Cryptography: A Very Short Introduction (Very Short by Fred Piper, Sean Murphy

By Fred Piper, Sean Murphy

This e-book is a transparent and informative creation to cryptography and knowledge protection--subjects of substantial social and political value. It explains what algorithms do, how they're used, the dangers linked to utilizing them, and why governments could be involved. vital parts are highlighted, resembling move Ciphers, block ciphers, public key algorithms, electronic signatures, and functions reminiscent of e-commerce. This ebook highlights the explosive influence of cryptography on glossy society, with, for instance, the evolution of the web and the creation of extra subtle banking equipment.

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Example text

For instance, if f (x) = 4x4 + 6x3 − 5x2 + x − 1 is a polynomial with real coefficients, then f (x) = 16x3 + 18x2 − 10x + 1. 23) 28 • • 1 A round-up on numbers linearity: D(f (x) + g(x)) = D(f (x)) + D(g(x)), for each pair of polynomials f (x), g(x) ∈ K[x]; Leibniz’s law : D(f (x) · g(x)) = D(f (x)) · g(x) + D(g(x)) · f (x), for each pair of polynomials f (x), g(x) ∈ K[x]. The derivative composed with itself h times, with h ≥ 2, applied to a polynomial f (x), is denoted by the symbol f (h) (x), or D(h) (f (x)), and is called the h-th derivative of f (x).

The set of all polynomials with coefficients in A is denoted by A[x]. In A[x] two operation are defined: an addition and a multiplication, as follows. Let p(x) = n m i j i=0 ai x and q(x) = j=0 bj x in A[x] be two polynomials, with n ≤ m. We put m def p(x) + q(x) = n+m (ah + bh )xh , h=0 def a i bj x h . p(x)q(x) = h=0 i+j=h The zero polynomials is the identity element for addition, and the opposite (or i additive inverse) of the polynomial p(x) = n i=0 ai x is the polynomial having as its coefficients the opposite of the coefficients ai , for each i.

H are distinct roots of f (x), Ruffini’s theorem implies that f (x) is divisible by the polynomial (x − α1 ) · · · (x − αh ) of degree h and so n ≥ h. Given a polynomial f (x) ∈ K[x], we may consider the function ϕf : x ∈ K → f (x) ∈ K, called the polynomial function determined by f (x). 21. If K is infinite, and if f (x), g(x) are polynomials on K, then f (x) = g(x) if and only if ϕf = ϕg . Proof. We have ϕf = ϕg if and only if each α ∈ K is a root of the polynomial f (x) − g(x). 20 this happens if and only if f (x) − g(x) = 0 and so if and only if f (x) = g(x).

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