any plaintext he wantsCan choose any plaintext he wants

ciphertext isciphertext is



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Public key functions: two keysPublic key functions: two keys

Hash functions: zero keyHash functions: zero key



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Involves the use ofInvolves the use ofa sin le kea sin le ke

Given a message (plaintext) and a keyGiven a message (plaintext) and a key Encryption producesEncryption produces

,,waswas

Decryption is the reverseDecryption is the reverse Usin the same ke as encr tionUsin the same ke as encr tion

Also calledAlso called Conventional cryptographyConventional cryptography

ymme r c cryp ograp yymme r c cryp ograp y Example?Example?



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Security Uses of Secret KeySecurity Uses of Secret Key

CryptographyCryptography

Secure storage on insecure mediaSecure storage on insecure media

Aut ent cat onAut ent cat on

Integrity checkIntegrity check



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Authentication Using Secret KeyAuthentication Using Secret Key


Stron authenticationStron authentication

Someone can prove knowledge of a secret without revealingSomeone can prove knowledge of a secret without revealingitit

oss e w t cryptograp y oss e w t cryptograp y

Particularly useful when 2 computers trying to communicateParticularly useful when 2 computers trying to communicate

over an insecure networkover an insecure network

Alice Bob

rA

rA encrypted with KAB

rBProblem?


rB encrypted with KAB


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What is a checksum?What is a checksum?

Original derivation of checksumOriginal derivation of checksum

CRCCRC Only protect against faulty hardware, but not an intelligent attackerOnly protect against faulty hardware, but not an intelligent attacker CRC algorithms are publishedCRC algorithms are published attacker can reattacker can re--compute CRC after altering thecompute CRC after altering the

messagemessage needsneeds secretsecret checksum algorithmschecksum algorithms

Cryptographic checksumCryptographic checksum

Common (known) algorithmCommon (known) algorithm Secret keSecret ke

MAC (message authentication code) or MIC (message integrity code)MAC (message authentication code) or MIC (message integrity code) At least 48 bits longAt least 48 bits long chance is only one in 280 trillion to guess the MACchance is only one in 280 trillion to guess the MAC

ExampleExample

InterInter--bank electronic funds transfersbank electronic funds transfers Messages are not kept secret, but their integrity is insuredMessages are not kept secret, but their integrity is insured



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Involves the use of 2 keyInvolves the use of 2 key

A public key that is preferably known to the entireA public key that is preferably known to the entire

orldorld No shared key between the 2 communicating partiesNo shared key between the 2 communicating parties



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Public Key CryptographyPublic Key Cryptography

Encryption and Digital SignatureEncryption and Digital Signature

plaintext

encryp on

ciphertext

ecryp on

plaintext

public key private key

plaintextsigning

signedverification

plaintext

private key public key



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and a digital signature?and a digital signature?ww

digital signature?digital signature?



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Security Uses of Public KeySecurity Uses of Public Key


key cryptography can dokey cryptography can do

communication tocommunication to

AuthenticateAuthenticate Establish a temporary shared secret keyEstablish a temporary shared secret key

The secret key is used to encrypt the remainder ofThe secret key is used to encrypt the remainder of

the conversation using secret key technologythe conversation using secret key technologyWhy?Why?



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What should happen?What should happen?



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Transmitting Over an InsecureTransmitting Over an Insecure

ChannelChannel

message before transmittingmessage before transmittingww



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For performance reasonsFor performance reasons

se toget er w t secret ey cryptograp yse toget er w t secret ey cryptograp y



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entitiesentities

Remember lots of secret keysRemember lots of secret keys

Remember only his private keysRemember only his private keys

Know be able to obtain others ublic keKnow be able to obtain others ublic ke

Does not need to keep any secret in order toDoes not need to keep any secret in order to



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Also known asAlso known as messa e di estsmessa e di ests oror oneone-- aa

transformationstransformations Mathematical transformation that takes a message ofMathematical transformation that takes a message of

arbitrary length and computes from it a fixedarbitrary length and computes from it a fixed--lengthlength(short) number(short) number

,,propertiesproperties For any message m, relatively easy to compute h(m)For any message m, relatively easy to compute h(m)

Given h(m), no way to find an m that hashes to h(m)Given h(m), no way to find an m that hashes to h(m) It is computationally infeasible to find 2 values that hash toIt is computationally infeasible to find 2 values that hash to



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Used to be publicly readableUsed to be publicly readable



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generate a MAC to protect the integritygenerate a MAC to protect the integrity

Sent the messageSent the message

se t e as o t e message as ase t e as o t e message as a

Method 2?Method 2?



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has been modifiedhas been modified

Method 2: use a hash function (save storage)Method 2: use a hash function (save storage)

atc outatc out

Bad guy may changes both the data and the hashBad guy may changes both the data and the hash

programprogram



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--

intensiveintensiveuu

Digitally sign the hash result, instead of theDigitally sign the hash result, instead of thew o e messagew o e message


lecture 02 introduction cryptography

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