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Encryption Decryption Interview Questions

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The ElGamal system is a public-key cryptosystem based on the discrete logarithm problem. It consists of both encryption and signature algorithms. The encryption algorithm is similar in nature to the Diffie-Hellman key agreement protocol. 

Elliptic curves are mathematical constructions from number theory and algebraic geometry, which in recent years have found numerous applications in cryptography.

An elliptic curve can be defined over any field (e.g., real, rational, complex). However, elliptic curves used in cryptography are mainly defined over finite fields. An elliptic curve consists of elements (x, y) satisfying the equation

y2 = x3 + ax + b

together with a single element denoted O called the "point at infinity," which can be visualized as the point at the top and bottom of every vertical line. Addition of two points on a elliptic curve is defined according to a set of simple rules (e.g., point p1 plus point p2 is equal to point -p3 in Figure 2). The addition operation in an elliptic curve is the counterpart to modular multiplication in common public-key cryptosystems, and multiple addition is the counterpart to modular exponentiation. 

Elliptic curve cryptosystems are analogs of public-key cryptosystems such as RSA and ElGamal, in which modular multiplication is replaced by the elliptic curve addition operation.

The curves used in elliptic curve analogs of discrete logarithm cryptosystems are normally of the form

y2 = x3 + ax + b (mod p),

where p is prime. The problem tapped by the discrete logarithm analogs in elliptic curves is the elliptic curve logarithm problem, defined as follows: given a point G on an elliptic curve with order r (number of points on the curve) and another point Y on the curve, find a unique x (0 x r - 1) such that Y = xG, i.e., Y is the xth multiple of G. 

The Merkle-Hellman knapsack cryptosystem is a public-key cryptosystem that was first published in 1978. It is commonly referred to as the knapsack cryptosystem. It is based on the subset sum problem in combinatorics. The problem involves selecting a number of objects with given weights from a large set such that the sum of the weights is equal to a pre-specified weight. This is considered to be a difficult problem to solve in general, but certain special cases of the problem are relatively easy to solve, which serve as the "trapdoor" of the system. The-single iteration knapsack cryptosystem introduced in 1978 was broken by Shamir. Merkle then published the multiple-iteration knapsack problem which was broken by Brickell [Bri85]. Merkle offered a $100 reward for anybody able to crack the single iteration knapsack and a $1000 reward for anybody able to crack the multiple iteration cipher from his own pocket. When they were cracked, he promptly paid up. 

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