Lecture 24

Cryptography

CPE 401 / 601Computer Network Systems

slides are modified from Jim Kurose and Keith Ross and Dave Hollinger

Cryptography Encryption

Scramble data so that only someone with a secret can make sense of the data.

Decryption Descrambling encrypted data.

Cryptography 2

The language of cryptography

symmetric key crypto: sender, receiver keys identicalpublic-key crypto: encryption key public, decryption key

secret (private)

plaintext plaintextciphertext

KA

encryptionalgorithm

decryption algorithm

Alice’s encryptionkey

Bob’s decryptionkey

KB

Cryptography 3

Symmetric key cryptographysubstitution cipher: substituting one thing for another

monoalphabetic cipher: substitute one letter for another

plaintext: abcdefghijklmnopqrstuvwxyz

ciphertext: mnbvcxzasdfghjklpoiuytrewq

Plaintext: bob. i love you. aliceciphertext: nkn. s gktc wky. mgsbc

E.g.:

Q: How hard to break this simple cipher? brute force? other?

Cryptography 4

Symmetric key cryptography

symmetric key crypto: Bob and Alice share know same (symmetric) key: K

e.g., key is knowing substitution pattern in mono alphabetic substitution cipher

Q: how do Bob and Alice agree on key value?

plaintextciphertext

KA-B

encryptionalgorithm

decryption algorithm

A-B

KA-B

plaintextmessage, m K (m)A-B K (m)A-Bm = K ( ) A-B

Cryptography 5

DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64-bit plaintext input

How secure is DES? DES Challenge: 56-bit-key-encrypted phrase

“Strong cryptography makes the world a safer place” decrypted (brute force) in 4 months

no known “backdoor” decryption approach

making DES more secure: use three keys sequentially (3-DES) on each datum use cipher-block chaining

Cryptography 6

DES

initial permutation 16 identical “rounds” of

function application, each using different 48 bits of key

final permutation

DES operation

Cryptography 7

AES: Advanced Encryption Standard symmetric-key NIST standard

replacing DES Nov 2001

processes data in 128 bit blocks 128, 192, or 256 bit keys

brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES

Cryptography 8

Block Cipher64-bit input

T1

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

64-bit scrambler

64-bit output

loop for n rounds T2 T3 T4 T6T5 T7 T8

one pass through: input bit affects eight output bits

multiple passes: each input bit afects all output bits

block ciphers: DES, 3DES, AES Cryptography 9

public key cryptography radically different approach

Diffie-Hellman76, RSA78 sender, receiver do not share secret key public encryption key known to all private decryption key known only to

receiver

Public key cryptographysymmetric key crypto requires sender, receiver know shared secret key Q: how to agree on key in first place

particularly if never “met”?

Cryptography 10

Public key cryptography

plaintextmessage, m

ciphertextencryptionalgorithm

decryption algorithm

Bob’s public key

plaintextmessageK (m)B

+

K B+

Bob’s privatekey K B

-

m = K (K (m))B+

B-

Cryptography 11

Public key encryption algorithms

need K ( ) and K ( ) such that

B B. .

given public key K , it should be impossible to compute private key K B

B

Requirements:

1

2

RSA: Rivest, Shamir, Adleman algorithm

+ -

K (K (m)) = m BB- +

+

-

Cryptography 12

RSA: Choosing keys1. Choose two large prime numbers p, q. (e.g., 1024 bits each)2. Compute n = pq, z = (p-1)(q-1)

3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”)4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 )

5. Public key is (n,e). Private key is (n,d).

K B+ K B

-

Cryptography 13

RSA: Encryption, decryption0. Given (n,e) and (n,d) as computed above

1. To encrypt bit pattern, m, computec = m mod n

e (i.e., remainder when m is divided by n)e

2. To decrypt received bit pattern, c, computem = c mod n

d (i.e., remainder when c is divided by n)d

m = (m mod n)

e mod n

dMagichappens!

c

Cryptography 14

RSA example:Bob chooses p=5, q=7. Then n=35, z=24.

e=5 (so e, z relatively prime).d=29 (so ed-1 exactly divisible by z.

letter m me c = m mod nel 12 1524832 17

c m = c mod nd17 481968572106750915091411825223071697 12

cd letterl

encrypt:

decrypt:

Cryptography 15

RSA: Why is that m = (m mod n)

e mod n

d

(m mod n)

e mod n = m mod n

d ed

Useful number theory result: If p,q prime and n = pq, then:

x mod n = x mod ny y mod (p-1)(q-1)

= m mod n

ed mod (p-1)(q-1)

= m mod n1

= m

(using number theory result above)

(since we chose ed to be divisible by(p-1)(q-1) with remainder 1 )

Cryptography 16

RSA: another important propertyThe following property will be very useful later:

K (K (m)) = m BB- + K (K (m)) BB

+ -=

use public key first, followed by private key

use private key first, followed by public key

Result is the same!

Cryptography 17

Using Keys Private keys are used for decrypting Public keys are used for encrypting

encryptionplaintext ciphertextpublic key

decryptionciphertext plaintextprivate key

Cryptography 18

Transmitting over an insecure channelAlice wants to send Bob a private

message.

Apublic is Alice’s public key.Aprivate is Alice’s private key.Bpublic is Bob’s public key.Bprivate is Bob’s private key.

Cryptography 19

Cryptography 20

Hello Bob,Wanna get together?

AliceAlice BobBob

encrypt using Bpublic decrypt using Bprivate

Cryptography 21

AliceAlice BobBob

decrypt using Aprivate encrypt using Apublic

OK Alice,Your place or mine?

Bob’s Dilemma Nobody can read the message from

Alice, but anyone could produce it.

How does Bob know that the message was really sent from Alice?

Bob may be comforted to know that only Alice can read his reply.

Cryptography 22

Alice can sign her message! Alice can create a digital signature and

prove she sent the message or someone with knowledge of her private

key

The signature can be a message digest encrypted with Aprivate.

Cryptography 23

Message Digest Also known as “hash function” or “one-

way transformation”.

Transforms a message of any length and computes a fixed length string.

We want it to be hard to guess what the message was given only the digest. Guessing is always possible.

Cryptography 24

Digital Signature Public key cryptography is also used to

provide digital signatures

signingplaintext signed message private key

verificationsigned message plaintext public key

Cryptography 25

Alice’s Signature Alice feeds her original message through a

hash function and encrypts the message digest with Aprivate.

Bob can decrypt the message digest using Apublic.

Bob can compute the message digest himself.

If the 2 message digests are identical, Bob knows Alice sent the message.

Cryptography 26

Revised Scheme

Cryptography 27

AliceAlice BobBob

Sign with Aprivate check signature using Apublic

encrypt using Bpublic decrypt using Bprivate

Why the digest? Alice could just encrypt her name, and

then Bob could decrypt it with Apublic. Why wouldn’t this be sufficient?

Suppose Alice denies she sent the message?

Bob can prove that only someone with Alice’s key could have produced the message.

Cryptography 28

Solution? Always start your messages with:

Dear Mehmet,

Create a digest from the encrypted message and sign that digest.

There are many other schemes as well.

Cryptography 29

Speed Secret key encryption/decryption

algorithms are much faster than public key algorithms.

Many times a combination is used: use public key cryptography to share a secret

key. use the secret key to encrypt the bulk of the

communication.

Cryptography 30

Goal: Bob wants Alice to “prove” her identity to him

Authentication

Protocol 1.0: Alice says “I am Alice”

Failure scenario??“I am Alice”

Cryptography 32

Goal: Bob wants Alice to “prove” her identity to him

Authentication

Protocol 1.0: Alice says “I am Alice”

in a network,Bob cannot “see” Alice, so Trudy simply declaresherself to be Alice

“I am Alice”

Cryptography 33

Authentication: another tryProtocol 2.0: Alice says “I am Alice” in an IP packet

containing her source IP address

Failure scenario??“I am Alice”Alice’s

IP address

Cryptography 34

Authentication: another try

Trudy can createa packet “spoofing”Alice’s address“I am Alice”Alice’s

IP address

Protocol 2.0: Alice says “I am Alice” in an IP packetcontaining her source IP address

Cryptography 35

Authentication: another tryProtocol 3.0: Alice says “I am Alice” and sends her

secret password to “prove” it.

Failure scenario??

“I’m Alice”Alice’s IP addr

Alice’s password

OKAlice’s IP addr

Cryptography 36

Authentication: another try

playback attack: Trudy records Alice’s packet and laterplays it back to Bob

“I’m Alice”Alice’s IP addr

Alice’s password

“I’m Alice”Alice’s IP addr

Alice’s password

Protocol 3.0: Alice says “I am Alice” and sends her secret password to “prove” it.

Cryptography 37

OKAlice’s IP addr

Authentication: yet another try

Protocol 3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.

Failure scenario??

“I’m Alice”Alice’s IP addr

encrypted password

OKAlice’s IP addr

Cryptography 38

Authentication: yet another try

Record andplaybackstill works!

“I’m Alice”Alice’s IP addr

encryptedpassword

“I’m Alice”Alice’s IP addr

encryptedpassword

Protocol 3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.

Cryptography 39

OKAlice’s IP addr

Authentication: yet another try

Goal: avoid playback attackNonce: number (R) used only once–in-a-lifetime

4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice

must return R, encrypted with shared secret key“I am Alice”

RK (R)A-B

Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!

Cryptography 40

Authentication: ap5.0ap4.0 requires shared symmetric key can we authenticate using public key techniques?ap5.0: use nonce, public key cryptography

“I am Alice”R

Bob computes

K (R)A-

“send me your public key”K A

+

(K (R)) = RA-K A

+

and knows only Alice could have the private key, that encrypted R such that

(K (R)) = RA-K A

+

Cryptography 41

ap5.0: security holeMan (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice)

I am Alice I am AliceR

TK (R)-

Send me your public key

TK +AK (R)-

Send me your public key

AK +

TK (m)+

Tm = K (K (m))+T

-Trudy gets

sends m to Alice encrypted with Alice’s public key

AK (m)+

Am = K (K (m))+A

-

R

Cryptography 42

ap5.0: security holeMan (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice)

Difficult to detect: Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet one week later and recall conversation) problem is that Trudy receives all messages as well!

Cryptography 43