mirror of https://github.com/cheat/cheat.git
35 lines
954 B
Go
35 lines
954 B
Go
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package math
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import (
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"crypto/rand"
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"io"
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"math/big"
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)
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// IsSafePrime reports whether p is (probably) a safe prime.
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// The prime p=2*q+1 is safe prime if both p and q are primes.
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// Note that ProbablyPrime is not suitable for judging primes
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// that an adversary may have crafted to fool the test.
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func IsSafePrime(p *big.Int) bool {
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pdiv2 := new(big.Int).Rsh(p, 1)
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return p.ProbablyPrime(20) && pdiv2.ProbablyPrime(20)
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}
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// SafePrime returns a number of the given bit length that is a safe prime with high probability.
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// The number returned p=2*q+1 is a safe prime if both p and q are primes.
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// SafePrime will return error for any error returned by rand.Read or if bits < 2.
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func SafePrime(random io.Reader, bits int) (*big.Int, error) {
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one := big.NewInt(1)
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p := new(big.Int)
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for {
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q, err := rand.Prime(random, bits-1)
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if err != nil {
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return nil, err
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}
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p.Lsh(q, 1).Add(p, one)
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if p.ProbablyPrime(20) {
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return p, nil
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}
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}
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}
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