Algorithm Types & Algorithm Modes Lecture slides by Lawrie Brown for “Cryptography and Network Security”, 4/e, by William Stallings, Chapter 2 – “Classical Encryption Techniques”.

2DES and 3DES Lecture slides by Lawrie Brown for “Cryptography and Network Security”, 4/e, by William Stallings, Chapter 2 – “Classical Encryption Techniques”. 2 2

Meet In the Middle attack Lecture slides by Lawrie Brown for “Cryptography and Network Security”, 4/e, by William Stallings, Chapter 2 – “Classical Encryption Techniques”. 3 3

Must know about: Algorithm types Confusion Diffusion Stream Cipher Block Cipher Confusion The idea of confusion is to hide the relationship between the cipher text and the key Confusion is achieved by means of the substitution techniques. Diffusion Diffusion hide the relationship between the cipher text and plain text. diffusion is achieved by means of the transposition techniques. Lecture slides by Lawrie Brown for “Cryptography and Network Security”, 4/e, by William Stallings, Chapter 2 – “Classical Encryption Techniques”. 4 4

Block cipher modes of operation The algorithm modes defines the details of the cryptographic algorithm, once the types is decided. There are four modes of operation (algorithm modes) of block cipher have been defined. It is a technique for enhancing the effect of a cryptographic algorithm. Briefly review some terminology used throughout the course. 5 5

Block cipher modes Electronic Code Book(ECB) Cipher Block Chaining(CBC) Cipher Feedback(CFB) Output Feedback(OFB) Also discuss a variation of the OFB mode, called as Counter(CTR)

Block cipher modes of operation Algorithm modes Electronic code Book(ECB) Cipher Block Chaining(CBC) Cipher feedback (CFB) Output Feedback (OFB) Briefly review some terminology used throughout the course. This two modes work on block Ciphers. These two modes work on block Ciphers acting as stream ciphers 7 7

Electronic code book (ECB) Simplest mode of operation. Plain text message is divided into blocks of 64 bits each. Each block then encrypted independently of the other blocks. For each blocks same key used for encryption. Briefly review some terminology used throughout the course. 8 8

Electronic code book (ECB) Encryption Reverse process is used for Decryption. Briefly review some terminology used throughout the course. 9 9

Electronic code book (ECB) ECB is useful only for small message. Characteristics of ECB The same b bit block of plaintext , if it appears more than once in message , always produce the same cipher text. For lengthy message ECB is not secure Briefly review some terminology used throughout the course. 10 10

Cipher Block Chaining (CBC) message is broken into blocks linked together in encryption operation each previous cipher blocks is chained with current plaintext block, hence name use Initial Vector (IV) to start process uses: bulk data encryption, authentication To overcome the problems of repetitions and order independence in ECB, want some way of making the ciphertext dependent on all blocks before it. This is what CBC gives us, by combining the previous ciphertext block with the current message block before encrypting. To start the process, use an Initial Value (IV), which is usually well known (often all 0's), or otherwise is sent, ECB encrypted, just before starting CBC use. CBC mode is applicable whenever large amounts of data need to be sent securely, provided that all data is available in advance (eg email, FTP, web etc).

Cipher block chaining mode(CBC) Briefly review some terminology used throughout the course. 12 12

Advantages and Limitations of CBC a ciphertext block depends on all blocks before it any change to a block affects all following ciphertext blocks need Initialization Vector (IV) which must be known to sender & receiver if sent in clear, attacker can change bits of first block, and change IV to compensate hence IV must either be a fixed value (as in EFTPOS) or must be sent encrypted in ECB mode before rest of message CBC is the block mode generally used. The chaining provides an avalanche effect, which means the encrypted message cannot be changed or rearranged without totally destroying the subsequent data. However there is the issue of ensuring that the IV is either fixed or sent encrypted in ECB mode to stop attacks on 1st block.

Cipher feedback mode(CFB) Briefly review some terminology used throughout the course. 14

Cipher feedback mode(CFB) C1 = P1 (XOR) MSBs [E(k,IV)]. P1 = C1 (XOR) MSBs [E(k,IV)]. For DES, b = 64 bits. For AES, b = 128 bits. Initially S = 8 bits. Briefly review some terminology used throughout the course. 15 15

Output feedback mode(OFB) Briefly review some terminology used throughout the course. 16 16

Output feedback mode(OFB) Extremely similar to the CFB. Only difference is that in the case of OFB, the output of the IV encryption process is fed into the next stage of encryption process. Briefly review some terminology used throughout the course. 17 17

Counter Mode(CTR) Similar to the OFB with one variation. It uses sequence number called counters as the input to the algorithm. After each block is encrypted to fill the register, the next counter value is used. Size of counter value is same as plain text block. Briefly review some terminology used throughout the course. 18 18

Counter Mode(CTR) Briefly review some terminology used throughout the course. 19 19

Counter Mode(CTR)