passgen.go
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passgen.go
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	// Package passgen generates random, unmemorable passwords.
	//
	// Example usage:
	//
	// p := passgen.New() // p is "6NX(W`GD]4:Tqk};Y@A-"
	//
	// Logic originally from dchest's uniuri library:
	// https://github.com/dchest/uniuri
	//
	// Functions read from crypto/rand random source, and panic if they fail to
	// read from it.
	package passgen

	import "crypto/rand"

	// DefLen is the default password length returned.
	const DefLen = 20

	// DefChars is the default set of characters used in the password.
	var DefChars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*()- _=+,.?/:;{}[]`~")

	// New returns a random password of the default length with the default set of
	// characters.
	func New() string {
	return NewLenChars(DefLen, DefChars)
	}

	// NewLen returns a random password of the given length with the default set
	// of characters.
	func NewLen(length int) string {
	return NewLenChars(length, DefChars)
	}

	// NewLenChars returns a random password of the given length with the given
	// set of characters.
	func NewLenChars(length int, chars []byte) string {
	if length == 0 {
	return ""
	}
	clen := len(chars)
	maxrb := 255 - (256 % clen)
	b := make([]byte, length)
	r := make([]byte, length+(length/4)) // storage for random bytes.
	i := 0
	for {
	if _, err := rand.Read(r); err != nil {
	panic("passgen: error reading random bytes: " + err.Error())
	}
	for _, rb := range r {
	c := int(rb)
	if c > maxrb {
	// Skip this number to avoid modulo bias.
	continue
	}
	b[i] = chars[c%clen]
	i++
	if i == length {
	return string(b)
	}
	}
	}
	}