Add XKCD style password generator

Signed-off-by: Knut Ahlers <knut@ahlers.me>

Makefile

@@ -10,6 +10,7 @@ debug: compile_coffee
 
 pack: compile_coffee
 	go-bindata frontend/...
+	bash generateXKCDWordList.sh
 
 publish:
 	curl -sSLo golang.sh https://raw.githubusercontent.com/Luzifer/github-publish/master/golang.sh

generateXKCDWordList.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+set -eu
+set -o pipefail
+
+VERSION="8744120d"
+SOURCE="https://cdn.rawgit.com/leonardr/olipy/${VERSION}/data/more-corpora/scribblenauts_words.txt"
+
+WORDS=$(curl -sL "${SOURCE}" | awk '/^[a-z]{4}[a-z]*$/{ print "\""$1"\"," }')
+
+cat -s <<EOF > lib/xkcd_words.go
+package securepassword
+
+// xkcdWordList contains a list of words derived from the scribblenauts
+// word list inside the olipy library by leonardr
+// https://github.com/leonardr/olipy
+var xkcdWordList = []string{
+  ${WORDS}
+}
+EOF
+
+gofmt -s -w lib/xkcd_words.go

lib/xkcd.go

@@ -0,0 +1,56 @@
+package securepassword
+
+import (
+	"errors"
+	"math/rand"
+	"strings"
+	"time"
+
+	"github.com/Luzifer/go_helpers/str"
+)
+
+type XKCD struct{}
+
+var (
+	// ErrTooFewWords represents an error thrown if the password will
+	// have fewer than four words and are not considered to be safe
+	ErrTooFewWords = errors.New("XKCD passwords with less than 4 words makes no sense")
+	// DefaultXKCD contains an default instance of the XKCD password
+	// generator
+	DefaultXKCD = NewXKCDGenerator()
+)
+
+// NewXKCDGenerator initializes a new XKCD password generator
+// https://xkcd.com/936/
+func NewXKCDGenerator() *XKCD { return &XKCD{} }
+
+// GeneratePassword generates a password with the number of words
+// given and optionally the current date prepended
+func (x XKCD) GeneratePassword(length int, addDate bool) (string, error) {
+	if length < 4 {
+		return "", ErrTooFewWords
+	}
+
+	var (
+		password  string
+		usedWords []string
+	)
+
+	if addDate {
+		password = time.Now().Format("20060102.")
+	}
+
+	rand.Seed(time.Now().UnixNano())
+	for len(usedWords) < length {
+		word := strings.Title(xkcdWordList[rand.Intn(len(xkcdWordList))])
+		if str.StringInSlice(word, usedWords) {
+			// Don't use a word twice
+			continue
+		}
+
+		password = password + word
+		usedWords = append(usedWords, word)
+	}
+
+	return password, nil
+}

lib/xkcd_test.go

@@ -0,0 +1,50 @@
+package securepassword
+
+import (
+	"fmt"
+	"regexp"
+	"testing"
+)
+
+func TestXKCDWordList(t *testing.T) {
+	if w := len(xkcdWordList); w < 1000 {
+		t.Fatalf("Word list is expected to contain at least 1000 words, has %d", w)
+	}
+}
+
+func TestXKCDGeneratePassword(t *testing.T) {
+	for i := 4; i < 20; i++ {
+		pwd, err := DefaultXKCD.GeneratePassword(i, false)
+
+		if err != nil {
+			t.Fatalf("Generated had an error: %s", err)
+		}
+
+		if !regexp.MustCompile(fmt.Sprintf("^([A-Z][a-z]+){%d}$", i)).MatchString(pwd) {
+			t.Errorf("Password %q is expected to contain %d words, did not match expected RegEx", pwd, i)
+		}
+	}
+}
+
+func TestXKCDDatePrepend(t *testing.T) {
+	pwd, err := DefaultXKCD.GeneratePassword(4, true)
+	if err != nil {
+		t.Fatalf("Generated had an error: %s", err)
+	}
+
+	if !regexp.MustCompile(`^[0-9]{8}\.([A-Z][a-z]+){4}$`).MatchString(pwd) {
+		t.Errorf("Password %q did not match expected RegEx", pwd)
+	}
+}
+
+func BenchmarkGeneratePasswords4Words(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		DefaultXKCD.GeneratePassword(4, false)
+	}
+}
+
+func BenchmarkGeneratePasswords20Words(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		DefaultXKCD.GeneratePassword(20, false)
+	}
+}