code: function(s1, s2) {
      var n = s1.length;
      if (s1.length > s2.length) n = s2.length;
      for (var i = 0; i < n; i++) {
  	if (s1.charCodeAt(i) != s2.charCodeAt(i)) return i;
      }
      if (s1.length != s2.length) return n;
      return -1; // same
  }
docs: find index of string where two string differs
  @name strdiffidx
  @function
  @param {String} s1 string to compare
  @param {String} s2 string to compare
  @return {Number} string index of where character differs. Return -1 if same.
  @since jsrsasign 4.9.0 base64x 1.1.5
  @example
  strdiffidx("abcdefg", "abcd4fg") -> 4
  strdiffidx("abcdefg", "abcdefg") -> -1
  strdiffidx("abcdefg", "abcdef") -> 6
  strdiffidx("abcdefgh", "abcdef") -> 6