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EEC-484/584Computer Networks

- Lecture 17
- Wenbing Zhao
- wenbing_at_ieee.org
- (Part of the slides are based on materials

supplied by Dr. Louise Moser at UCSB and

Prentice-Hall)

Outline

- Quiz4 results
- Introduction to cryptography

EEC484

- Max 100
- Min 93
- Average 97
- Q1 avg 19/20
- Q2 avg 30/30
- Q3 avg 19/20
- Q4 avg 10/10
- Q5 avg 19/20

EEC584 (MW)

- Max 97
- Min 48
- Average 79
- Q1 avg 19/20
- Q2 avg 29/30
- Q3 avg 10/20
- Q4 avg 8/10
- Q5 avg 13/20

EEC584 (TTh)

- Max 98
- Min 60
- Average 88
- Q1 avg 19/20
- Q2 avg 29/30
- Q3 avg 16/20
- Q4 avg 5/10
- Q5 avg 18/20

Cryptography Terminology

- Encryption is the process of encoding a message

so that its meaning is not obvious - Equivalent terms encode, encipher
- Encryption addresses the need for confidentiality

of data - Encryption can also be used to ensure integrity

(i.e., unauthorized change can be detected) - Encryption is the basis of protocols that enable

us to provide security while accomplishing system

or network tasks

Cryptography Terminology

- Decryption is the reverse process, transforming

an encrypted message back into its normal,

original form - Equivalent terms decode, decipher
- A system for encryption and decryption is called

a cryptosystem

Cryptography Terminology

- The encryption and decryption rules are called

encryption and decryption algorithms - Encryption/decryptions algorithms often use a

device called a key, denoted by K, so that the

resulting ciphertext depends on the original

plaintext message, the algorithm, and the key

value - An encryption scheme that does not require the

use of a key is called a keyless cipher

Cryptography Terminology

- Plaintext message to be encrypted
- Ciphertext encrypted message
- DK(EK(P)) P

Symmetric Encryption

- The encryption and decryption keys are the same,

so P D(K, E(K,P)) - D and E are closely related. They are

mirror-image processes - The symmetric systems provide a two-way channel

to their users - The symmetry of this situation is a major

advantage of this type of encryption, but it also

leads to a problem key distribution

Asymmetric Encryption

- Encryption and decryption keys come in pairs. The

decryption key, KD, inverts the encryption of key

KE, so that P D(KD, E(KE,P)) - Asymmetric encryption systems excel at key

management

Cryptology

- Cryptology is the research into and study of

encryption and decryption it includes both

cryptography and cryptanalysis - Cryptography art of devising ciphers
- Comes from Greek words for secret writing. It

refers to the practice of using encryption to

conceal text - Cryptanalysis art of breaking ciphers
- Study of encryption and encrypted messages,

hoping to find the hidden meanings

Basic Encryption Methods

- Substitution ciphers one letter is exchanged for

another - Transposition ciphers order of letters is

rearranged

Substitution Ciphers

- Idea each letter or group of letters is replaced

by another letter or group of letters - Caesar cipher circularly shift by 3 letters
- a -gt D, b -gt E, z -gt C
- More generally, shift by k letters, k is the key
- Monoalphabetic cipher map each letter to some

other letter - A b c d e f w x y z
- Q W E R T Y V B N M lt the key

Substitution Ciphers

- Not difficult to determine the key using

frequencies of letters, pairs of letter etc., or

by guessing a probable word or phrase - Most frequently occurred
- Letters e, t, o, a, n,
- Digrams th, in, er, re, an,
- Trigrams the, ing, and, ion, ent
- Words the, of, and, to, a, in, that,

Transposition Ciphers

- Transposition cipher reorders (rearrange)

symbols but does not disguise them. It is also

called permutation - Transpositions try to break established patterns
- Both substitution and transport ciphers can be

broken using language statistical information

Columnar Transposition

- Plaintext written in rows, number of columns

key length - Key is used to number the columns
- Ciphertext read out by columns, starting with

column whose key letter is lowest

Columnar Transposition

- A transposition cipher example

One-Time Pads

- One-time pad construct an unbreakable cipher
- Choose a random bit string as the key
- Convert the plaintext into a bit string
- Compute the XOR of these two strings, bit by bit
- The resulting ciphertext cannot be broken,

because in a sufficiently large sample of

ciphertext, each letter will occur equally often - gt there is simply no information in the message

because all possible plaintexts of the given

length are equally likely

One-Time Pads

Original one-time pad used

I L O V E

Y O U .

E L V I S

L I V E S

If someone tries to decrypt using another

one-time pad

One-Time Pads

- Disadvantages
- The key cannot be memorized, both sender and

receiver must carry a written copy with them - Total amount of data can be transmitted is

limited by the amount of key available - Sensitive to lost or inserted characters

Stream Ciphers

- Stream ciphers convert one symbol of plaintext

immediately into a symbol of ciphertext - The transformation depends only on the symbol,

the key, and the control information of the

encryption algorithm

Some kinds of errors affect the encryption of all

future characters

Block Ciphers

- Block cipher encrypts a group of plaintext

symbols as one block - Block ciphers work on blocks of plaintext and

produce blocks of ciphertext - The columnar transposition is an example of block

ciphers

Cryptanalysis Breaking Encryption Schemes

- Ciphertext-only cryptanalyst has a quantity of

ciphertext and no plaintext - Known plaintext cryptanalyst has some matched

ciphertext and plaintext - Chosen plaintext cryptanalyst has the ability to

encrypt pieces of plaintext of his own choosing

Symmetric-Key Algorithms

- DES The Data Encryption Standard
- AES The Advanced Encryption Standard
- Cipher Modes
- Other Ciphers

Data Encryption Standard

- Developed by IBM. US standard for unclassified

info (1977) - Same key for encryption as for decryption
- Encrypts in 64-bit blocks
- Uses 56-bit key
- Has 19 stages, 16 parameterized by different

functions of the key

Data Encryption Standard

- Building blocks
- P-box (permutation box) used to implement

transposition in hardware - S-box (substitution box) used to implement

substitution in hardware

Triple DES

- Triple DES effectively increases the key

length. It uses two keys and three stages - In first stage, the plaintext is encrypted using

DES in the usual way with K1 - In second stage, DES is run in decryption mode,

using K2 as the key - In third stage, another DES encryption is done

with K1

Triple DES encryption

Triple DES decryption

AES The Advanced Encryption Standard

- AES is a result of a cryptographic contest
- Organized by NIST in 1997
- Rules for AES proposals
- The algorithm must be a symmetric block cipher
- The full design must be public
- Key lengths of 128, 192, and 256 bits supported
- Both software and hardware implementations

required - The algorithm must be public or licensed on

nondiscriminatory terms - Winner Rijndael (from two Belgian

cryptographers Joan Daemen and Vincent Rijmen)

AES

- Creating of the state and rk arrays

Cipher Modes

- Despite all the complexity, AES and DES (or any

block cipher) is basically a monoalphabetic

substitution cipher using big characters - Whenever the same plaintext block goes in the

front end, the same ciphertext block comes out

the back end - If you encrypt the plaintext abcdefgh 100 times

with same DES key, you get the same ciphertext

100 times - An intruder can exploit this property to help

subvert the cipher

Electronic Code Book Mode

- In ECB mode, each plaintext block is encrypted

independently with the block cipher - ECB allows easy parallelization to yield higher

performance. However, no processing is possible

before a block is seen

Electronic Code Book Mode - Problems

- In ECB, plaintext patterns are not concealed
- Each identical block of plaintext gives an

identical block of ciphertext. The plaintext can

be easily manipulated by removing, repeating, or

interchanging blocks - Example

Cipher Block Chaining Mode

- To avoid the ECB mode problem replacing a block

will cause the plaintext decrypted starting at

the replaced to become garbage - Exclusive OR the encrypted text with the next

block of plaintext before encryption C0 E(P0

XOR IV), C1 E(P1 XOR C0), etc. - Drawback must wait until full 64-bit (128-bit)

block to arrive to decrypt

Cipher Block Chaining Mode

- Exclusive OR the encrypted text with the next

block of plaintext before encryption C0 E(P0

XOR IV), C1 E(P1 XOR C0), etc.

Initialization Vector

Encryption

Decryption

Cipher Feedback Mode

- To enable byte-by-byte encryption
- When plaintext byte n (Pn) arrives, DES algorithm

operates a 64-bit register to generate a 64-bit

ciphertext (128-bit register needed for AES) - Leftmost byte of that ciphertext is extracted and

XORed with Pn - That byte is transmitted on the transmission line
- The shift register is shifted left 8 bits,

causing Cn-8 to fall off the left end, and Cn is

inserted in the position just vacated at the

right end by C9 - Drawback One byte of transmission error will

ruin 8 bytes of data

Cipher Feedback Mode

Decryption

Encryption

Stream Cipher Mode

- To be insensitive to transmission error, an

arbitrarily large sequence of output blocks,

called the keystream, is treated like a one-time

pad and XORed with the plaintext to get the

ciphertext - It works by encrypting an IV, using a key to get

an output block - The output block is then encrypted, using the key

to get a second output block - This block is then encrypted to get a third

block, and so on

Stream Cipher Mode

- The keystream is independent of the data
- It can be computed in advance
- It is completely insensitive to transmission

errors

Encryption

Decryption

Stream Cipher Mode

- It is essential never to use the same (key, IV)

pair twice with a stream cipher because doing so

will generate the same keystream each time - Using the same keystream twice exposes the

ciphertext to a keystream reuse attack - Stream cipher mode is also called output feedback

mode

Keystream Reuse Attack

- Plaintext block, P0, is encrypted with the

keystream to get P0 XOR K0 - Later, a second plaintext block, Q0, is encrypted

with the same keystream to get Q0 XOR K0 - An intruder who captures both ciphertext blocks

can simply XOR them together to get P0 XOR Q0,

which eliminates the key - The intruder now has the XOR of the two plaintext

blocks - If one of them is known or can be guessed, the

other can also be found - In any event, the XOR of two plaintext streams

can be attacked by using statistical properties

of the message

Counter Mode

- To allow random access to encrypted data
- The IV plus a constant is encrypted, and the

resulting ciphertext XORed with the plaintext - By stepping the IV by 1 for each new block, it is

easy to decrypt a block anywhere in the file

without first having to decrypt all of its

predecessors