Crypto B/B mindmeld history symmetric hashes/macs/1-way etc public key summary November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-1 Crypto B/B mindmeld

history symmetric hashes/macs/1-way etc public key summary November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-2 a little bibliography please Zimmerman Telegram Barbara Tuchman

how one nation state can go wrong with crypto twice in one century (this is only WWI) Secrets and Lies. Bruce Schneier why crypto may not solve your problems The Codebreakers. David Kahn. the book (WW II got added post declassification) Cryptography Decrypted. Mel/Baker or if you are a hard case, Applied Crypto, although November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-3 history up to WW II it was alphanumeric in the west

stream ciphers (a letter at a time) block ciphers (a block of letters at a time) WW II changed everything (computers) but there are still some very basic principles to cryptoanalysis that have lasted November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-4 fundamental defs cryptography sometimes secret writing wasnt meant to be secret sometimes it is alphabetic historically, now bits/bytes/blocks cryptoanalysis decoding the secret writing without the keys

hey! chocolate for your password? November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-5 policy considerations think about this as we talk about what are basically mechanisms what threats exist in this space? what might policy considerations thus be for: a. govt. spy agency (NSA or CIA or MI5?) bond, james bond AND his laptop? b. hospital c. computer technology company d. university November 1, 2004

Introduction to Computer Security 2004 Matt Bishop Slide crypto1-6 Rosetta stone - solved November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-7 linear A not solved November 1, 2004 Introduction to Computer Security 2004 Matt Bishop

Slide crypto1-8 oh yes the enigma machine November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-9 and its natural enemy the bombe November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-10 brought to you by this man (and

friends) Hmmm any impact on Computer Science? November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-11 Cryptosystem Quintuple (E, D, M, K, C) M set of plaintexts K set of keys C set of ciphertexts

E set of encryption functions e: M K C D set of decryption functions d: C K M November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-12 Example Example: Csar cipher M = { sequences of letters } K = { i | i is an integer and 0 i 25 } E = { Ek | k K and for all letters m, Ek(m) = (m + k) mod 26 } D = { Dk | k K and for all letters c, Dk(c) = (26 + c k) mod 26 } C=M November 1, 2004

Introduction to Computer Security 2004 Matt Bishop Slide crypto1-13 J. Caesar total idiot? great Caesars ghost isnt this the same as the legendary rot13? what was he thinking anyway? November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-14 Attacks Opponent whose goal is to break cryptosystem is the adversary Assume adversary knows algorithm used, but not key

Three types of attacks: ciphertext only: adversary has only ciphertext; goal is to find plaintext, possibly key known plaintext: adversary has ciphertext, corresponding plaintext; goal is to find key chosen plaintext: adversary may supply plaintexts and obtain corresponding ciphertext; goal is to find key November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-15 Basis for Attacks Mathematical attacks Based on analysis of underlying mathematics Statistical attacks Make assumptions about the distribution of

letters, pairs of letters (digrams), triplets of letters (trigrams), etc. Called models of the language Examine ciphertext, correlate properties with the assumptions. November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-16 Classical Cryptography Sender, receiver share common key Keys may be the same, or trivial to derive from one another Sometimes called symmetric cryptography Two basic types Transposition ciphers

Substitution ciphers Combinations are called product ciphers November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-17 modern functional version M is a message K is the shared secret key we have 2 functions e(K,M) -> cybermsg -> d(K,C) -> plaintext so the sticky wicket is what? November 1, 2004 Introduction to Computer Security 2004 Matt Bishop

Slide crypto1-18 Transposition Cipher Rearrange letters in plaintext to produce ciphertext Example (Rail-Fence Cipher) Plaintext is HELLO WORLD Rearrange as HLOOL ELWRD Ciphertext is HLOOL ELWRD November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-19 Attacking the Cipher Anagramming (rearrange letters in word)

If 1-gram frequencies match English frequencies, but other n-gram frequencies do not, probably transposition Rearrange letters to form n-grams with highest frequencies November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-20 Example Ciphertext: HLOOLELWRD Frequencies of 2-grams beginning with H HE 0.0305 HO 0.0043 HL, HW, HR, HD < 0.0010 Frequencies of 2-grams ending in H

WH 0.0026 EH, LH, OH, RH, DH 0.0002 Implies E follows H November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-21 Example Arrange so the H and E are adjacent HE LL OW OR LD Read off across, then down, to get original plaintext

November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-22 Substitution Ciphers Change characters in plaintext to produce ciphertext Example (Csar cipher) Plaintext is HELLO WORLD Change each letter to the third letter following it (X goes to A, Y to B, Z to C) Key is 3, usually written as letter D Ciphertext is KHOOR ZRUOG November 1, 2004 Introduction to Computer Security 2004 Matt Bishop

Slide crypto1-23 Attacking the Cipher Exhaustive search If the key space is small enough, try all possible keys until you find the right one Csar cipher has 26 possible keys Statistical analysis Compare to 1-gram model of English November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-24 Statistical Attack Compute frequency of each letter in

ciphertext: G 0.1 R 0.2 H 0.1 U 0.1 K 0.1 Z 0.1 O 0.3 Apply 1-gram model of English Frequency of characters (1-grams) in English is on next slide November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-25

Character Frequencies a 0.080 h 0.060 n 0.070 t 0.090 b 0.015

i 0.065 o 0.080 u 0.030 c 0.030 j 0.005

p 0.020 v 0.010 d 0.040 k 0.005 q 0.002 w 0.015

e 0.130 l 0.035 r 0.065 x 0.005 f 0.020

m 0.030 s 0.060 y 0.020 g 0.015 z 0.002 November 1, 2004

Introduction to Computer Security 2004 Matt Bishop Slide crypto1-26 Statistical Analysis f(c) frequency of character c in ciphertext (i) correlation of frequency of letters in ciphertext with corresponding letters in English, assuming key is i (i) = 0 c 25 f(c)p(c i) so here, (i) = 0.1p(6 i) + 0.1p(7 i) + 0.1p(10 i) + 0.3p(14 i) + 0.2p(17 i) + 0.1p(20 i) + 0.1p(25 i) p(x) is frequency of character x in English November 1, 2004 Introduction to Computer Security 2004 Matt Bishop

Slide crypto1-27 Correlation: (i) for 0 i 25 i 0 1 2 3 4 5 6 (i) 0.0482 0.0364 0.0410 0.0575 0.0252 0.0190 0.0660

November 1, 2004 i 7 8 9 10 11 12 (i) 0.0442 0.0202 0.0267 0.0635 0.0262 0.0325 i 13 14

15 16 17 18 (i) 0.0520 0.0535 0.0226 0.0322 0.0392 0.0299 Introduction to Computer Security 2004 Matt Bishop i 19 20 21 22

23 24 25 (i) 0.0315 0.0302 0.0517 0.0380 0.0370 0.0316 0.0430 Slide crypto1-28 The Result Most probable keys, based on : i = 6, (i) = 0.0660 plaintext EBIIL TLOLA i = 10, (i) = 0.0635 plaintext AXEEH PHKEW

i = 3, (i) = 0.0575 plaintext HELLO WORLD i = 14, (i) = 0.0535 plaintext WTAAD LDGAS Only English phrase is for i = 3 Thats the key (3 or D) November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-29 Csars Problem Key is too short Can be found by exhaustive search Statistical frequencies not concealed well They look too much like regular English letters

So make it longer Multiple letters in key Idea is to smooth the statistical frequencies to make cryptanalysis harder November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-30 Vignere Cipher Csar cipher, but use a phrase Example Message THE BOY HAS THE BALL Key VIG Encipher using Csar cipher for each letter: key VIGVIGVIGVIGVIGV plain THEBOYHASTHEBALL

cipher OPKWWECIYOPKWIRG November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-31 Relevant Parts of Tableau A B E H L O S T Y November 1, 2004 G

G H L N R U Y Z E I I J M P T W A B H

V V W Z C G J N O T Tableau shown has relevant rows, columns only Example encipherments: key V, letter T: follow V column down to T row (giving O) Key I, letter H: follow I column down to H row (giving P)

Introduction to Computer Security 2004 Matt Bishop Slide crypto1-32 Useful Terms period: length of key In earlier example, period is 3 tableau: table used to encipher and decipher Vignere cipher has key letters on top, plaintext letters on the left polyalphabetic: the key has several different letters Csar cipher is monoalphabetic November 1, 2004 Introduction to Computer Security 2004 Matt Bishop

Slide crypto1-33 Attacking the Cipher Approach Establish period; call it n Break message into n parts, each part being enciphered using the same key letter Solve each part You can leverage one part from another November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-34 Establish Period Kaskski: repetitions in the ciphertext occur when characters of the key appear over the same

characters in the plaintext Example: key VIGVIGVIGVIGVIGV plain THEBOYHASTHEBALL cipher OPKWWECIYOPKWIRG Note the key and plaintext line up over the repetitions (underlined). As distance between repetitions is 9, the period is a factor of 9 (that is, 1, 3, or 9) November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-35 One-Time Pad A Vigenre cipher with a random key at least as long as the message Provably unbreakable Why? Look at ciphertext DXQR. Equally likely to

correspond to plaintext DOIT (key AJIY) and to plaintext DONT (key AJDY) and any other 4 letters Warning: keys must be random, or you can attack the cipher by trying to regenerate the key Approximations, such as using pseudorandom number generators to generate keys, are not random November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-36 so at this point we have certain algorithms for symmetric encryption

typically these algorithms do the bulk work as public key is too slow DES, 3-DES IDEA BLOWFISH SKIPJACK AES keys are different lengths November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-37 Overview of the DES A block cipher:

encrypts blocks of 64 bits using a 64 bit key outputs 64 bits of ciphertext A product cipher basic unit is the bit performs both substitution and transposition (permutation) on the bits Cipher consists of 16 rounds (iterations) each with a round key generated from the user-supplied key November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-38 Controversy Considered too weak from day one Diffie, Hellman said in a few years technology would allow DES to be broken in days

Design using 1999 technology published Design decisions not public S-boxes may have backdoors November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-39 DES Modes Electronic Code Book Mode (ECB) Encipher each block independently Cipher Block Chaining Mode (CBC) Xor each block with previous ciphertext block Requires an initialization vector for the first one Encrypt-Decrypt-Encrypt Mode (2 keys: k, k)

c = DESk(DESk1(DESk(m))) Encrypt-Encrypt-Encrypt Mode (3 keys: k, k, k) c = DESk(DESk (DESk(m))) November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-40 CBC Mode Encryption init. vector m1

DES DES c1 c2 sent November 1, 2004 m2 Introduction to Computer Security 2004 Matt Bishop

sent Slide crypto1-41 CBC Mode Decryption init. vector November 1, 2004 c1 c2 DES DES

m1 m2 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-42 Self-Healing Property Initial message 3231343336353837 3231343336353837 3231343336353837 3231343336353837

Received as (underlined 4c should be 4b) ef7c4cb2b4ce6f3b f6266e3a97af0e2c 746ab9a6308f4256 33e60b451b09603d Which decrypts to efca61e19f4836f1 3231333336353837 3231343336353837 3231343336353837 Incorrect bytes underlined Plaintext heals after 2 blocks November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-43 Current Status of DES Design for computer system, associated software that could break any DES-enciphered message in a few days published in 1998

Several challenges to break DES messages solved using distributed computing NIST selected Rijndael as Advanced Encryption Standard, successor to DES Designed to withstand attacks that were successful on DES November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-44 pros/cons of symm. encryption pros faster than public key/asymmetric usually cons key distribution is not scalable more people know secret, less of a secret Ben Franklin rule

export laws have been a problem Moores law may eat a few bits a year November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-45 let us generalize a bit if you do cryptoanalysis, you dont know Key, you only know ciphertext (CC) we assume you know E and D (know alg) there is some set of K1 KN that you can guess any function that say reduces the odds is good if you get the odds down you can bruteforce solve it with a computer one common design flaw: reduce the entropy of the system by making keys easy for users

November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-46 Cryptographic Checksums Mathematical function to generate a set of k bits from a set of n bits (where k n). k is smaller then n except in unusual circumstances Example: ASCII parity bit ASCII has 7 bits; 8th bit is parity Even parity: even number of 1 bits Odd parity: odd number of 1 bits November 1, 2004 Introduction to Computer Security 2004 Matt Bishop

Slide crypto1-47 Example Use Bob receives 10111101 as bits. Sender is using even parity; 6 1 bits, so character was received correctly Note: could be garbled, but 2 bits would need to have been changed to preserve parity Sender is using odd parity; even number of 1 bits, so character was not received correctly November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-48 Definition

Cryptographic checksum h: AB: 1. 2. 3. For any x A, h(x) is easy to compute For any y B, it is computationally infeasible to find x A such that h(x) = y It is computationally infeasible to find two inputs x, x A such that x x and h(x) = h(x) November 1, 2004 Alternate form (stronger): Given any x A, it is computationally infeasible to find a different x A such that h(x) = h(x). Introduction to Computer Security

2004 Matt Bishop Slide crypto1-49 functional forms md(msg) -> bit string of length N (128, 160) md(shared secret, msg) -> bit string Alice can send the bits to Bob can use the shared secret to prove what exactly? consider M where M = M1, M2, M3 we can skip M2 and generate a bit string md(M1), md(M3) -> bit string and expect Bob to know the same order November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-50

Collisions If x x and h(x) = h(x), x and x are a collision Pigeonhole principle: if there are n containers for n+1 objects, then at least one container will have 2 objects in it. Application: if there are 32 files and 8 possible cryptographic checksum values, at least one value corresponds to at least 4 files November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-51 some example uses a MD is used as a hash reduce message M of arbitrary length to N bits

an integrity check file F has an integrity check published for it with public-key, we sign the integrity check with a shared secret we get a authentication system November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-52 note a very interesting idea lurking a one-way function given some math function we can compute and not be able to figure out the inputs

MD functions are not the only examples given x, and f(x)->z and you have z good luck figuring out x this is fundamentally important November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-53 Keys Keyed cryptographic checksum: requires cryptographic key DES in chaining mode: encipher message, use last n bits. Requires a key to encipher, so it is a keyed cryptographic checksum.

Keyless cryptographic checksum: requires no cryptographic key MD5 and SHA-1 are best known; others include MD4, HAVAL, and Snefru November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-54 HMAC Make keyed cryptographic checksums from keyless cryptographic checksums h keyless cryptographic checksum function that takes data in blocks of b bytes and outputs blocks of l bytes. k is cryptographic key of length b bytes If short, pad with 0 bytes; if long, hash to length b ipad is 00110110 repeated b times opad is 01011100 repeated b times

HMAC-h(k, m) = h(k opad || h(k ipad || m)) exclusive or, || concatenation November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-55 Public Key Cryptography Two keys Private key known only to individual Public key available to anyone Public key, private key inverses Idea Confidentiality: encipher using public key, decipher using private key Integrity/authentication: encipher using private key, decipher using public one November 1, 2004

Introduction to Computer Security 2004 Matt Bishop Slide crypto1-56 there are really 5 possible functions here encryption, decryption signing and verification public key certificates (later chapter) interesting because public key is PUBLIC session-key generation generate a key to use for awhile avoid distribution of shared secrets November 1, 2004 Introduction to Computer Security 2004 Matt Bishop

Slide crypto1-57 Requirements 1. It must be computationally easy to encipher or decipher a message given the appropriate key 2. It must be computationally infeasible to derive the private key from the public key 3. It must be computationally infeasible to determine the private key from a chosen plaintext attack November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-58 RSA Exponentiation cipher thus 1-way

Relies on the difficulty of determining the number of numbers relatively prime to a large integer n November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-59 Security Services Confidentiality Only the owner of the private key knows it, so text enciphered with public key cannot be read by anyone except the owner of the private key Authentication Only the owner of the private key knows it, so text enciphered with private key must have been generated by the owner

November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-60 More Security Services Integrity Enciphered letters cannot be changed undetectably without knowing private key Non-Repudiation Message enciphered with private key came from someone who knew it November 1, 2004 Introduction to Computer Security 2004 Matt Bishop

Slide crypto1-61 Warnings Encipher message in blocks considerably larger than the examples here If 1 character per block, RSA can be broken using statistical attacks (just like classical cryptosystems) Attacker cannot alter letters, but can rearrange them and alter message meaning Example: reverse enciphered message of text ON to get NO November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-62 Diffie-Hellman public key but doesnt do

signing/encryption allows 2 sides to create shared secrets that can be used with MD and bulk sym. enc. to encode messages/pkts basis of many session key algorithms DH exchange however must be authenticated a priori to prevent MITM November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-63 like so: Alice/Bob a priori agree on 2 public #s p: a large prime g, where g < p pre-compute:

Alice S(a) = f(random) T(a) = g**S(a) mod p Bob S(b) = f(random) T(b) = g**S(b) mod p Alice sends T(a) to Bob, and Bob sends T(b) to A November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-64 DH, part 2 post-compute of shared secret key material Alice Bob

S(secret) T(b) ** S(a) mod p S(secret) = T(a) ** S(b) mod p never mind the proof: S(secret) gives the same number of shared secret bits on both sides can be used with MD or symmetric enc. algorithm November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-65 pros/cons of public-key crypto usually quite slow in software

more likely do this: create random key N for bulk encryption encrypt M with N using E(M,N) now encrypt N with public key crypto thus in networking protocols some combo of algorithms is likely AES, SHA, RSA (or something) existence of session-key alg, or signatures a PRO! November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-66 Key Points Classical cryptosystems encipher and decipher using the same key

Or one key is easily derived from the other Public key cryptosystems encipher and decipher using different keys Computationally infeasible to derive one from the other Cryptographic checksums provide a check on integrity used for authentication, session-key generation and in point of fact are very useful November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-67 policy considerations .so given an enterprise a govt. security agency

its WWII: the US 101st Airborne Division in Bastogne during the Battle of the Bulge a hospital worried about S/OX or HIPPA a university what policy considerations may exist re say crypto in and of itself? November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-68 and furthermore what can crypto do and what can it NOT do? what is key escrow and did you think about that in your policy considerations? do you allow users to bring laptops on site, and insist on encryption between your home

and branch campuses? November 1, 2004 Introduction to Computer Security 2004 Matt Bishop Slide crypto1-69