Merge pull request #75 from wneessen/fix/74-messageid-generation-not-random-enough

Fixed SetMessageID message ID generation

msg.go

@@ -12,7 +12,6 @@ import (
 	"fmt"
 	ht "html/template"
 	"io"
-	"math/rand"
 	"mime"
 	"net/mail"
 	"os"
@@ -357,12 +356,11 @@ func (m *Msg) SetMessageID() {
 	if err != nil {
 		hn = "localhost.localdomain"
 	}
-	ct := time.Now().Unix()
-	r := rand.New(rand.NewSource(ct))
-	rn := r.Int()
-	pid := os.Getpid()
-
-	mid := fmt.Sprintf("%d.%d.%d@%s", pid, rn, ct, hn)
+	rn, _ := randNum(100000000)
+	rm, _ := randNum(10000)
+	rs, _ := randomStringSecure(25)
+	pid := os.Getpid() * rm
+	mid := fmt.Sprintf("%d.%d%d.%s@%s", pid, rn, rm, rs, hn)
 	m.SetMessageIDWithValue(mid)
 }
 

msg_test.go

@@ -695,6 +695,26 @@ func TestMsg_SetMessageIDWithValue(t *testing.T) {
 	}
 }
 
+// TestMsg_SetMessageIDRandomness tests the randomness of Msg.SetMessageID methods
+func TestMsg_SetMessageIDRandomness(t *testing.T) {
+	var mids []string
+	for i := 0; i < 100; i++ {
+		m := NewMsg()
+		m.SetMessageID()
+		mid := m.GetGenHeader(HeaderMessageID)
+		mids = append(mids, mid[0])
+	}
+	c := make(map[string]int)
+	for i := range mids {
+		c[mids[i]]++
+	}
+	for k, v := range c {
+		if v > 1 {
+			t.Errorf("MessageID randomness not given. MessageID %q was generated %d times", k, v)
+		}
+	}
+}
+
 // TestMsg_FromFormat tests the FromFormat and EnvelopeFrom methods for the Msg object
 func TestMsg_FromFormat(t *testing.T) {
 	tests := []struct {

random.go

@@ -0,0 +1,75 @@
+// SPDX-FileCopyrightText: 2022 Winni Neessen <winni@neessen.dev>
+//
+// SPDX-License-Identifier: MIT
+
+package mail
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"fmt"
+	"math/big"
+	"strings"
+)
+
+// Range of characters for the secure string generation
+const cr = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
+
+// Bitmask sizes for the string generators (based on 93 chars total)
+const (
+	letterIdxBits = 7                    // 7 bits to represent a letter index
+	letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
+	letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
+)
+
+// randomStringSecure returns a random, n long string of characters. The character set is based
+// on the s (special chars) and h (human readable) boolean arguments. This method uses the
+// crypto/random package and therfore is cryptographically secure
+func randomStringSecure(n int) (string, error) {
+	rs := strings.Builder{}
+	rs.Grow(n)
+	crl := len(cr)
+
+	rp := make([]byte, 8)
+	_, err := rand.Read(rp)
+	if err != nil {
+		return rs.String(), err
+	}
+	for i, c, r := n-1, binary.BigEndian.Uint64(rp), letterIdxMax; i >= 0; {
+		if r == 0 {
+			_, err := rand.Read(rp)
+			if err != nil {
+				return rs.String(), err
+			}
+			c, r = binary.BigEndian.Uint64(rp), letterIdxMax
+		}
+		if idx := int(c & letterIdxMask); idx < crl {
+			rs.WriteByte(cr[idx])
+			i--
+		}
+		c >>= letterIdxBits
+		r--
+	}
+
+	return rs.String(), nil
+}
+
+// randNum returns a random number with a maximum value of n
+func randNum(n int) (int, error) {
+	if n <= 0 {
+		return 0, fmt.Errorf("provided number is <= 0: %d", n)
+	}
+	mbi := big.NewInt(int64(n))
+	if !mbi.IsUint64() {
+		return 0, fmt.Errorf("big.NewInt() generation returned negative value: %d", mbi)
+	}
+	rn64, err := rand.Int(rand.Reader, mbi)
+	if err != nil {
+		return 0, err
+	}
+	rn := int(rn64.Int64())
+	if rn < 0 {
+		return 0, fmt.Errorf("generated random number does not fit as int64: %d", rn64)
+	}
+	return rn, nil
+}

random_test.go

@@ -0,0 +1,70 @@
+// SPDX-FileCopyrightText: 2022 Winni Neessen <winni@neessen.dev>
+//
+// SPDX-License-Identifier: MIT
+
+package mail
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestRandomStringSecure tests the randomStringSecure method
+func TestRandomStringSecure(t *testing.T) {
+	tt := []struct {
+		testName     string
+		length       int
+		mustNotMatch string
+	}{
+		{"20 chars", 20, "'"},
+		{"100 chars", 100, "'"},
+		{"1000 chars", 1000, "'"},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.testName, func(t *testing.T) {
+			rs, err := randomStringSecure(tc.length)
+			if err != nil {
+				t.Errorf("random string generation failed: %s", err)
+			}
+			if strings.Contains(rs, tc.mustNotMatch) {
+				t.Errorf("random string contains unexpected character. got: %s, not-expected: %s",
+					rs, tc.mustNotMatch)
+			}
+			if len(rs) != tc.length {
+				t.Errorf("random string length does not match. expected: %d, got: %d", tc.length, len(rs))
+			}
+		})
+	}
+}
+
+// TestRandomNum tests the randomNum method
+func TestRandomNum(t *testing.T) {
+	tt := []struct {
+		testName string
+		max      int
+	}{
+		{"Max: 1", 1},
+		{"Max: 20", 20},
+		{"Max: 50", 50},
+		{"Max: 100", 100},
+		{"Max: 1000", 1000},
+		{"Max: 10000", 10000},
+		{"Max: 100000000", 100000000},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.testName, func(t *testing.T) {
+			rn, err := randNum(tc.max)
+			if err != nil {
+				t.Errorf("random number generation failed: %s", err)
+			}
+			if rn < 0 {
+				t.Errorf("random number generation failed: %d is smaller than zero", rn)
+			}
+			if rn > tc.max {
+				t.Errorf("random number generation failed: %d is bigger than given value %d", rn, tc.max)
+			}
+		})
+	}
+}