# Lecture02 - Crypto Lecture#2 1 Hand out solution to class...

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Crypto Lecture #2 1. Hand out solution to class exercise Extended Euclidean Algorithm Euclid’s algorithm says to write out the following: a = q 1 b + r 1 , where 0 < r < b b = q 2 r 1 + r 2 , where 0 < r 2 < r 1 r 1 = q 3 r 2 + r 3 , where 0 < r 3 < r 2 . . r i = q i+2 r i+1 + r i+2 , where 0 < r i+2 < r i+1 . . r k-1 = q k+1 r k Euclid’s algorithm says that the GCD(a,b) = r k This might make more sense if we look at an example: Consider computing GCD(125, 87) 125 = 1*87 + 38 87 = 2*38 + 11 38 = 3*11 + 5 11 = 2*5 + 1 5 = 5*1 Thus, we find that GCD(125,87) = 1. Let’s look at one more quickly, GCD(125, 20) 125 = 6*20 + 5 20 = 4*5, thus, the GCD(125,20) = 5 Proof That Euclid’s Algorithm Works Now, we should prove that this algorithm really does always give us the GCD of the two numbers “passed to it”. First I will show that the number the algorithm produces is indeed a divisor of a and b. a = q 1 b + r 1 , where 0 < r < b b = q 2 r 1 + r 2 , where 0 < r 2 < r 1 r 1 = q 3 r 2 + r 3 , where 0 < r 3 < r 2 . . r i = q i+2 r i+1 + r i+2 , where 0 < r i+2 < r i+1 . . r k-1 = q k+1 r k From the last equation, we know that r k | r k-1 . So, we know that we can express r k-1 = cr k , where c is an integer. Now consider the previous equation:

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r k-2 = q k r k-1 + r k = q k cr k , + r k = r k (q k c + 1) Thus, we have that r k | r k-2 . In our equation previous to that one, we have: r k-3 = q k-1 r k-2 + r k-1 From here , since r k | r k-1 and r k | r k-2 , using our rules of divisibility we have that r k | r k-3 . As you can see, we can continue this process, considering each previous equation until we get to the last two, where we will
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Lecture02 - Crypto Lecture#2 1 Hand out solution to class...

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