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# Source file src/crypto/rand/util.go

## Documentation: crypto/rand

```  // Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style

package rand

import (
"errors"
"io"
"math/big"
)

// smallPrimes is a list of small, prime numbers that allows us to rapidly
// exclude some fraction of composite candidates when searching for a random
// prime. This list is truncated at the point where smallPrimesProduct exceeds
// a uint64. It does not include two because we ensure that the candidates are
// odd by construction.
var smallPrimes = []uint8{
3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53,
}

// smallPrimesProduct is the product of the values in smallPrimes and allows us
// to reduce a candidate prime by this number and then determine whether it's
// coprime to all the elements of smallPrimes without further big.Int
// operations.
var smallPrimesProduct = new(big.Int).SetUint64(16294579238595022365)

// Prime returns a number, p, of the given size, such that p is prime
// with high probability.
// Prime will return error for any error returned by rand.Read or if bits < 2.
func Prime(rand io.Reader, bits int) (p *big.Int, err error) {
if bits < 2 {
err = errors.New("crypto/rand: prime size must be at least 2-bit")
return
}

b := uint(bits % 8)
if b == 0 {
b = 8
}

bytes := make([]byte, (bits+7)/8)
p = new(big.Int)

bigMod := new(big.Int)

for {
if err != nil {
return nil, err
}

// Clear bits in the first byte to make sure the candidate has a size <= bits.
bytes[0] &= uint8(int(1<<b) - 1)
// Don't let the value be too small, i.e, set the most significant two bits.
// Setting the top two bits, rather than just the top bit,
// means that when two of these values are multiplied together,
// the result isn't ever one bit short.
if b >= 2 {
bytes[0] |= 3 << (b - 2)
} else {
// Here b==1, because b cannot be zero.
bytes[0] |= 1
if len(bytes) > 1 {
bytes[1] |= 0x80
}
}
// Make the value odd since an even number this large certainly isn't prime.
bytes[len(bytes)-1] |= 1

p.SetBytes(bytes)

// Calculate the value mod the product of smallPrimes. If it's
// a multiple of any of these primes we add two until it isn't.
// The probability of overflowing is minimal and can be ignored
// because we still perform Miller-Rabin tests on the result.
bigMod.Mod(p, smallPrimesProduct)
mod := bigMod.Uint64()

NextDelta:
for delta := uint64(0); delta < 1<<20; delta += 2 {
m := mod + delta
for _, prime := range smallPrimes {
if m%uint64(prime) == 0 && (bits > 6 || m != uint64(prime)) {
continue NextDelta
}
}

if delta > 0 {
bigMod.SetUint64(delta)
}
break
}

// There is a tiny possibility that, by adding delta, we caused
// the number to be one bit too long. Thus we check BitLen
// here.
if p.ProbablyPrime(20) && p.BitLen() == bits {
return
}
}
}

// Int returns a uniform random value in [0, max). It panics if max <= 0.
func Int(rand io.Reader, max *big.Int) (n *big.Int, err error) {
if max.Sign() <= 0 {
panic("crypto/rand: argument to Int is <= 0")
}
n = new(big.Int)
n.Sub(max, n.SetUint64(1))
// bitLen is the maximum bit length needed to encode a value < max.
bitLen := n.BitLen()
if bitLen == 0 {
// the only valid result is 0
return
}
// k is the maximum byte length needed to encode a value < max.
k := (bitLen + 7) / 8
// b is the number of bits in the most significant byte of max-1.
b := uint(bitLen % 8)
if b == 0 {
b = 8
}

bytes := make([]byte, k)

for {
if err != nil {
return nil, err
}

// Clear bits in the first byte to increase the probability
// that the candidate is < max.
bytes[0] &= uint8(int(1<<b) - 1)

n.SetBytes(bytes)
if n.Cmp(max) < 0 {
return
}
}
}

```

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