is 302: information security and trust week 3: from des to aes 2012

IS 302: Information Security and TrustWeek 3: From DES to AES

2012

© Yingjiu Li 2007 2

Review– Kerckhoff principle– Attacks to cryptosystem

– Caesar, Vigenere, Zimmerman, Vernam cipher– Confusion and diffusion


Modern Symmetric Ciphers

• DES

• AES


Block Ciphers vs Stream Ciphers

• Block ciphers– DES and AES…– For each block, perform multiple rounds of

confusion and diffusion operations

• Stream ciphers– Vernam,…– Bit by bit operations


DES• 1972-1974 NBS call for proposal• IBM’s DES Horst Feistel’s Lucifer cipher• 1976 US Federal standard

• 1990 DES design is optimal – Almost any change to DES weakens it

• May 26, 2002, DES was superseded by AES – brute force attack can easily break 56-bit DES key

• 1998 3DES valid till 2030 – extensively used in banking industry


Jeff Moser: http://www.moserware.com/2009/09/stick-figure-guide-to-advanced.html

http://1.bp.blogspot.com/_Zfbv3mHcYrc/SrebavzxZHI/AAAAAAAABi8/KJhKmQTtqi4/s1600-h/aes_act_1_scene_05_judge_1100.png


http://2.bp.blogspot.com/_Zfbv3mHcYrc/Sreh0fsDjRI/AAAAAAAABjM/bNXFrxuBY8Y/s1600-h/aes_act_1_scene_06_nbs_decree_1100.png


http://4.bp.blogspot.com/_Zfbv3mHcYrc/SrektFJi7oI/AAAAAAAABjc/wp0BKrW50h0/s1600-h/aes_act_1_scene_07_lucifer_1100.png


http://3.bp.blogspot.com/_Zfbv3mHcYrc/SremXGo1_9I/AAAAAAAABjs/iaUL3R8V0AQ/s1600-h/aes_act_1_scene_08_anoint_des_1100.png


http://3.bp.blogspot.com/_Zfbv3mHcYrc/SreoBBfhLJI/AAAAAAAABj8/bg2juCimsl0/s1600-h/aes_act_1_scene_09_des_ruled_1100.png


One Round in Encoding• 56-bits key 16 48-bits

sub-key• 64-bits blocks

– Right half left half

– Left half mixed with encrypted right half right half

A round of encoding a block in DES (repeat 16 times)

Left half block Right half block

New left half block

New right half block

substitution

permutation

Sub key

f


Overall Diagram

f function

http://en.wikipedia.org/wiki/Data_Encryption_Standard


DES procedure visualization

• Cryptool:– Indiv. procedures visualization of algorithms

DES

– You need Java runtime environment at least version 1.6 http://java.sun.com

– You also need to download and install the current version of ANIMAL animation software http://www.algoanim.info/Animal2/

http://java.sun.com/

http://www.algoanim.info/Animal2/


Multiple DESes• Two-Key DES

– Total key size is 56x2=112 bits; but the effective key size is only 57 bits only!

• Triple DES (3DES)

– This is a secure solution with effective key size of 112 bits

EDataK1 EK2 C

EDataK1 DK2

CEK1


DES encryption demo

• Cryptoolencrypt/decryptsymmetric (modern)– DES (CBC) – 3-DES (CBC)


Security Concerns

56 bit key is too short– Can be broken on average in 2^55 ≈3.6*10^16

trials– Moore’s law: speed of processor doubles per

1.5 yr– 1997: 3500 machines broke DES in about 4

months– 1998: 1M dollar machine broke DES in about 4

days


http://4.bp.blogspot.com/_Zfbv3mHcYrc/SreoJQaJCmI/AAAAAAAABkM/g3HmEDLRJws/s1600-h/aes_act_1_scene_10_des_defeated_1100.png


http://1.bp.blogspot.com/_Zfbv3mHcYrc/SreojET_QBI/AAAAAAAABkc/knYwS6AsM04/s1600-h/aes_act_1_scene_11_triple_des_1100.png


http://4.bp.blogspot.com/_Zfbv3mHcYrc/Sreqlu0gkOI/AAAAAAAABks/TFPgQMW2KJI/s1600-h/aes_act_1_scene_12_nist_decree_1100.png


http://2.bp.blogspot.com/_Zfbv3mHcYrc/SreqryYxgTI/AAAAAAAABk8/gkd3ibNVQyc/s1600-h/aes_act_1_scene_13_rallied_1100.png


http://3.bp.blogspot.com/_Zfbv3mHcYrc/SrezP7Vek8I/AAAAAAAABlM/vY8BCz_xZ_M/s1600-h/aes_act_1_scene_14_rijndael_1100.png


http://1.bp.blogspot.com/_Zfbv3mHcYrc/SrezVYrKN3I/AAAAAAAABlc/QV7SKeqbjYw/s1600-h/aes_act_1_scene_15_vote_1100.png


http://2.bp.blogspot.com/_Zfbv3mHcYrc/SrezjnfyXnI/AAAAAAAABls/cq-87oH48Tg/s1600-h/aes_act_1_scene_16_won_1100.png


AES• 1997 NIST call • Final five

– Rijndael(Joan Daemen and Vincent Rijmen),– Serpent(Ross Anderson), – Twofish(Bruce Schneier), – RC6(Don Rivest, Lisa Yin), – MARS (Don Coppersmith, IBM)

• 2000 Rijndael won • 2002 Rijndael became AES


AES vs DESDES AES

Date 1976 1999

Block size 64 128

Key length 56 128, 192, 256

Number of rounds 16 9,11,13

Encryption primitives Substitution, permutation Substitution, shift, bit mixing

Cryptographic primitives Confusion, diffusion Confusion, diffusion

Design Open Open

Design rationale Closed Open

Selection process Secret Secret, but accept open public comment

Source IBM, enhanced by NSA Independent cryptographers

© Yingjiu Li 2007 http://en.wikipedia.org/wiki/Advanced_Encryption_Standard

27

High-Level Cipher Algorithm

• KeyExpansion (one 128/192/256-biy key to 10/12/14 128-bit subkeys)• Initial Round

– AddRoundKey (cipher key)• Steps in each of 9/11/13 rounds (state: 4*4=16-array of bytes = 128 bit-

block)– SubBytes — a non-linear substitution step where each byte is replaced with

another according to a lookup table.– ShiftRows — a transposition step where each row of the state is shifted

cyclically a certain number of steps. – MixColumns — a mixing operation which operates on the columns of the

state, combining the four bytes in each column– AddRoundKey — each byte of the state is combined with the round key;

each round key is derived from the cipher key using a key schedule. • Final Round (no MixColumns)

– SubBytes – ShiftRows – AddRoundKey

• How many rounds in total? How many round keys?


Follow Me

• Cryptool: AES procedures – Invid. Procedures visualization of

algorithms AES– Web version: http://www.ccna-security.net/wp-

content/uploads/2008/10/rijndael_ingles2004.swf

• AES-128-CBC encryption– Encrypt/decrypt symmetric (modern)

AES


Four Modes of Block Ciphers

• How are multiple blocks processed?– ECB: Electronic Code Book

– CBC: Cipher Block Chaining

– CFB: Cipher Feedback

– OFB: Output Feedback


Electronic codebook (ECB) mode

P1 P2 P3

Enc

C1 C2 C3

K

C1 C2 C3

Dec

P1 P2 P3

K

© Yingjiu Li 2007

Cipher-block Chaining (CBC) Mode

P1 IV P2 C1 P3 C2

Enc

C1 C2 C3

K

C1 C2 C3

Dec

P1 IV P2 C1 P3 C2

K

P1 P2 P3

IV C1 C2


Original image

ECB vs CBC

• Which mode would you choose?

• Which one is semantically secure?

Encrypted with ECB Encrypted with CBC


Hands-On Exercise

• AES Encryption and Decryption– OpenSSL– JCE

• Download Lab.doc and follow instructions


Introduction:Javax.crypto.Cipher

• This class provides the functionality of a cryptographic cipher for encryption and decryption

• Methods:– getInstance(String algorithm )

• Generates a Cipher object that implements the specified algorithm.

– init(int opmode, Key key )• The cipher is initialized with a key for either encryption or

decryption.

– doFinal(byte[] input )• Encrypts or decrypts data depending on how this cipher was

initialized.

Further details: http://java.sun.com/j2se/1.4.2/docs/api/javax/crypto/Cipher.htmlhttp://java.sun.com/j2se/1.5.0/docs/api/javax/crypto/Cipher.html


Example: AES Encryption

• The following sample encrypts a file “clear.txt”and save the output as a file named “encryptedfile”.

// Create a cipher object with algorithm “AES”.Cipher cipher = Cipher.getInstance("AES");

//Set the Cipher object to ENCRYPT MODE//Initialise it with the encryption key “mykey”. //Previous example demostrated how to retrieve this key from a keystore.cipher.init(Cipher.ENCRYPT_MODE, mykey);

//Create an input stream to read the file.File clr=new File(“clear.txt”);FileInputStream fi = new FileInputStream(clr);

//Get the size of the file.long length = clr.length(); …..cont’d


Example: AES Encryption cont’d

//Create a byte array with the size of the file.byte[] plaintext = new byte[(int) length];

//Read data into the byte arrayfi.read(plaintext);

//Close file fi.close();

// Now encrypt the text and store it in the byte array ciphertext byte[] ciphertext = cipher.doFinal(plaintext);

//Write encrypted text into the output fileFile enc=new File(“encryptedfile”)FileOutputStream fo = new FileOutputStream(enc);fo.write(ciphertext);fo.close();


Example: AES Decryption

• The following sample decrypts a file “encryptedfile”and save the output as a file named “decryptedfile”.

// Create a cipher object with algorithm “AES”Cipher cipher = Cipher.getInstance("AES");

//Set the Cipher object to DECRYPT MODE//Initialise it with the decryption key “mykey”. //Previous example demostrated how to retrieve this key from a keystore.cipher.init(Cipher.DECRYPT_MODE, mykey);

//Create an input stream to read the file.File enc=new File(“encryptedfile”);FileInputStream fi = new FileInputStream(enc);

//Get the size of the file.long length = enc.length(); …..cont’d


Example: AES Decryption cont’d

//Create a byte array with the size of the file.byte[] ciphertext = new byte[(int) length];

//Read data into the byte arrayfi.read(ciphertext);

//Close file fi.close();

// Now decrypt the text and store it in the byte array ciphertext byte[] plaintext = cipher.doFinal(ciphertext);

//Write encrypted text into the output fileFile dec=new File(“decryptedfile”)FileOutputStream fo = new FileOutputStream(dec);fo.write(plaintexttext);fo.close();


Review Questions• Which of the following is stream cipher

1) DES 2) AES 3) Vernam

• What is effective key size for 4-DES1) 112 bits 2) 113 bits 3) 168 bits

• Increasing key size from 56 bits to 128 bits, how many times more effort an attacker needs to spend in brute force attack?

1) 72 2) 2^72 3) 2^184


Individual Assignment 1 (5%)• Due in week 4 (please submit hardcopy during

week 4 class)– Textbook 1.11 Exercises 3, 12, 15 (pages 32-33 in 3rd

edition, pages 34-36 in 4th edition)– Textbook 2.13 Exercises 1, 13, 17, 19 (pages 91-93 in

3rd edition, pages 94-97 in 4th edition)

is 302: information security and trust week 3: from des to aes 2012

Documents