45 lines
788 B
Plaintext
45 lines
788 B
Plaintext
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# recursive routine to calculate c^d%n
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# by reaking it down. Works only where
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# d is a power of 2.
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#
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define x ( c,d,n) {
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#-1; d; -1;
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if ( d == 1 ) return (c^d%n);
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return x(c^2,d/2,n)%n;
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}
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r = 1;
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# recursive routine to break d into powers of 2
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# and then to collect multiply the results
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# of the 2^n exponents.
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#
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define t(c,d,n) {
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auto i
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i = 0;
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# find biggest power of 2 left in exponentiation
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#
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while(d >= (2^i)) { i = i+1; }
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#
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# subtract from exponent
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#
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d = (d - (2^(i-1)));
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if (d>0) {
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r=(r*x(c,2^(i-1),n))%n; # calculate this large exponentiation
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t(c,d,n); # recurse with new d value
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return (2^(i-1));
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}
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if (d == 0) {
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r = (r*x(c,2^(i-1),n))%n; # last one
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return (2^(i-1));
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}
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}
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