Computer security principles and practice 3rd by williams stallings and brown ch20

Symmetric Encryption • Also referred to as: • Only alternative before public-key encryption in 1970’s • Has five ingredients:... • Sender and receiver use same key – symmetric • Sender

Chapter 20

Symmetric Encryption and Message Confidentiality

Symmetric Encryption

• Also referred to as:

• Only alternative before public-key

encryption in 1970’s

• Has five ingredients:

• Sender and receiver use same key –

symmetric

• Sender and receiver each use a different key - asymmetric

The number of keys used

• Sender and receiver use same key –

symmetric

• Sender and receiver each use a different key - asymmetric

The way in which the plaintext is processed

• Block cipher – processes input one block of elements at a time

• Stream cipher – processes the input elements

continuously

The way in which the plaintext is processed

• Block cipher – processes input one block of elements at a time

• Stream cipher – processes the input elements

continuously

Computationally Secure

Encryption Schemes

• Encryption is computationally secure if:

o Cost of breaking cipher exceeds value of information

o Time required to break cipher exceeds the useful lifetime of the information

• Usually very difficult to estimate the

amount of effort required to break

• Can estimate time/cost of a brute-force

attack

Block Cipher Structure

• Symmetric block cipher consists of:

o A sequence of rounds

o With substitutions and permutations controlled by key

• Parameters and design features:

Block size Key size Number of Number of rounds rounds

Subkey generatio

n algorithm

Subkey generatio

n algorithm

Round function Round function

Fast software encryption /decryptio

n

Fast software encryption /decryptio

n

Ease of analysis Ease of analysis

 Most widely used

(DES)

Add round key

Inverse mix cols Add round key Inverse sub bytes Inverse shift rows Add round key Ciphertext (b) Decryption w[36, 39]

w[40, 43]

S SubBytes

Table 20.2 AES S-Boxes

Shift Rows

On encryption left rotate each row of State by 0,1,2,3 bytes respectively

On encryption left rotate each row of State by 0,1,2,3 bytes respectively

Decryption does

reverse

Decryption does

reverse

Mix Columns and Add Key

• Mix columns

o Operates on each column individually

o Mapping each byte to a new value that is a function of all four bytes in the column

o Use of equations over finite fields

o To provide good mixing of bytes in column

• Add round key

o Simply XOR State with bits of expanded key

o Security from complexity of round key expansion and other stages of AES

Key input to a pseudorandom bit generator

• Produces stream of random like

numbers

• Unpredictable without knowing input key

• XOR keystream output with plaintext bytes

Table 20.3 Speed Comparisons of Symmetric Ciphers on a Pentium 4

255 253 253 4

3 2 1

Table 20.4 Block Cipher Modes of Operation

Electronic Codebook

(ECB)

block is encrypted using the same key

value for each plaintext block

o Not secure for long messages since repeated plaintext is seen

in repeated ciphertext

technique where the same plaintext block, if

repeated, produces different ciphertext blocks

Frame relay network

= end-to-end encryption device

= link encryption device

FRN = frame relay node

Figure 20.9 Encryption Across a Frame Relay Network

F R N

F R N

F R N

F R N

Key Distribution

wish to exchange data without allowing others to see the key

1 •• A key could be selected by A and physically delivered to BA key could be selected by A and physically delivered to B

2 •• A third party could select the key and physically deliver it to A and BA third party could select the key and physically deliver it to A and B3

• If A and B have previously and recently used a key, one party could transmit the new key to the other, encrypted using the old key

• If A and B have previously and recently used a key, one party could transmit the new key to the other, encrypted using the old key

4

• If A and B each have an encrypted connection to a third party C,

C could deliver a key on the encrypted links to A and B

• If A and B each have an encrypted connection to a third party C,

C could deliver a key on the encrypted links to A and B

Key distribution center

Network

1 Host sends packet requesting connection.

2 Security service buffers packet; asks

KDC for session key.

3 KDC distributes session key to both hosts.

4 Buffered packet transmitted.

Figure 20.10 Automatic Key Distribution for Connection-Oriented Protocol

2

3

4 1

• Stream ciphers and RC4

o Stream cipher structure

o The RC4 algorithm

• Cipher block modes of

operation

o Electronic codebook mode

o Cipher block chaining mode

o Cipher feedback mode

o Counter mode

• Location of symmetric encryption devices