Fix secretBytes adding unintended padding (#556)

* Fix secretBytes adding unintended padding

* Add more SecretBytes test scenarios

* Add CHANGELOG entry about breaking secret padding change

* Add SecretBytes tests explanation comments

pkg/encryption/cipher.go

@@ -19,27 +19,22 @@ import (
 
 // SecretBytes attempts to base64 decode the secret, if that fails it treats the secret as binary
 func SecretBytes(secret string) []byte {
-	b, err := base64.URLEncoding.DecodeString(addPadding(secret))
+	b, err := base64.RawURLEncoding.DecodeString(strings.TrimRight(secret, "="))
 	if err == nil {
-		return []byte(addPadding(string(b)))
+		// Only return decoded form if a valid AES length
+		// Don't want unintentional decoding resulting in invalid lengths confusing a user
+		// that thought they used a 16, 24, 32 length string
+		for _, i := range []int{16, 24, 32} {
+			if len(b) == i {
+				return b
+			}
+		}
 	}
+	// If decoding didn't work or resulted in non-AES compliant length,
+	// assume the raw string was the intended secret
 	return []byte(secret)
 }
 
-func addPadding(secret string) string {
-	padding := len(secret) % 4
-	switch padding {
-	case 1:
-		return secret + "==="
-	case 2:
-		return secret + "=="
-	case 3:
-		return secret + "="
-	default:
-		return secret
-	}
-}
-
 // cookies are stored in a 3 part (value + timestamp + signature) to enforce that the values are as originally set.
 // additionally, the 'value' is encrypted so it's opaque to the browser
 

pkg/encryption/cipher_test.go

@@ -1,14 +1,87 @@
 package encryption
 
 import (
+	"crypto/rand"
 	"crypto/sha1"
 	"crypto/sha256"
 	"encoding/base64"
+	"fmt"
+	"io"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 )
 
+func TestSecretBytesEncoded(t *testing.T) {
+	for _, secretSize := range []int{16, 24, 32} {
+		t.Run(fmt.Sprintf("%d", secretSize), func(t *testing.T) {
+			secret := make([]byte, secretSize)
+			_, err := io.ReadFull(rand.Reader, secret)
+			assert.Equal(t, nil, err)
+
+			// We test both padded & raw Base64 to ensure we handle both
+			// potential user input routes for Base64
+			base64Padded := base64.URLEncoding.EncodeToString(secret)
+			sb := SecretBytes(base64Padded)
+			assert.Equal(t, secret, sb)
+			assert.Equal(t, len(sb), secretSize)
+
+			base64Raw := base64.RawURLEncoding.EncodeToString(secret)
+			sb = SecretBytes(base64Raw)
+			assert.Equal(t, secret, sb)
+			assert.Equal(t, len(sb), secretSize)
+		})
+	}
+}
+
+// A string that isn't intended as Base64 and still decodes (but to unintended length)
+// will return the original secret as bytes
+func TestSecretBytesEncodedWrongSize(t *testing.T) {
+	for _, secretSize := range []int{15, 20, 28, 33, 44} {
+		t.Run(fmt.Sprintf("%d", secretSize), func(t *testing.T) {
+			secret := make([]byte, secretSize)
+			_, err := io.ReadFull(rand.Reader, secret)
+			assert.Equal(t, nil, err)
+
+			// We test both padded & raw Base64 to ensure we handle both
+			// potential user input routes for Base64
+			base64Padded := base64.URLEncoding.EncodeToString(secret)
+			sb := SecretBytes(base64Padded)
+			assert.NotEqual(t, secret, sb)
+			assert.NotEqual(t, len(sb), secretSize)
+			// The given secret is returned as []byte
+			assert.Equal(t, base64Padded, string(sb))
+
+			base64Raw := base64.RawURLEncoding.EncodeToString(secret)
+			sb = SecretBytes(base64Raw)
+			assert.NotEqual(t, secret, sb)
+			assert.NotEqual(t, len(sb), secretSize)
+			// The given secret is returned as []byte
+			assert.Equal(t, base64Raw, string(sb))
+		})
+	}
+}
+
+func TestSecretBytesNonBase64(t *testing.T) {
+	trailer := "equals=========="
+	assert.Equal(t, trailer, string(SecretBytes(trailer)))
+
+	raw16 := "asdflkjhqwer)(*&"
+	sb16 := SecretBytes(raw16)
+	assert.Equal(t, raw16, string(sb16))
+	assert.Equal(t, 16, len(sb16))
+
+	raw24 := "asdflkjhqwer)(*&CJEN#$%^"
+	sb24 := SecretBytes(raw24)
+	assert.Equal(t, raw24, string(sb24))
+	assert.Equal(t, 24, len(sb24))
+
+	raw32 := "asdflkjhqwer)(*&1234lkjhqwer)(*&"
+	sb32 := SecretBytes(raw32)
+	assert.Equal(t, raw32, string(sb32))
+	assert.Equal(t, 32, len(sb32))
+}
+
 func TestSignAndValidate(t *testing.T) {
 	seed := "0123456789abcdef"
 	key := "cookie-name"