Network Security Unit 2

NETWORK SECURITY

Unit-2

Outline

Conventional Encryption Principles Conventional encryption algorithms Cipher block modes of operation Location of encryption devices Key distribution Approaches of Message Authentication Secure Hash Functions and HMAC.

CONVENTIONAL ENCRYPTION PRINCIPLES

Plain text

Secret Key

Encryption Algorithm

Cipher text

Decryption Algorithm

Security depends on the secrecy of the key, not the secrecy of the algorithm.

Encryption

Algorithm

Decryption Algorithm

Shared by sender and

receiver only

SIMPLIFIED MODEL OF CONVENTIONAL ENCRYPTION

Cryptography

Classified along three independent dimensions:

– The type of operations used for transforming plaintext to cipher text

– The number of keys used

• symmetric (single key)

• asymmetric (two-keys, or public-key encryption)

– The way in which the plaintext is processed

Cryptanalysis The process of discovering the key or the

plaintext. Types of Attacks:

1. Cipher text only

2. Known plaintext

3. Chosen plaintext

4. Chosen cipher text

5. Chosen text.

//It is assumed that the encryption algorithm

is known to a cryptanalyst.//

Types of AttacksTypes of Attacks Info. Known to Cryptanalyst

Cipher text only Cipher text to be decoded

Known plaintext Cipher text to be decoded.

Corresponding plaintext.

Chosen plaintext Plain text message of the

choice of cryptanalyst and

corresponding cipher text.

Types of Attacks

Chosen cipher text Cipher text to be decoded.

Purported cipher text chosen by cryptanalyst.

Along with corresponding plaintext.

Chosen text Chosen plaintext + Chosen cipher text

Average time required for exhaustivekey search

Key Size(bits)

Number of AlternativeKeys

Time required at 106Decryption/μs

32 2 32 =4.3*109 2.15 milliseconds

56 256 = 7.2 x 1016 10 hours

128 2128 = 3.4 x 1038 5.4 x 1018 years

168 2168 = 3.7 x 1050 5.9 x 1030 years

Feistel Cipher Structure Virtually all conventional block encryption

algorithms, including DES have a structure first

described by Horst Feistel of IBM in 1973.

Feistel Network depends on the choice of the following parameters and design features

Plain text

+ F

+ F

Sub key generation algorithm

2W bits

K1

K2

L0 R0

Cipher Text

Block size

Key size

Number of Rounds

Sub key generation Algorithm

Round function

Conventional Encryption algorithms DES

Triple DES

IDEA

Blowfish

Cast-128

Data Encryption Standard Adopted by NIST in 1977 Based on a cipher (Lucifer) developed

earlier by IBM for Lloyd’s of London for cash transfer.

DES uses the Feistel cipher structure with 16 rounds of processing.

DES is a block cipher. The plaintext is processed in 64-bit blocks. The key is 56-bits in length.

Phase1

•Initial permutation that rearranges the bits

Phase2

•16 iterations of the same function

Phase3

•Swapping to produce the pre-output and inverse of initial permutation.

DES Cracker

DES encryption was broken in 1999 by Electronics.

1998 – DES cracker machine built

– cost = $250,000

– attack length = 3 days

Time to break a code

Triple DES

Use three keys and three executions of the DES algorithm (encrypt-decrypt-encrypt)

• Effective key length of 168 bits.

C = EK3[DK2[EK1[P]]]

Conventional Encryption Algorithms

Algorithm Key size(bits)

Number ofRounds

Mathematical operations

Applications

DES 56 16 XOR, S-box SET,KERBEROS

TDEA 112 or 168 48 XOR, S-box PGP,SMIME

IDEA 126 16 XOR, S-box, add

Blowfish 40-448 16 Add, Sub, XOR, Rotation

Cast-128 40-128 16 Add, Sub, XOR, Rotation, S-box

PGP

 Electronic Code Book Mode

Message is broken into independent blocks which are encryptedEach block is a value which is substituted, like a codebook, hence nameEach block is independent of all others

Cipher Feedback mode

Location of Encryption Device Link encryption:

– A lot of encryption devices

– High level of security

– Decrypt each packet at every switch

• End-to-end encryption

– The source encrypt and the receiver decrypts

– Payload encrypted

– Header in the clear

• High Security: Both link and end-to-end

encryption are needed

Key Distribution

1. A 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 B.

3. If A and B have previously 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.

Key Distribution

• Session key:

– Data encrypted with a one-time session key. At the conclusion of the session the key is destroyed

• Permanent key:

– Used between entities for the purpose of distributing session keys

Authentication

Requirements - must be able to verify that:

1. Message came from apparent source or author,

2. Contents have not been altered,3. Sometimes, it was sent at a

certain time or sequence.

Protection against active attack (falsification of data and transactions)

Approaches to Message Authentication

Authentication Using Conventional EncryptionOnly the sender and receiver should share a key

Message Authentication without Message EncryptionAn authentication tag is generated and

appended to each message Message Authentication Code

Calculate the MAC as a function of the message and the key. MAC = F(K, M)

One-way HASH function

Secure HASH Functions Properties of a HASH function H :

1. H can be applied to a block of data at any size2. H produces a fixed length output3. H(x) is easy to compute for any given x.4. For any given block x, it is computationally

infeasible to find x such that H(x) = h5. For any given block x, it is computationally

infeasible to find with H(y) = H(x).6. It is computationally infeasible to find any pair

(x, y) such that H(x) = H(y)

Message Digest Generation Using SHA

Processing of single 512-Bit Block

Other Secure HASH functionsSHA-1 MD5 RIPEMD-

160Digest length 160 bits 128 bits 160 bitsBasic unit of processing

512 bits 512 bits 512 bits

Number of steps

80 (4 rounds of 20)

64 (4 rounds of 16)

160 (5 paired rounds of 16)

Maximum message size

264-1 bits

HMAC

ANY QUERIES