Add passgen package

Generates random passwords.

passgen/passgen.go

@@ -0,0 +1,62 @@
+// 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)
+			}
+		}
+	}
+}