cisc 210 - class today

March 2005 1R. Smith - University of St Thomas - Minnesota

CISC 210 - Class TodayCISC 210 - Class Today

• Homework: Chapter 5, exercises E6-E17Homework: Chapter 5, exercises E6-E17• RecapRecap• Keystream reuseKeystream reuse• File EncryptionFile Encryption


RecapRecap

• Trojan horse – a reason for file encryptionTrojan horse – a reason for file encryption– Bob’s defenseBob’s defense– What about Chain of Control?What about Chain of Control?

• Encryption termsEncryption terms– Algorithm, plaintext, ciphertext, keyAlgorithm, plaintext, ciphertext, key


Bob’s defense of his filesBob’s defense of his files

• Block everyones’ accessBlock everyones’ access• Allow his own accessAllow his own access• What does the game do?What does the game do?


The attack on BobThe attack on Bob

• Game does the work under Bob’s processGame does the work under Bob’s process


The Encryption ProcessThe Encryption Process

• Convert plaintext to ciphertext with a keyConvert plaintext to ciphertext with a key


CryptanalysisCryptanalysis

• Known ciphertext attackKnown ciphertext attack– a.k.a. ciphertext-only attack – classic attacka.k.a. ciphertext-only attack – classic attack– Newspaper cryptogramsNewspaper cryptograms– You have ciphertext, no plaintextYou have ciphertext, no plaintext

• Known plaintext attackKnown plaintext attack– You have You have somesome plaintext for some intercepted ciphertext plaintext for some intercepted ciphertext– The attack used against ENIGMA to reduce the problemThe attack used against ENIGMA to reduce the problem


Looking more at Stream CiphersLooking more at Stream Ciphers

• Same for streams and one time padsSame for streams and one time pads


The One Time PadThe One Time Pad

• ““Provably secure”Provably secure”

• We use 1 bit of random key for EVERY BIT of We use 1 bit of random key for EVERY BIT of data we transmit. data we transmit.

• Use XOR to encrypt and decryptUse XOR to encrypt and decrypt

• Sender and recipient must share exactly the Sender and recipient must share exactly the same stream of random bits, and use them in same stream of random bits, and use them in exactly the same orderexactly the same order– NO REUSE!!NO REUSE!!


One time pads in DecimalOne time pads in Decimal

• To Encrypt:To Encrypt:– Put plaintext in numeric formPut plaintext in numeric form– Write decimal numbers from pad underneathWrite decimal numbers from pad underneath– Encrypt by doing add-without-carryEncrypt by doing add-without-carry

• We discard the carry without changing the neighboring We discard the carry without changing the neighboring digit.digit.

• To decrypt:To decrypt:– Write ciphertext digits in a rowWrite ciphertext digits in a row– Write decimal numbers from pad underneathWrite decimal numbers from pad underneath– Decrypt by doing subtract-without-borrowDecrypt by doing subtract-without-borrow

• We ‘pretend’ to borrow but don’t change the neighboring We ‘pretend’ to borrow but don’t change the neighboring digit when we do.digit when we do.


The XOR Reuse ProblemThe XOR Reuse Problem

• If a xor k = AIf a xor k = A• and b xor k = Band b xor k = B• then A xor B = a xor bthen A xor B = a xor b

• We can do this with Matlab...We can do this with Matlab...


Using MatlabUsing Matlab

• im = imread(‘file.gif);im = imread(‘file.gif);– reads the gif file and converts it to a matrixreads the gif file and converts it to a matrix– Assigns the matrix to variable ‘im’Assigns the matrix to variable ‘im’– the “;” at the end prevents Matlab from the “;” at the end prevents Matlab from printing the matrixprinting the matrix

• image(im)image(im)– Displays the image. Enable plot tools from the toolbarDisplays the image. Enable plot tools from the toolbar

• Especially the ‘property editor’Especially the ‘property editor’– Colormap setting – First choose HSV. Then choose customColormap setting – First choose HSV. Then choose custom

• Under custom, change the arrow to increase bit contrastUnder custom, change the arrow to increase bit contrast

• eiv = xor (im, key);eiv = xor (im, key);– xors the iv matrix with the key matrix – both same sizexors the iv matrix with the key matrix – both same size– eiv gets the result – the ‘;’ suppresses printing the matrixeiv gets the result – the ‘;’ suppresses printing the matrix


Remember this example:Remember this example:

• Use 128x128 image to hold the messageUse 128x128 image to hold the message• XOR with 128x128 random set of bitsXOR with 128x128 random set of bits

• SC xor KEY = SCESC xor KEY = SCE


Let’s reuse the bitsLet’s reuse the bits

• Trust me, it’s the same key streamTrust me, it’s the same key stream

• SM xor KEY = SMESM xor KEY = SME


Look what happens...Look what happens...

• We combine the 2 ciphertexts with XORWe combine the 2 ciphertexts with XOR• Not so secure, eh?Not so secure, eh?

• SME xor SCE == SM xor SCSME xor SCE == SM xor SC


File encryption issuesFile encryption issues

• Entropy in the passphrasesEntropy in the passphrases– No truncationNo truncation– Don’t waste bitsDon’t waste bits

• Reusing the same passwordReusing the same password– All things being equal, if we use the same password twice as All things being equal, if we use the same password twice as

the encryption key, we generate an identical keystreamthe encryption key, we generate an identical keystream– Don’t want that problem againDon’t want that problem again


File Encryption ProcessFile Encryption Process

• Here’s the perimeterHere’s the perimeter


Simple password handlingSimple password handling

•


Using a hash – betterUsing a hash – better

• Makes longer passphrases effectiveMakes longer passphrases effective


Nonce and HashNonce and Hash

• Lets us reuse the same passwordLets us reuse the same password


Trying file encryptionTrying file encryption

• Let’s download some stuffLet’s download some stuff– Sourceforge.netSourceforge.net

• Neocrypt – file encryptionNeocrypt – file encryption

• Frhed – binary file editorFrhed – binary file editor


Fundamentals of CryptoFundamentals of Crypto

Here is a basic example of a ‘crypto protocol’ Here is a basic example of a ‘crypto protocol’


Elements of crypto protocolsElements of crypto protocols

• Encryption functionsEncryption functions• Hash functionsHash functions• Random values (encryption keys)Random values (encryption keys)• NoncesNonces

• Apply the functions to inputsApply the functions to inputs• Save extra data with the protected outputSave extra data with the protected output• Use the extra data to retrieve or verify the Use the extra data to retrieve or verify the

protected outputprotected output


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