Cryptography is a vital technology that underpins the security of information in computer networks. This book presents a comprehensive introduction to the role that cryptography plays in providing information security for technologies such as the Internet, mobile phones, payment cards, and wireless local area networks. Focusing on the fundamental principles that ground modern cryptography as they arise in modern applications, it avoids both an over-reliance on transient current technologies and over-whelming theoretical research.

are redundant and remove them to recover NATTERIACKTOAD. A knowledge of European amphibians should then allow the correct plaintext to be obtained! 61 2 HISTORICAL CRYPTOSYSTEMS CRYPTANALYSIS OF THE PLAYFAIR CIPHER Is the Playfair Cipher a secure cryptosystem? The ﬁrst thing to check is the size of the keyspace. The key is an arrangement of the letters A to Z (excluding J) in a square. Since every arrangement of these letters corresponds to a different key, the number of possible keys is the

information about the value of the PIN. On the other hand, had a symmetric encryption algorithm with just 50 keys been used, then, on seeing the ciphertext, the interceptor would be able to deduce that the PIN was one of 50 different possible PINs simply by trying out the 50 possible decryption keys. In this case, by seeing the ciphertext, the interceptor would have learnt some useful information about the plaintext. 77 3 THEORETICAL VERSUS PRACTICAL SECURITY • If a cryptosystem has perfect

1. The notation EK (data) means the ciphertext created by encrypting the data using key K. The cryptosystem works as follows: 1. Investor and broker agree on a randomly chosen key in advance (either K1 or K2 ). 2. Once the investor makes his investment decision, he looks up Table 3.1 and reads the ciphertext in the row corresponding to the chosen key and the column corresponding to the chosen decision. For example, if the key is K1 and the investor is selling then the ciphertext selected will be

attacks on DES other than exhaustive key search. There have been some academic breaks, involving techniques known as differential and linear cryptanalysis, but these attacks have not threatened DES in practice. 120 4.4 THE DATA ENCRYPTION STANDARD DES has, however, been subjected to several different design criticisms over the years: Secret design criteria. Although the full description of DES, including the round function and key schedule, was published, their design criteria (in other

exponentiation, it is regarded as an easy operation. However, given the result of a modular exponentiation, and only the numbers a and n (where n is a prime), calculating the number b that was used to compute the modular exponentiation is believed to be a hard problem, assuming that n is large. This difﬁcult problem is often referred to as the discrete logarithm problem. In other words, given numbers a and n (where n is prime), the function f (b) = ab mod n, is believed to be a one-way function,