Note for Information Security and Cyber Law - ISCL by Karthik Reddy

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Smartzworld.com Smartworld.asia UNIT-2 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 A Symmetric encryption scheme has five ingredients 1. Plain Text: This is the original message or data which is fed into the algorithm as input. 2. Encryption Algorithm: This encryption algorithm performs various substitutions and transformations on the plain text. 3. Secret Key: The key is another input to the algorithm. The substitutions and transformations performed by algorithm depend on the key. 4. Cipher Text: This is the scrambled (unreadable) message which is output of the encryption algorithm. This cipher text is dependent on plaintext and secret key. For a given plaintext, two different keys produce two different cipher texts. 5. Decryption Algorithm: This is the reverse of encryption algorithm. It takes the cipher text and secret key as inputs and outputs the plain text. Two main requirements are needed for secure use of conventional encryption: (i). A strong encryption algorithm is needed. It is desirable that the algorithm should be in such a way that, even the attacker who knows the algorithm and has access to one or more cipher texts would be unable to decipher the cipher text or figure out the key. (ii).The secret key must be distributed among the sender and receiver in a very secured way. If in any way the key is discovered and with the knowledge of algorithm, all communication using this key is readable. 19 jntuworldupdates.org Specworld.in

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Smartzworld.com Smartworld.asia Cryptography A cipher is a secret method of writing, as by code. Cryptography, in a very broad sense, is the study of techniques related to aspects of information security. Hence cryptography is concerned with the writing (ciphering or encoding) and deciphering (decoding) of messages in secret code. Cryptographic systems are classified along three independent dimensions: The type of operations used for performing plaintext to ciphertext All the encryption algorithms make use of two general principles; substitution and transposition through which plaintext elements are rearranged. Important thing is that no information should be lost. The number of keys used If single key is used by both sender and receiver, it is called symmetric, single-key, secret-key or conventional encryption. If sender and receiver each use a different key, then it is called asymmetric, twokey or public-key encryption. The way in which plaintext is processed A block cipher process the input as blocks of elements and generated an output block for each input block. Stream cipher processes the input elements continuously, producing output one element at a time as it goes along. Cryptanalysis The process of attempting to discover the plaintext or key is known as cryptanalysis. It is very difficult when only the cipher text is available to the attacker as in some cases even the encryption algorithm is not known. The most common attack under these circumstances is brute-force approach of trying all the possible keys. This attack is made impractical when the key size is considerably large. The table below gives an idea on types of attacks on encrypted messages. 20 jntuworldupdates.org Specworld.in

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Smartzworld.com Smartworld.asia Cryptology covers both cryptography and cryptanalysis. Cryptology is a constantly evolving science; ciphers are invented and, given time, are almost certainly breakable. Cryptanalysis is the best way to understand the subject of cryptology. Cryptographers are constantly searching for the perfect security system, a system that is both fast and hard and a system that encrypts quickly but is hard or impossible to break. Cryptanalysts are always looking for ways to break the security provided by a cryptographic system, mostly though mathematical understanding of the cipher structure. Cryptography can be defined as the conversion of data into a scrambled code that can be deciphered and sent across a public or a private network. A Ciphertext-only attack is an attack with an attempt to decrypt ciphertext when only the ciphertext itself is available. A Known-plaintext attack is an attack in which an individual has the plaintext samples and its encrypted version(ciphertext) thereby allowing him to use both to reveal further secret information like the key A Chosen- plaintext attack involves the cryptanalyst be able to define his own plaintext, feed it into the cipher and analyze the resulting ciphertext. A Chosen-ciphertext attack is one, where attacker has several pairs of plaintext-ciphertext and ciphertext chosen by the attacker. An encryption scheme is unconditionally secure if the ciphertext generated by the scheme does not contain enough information to determine uniquely the corresponding plaintext, no matter how much ciphertext and time is available to the opponent. Example for this type is One-time Pad. An encryption scheme is computationally secure if the ciphertext generated by the scheme meets the following criteria: Cost of breaking cipher exceeds the value of the encrypted information. Time required to break the cipher exceeds the useful lifetime of the information. The average time required for exhaustive key search is given below: Key Size Number of Time required at 1 Time required at (bits) Alternative Keys decryption/µs 106 decryptions/µs 32 232 56 256 128 2128 = 4.3 = 7.2 = 3.4 109 1016 1038 231 µs = 35.8 minutes 255 µs = 1142 years 2127 µs = 5.4 1024 years 2.15 milliseconds 10.01 hours 5.4 1018 years 21 jntuworldupdates.org Specworld.in

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Smartzworld.com Smartworld.asia 168 2168 = 3.7 1050 2167 µs = 5.9 1036 years 5.9 1030 years Feistel Cipher Structure Most symmetric block ciphers are based on a Feistel Cipher Structure. It was first described by Horst Feistel of IBM in 1973 and is still forms the basis for almost all conventional encryption schemes. It makes use of two properties namely diffusion and confusion; identified by Claude Shannon for frustrating statistical cryptanalysis. Confusion is basically defined as the concealment of the relation between the secret key and the cipher text. On the other hand, diffusion is regarded as the complexity of the relationship between the plain text and the cipher text. 22 jntuworldupdates.org Specworld.in