Handle base64 decoding correctly to avoid panic (#26483)

Fix the panic if the "base64 secret" is too long.

modules/util/util.go

@@ -6,6 +6,7 @@ package util
 import (
 	"bytes"
 	"crypto/rand"
+	"encoding/base64"
 	"fmt"
 	"math/big"
 	"strconv"
@@ -261,3 +262,13 @@ func ToFloat64(number any) (float64, error) {
 func ToPointer[T any](val T) *T {
 	return &val
 }
+
+func Base64FixedDecode(encoding *base64.Encoding, src []byte, length int) ([]byte, error) {
+	decoded := make([]byte, encoding.DecodedLen(len(src))+3)
+	if n, err := encoding.Decode(decoded, src); err != nil {
+		return nil, err
+	} else if n != length {
+		return nil, fmt.Errorf("invalid base64 decoded length: %d, expects: %d", n, length)
+	}
+	return decoded[:length], nil
+}

modules/util/util_test.go

@@ -4,6 +4,7 @@
 package util
 
 import (
+	"encoding/base64"
 	"regexp"
 	"strings"
 	"testing"
@@ -233,3 +234,16 @@ func TestToPointer(t *testing.T) {
 	val123 := 123
 	assert.False(t, &val123 == ToPointer(val123))
 }
+
+func TestBase64FixedDecode(t *testing.T) {
+	_, err := Base64FixedDecode(base64.RawURLEncoding, []byte("abcd"), 32)
+	assert.ErrorContains(t, err, "invalid base64 decoded length")
+	_, err = Base64FixedDecode(base64.RawURLEncoding, []byte(strings.Repeat("a", 64)), 32)
+	assert.ErrorContains(t, err, "invalid base64 decoded length")
+
+	str32 := strings.Repeat("x", 32)
+	encoded32 := base64.RawURLEncoding.EncodeToString([]byte(str32))
+	decoded32, err := Base64FixedDecode(base64.RawURLEncoding, []byte(encoded32), 32)
+	assert.NoError(t, err)
+	assert.Equal(t, str32, string(decoded32))
+}