util.go

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package rand
     6  
     7  import (
     8  	"crypto/internal/randutil"
     9  	"errors"
    10  	"io"
    11  	"math/big"
    12  )
    13  
    14  // Prime returns a number of the given bit length that is prime with high probability.
    15  // Prime will return error for any error returned by [rand.Read] or if bits < 2.
    16  func Prime(rand io.Reader, bits int) (*big.Int, error) {
    17  	if bits < 2 {
    18  		return nil, errors.New("crypto/rand: prime size must be at least 2-bit")
    19  	}
    20  
    21  	randutil.MaybeReadByte(rand)
    22  
    23  	b := uint(bits % 8)
    24  	if b == 0 {
    25  		b = 8
    26  	}
    27  
    28  	bytes := make([]byte, (bits+7)/8)
    29  	p := new(big.Int)
    30  
    31  	for {
    32  		if _, err := io.ReadFull(rand, bytes); err != nil {
    33  			return nil, err
    34  		}
    35  
    36  		// Clear bits in the first byte to make sure the candidate has a size <= bits.
    37  		bytes[0] &= uint8(int(1<<b) - 1)
    38  		// Don't let the value be too small, i.e, set the most significant two bits.
    39  		// Setting the top two bits, rather than just the top bit,
    40  		// means that when two of these values are multiplied together,
    41  		// the result isn't ever one bit short.
    42  		if b >= 2 {
    43  			bytes[0] |= 3 << (b - 2)
    44  		} else {
    45  			// Here b==1, because b cannot be zero.
    46  			bytes[0] |= 1
    47  			if len(bytes) > 1 {
    48  				bytes[1] |= 0x80
    49  			}
    50  		}
    51  		// Make the value odd since an even number this large certainly isn't prime.
    52  		bytes[len(bytes)-1] |= 1
    53  
    54  		p.SetBytes(bytes)
    55  		if p.ProbablyPrime(20) {
    56  			return p, nil
    57  		}
    58  	}
    59  }
    60  
    61  // Int returns a uniform random value in [0, max). It panics if max <= 0.
    62  func Int(rand io.Reader, max *big.Int) (n *big.Int, err error) {
    63  	if max.Sign() <= 0 {
    64  		panic("crypto/rand: argument to Int is <= 0")
    65  	}
    66  	n = new(big.Int)
    67  	n.Sub(max, n.SetUint64(1))
    68  	// bitLen is the maximum bit length needed to encode a value < max.
    69  	bitLen := n.BitLen()
    70  	if bitLen == 0 {
    71  		// the only valid result is 0
    72  		return
    73  	}
    74  	// k is the maximum byte length needed to encode a value < max.
    75  	k := (bitLen + 7) / 8
    76  	// b is the number of bits in the most significant byte of max-1.
    77  	b := uint(bitLen % 8)
    78  	if b == 0 {
    79  		b = 8
    80  	}
    81  
    82  	bytes := make([]byte, k)
    83  
    84  	for {
    85  		_, err = io.ReadFull(rand, bytes)
    86  		if err != nil {
    87  			return nil, err
    88  		}
    89  
    90  		// Clear bits in the first byte to increase the probability
    91  		// that the candidate is < max.
    92  		bytes[0] &= uint8(int(1<<b) - 1)
    93  
    94  		n.SetBytes(bytes)
    95  		if n.Cmp(max) < 0 {
    96  			return
    97  		}
    98  	}
    99  }
   100
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