From string validation (#75)

* chore: address linter issues

Perform changes suggested by linter; no functional changes.

* error.go: make ErrInvalidID a const

Make ErrInvalidID a constant instead of a variable. This prevent it from
being changed by external packages; a behavior that although allowed by 
the compiler, should probably be considered an invalid operation.

* add benchmark and new failing test for FromString

* fix: let decode look for additional base32 padding

Update FromString and XID.TextUnmarshal so that it looks for discarded
bits in the final source character. This ensures that XIDs that have
been manually tampered with in a way that's ignored by base32 decode,
will not pass as valid.

error.go

@@ -0,0 +1,11 @@
+package xid
+
+const (
+	// ErrInvalidID is returned when trying to unmarshal an invalid ID.
+	ErrInvalidID strErr = "xid: invalid ID"
+)
+
+// strErr allows declaring errors as constants.
+type strErr string
+
+func (err strErr) Error() string { return string(err) }

id.go

@@ -47,7 +47,6 @@ import (
 	"crypto/rand"
 	"database/sql/driver"
 	"encoding/binary"
-	"errors"
 	"fmt"
 	"hash/crc32"
 	"io/ioutil"
@@ -73,9 +72,6 @@ const (
 )
 
 var (
-	// ErrInvalidID is returned when trying to unmarshal an invalid ID
-	ErrInvalidID = errors.New("xid: invalid ID")
-
 	// objectIDCounter is atomically incremented when generating a new ObjectId
 	// using NewObjectId() function. It's used as a counter part of an id.
 	// This id is initialized with a random value.
@@ -242,7 +238,9 @@ func (id *ID) UnmarshalText(text []byte) error {
 			return ErrInvalidID
 		}
 	}
-	decode(id, text)
+	if !decode(id, text) {
+		return ErrInvalidID
+	}
 	return nil
 }
 
@@ -260,8 +258,8 @@ func (id *ID) UnmarshalJSON(b []byte) error {
 	return id.UnmarshalText(b[1 : len(b)-1])
 }
 
-// decode by unrolling the stdlib base32 algorithm + removing all safe checks
-func decode(id *ID, src []byte) {
+// decode by unrolling the stdlib base32 algorithm + customized safe check.
+func decode(id *ID, src []byte) bool {
 	_ = src[19]
 	_ = id[11]
 
@@ -277,6 +275,16 @@ func decode(id *ID, src []byte) {
 	id[2] = dec[src[3]]<<4 | dec[src[4]]>>1
 	id[1] = dec[src[1]]<<6 | dec[src[2]]<<1 | dec[src[3]]>>4
 	id[0] = dec[src[0]]<<3 | dec[src[1]]>>2
+
+	// Validate that there are no discarer bits (padding) in src that would
+	// cause the string-encoded id not to equal src.
+	var check [4]byte
+
+	check[3] = encoding[(id[11]<<4)&0x1F]
+	check[2] = encoding[(id[11]>>1)&0x1F]
+	check[1] = encoding[(id[11]>>6)&0x1F|(id[10]<<2)&0x1F]
+	check[0] = encoding[id[10]>>3]
+	return bytes.Equal([]byte(src[16:20]), check[:])
 }
 
 // Time returns the timestamp part of the id.

id_test.go

@@ -5,8 +5,10 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"math/rand"
 	"reflect"
 	"testing"
+	"testing/quick"
 	"time"
 )
 
@@ -19,21 +21,21 @@ type IDParts struct {
 }
 
 var IDs = []IDParts{
-	IDParts{
+	{
 		ID{0x4d, 0x88, 0xe1, 0x5b, 0x60, 0xf4, 0x86, 0xe4, 0x28, 0x41, 0x2d, 0xc9},
 		1300816219,
 		[]byte{0x60, 0xf4, 0x86},
 		0xe428,
 		4271561,
 	},
-	IDParts{
+	{
 		ID{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00},
 		0,
 		[]byte{0x00, 0x00, 0x00},
 		0x0000,
 		0,
 	},
-	IDParts{
+	{
 		ID{0x00, 0x00, 0x00, 0x00, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0x00, 0x00, 0x01},
 		0,
 		[]byte{0xaa, 0xbb, 0xcc},
@@ -252,6 +254,60 @@ func BenchmarkFromString(b *testing.B) {
 	})
 }
 
+func TestFromStringQuick(t *testing.T) {
+	f := func(id1 ID, c byte) bool {
+		s1 := id1.String()
+		for i := range s1 {
+			s2 := []byte(s1)
+			s2[i] = c
+			id2, err := FromString(string(s2))
+			if id1 == id2 && err == nil && c != s1[i] {
+				t.Logf("comparing XIDs:\na: %q\nb: %q (index %d changed to %c)", s1, s2, i, c)
+				return false
+			}
+		}
+		return true
+	}
+	err := quick.Check(f, &quick.Config{
+		Values: func(args []reflect.Value, r *rand.Rand) {
+			i := r.Intn(len(encoding))
+			args[0] = reflect.ValueOf(New())
+			args[1] = reflect.ValueOf(byte(encoding[i]))
+		},
+		MaxCount: 1000,
+	})
+	if err != nil {
+		t.Error(err)
+	}
+}
+
+func TestFromStringQuickInvalidChars(t *testing.T) {
+	f := func(id1 ID, c byte) bool {
+		s1 := id1.String()
+		for i := range s1 {
+			s2 := []byte(s1)
+			s2[i] = c
+			id2, err := FromString(string(s2))
+			if id1 == id2 && err == nil && c != s1[i] {
+				t.Logf("comparing XIDs:\na: %q\nb: %q (index %d changed to %c)", s1, s2, i, c)
+				return false
+			}
+		}
+		return true
+	}
+	err := quick.Check(f, &quick.Config{
+		Values: func(args []reflect.Value, r *rand.Rand) {
+			i := r.Intn(0xFF)
+			args[0] = reflect.ValueOf(New())
+			args[1] = reflect.ValueOf(byte(i))
+		},
+		MaxCount: 2000,
+	})
+	if err != nil {
+		t.Error(err)
+	}
+}
+
 // func BenchmarkUUIDv1(b *testing.B) {
 // 	b.RunParallel(func(pb *testing.PB) {
 // 		for pb.Next() {
@@ -329,7 +385,7 @@ func TestFromBytes_InvalidBytes(t *testing.T) {
 		{13, true},
 	}
 	for _, c := range cases {
-		b := make([]byte, c.length, c.length)
+		b := make([]byte, c.length)
 		_, err := FromBytes(b)
 		if got, want := err != nil, c.shouldFail; got != want {
 			t.Errorf("FromBytes() error got %v, want %v", got, want)