99 lines
2.3 KiB
Go
99 lines
2.3 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package rand
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/* TODO(xsw):
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import (
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"errors"
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"io"
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"math/big"
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)
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// Prime returns a number of the given bit length that is prime with high probability.
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// Prime will return error for any error returned by rand.Read or if bits < 2.
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func Prime(rand io.Reader, bits int) (*big.Int, error) {
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if bits < 2 {
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return nil, errors.New("crypto/rand: prime size must be at least 2-bit")
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}
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b := uint(bits % 8)
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if b == 0 {
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b = 8
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}
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bytes := make([]byte, (bits+7)/8)
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p := new(big.Int)
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for {
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if _, err := io.ReadFull(rand, bytes); err != nil {
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return nil, err
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}
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// Clear bits in the first byte to make sure the candidate has a size <= bits.
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bytes[0] &= uint8(int(1<<b) - 1)
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// Don't let the value be too small, i.e, set the most significant two bits.
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// Setting the top two bits, rather than just the top bit,
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// means that when two of these values are multiplied together,
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// the result isn't ever one bit short.
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if b >= 2 {
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bytes[0] |= 3 << (b - 2)
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} else {
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// Here b==1, because b cannot be zero.
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bytes[0] |= 1
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if len(bytes) > 1 {
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bytes[1] |= 0x80
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}
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}
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// Make the value odd since an even number this large certainly isn't prime.
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bytes[len(bytes)-1] |= 1
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p.SetBytes(bytes)
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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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// Int returns a uniform random value in [0, max). It panics if max <= 0.
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func Int(rand io.Reader, max *big.Int) (n *big.Int, err error) {
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if max.Sign() <= 0 {
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panic("crypto/rand: argument to Int is <= 0")
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}
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n = new(big.Int)
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n.Sub(max, n.SetUint64(1))
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// bitLen is the maximum bit length needed to encode a value < max.
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bitLen := n.BitLen()
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if bitLen == 0 {
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// the only valid result is 0
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return
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}
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// k is the maximum byte length needed to encode a value < max.
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k := (bitLen + 7) / 8
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// b is the number of bits in the most significant byte of max-1.
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b := uint(bitLen % 8)
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if b == 0 {
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b = 8
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}
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bytes := make([]byte, k)
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for {
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_, err = io.ReadFull(rand, bytes)
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if err != nil {
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return nil, err
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}
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// Clear bits in the first byte to increase the probability
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// that the candidate is < max.
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bytes[0] &= uint8(int(1<<b) - 1)
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n.SetBytes(bytes)
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if n.Cmp(max) < 0 {
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return
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}
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}
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}
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*/
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