Information and network security 18 modern techniques block ciphers
- 1. Information and Network Security:18 Modern Techniques: Block Ciphers Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Block Ciphers and the Data Encryption Standard All the afternoon Mungo had been working on Stern's code, principally with the aid of the latest messages which he had copied down at the Nevin Square drop. Stern was very confident. He must be well aware London Central knew about that drop. It was obvious that they didn't care how often Mungo read their messages, so confident were they in the impenetrability of the code. —Talking to Strange Men, Ruth Rendell
- 3. Modern Block Ciphers now look at modern block ciphers one of the most widely used types of cryptographic algorithms provide secrecy /authentication services focus on DES (Data Encryption Standard) to illustrate block cipher design principles
- 4. Modern Block Ciphers • This ppt set discusses the principles of modern symmetric ciphers. • For this purpose, we focus on the most widely used symmetric cipher: the Data Encryption Standard (DES). • Although numerous symmetric ciphers have been developed since the introduction of DES, and although it is destined to be replaced by the Advanced Encryption Standard (AES), DES remains the most important such algorithm. • Further, a detailed study of DES provides an understanding of the principles used in other symmetric ciphers.
- 5. Block vs Stream Ciphers • Block ciphers work a on block / word at a time, which is some number of bits. • All of these bits have to be available before the block can be processed. • block ciphers process messages in blocks, each of which is then en/decrypted like a substitution on very big characters • 64-bits or more • stream ciphers process messages a bit or byte at a time when en/decrypting • many current ciphers are block ciphers • better analysed • broader range of applications
- 6. Block vs Stream Ciphers
- 7. • A block cipher is one in which a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length. • Typically, a block size of 64 or 128 bits is used. • As with a stream cipher, the two users share a symmetric encryption key (Figure 3.1b). • A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. • In the ideal case, a one-time pad version of the Vernam cipher would be used (Figure 2.7), in which the keystream (k ) is as long as the plaintext bit stream (p).
- 8. Block Cipher Principles • most symmetric block ciphers are based on a Feistel Cipher Structure • needed since must be able to decrypt ciphertext to recover messages efficiently • block ciphers look like an extremely large substitution • would need table of 264 entries for a 64-bit block • instead create from smaller building blocks • using idea of a product cipher
- 9. • Most symmetric block encryption algorithms in current use are based on a structure referred to as a Feistel block cipher. • A block cipher operates on a plaintext block of n bits to produce a ciphertext block of n bits. • An arbitrary reversible substitution cipher for a large block size is not practical, however, from an implementation and performance point of view. In general, for an n-bit general substitution block cipher, the size of the key is n x 2n. • For a 64-bit block, which is a desirable length to thwart statistical attacks, the key size is 64x 264 = 270 = 1021 bits. • In considering these difficulties, Feistel points out that what is needed is an approximation to the ideal block cipher system for large n, built up out of components that are easily realizable.
- 11. • Feistel refers to an n-bit general substitution as an ideal block cipher, because it allows for the maximum number of possible encryption mappings from the plaintext to ciphertext block. • A 4-bit input produces one of 16 possible input states, which is mapped by the substitution cipher into a unique one of 16 possible output states, each of which is represented by 4 ciphertext bits. • The encryption and decryption mappings can be defined by a tabulation, as shown in Stallings Figure 3.2. It illustrates a tiny 4-bit substitution to show that each possible input can be arbitrarily mapped to any output - which is why its complexity grows so rapidly.
- 12. Assignment • What do you understand by block cipher. Discuss the Block Cipher Principals.